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Gadolinium zirconate (GZO) with pyrochlore phase exhibits exceptional promise for thermal barrier coatings due to its notable low thermal conductivity. Current synthesis methods for GZO, however, are frequently complex, energy-intensive, and environmentally hazardous. A more sustainable and eco-friendly synthesis route for GZO is thus imperative. This research introduces a green synthesis process for pyrochlore GZO utilizing the molten salt method. Experimental synthesis at varying temperatures revealed changes in the phase composition of GZO via the molten salt method. Fluorite GZO formed at 1200 °C, while pyrochlore GZO appeared at 1400 °C. XRD refinement, TEM, and Raman spectroscopy confirmed these results. Density Functional Theory (DFT) calculations established the minimum theoretical synthesis temperature for pyrochlore GZO at 1370 °C. Limiting factors for reactions at different temperatures during the molten salt synthesis process were identified. DFT calculations and experiments verified that the fluorite structure GZO transitions to pyrochlore GZO at temperatures exceeding 1370 °C. Property tests on pyrochlore GZO samples synthesized through two pathways showed minimal differences. This study thus lays a theoretical foundation for the eco-friendly synthesis of pyrochlore GZO material using the molten salt method and offers insights for future process development. [ABSTRACT FROM AUTHOR]

Zirconolite and pyrochlore have been considered as the potential immobilization matrix materials of minor actinides due to their high solubility, radiation tolerance and chemical durability. In this work, a series of Ca 1-x Ho 2x Zr 1-x Ti 2 O 7 (0.1 ≤ x ≤ 1.0) ceramics were prepared by solid state synthesis and sintering at 1400 °C for 48 h twice. Powder X-ray Diffraction (XRD), Backscattered Scanning Electron Microscopy with Energy-Dispersive X-ray spectroscopy (BSEM-EDX), Transmission Electron Microscopy (TEM) were employed to investigate the phase evolution, solubility, and substitution mechanism of Ho. The results displayed that zirconolite-2M would be transformed to zirconolite-4M and then to pyrochlore with increasing Ho concentration. The initial phase transformation of zirconolite-2M to zirconolite-4M, subsequently zirconolite-4M to pyrochlore were observed in x = 0.2 and 0.4 samples, respectively. Zirconolite-4M could be synthesised at x = 0.38 sample with trace perovskite. Moreover, the semi-quantitively solubility limits of Ho in zirconolite-2M and zirconolite-4M were determined to be 27 and 41.5 at.%, respectively. Single-phase pyrochlore were fabricated after 70 at.%. In addition, the novel substitution mechanism was revealed by Rietveld refinement with Ho preferentially replacing the Ca of zirconolite-2M so that some replaced Ca occupied vacant Zr site. For pyrochlore, Ho, Ca and Zr occupied A site (16d) and Ti entered in B site (16c). The findings would provide more insights into zirconolite–pyrochlore ceramic system. [ABSTRACT FROM AUTHOR]

A single high-entropy pyrochlore-type compound (A 2 B 2 O 7) was successfully synthesized, incorporating 8 different rare-earth cations at the A site and 2 different metal cations at the B site in equiatomic amounts [(8A 1/8) 2 (2B 1/2) 2 O 7 ]. Powders with a nominal composition of (La 1/8 Sm 1/8 Nd 1/8 Pr 1/8 Y 1/8 Gd 1/8 Dy 1/8 Yb 1/8) 2 (Hf 1/2 Zr 1/2) 2 O 7 were fabricated by glycine nitrate procedure (GNP). The GNP process yielded powders with low crystallinity and after subsequent calcination, a well crystalline ceramic powders were formed. The phase evolution was initially analyzed using X-ray diffraction (XRD). In situ high-pressure X-ray diffraction was employed to investigate the structural stability and high-pressure behavior of the A 2 B 2 O 7 pyrochlore up to 25.8 GPa. Theoretical investigations of the high-entropy pyrochlore systems were performed and showed good agreement with the experimentally obtained results. [ABSTRACT FROM AUTHOR]

Pyrochlore-based high-entropy ceramics can encapsulate multiple elements with different valences, making them promising for immobilizing high-level radioactive nuclides in molten salt radwaste. This approach addresses key challenges in dry reprocessing and molten salt reactor waste management. In this study, a single-phase pyrochlore-based high-entropy ceramic is synthesized to incorporate NdF 3 and CeF 3 as simulated molten salt radwastes. Various synthesis components have been explored, and the ceramics were characterized using x-ray diffraction (XRD) and Raman spectroscopy. Comparative analysis revealed that the size disorder is the critical factor to synthesize single-phase pyrochlore-based high-entropy ceramics. The effects of elemental ratio and the presence of variable valence elements on the synthesis of single-phase pyrochlore-based high-entropy ceramics are elucidated through the concept of size disorder. Two single-phase pyrochlore compositions have been successfully synthesized: #HE4 (Nd 2 (Ti 0.25 Zr 0.25 Hf 0.25 Sn 0.25) 2 O 7) and #HECe0.1 (Nd 2 (Zr 0.3 Hf 0.2 Sn 0.2 Nb 0.2 Ce 0.1) 2 O 7). These samples were tested with a 7-day leaching experiment using the Product Consistency Test Method PCT-B. The normalized release rates of all elements in #HE4 ranged from 10−7 to 10−9 g m−2 d−1, indicating excellent chemical durability. These findings suggest that medium- and high-entropy pyrochlore-based ceramics are effective for immobilizing radioactive nuclides in molten salt radwastes. [ABSTRACT FROM AUTHOR]

A Gd 2 Hf 2 O 7 ceramic with pyrochlore structure has been prepared by the sol-gel and cold-pressing sintering, the corrosion behavior and mechanism of CFAS powder (33CaO:10FeO 1.5 :13AlO 1.5 :44SiO 2) on the Gd 2 Hf 2 O 7 ceramic at 1673 K was investigated. Results showed that the average grain size of Gd 2 Hf 2 O 7 ceramic was 3.37 μm. The CFAS melt rapidly flowed and diffused into the interior of Gd 2 Hf 2 O 7 ceramics and reacted with Gd 2 Hf 2 O 7 to generate high melting point rod-shaped Ca 2 Gd 8 (SiO 4) 6 O 2 apatite and Hf-Gd oxides, which can effectively hinder further flow and diffusion of CFAS melt, resulting in a slow increased in corrosion depth with corrosion time. The penetration depth of the Gd 2 Hf 2 O 7 ceramic wafer after 30 h of CFAS corrosion at 1673 K was only 337.7 μm, showing excellent resistance to CFAS corrosion at high temperatures. [ABSTRACT FROM AUTHOR]

The Pechini method was used for the first time to synthesize the phase-pure nickel-containing cubic pyrochlore Bi 2 NiNb 2 O 9 (space group Fd-3m, a = 10.5371(5) Å), while the solid-phase reaction method provided no possibility to obtain impurities-free pyrochlore. At a synthesis temperature of 950°С, low-porosity ceramics with indistinct grain boundaries were formed. The results of elemental mapping indicated a uniform distribution of metal atoms on the surface of the sample. The X-ray energy dispersive analysis showed that the chemical composition of the synthesized sample corresponded to the predetermined theoretical composition. The calcination of the sample synthesized by the Pechini method was studied by the thermal analysis up to 1100°С. The functional composition of intermediate synthesis products was analyzed by vibrational spectroscopy (FTIR). In the FTIR spectrum of pyrochlore (4000–400 cm−1), absorption bands were identified at 829, 540, 654, and 490 cm−1. The dielectric properties of the pyrochlore were studied using an impedance spectroscopy. Pyrochlore Bi 2 NiNb 2 O 9 has the high relative dielectric permittivity of 145 and low dielectric losses of 0.001 (at 24°С, 1 MHz), which make it a promising for dielectric in multilayer ceramic capacitors. [ABSTRACT FROM AUTHOR]

This study investigated the potential of A 2 Zr 2 O 7 pyrochlore phase oxides (A = La, Ce, Nd, and Gd) as solid oxide fuel cell (SOFC) electrolyte materials. A 2 Zr 2 O 7 ceramics were synthesized using the sol-gel process, and their disordered pyrochlore structure was confirmed by X-ray Diffraction and Rietveld refinement. Field Emission Scanning Electron Microscopy (FE-SEM) was used to examine the morphology of the materials, while X-Ray Photoelectron Spectroscopy (XPS) confirmed a high oxygen ion concentration. UV–Vis Diffuse Reflectance Spectroscopy (DRS) and density of states calculations consistently showed similar bandgap patterns in all zirconate pyrochlore. Density Functional Theory (DFT) simulations provided insights into the electronic structure and migration pathways. A two-probe conductivity study confirmed the relation between the ionic conductivity and ordering degree of pyrochlore. At 750 °C, LZO, CZO, and GZO exhibited ionic conductivity values of 5.44 × 10−3 S/cm, 3.00 × 10−3 S/cm, and 2.01 × 10−3 S/cm respectively, while NZO demonstrated a conductivity of 1.27 × 10−2 S/cm at 700 °C, which is higher than that of the other samples. This study favours the use of the sol-gel route for processing zirconate pyrochlore materials, highlighting its ability to achieve high densities at lower temperatures and exhibit enhanced ionic conductivity. A 2 Zr 2 O 7 has emerged as a promising oxide-ion conducting solid electrolyte for intermediate temperature SOFC applications within the 600–750 °C temperature range. [Display omitted] [ABSTRACT FROM AUTHOR]

Among different pollutants, organic dyes, phenolic compounds, and antibiotics are three types of hazardous contaminants that have become serious environmental issues. This work deals with the synthesis of new visible-light responsive CsM 0.25 W 1.75 O 6 (M = Ni, Co, Mn, Cu) compounds with a solid-state reaction induced defect pyrochlore structure followed by crushing in a ball mill. The prepared materials were characterized using scanning electron microscopy, X-ray diffraction with Rietveld refinement, X-ray photo-electron spectroscopy, differential thermal analysis, and nitrogen adsorption-desorption measurements at low-temperature. The optimized CsNi 0.25 W 1.75 O 6 (CsNiWO-2) compound was tested for its photocatalytic abatement efficient for several contaminants including methylene blue, phenol, as well as levofloxacin and isoniazid antibiotics. The influence of the various key aspects, such as photocatalyst dosage and hydrogen peroxide concentration, on the degradation efficiency of CsNiWO-2 was examined. The degradation efficiency of CsNiWO-2 (0.6 g L−1) towards methylene blue, phenol, levofloxacin, and isoniazid in the presence of H 2 O 2 (0.06 mol L−1) was 81.4, 47.9, 84.6, and 37.4 %, respectively. Thus, the developed material demonstrates promising properties to degrade methylene blue and levofloxacin. Also, the mechanism and comprehensive schemes for the photooxidation of methylene blue and levofloxacin were elucidated over CsNiWO-2. The present research potentially contributes to designing efficient defect pyrochlores for wastewater treatment under visible light irradiation. [Display omitted] • CsM 0.25 W 1.75 O 6 (M = Ni, Co, Mn, Cu) materials were prepared by a solid-state method. • First report of CsM 0.25 W 1.75 O 6 (M = Ni, Co, Mn, Cu) photodegradation activity. • Grinded CsNi 0.25 W 1.75 O 6 displayed highest activity in the degradation of MB and LFX. • Potential of CsNi 0.25 W 1.75 O 6 photocatalyst for wastewater treatment under LED light. [ABSTRACT FROM AUTHOR]

Pyrochlore oxides, specifically G d 2 Z r 2 O 7 , have been a subject of interest due to their unique structural, physical, and magnetic properties. These properties make pyrochlore oxides potential candidates for various technological applications, including magnetocaloric refrigeration. Magnetocaloric refrigeration is a promising alternative to conventional refrigeration methods, as it is more energy-efficient and environmentally friendly. By studying the magnetocaloric effect of pyrochlore oxides like G d 2 Z r 2 O 7 , researchers can gain a deeper understanding of the underlying mechanisms and potential applications of these materials in magnetocaloric refrigeration systems. By employing advanced computational methods such as Monte Carlo simulations and first-principles calculations based on the full-potential linearized augmented plane wave (FP-LAPW) a technique is employed to examine the magnetic structure, researchers can investigate the magnetocaloric effect of pyrochlore oxides with high accuracy and precision. This information has the potential to aid in the advancement of more effective magnetocaloric systems materials and ultimately lead to the design and optimization of advanced magnetocaloric refrigeration systems. [ABSTRACT FROM AUTHOR]

In this study, La 2 (TiZrSnHfGe) 2 O 7 high entropy oxides were firstly prepared by solid state reaction method. The effects of different sintering temperatures on the structure, morphology and dielectric properties were studied. When the pre-calcination temperature is 1300 °C, the powder exhibits a single pyrochlore phase instead of defective fluorite phase. The existence of high entropy affects the changes in the crystal structure of A 2 B 2 O 7 type oxides, resulting in lattice distortion. The dielectric results showed that the ceramics have certain frequency dispersion, and the dielectric properties are affected by temperature and frequency. Due to the high entropy effect, the La 2 (TiZrSnHfGe) 2 O 7 ceramic exhibits good dielectric stability at both high and low frequencies. The formation of defect dipole clusters in the system results in the increase of dielectric constant. It is feasible to change the dielectric properties of ceramics by designing high entropy oxides, which showed the potential value in microelectronics field. [ABSTRACT FROM AUTHOR]

Dry reforming with CO 2 is one potential method for tar removal and further production of syngas in biomass gasification, but the crucial catalysts with good reaction characteristics are still being developed. In the work, Ni/pyrochlore was proposed to be utilized as catalysts in the dry reforming of tar, and thermodynamic calculation, reaction progress test, and materials characterization were conducted to reveal dry reforming characteristics over Ni/pyrochlore catalysts and the reaction mechanism behind. Results show that YS 0.2 C catalyst exhibits the best activity (toluene and CO 2 conversion of 70.1% and 79.5%) and stability among all catalysts under the optimized conditions of 700 °C with a CO 2 /toluene molar ratio of 7. Analysis on physicochemical properties of the catalysts indicate Sr doping can effectively promote catalytic reforming by enhanced interaction between Ni and support, CO 2 adsorption and oxygen mobility. The work offers a promising catalyst type in improving tar removal by dry reforming. • Systematic study on pyrochlore supported Ni catalysts in the dry reforming of tar. • Ni/Y 1.8 Sr 0.2 Ce 2 O 7 exhibits superior activity and stability during dry reforming. • Reaction conditions were optimized to be at 700 °C with CO 2 /toluene = 7. • Sr doping contributes to the interaction, CO 2 adsorption and oxygen mobility. [ABSTRACT FROM AUTHOR]

A comprehensive study of the structural properties of the heavily investigated rare-earth A 2 Ir 2 O 7 iridate series under extreme conditions is presented. From Pr 2 Ir 2 O 7 to Lu 2 Ir 2 O 7 , the series is sufficiently covered by iridates with A = Pr, Sm, Dy, Ho, Er, Tm, Yb, and Lu; general trends and systematics within the series, including the understudied heavy-rare-earth members, are dependably followed. Temperature- and pressure-dependent synchrotron X-ray powder diffraction experiments reveal robustness of the pyrochlore structure throughout the series, down to 4 K and up to 20 GPa. The thermal expansivity of the pyrochlore lattice is determined, all falling in the Debye temperature range of θ D = 360–420 K. The pressure compressibility shows a systematic increase of the bulk modulus with the rare-earth atomic number from K = 180–210 GPa. Combining the results of thermal measurements (Debye temperature) and pressure measurements (bulk modulus) enables us to determine the Grüneisen parameter of selected members and compare it to previous studies. Temperature and pressure evolution of the fractional coordinate of oxygen at 48 f Wyckoff position, the sole free fractional coordinate in the crystal structure, is investigated and discussed regarding the antiferromagnetic ordering of the Ir magnetic moments. In addition to results on A 2 Ir 2 O 7 iridates, the temperature and pressure evolution of the crystal structure of an IrO 2 minority phase is followed. The tetragonal rutile-type structure is stable down to the lowest temperature. However, an application of pressure of approximately 15 GPa induces a structural transition: The tetragonal structure is orthorhombically distorted. The orthorhombic structure is still not fully stabilised at 20 GPa, and further distortion of the lattice (or subsequent structural transformations) is expected with increasing external pressure. [ABSTRACT FROM AUTHOR]

A new series of 20-component fluorite-based compositionally complex oxides (20CCFBO x Nb/Ta) with the general chemical formula (15RE 1/15) 2 x +1 (Ce 1/3 Zr 1/3 Hf 1/3) 3–3 x (Nb 1/2 Ta 1/2) x O 8- δ (0 ≤ x ≤ 1, where 15RE 1/15 = La 1/15 Pr 1/15 Nd 1/15 Sm 1/15 Eu 1/15 Gd 1/15 Tb 1/15 Dy 1/15 Y 1/15 Ho 1/15 Er 1/15 Tm 1/15 Yb 1/15 Lu 1/15 Sc 1/15) are synthesized. Despite that the Gibbs phase rule allows for the existence of up to 20 phases at the thermodynamic equilibrium, 17 of the 20CCFBO x Nb/Ta compositions synthesized in this study all possess single ultrahigh-entropy phases in fluorite, pyrochlore, or weberite structure, as shown by X-ray diffraction (XRD). Only < 1 vol% of secondary phases are observed in two compositions near the phase-transition points. With changing compositional variable x , this series of 20CCFBO x Nb/Ta undergoes an abrupt fluorite-pyrochlore transition at x = ∼0.27 and an abrupt pyrochlore-weberite transition at x = ∼0.87. Careful characterization reveals abrupt changes of order parameters at both phase transitions. In addition, weberite short-range ordering can persist into the long-range pyrochlore phase, which leads to the lowest thermal conductivities. [ABSTRACT FROM AUTHOR]

When exposed to severe environments, the short-range order in pyrochlores significantly influences their physicochemical properties. Using the DFT + U method, this study investigates the effect of short-range order on the solubility of actinides (An = Th, U, Np, Pu, Am, and Cm) in Gd 2 Zr 2 O 7 with distinct coordinated Gd and Zr sites. Our findings indicate that all the actinides under consideration are susceptible to being incorporated into the 7-fold Zr-site of quasi -weberite Gd 2 Zr 2 O 7. The weberite-like short-range order also favors immobilization of these actinides in the 6-fold Zr-site. The effect of the weberite-like motif on the solubility of actinides in the Gd-site of quasi -WB Gd 2 Zr 2 O 7 depends on the chemistry of the actinides. Furthermore, the short-range order results in a lower solution energy for An incorporation into the Gd and Zr sites of the quasi -weberite Gd 2 Zr 2 O 7 with cation antisites, thereby favoring An immobilization in a disordered environment. These results strongly demonstrate the importance of the short-range order on the solubility of actinides in disordered pyrochlores. [ABSTRACT FROM AUTHOR]

The challenge of producing dense fine-grained high-entropy ceramics is hereby addressed by a combined cold and conventional pressureless sintering method for the first time. It provides nearly fully dense high-entropy pyrochlores with better microstructural homogeneity, and only half the grain size of conventional pressurelessly sintered samples with identical density. Both the finer and more homogeneous microstructures benefit better mechanical properties in hardness and reduced Young's modulus. The kinetics and activation energies of both densification and grain growth were quantified and discussed to understand how cold sintering affects the microstructural evolution during the following pressureless sintering. More importantly, cathodoluminescence analysis suggested that cold sintering could cause more defects on the surface of the original particles, which could promote the grain boundary diffusion and have drag effects on grain boundary motion. The lessons learned here offer scientific understanding and technological guidance towards pressureless sintering of dense bulk high-entropy ceramics. [ABSTRACT FROM AUTHOR]

Gd 2 Zr 2 O 7 is considered a promising host material for immobilizing high-level radioactive waste. However, its interplay between grain sizes, radiation resistance and chemical stability is not well understood yet. Herein, Gd 1.7 Nd 0.3 Zr 0.5 Ce 1.5 O 7 ceramics with defect-fluorite phase and three different grain sizes (96, 390 and 1959 nm) were successfully fabricated and irradiated by 140 keV He ion beam up to a fluence of 8 × 1017 ions/cm2. Phase evolutions, microstructure changes and chemical stabilities were investigated. The nano-grained ceramic exhibits the lowest degree of amorphization and a delayed irradiation-induced lattice expansion and damage evolution process, demonstrating an enhanced ability in suppressing the coalescence of He bubbles inside the grain. Grain boundaries parallel to the surface were found to be more favorable sinks for irradiation-induced defects. Chemical stability tests showed that the 42 days leaching rates of all elements were between 10−7∼10−5 g/m2·d. All samples have maintained this superior chemical stability even after irradiation. Interestingly, the leaching test results also suggested that grain size may be not the primary factor influencing the long-term leaching rate. [ABSTRACT FROM AUTHOR]

The new generation of T/EBCs requires their top layer materials with good resistance to severe corrosion by CMAS. Rare-earth zirconates/hafnates with pyrochlore structure are regarded as promising candidates for T/EBC. In this work, the corrosion behaviours and mechanisms of plasma-sprayed Gd 2 M 2 O 7 (M = Zr, Hf) coatings at 1300 and 1500 °C for 4–25 h were investigated. The Gd 2 M 2 O 7 coating showed good corrosion resistance under long-term corrosion at 1300 °C. The Gd 2 M 2 O 7 coatings reacted with CMAS and precipitated apatite and fluorite, forming a dense layer that can prevent further penetration of CMAS. However, the CMAS penetrated into the Gd 2 M 2 O 7 coatings because of the sharply decreased viscosity of the CMAS as the temperature raised to 1500°C, forming another infiltration layer. The results also suggested that the Gd 2 Hf 2 O 7 possessed better resistance to CMAS corrosion than Gd 2 Zr 2 O 7. [ABSTRACT FROM AUTHOR]

A multi-crystalline specimen of gadolinium zirconate (Gd 2 Zr 2 O 7) has been synthesized by solid-state reaction method (un-milled sample). The sample underwent high-energy ball-milling for 6 h, 12 h, 18 h, and 24 h durations (milled samples). An investigation has been conducted to examine the effects of mechanical attrition on structural, microstructural, vibrational, and dielectric properties. Notably, the elemental analysis of the samples reveals a substantial increase in the atomic percentage of oxygen. This indicates the partial development of an oxygen-enriched phase during the milling process. The Rietveld refinement of X-ray diffraction profiles indicates that all the samples have a disordered fluorite structure however, in the 18 h and 24 h milling samples in addition to this main phase, a minor ordered phrochlore-phase has been formed. The nano-phasic character of all the milled samples has been confirmed by the SEM, TEM, and BET surface area analysis. The BET isotherms displayed a type –II hysteresis loop, indicating that all the samples are macro-porous with a pore diameter exceeding 50 nm. The FTIR and Raman spectrum studies indicate the presence of both defect fluorite and pyrochlore phases in the samples. An in-depth investigation has been conducted on the impact of mechanical milling-induced reduction in particle size and micro-strain on conduction mechanism, dielectric relaxation, and variation of various dielectric properties with frequency. [ABSTRACT FROM AUTHOR]

The pyrochlore oxide La 2 Hf 2 O 7 (LHO) has attracted extensive attention in thermal management applications. Accurate determination and comprehensive understanding of thermal conductivity (κ) stand as imperative prerequisites for the effective utilization and manipulation of LHO in various applications. In this study, with the assistance of machine learning interatomic potential, we investigate the microscopic mechanisms of phonon thermal transport in LHO crystals. Our findings show that the thermal transport in LHO defies a precise explication through the conventional framework of the Boltzmann transport equation (BTE) due to the emergence of diffusive phonon modes, engendered by strong anharmonicity. Using the state-of-the-art dual-phonon model incorporating both propagating and diffusive modes, we successfully reproduce the experimentally measured results in both magnitude and temperature dependence. It is also shown that the thermal conduction in LHO crystals is dominated by the propagating modes at low temperatures while by the diffusive modes at high temperatures, which synergistically lead to a low κ for LHO crystals. The novel insights into phonon thermal transport provide a stepping stone for regulating the thermal transport properties of LHO crystals. [ABSTRACT FROM AUTHOR]

Many factors impact the formability of single phase high-entropy pyrochlore ceramics (HEPCs) which become a research hotspot owing to their excellent properties. To investigate the effect of size disorder, two combinations with same mixing entropy and different size disorder were designed, thermodynamic analysis of first-principles calculations and experimental research were conducted in this paper. The calculation results reveal that the enthalpy change decreases significantly with the decrease of size disorder, promoting the synthesis of single-phase high-entropy pyrochlore. The experimental results prove that the combination with lower size disorder leads to single-phase HEPCs, and the prepared material exhibits low thermal conductivity (1.68 W/(m·K)) and low theoretical density (5.73 g/cm3). [ABSTRACT FROM AUTHOR]

In this work, the irradiation response of A 2 Ti 2 O 7 (A represents rare earth elements of Gd, Dy, Ho, Er, Yb and Nd) series ceramics were systematically studied under 3 MeV Au2+ ions. The structural changes pre- and post-irradiation were characterized using X-ray diffraction (XRD), grazing incidence X-ray diffraction (GIXRD) and Raman spectroscopy. After heavy ion irradiation, all the samples undergo structural transformations from ordered pyrochlore to disordered fluorite and then to amorphous phase. The sample surface became fully amorphous when exposed to the maximum irradiation dose. The 5RETO high-entropy pyrochlore exhibits the lowest rate of amorphization during the process of structural transformation. The cation radius ratio and mixed entropy are the primary factors influencing amorphization rate. The cation radius ratio shows a strong positive correlation with the amorphization rate, while the mixed entropy demonstrates a weak negative correlation with the amorphization rate. The cation radius ratio can thus serve as a reliable indicator for predicting the radiation resistance of high-entropy pyrochlore ceramics. [ABSTRACT FROM AUTHOR]

The study investigated the formation of inhomophase ceramics through Zn doping in Bi 2 СrTa 2 O 9.5–Δ. It was found that all Bi 2 Zn x Cr 1– x Ta 2 O 9.5–Δ (0.3 ≤ x ≤ 0.7) investigated samples crystallized in the pyrochlore structural type (sp. Gr. Fd-3m) and contained an admixture of triclinic β-BiTaO 4 (up to 24 wt %, sp. gr. P-1), with the admixture concentration proportional to the degree of zinc doping. It was shown that impurities in the samples could be eliminated by creating a defective bismuth sublattice proportional to the impurity content, resulting in single-phase Bi 2– y Zn x Cr 1– x Ta 2 O 9.5–Δ (y ≤ 0.5, 0.3 ≤ x ≤ 0.7) samples with the pyrochlore structure. The unit cell parameter of pyrochlore increased with an increase in zinc ion content in the samples from 10.4493 Å (x = 0.3) to 10.4976 Å (x = 0.7). The microstructure of ceramics was represented by individual slightly melted grains 0.5–2 μm in size, which increased in size and fuse with each other with an increase in zinc content. The near edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) data showed that zinc doping did not change the oxidation degree of bismuth and tantalum in pyrochlore, with the ions in charge states Bi(+3), Zn(+2) and Ta(+5). The study also observed an energy shift of the absorption band in the Ta 4f spectrum toward lower energies (Δ E = 0.55 eV) characteristic of tantalum ions with an effective charge (+5–δ) and a shift of the Bi 4f 7/2 and Bi 4f 5/2 bands shift to lower energies with increase in x (Zn), due to the distribution of some Zn(II) ions in the bismuth position. The oxidation degree of chromium ions in the samples was mainly (+3), with some chromium ions oxidized and having an oxidation degree close to (+6) according to NEXAFS data. The studied ceramics showed promise as dielectrics for multilayer ceramic capacitors and devices for microwave applications due to their low sintering temperature and high porosity. [ABSTRACT FROM AUTHOR]

The increasing anthropogenic emissions of greenhouse gases (GHG) is encouraging extensive research in CO 2 utilisation. Dry reforming of methane (DRM) depicts a viable strategy to convert both CO 2 and CH 4 into syngas, a worthwhile chemical intermediate. Among the different active phases for DRM, the use of nickel as catalyst is economically favourable, but typically deactivates due to sintering and carbon deposition. The stabilisation of Ni at different loadings in cerium zirconate inorganic complex structures is investigated in this work as strategy to develop robust Ni-based DRM catalysts. XRD and TPR-H 2 analyses confirmed the existence of different phases according to the Ni loading in these materials. Besides, superficial Ni is observed as well as the existence of a CeNiO 3 perovskite structure. The catalytic activity was tested, proving that 10 wt.% Ni loading is the optimum which maximises conversion. This catalyst was also tested in long-term stability experiments at 600 and 800°C in order to study the potential deactivation issues at two different temperatures. At 600°C, carbon formation is the main cause of catalytic deactivation, whereas a robust stability is shown at 800°C, observing no sintering of the active phase evidencing the success of this strategy rendering a new family of economically appealing CO 2 and biogas mixtures upgrading catalysts. [Display omitted] [ABSTRACT FROM AUTHOR]

The order-disorder transition (ODT) of RE zirconate (RE 2 Zr 2 O 7) has received much attention due to its potential applications in the design of thermal barrier coatings (TBCs). A series of high entropy RE zirconates (RE = La, Sm, Gd, Er and Lu) were designed and synthesized in order to regulate the fluorite-pyrochlore phases, and to investigate the effect of phase composition on the microstructures and performances. The results showed that the sintering temperature, as well as equivalent ion radius, played an important role in regulating the phase transformation of fluorite-pyrochlore. The coexistence of pyrochlore-fluorite phases not only can result in the decrease in grain size but also increased the lattice strain of each phase, thereby enhancing the phonon scattering and contributing to the reduction in the thermal conductivity. The CMAS corrosion resistance was mainly related to phase compositions, and the thicknesses of reaction layer decreased gradually with the increasing fluorite phase content. The results will provide theoretical guidance for the microstructural design of high entropy RE zirconates as next generation TBC materials in future. [ABSTRACT FROM AUTHOR]

The (Sm 1-x Yb x) 3 TaO 7 compounds were prepared using high-temperature calcining technology to evaluate the impact of Yb 2 O 3 on the crystal-type and thermal-physical performances for Sm 3 TaO 7. Final analytical conclusions show that the synthesised compounds have a sole pyrochlore or fluorite crystal structure, and the lattice variation from pyrochlore to fluorite is found with an increase in Yb 2 O 3 fraction. Owing to the phonon scattering caused by substituting cation and disorder atomic arrangement, the heat insulation ability of (Sm 1-x Yb x) 3 TaO 7 oxides is improved, which meets the requirements for thermal barrier coatings. The synthesised (Sm 1-x Yb x) 3 TaO 7 oxides also present relatively high linear thermal expansion coefficients owing to the combined influence of ionic radius and crystal lattice energy. The addition of Yb 2 O 3 also improves the crystal constancy until 1200 °C. [ABSTRACT FROM AUTHOR]

Pyrochlore structures with composition Pr 2−x Sm x Zr 2 O 7 (x = 0, 1, and 2) were synthetized and studied. Spark Plasma Sintering (SPS) was employed to produce ceramics with over 95% of relative density. The reoxidation behavior and the influence of the sintering process on the electrical properties were studied by Electrochemical Impedance Spectroscopy. The overall conductivity of the materials is higher for the samples with Pr, while the introduction of Sm induced an additional electrical contribution. The reoxidation of the samples presented fast kinetics and increased conductivity, particularly in the PrSmZr 2 O 7 (PSZO) sample. PSZO also exhibits a total conductivity comparable to that of the reference materials used as ionically conductive solid electrolytes. The prepared ceramics, especially the PSZO sample, demonstrate excellent relative density, good conductivity, and fast oxidation, holding promise for applications such as Solid Oxide Fuel Cells, gas sensors, and clean energy technologies that require thermal and physical stability, as well as rapid oxidation-reduction processes. [ABSTRACT FROM AUTHOR]

Ideally, a solid electrolyte which is a central component of SOFC should exhibit high anionic or cationic ionic conductivity at the proposed operating temperatures. In most cases, the performance is compromised when operating above 1000 °C due to poor mechanical, thermal and chemical stability of selected functional and non-functional materials. In this context, pyrochlores are one of the potential candidates due to their high conductivity, flexibility to accommodate large cations, and high mechanical, thermal and chemical stability. In this study, we report the synthesis of nano-powders of Ho 2 Hf 2 O 7 (HH) and Ho 2 Zr 2 O 7 (HZ) pyrochlore ceramics by eco-friendly alginate mediated ion-exchange process also known as Leeds Alginate Process (LAP) and provide further insight into the structure - conductivity relationship of HH and HZ compounds by EXAFS studies. Both the compositions were sintered at temperatures, ranging from 1100 °C-1500 °C at 2 h dwell time to achieve the desired high-density ceramic with stable pyrochlore structure. X-ray diffraction and Extended X-ray absorption fine structural (EXAFS) analysis showed superlattice reflections and complex coordination geometry of these oxides. The coordination number and disorder factor in the case of HZ were found to be more stable than the HH sample, as evident from the EXAFS. Impedance spectroscopy and dc-conductivity analysis showed a better charge transport behavior in HZ ceramics than in HH making HZ as a preferred solid electrolyte for SOFC. The conductivity of Ho 2 Zr 2 O 7 is comparable with the best-known fluorite oxide-ion conductor such as Sc 2 O 3 -stabilized ZrO 2 at 500 °C. [ABSTRACT FROM AUTHOR]

In this study, a series of pyrochlore-type compounds (Y 2-x Zn x)Ti 2 O 7- δ with x = 0.0, 0.1, and 0.2 were synthesized using the molten salt method. The samples were studied by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and UV–visible spectroscopy. Rietveld refinement of the XRD patterns shows that Zn2+ ions occupy Y3+ sites. Consequently, a lattice parameter and ionic radius ratio r A /r B decrease; consequently, the crystal structure is less ordered. Raman spectroscopy shows that the disorder manifests by broadening of FWHM Raman modes and increasing the A 1g Raman mode frequency as the Zn content (x) rises. Another repercussion arising from the disorder observed in the pyrochlore is the occurrence of oxygen vacancies, which are associated with ionic conductivity and are localized at 531 eV in the high-resolution O1s XPS spectra. First-principles studies using LDA + U show that Y 2 Ti 2 O 7 has a 3.07 eV electronic band gap, slightly lower than the 3.12 eV experimentally reported. The valence band (VB) is a contribution of O 2p hybridized states with Ti-3d and Y-4d, while the conduction band (CB) has Ti-3d states hybridized with O-2p states. The VB XPS spectrum and calculated DOS for Y 2 Ti 2 O 7 are well correlated. As the Zn content (x) increases, the VB XPS spectra shift to low energies, with an increment in partial states at zero eV, which induces an increase in the carrier charge. UV–visible absorption measurements using the Tauc plot approximation show a decrease in the optical band gap due to an increased concentration of Zn content. [ABSTRACT FROM AUTHOR]

A set of medium-/high-entropy pyrochlore oxides (M-HEPOs) with different compositions were designed and synthesized. The phase compositions of the prepared materials were determined through X-ray diffraction and Raman spectroscopy. Among the 24 designed samples, two high-entropy and six medium-entropy samples formed single-phase pyrochlore materials, two medium-entropy samples formed dual-phase pyrochlore materials, and the others formed multiphase materials dominated by the pyrochlore phase. The factors affecting the formation of single-phase pyrochlore structure were studied. The results revealed that size disorder is a major element determining the formation of single-phase pyrochlore, whereas mixing entropy has minimal effect. It is anticipated that this study will provide significant insights into the effects of size disorder and mixing entropy on the formation of M-HEPOs. [ABSTRACT FROM AUTHOR]

In this study, a secondary sintering process was adopted to optimize the preparation process of single-phase high entropy A 2 Zr 2 O 7 pyrochlore-based waste forms by spark plasma sintering (SPS). This method eliminates the significant internal stress when preparing samples directly by SPS, and thus well preserves the integrity of the samples after annealing. The phase composition, microstructure, and leaching properties of high entropy A 2 Zr 2 O 7 pyrochlore were systematically investigated. Owning to the low sintering temperature (1450 °C) and short held time (3 min), the high entropy A 2 Zr 2 O 7 pyrochlore exhibited a fine average grain size of 0.8 μm. The high entropy A 2 Zr 2 O 7 pyrochlore oxides exhibit a reliable leaching resistance with low normalized release rates around 10−5 g/(m2∙d) for all elements. For the high entropy systems, the aqueous durability is mainly determined by the average grain size of ceramics and the temperature of the leaching test as well as traditional pyrochlore ceramics solidification. [ABSTRACT FROM AUTHOR]

As a quick and affordable experimental technique, the heavy ion irradiation method may quickly learn about the correlation between the degree of amorphization and structural modification of samples with varying ion implantation. Using the heavy ion irradiation technique, the irradiation stability of multiphase ceramic waste made of Ln 2 Zr 2 O 7 /SrZrO 3 was assessed. Here, we found that multiphase ceramic waste forms still exhibit excellent radiation resistance while achieving high capacity and weak selectivity for immobilization of actinide nuclides (simulated by lanthanide elements) and fission products 90Sr (replaced by 89Sr). The analysis of grazing incidence and Raman spectroscopy provides evidence for explaining the radiation damage mechanism caused by 84Kr15+ heavy ions. Analysis of grazing incidence offers proof of the manner and extent of radiation damage brought on by 84Kr15+ ions. The impact and mechanism of crystal structure on radiation resistance in multiphase ceramics are discussed. [ABSTRACT FROM AUTHOR]

The demand for efficient engineering components operating in harsh environments has necessitated the exploration of advanced protective measures. Thermal barrier coatings (TBCs) have emerged as a promising solution for improving the performance, durability, and lifespan of components exposed to high temperatures, corrosive conditions, and neutron irradiation. Advanced ceramic materials specially pyrochlore of zirconates significance are enhancing the performance and longevity of components exposed to extreme thermal conditions. Dual-phase innovative zirconate coatings and their hot corrosion mechanism, a complex phenomenon involving the simultaneous presence of multiple corrosive species, is extensively explored. The interplay between different corrosion mechanisms and their impact on TBCs are elucidated, providing valuable insights for the development of effective mitigation strategies. This article addresses the understanding the hot corrosion mechanism and corrosion types, new innovative dual phase composite systems with improved properties, their experience with molten salt and pure oxidation environments followed by the stresses experienced by TBCs, highlighting the importance of understanding and managing mechanical and thermal stresses during operation. Coatings design, material selection, and advanced manufacturing techniques are discussed as key factors in alleviating these stresses and enhancing TBC performance. To overcome the challenges associated with hot corrosion and stresses, innovative strategies are being investigated. These include the development of novel coatings compositions, the incorporation of protective layers, advanced characterization techniques, and the exploration of alternative materials. This piece of review article provides a thorough analysis of these techniques and how they might increase the robustness and dependability of TBCs. Finally, we highlight the contemporary applications of TBCs in gas turbines, aero engines, nuclear industries, and other high-temperature situations. • The current research conducts a thorough examination of degradation mechanisms in dual-phase thermal barrier coatings (TBCs) based on YSZ and zirconates of (gadolinium, samarium, neodymium and lanthanum). • Highlights the formation of new zirconate phases with varying gadolinium oxide, lanthanum oxide, samarium oxide, and neodymium oxide content. • Addresses the unique challenges and benefits associated with these advanced coatings. • Introduces new modes of degradation in dual-phase TBCs. [ABSTRACT FROM AUTHOR]

This paper reported the irradiation response of a 55 wt.% MgO-45 wt.% Nd 2 (Zr 1- x Ce x) 2 O 7 (abbreviated to M-NZC x , x = 0, 0.3 and 1) composite ceramics used as candidate inert matrix fuel under 500 keV He2+ ions with fluence of 1 × 1016–5 × 1017 ions/cm2. The irradiation-induced lattice expansion, amorphization of MgO and NZC x phases and order-disorder transition of Nd 2 (Zr 1- x Ce x) 2 O 7 pyrochlore were systemically explored via GIXRD, Raman and TEM techniques. And the detailed irradiation response at different depths of the typical M-NZC 0.3 sample was especially investigated via GIXRD and TEM. The results showed that the lattice expansion ratios (Rs) of MgO (M), ordered pyrochlore (P) and disordered defect-fluorite (F) structures of NZC x pyrochlore in the samples under the same dose of He2+ irradiation were regular, which were ranged as the following order: Rs M < Rs P < Rs F. And there was a belt-shaped damage aggregation zone located near interphase boundary at the maximum damage depth of M-NZC 0.3 sample, which could be explained by the defect-absorption of boundary. And this defect-absorption of boundary was also found at any depth of M-NZC 0.3 sample. This work systematically analyzed α-decay irradiation response of different phases in M-NZC x composite ceramics applying for inert matrix fuel. [ABSTRACT FROM AUTHOR]

The advance of portable and smart near-infrared (NIR) light sources has sparked significant interest. However, achieving high-performance NIR emission at longer wavelengths remains a formidable challenge. In this study, we prepared broadband NIR phosphors by codoping pyrochlore oxide Y 2 GaSbO 7 (YGSO) with Cr3+ and Yb3+ ions using a conventional high-temperature solid-state reaction. Upon excitation by blue light, YGSO: 0.03Cr3+ exhibited a broadband NIR emission centered at 765 nm with a full width at half maximum (FWHM) of 160 nm. The purposeful incorporation of Yb3+ ions facilitated efficient energy transfer from Cr3+ to Yb3+, resulting in a newly observed NIR emission peak at 970 nm. Notably, the addition of Yb3+ improved the overall thermal quenching resistance owing to its inherent thermal stability. Furthermore, we encapsulated the as-obtained phosphors with commercial blue LED chips to create prototype phosphor-converted light-emitting diodes (pc-LEDs). The Cr3+ and Cr3+-Yb3+ codoped pc-LEDs achieved output powers of 120 mW@660 mA with a photoelectric conversion efficiency of 12.66%@30 mA, and 131.7 mW@1020 mA with a photoelectric conversion efficiency of 13.93%@30 mA, respectively. These broadband NIR pc-LEDs hold promise for optical imaging applications. [ABSTRACT FROM AUTHOR]

In this work, an efficient method for the synthesis of high-entropy pyrochlore ceramic (La 1/7 Nd 1/7 Sm 1/7 Eu 1/7 Gd 1/7 Dy 1/7 Ho 1/7) 2 Zr 2 O 7 (7RE 2 Zr 2 O 7) was developed via reactive spark plasma sintering. The sintering reactivity, phase composition, microstructure, and mechanical properties of 7RE 2 Zr 2 O 7 were systematically investigated. With the aid of uniaxial pressure and direct current (DC) pulse voltage, the entire sintering process was greatly shortened to less than 1 h. Owing to the synergistic effects of the compositional complexity and severe lattice distortion, 7RE 2 Zr 2 O 7 exhibited a reduced average grain size of only 1.45 μm, which is approximately 1/6 of that for Gd 2 Zr 2 O 7 prepared using the same sintering process. Furthermore, 7RE 2 Zr 2 O 7 possessed a high hardness of up to 13.86 GPa, while a low fracture toughness of 1.1 MPa m1/2, which can be mainly attributed to the severe internal stress and defects originated from the fast-sintering process. This work is expected to provide a guidance for the highly efficient and rapid preparation of high-entropy A 2 B 2 O 7 oxides, especially for those compositions that are difficult to densify. [ABSTRACT FROM AUTHOR]

The high-entropy ceramic solidified body ((LaPrEuGdYb) y U x) 2 Zr 2 O 7+δ (y = (1− x)/5, x = 0.05, 0.10, 0.15, 0.20, 0.25) was prepared by immobilizing uranium (U) on the A-site of the pyrochlore in this study. The phase composition and evolution of solidified body were determined by XRD, Raman and SEM analysis. The phase of (LaPrEuGdYb) y U x) 2 Zr 2 O 7+δ ceramic was varied with U doping amounts: a dual-phase of pyrochlore and fluorite with no U doping. A single-phase pyrochlore with U doping amounts from x = 0.15 to x = 0.20, and the U 3 O 8 phase was precipitated at doping levels above x = 0.25. The solid solution limit for high entropy pyrochlore in relation to uranium is roughly x = 0.2. The results should be linked to the increase in configurational entropy and the variation of U valence in the ceramic bodies, as evidenced by XPS. The normalized leaching rate of U from high-entropy ceramic waste forms (x = 0.20) was about 1.65 × 10−5 g/m2⋅d after 42 d leaching. It performed well in the leaching tests. Our study is expected to provide candidate materials for solidifying complex high-level radioactive waste (HLW). [ABSTRACT FROM AUTHOR]

High entropy pyrochlore oxide ceramics with hierarchical porous structures were prepared by coprecipitation, freeze-casting, and press-less sintering methods. In the fabrication process, a Ca2+-crosslinked alginate network was utilized to control the growth of ice crystals, resulting in uniformly dispersed micro-scale circular pores. The submicron pores were formed in the high entropy oxide ceramic during the press-less sintering method. The XRD was employed to explore the phase of the high entropy pyrochlore oxide ceramic. The porous structures of high-entropy ceramic were analyzed through the discussion of microstructure and pore size distribution. The tests show that the prepared series of ceramics with high porosity have optional compressive strength and thermal insulation properties. The high entropy oxide ceramic (La 0.2 Y 0.2 Gd 0.2 Sm 0.2 Nd 0.2) 2 Zr 2 O 7 with hierarchical porous structures possess moderate density (0.4 g/cm3) and low thermal conductivity (0.04 W m−1 K−1). The excellent comprehensive performance makes the porous high entropy oxide ceramic could be applied as a thermal insulating material. [ABSTRACT FROM AUTHOR]

Chemical durability of compositionally-complex rare-earth titanate pyrochlore solid solutions with or without uranium incorporation was investigated by semi-dynamic leaching testing. A Ti-enriched amorphous passivation film formed on the surface due to the surface alteration-reorganization mechanism with preferential release of the weakly oxygen-bonded rare earth elements. Elemental release was found to be strongly correlated with chemical disorder, with a negligible correlation with entropy. The release rate of the uranium gradually decreased with increasing numbers of components, implying a possibility of designing and optimizing waste form performance with enhanced chemical durability by controlling composition complexity and chemical disorder for effective nuclear waste management. [ABSTRACT FROM AUTHOR]

The La 2 Sn 2 O 7 ceramic was synthesized using the solid−phase reaction method. The crystal structure, sintering characteristics, and microwave dielectric properties of the La 2 Sn 2 O 7 ceramics were studied. XRD analysis shows that La 2 Sn 2 O 7 ceramics possess a pyrochlore structure with a space group of F d 3 ¯ m. As the sintering temperature increases, the ε r and relative density show the same trend of change. The Q × f of ceramics has a strong correlation with the packing fraction. The P−V−L theory shows that La−O and Sn−O bonds have certain effects on the ε r and Q × f. The intrinsic dielectric properties of ceramics were analyzed using infrared reflection spectra. At 1350 °C, La 2 Sn 2 O 7 ceramic obtained the largest relative density and optimum microwave dielectric properties (ε r = 15.65, Q × f = 73,804 GHz and τ f = −18.58 ppm/°C). [ABSTRACT FROM AUTHOR]

High-entropy perovskite oxides have attracted significant attention owing to their excellent performance and diverse application potential. This study reports the synthesis of (Bi 0.2 Ln 0.2 Sr 0.2 K 0.2 Na 0.2)TiO 3 high-entropy perovskite oxides containing Ln element. The results indicate that seven of the 13 compositions with a larger ionic radius of Ln3+ can be prepared as single-phase high-entropy perovskites. During the formation of high-entropy perovskites, the Ln 2 Ti 2 O 7 pyrochlore or pyrochlore-like phase first forms as an intermediate phase. The formation of a single high-entropy perovskite phase depended on the complete dissolution of the Ln 2 Ti 2 O 7 pyrochlore or pyrochlore-like phase within the perovskite matrix. The pyrochlore phase content depends on the stability of this intermediate pyrochlore phase, related to the ionic radii ratio of Ln3+ and Ti4+. These findings provide guidance for judicious elemental selection in high-entropy perovskite oxides. [ABSTRACT FROM AUTHOR]

This paper focuses on the analysis of light scattering effects in pyrochlore transparent ceramics. The refractive indices of various pyrochlore materials are summarized, and the refractive index-wavelength dependences are fitted using the Sellmeier equation, enabling the extrapolation of refractive indices at different wavelengths. The study investigates the validity of different light scattering approximations compared to Mie theory, highlighting the effectiveness of the van de Hulst approximation in describing the transmittance-pore diameter-wavelength dependence for pyrochlore systems. The influence of pore size (distribution) on light scattering is explored and among other results, it is found that the critical pore size leading to the highest level of light scattering was approximately half the wavelength of light, with variations depending on the system's refractive index. The findings provide valuable insights into the optical properties of pyrochlore transparent ceramics and offer guidelines for reduction of light scattering. • Refractive indices of pyrochlores were summarized and fitted via Sellmeier equation. • Light scattering effects in pyrochlore transparent ceramics were investigated. • Effectiveness of the van de Hulst approximation for pyrochlores was validated. • Critical pore diameter for light scattering is approximately half of the wavelength. • Influence of pore size distribution on transmittance was explored. [ABSTRACT FROM AUTHOR]

The densification of Ga-doped Li 7 La 3 Zr 2 O 12 (LLZO) ceramics was optimized by controlling the phase chemistry and morphology of green powders after calcination. A solution-based approach was used to synthesize the green powders which were calcined at temperatures between 600 °C and 950 °C and free-sintered into dense pellets. The ionic conductivity and relative density were optimized at a calcination temperature of 700 °C to be 1.28×10−3 S/cm and 96.2%, respectively. At this condition, the green powder consisted primarily of La 2 Zr 2 O 7 pyrochlore nanoparticles (80–100 nm) and LLZO particles (0.5–1 µm), which promoted reaction-driven densification and enabled a favorable bimodal size distribution for improved packing density. Further increasing the calcination temperature was correlated with a reduction in density due to the detrimental effects of particle coarsening on sintering activity. These findings show that controlling the calcination conditions is an effective method for tailoring green body properties for improved densification of LLZO. [ABSTRACT FROM AUTHOR]

The carbonatite-related Nb deposits of the Alto Paranaíba Igneous Province (APIP) in central Brazil, currently account for ∼92 % of the global Nb production. In the APIP, pyrochlore is abundant in magnetite–apatite–tetraferriphlogopite ± carbonate rocks or phoscorites, occurring as interbedded layers with carbonatites in the lower hypogene zone, feeding dike swarms of phoscorite and calcite carbonatite, and late-stage carbothermal veins in the upper hypogene zone. The origin of the phoscorite-carbonatite association can be explained by three hypotheses: (1) crystal segregation from fractional crystallization, (2) liquid immiscibility, and/or (3) phoscoritic magma formation after basement metasomatism (fenitization). However, it is not well understood whether pyrochlore formation is limited to a carbonatitic event, carbohydrothermal, or both, and this gap of knowledge is addressed in this work. To investigate the petrogenesis of pyrochlore-rich phoscorite, cathodoluminescence (CL) images, chemical maps, and LA-ICP-MS data were acquired of pyrochlore crystals from magmatic and carbothermal rocks from the Boa Vista Nb mine, Catalão II Complex. In the Boa Vista mine, oscillatory and patchy zoning were identified as primary pyrochlore textures commonly recorded at the lower hypogene zone, while secondary dissolution, skeletal and zonation-free textures are registered at shallower depths in the upper hypogene zone. Calciopyrochlore is the dominant Nb phase at the Boa Vista mine, with only two kenopyrochlore outliers. The pyrochlore CI chondrite-normalized REE distribution is consistent with geochemical results of the carbonatite and phoscorite rocks, indicating a magmatic origin for pyrochlore and the presence of pyrochlore antecrysts in carbothermal veins. The Sr/Y vs La and Na vs Ce diagrams in pyrochlore indicate a continuous fractionation pattern, with some mixtures of antecrysts and primary phases. An examination of intercumulus calcite using CL provide evidence of carbonatitic magma residues within tetraferriphlogopite phoscorite dikes and suggests that alkaline–carbonate-rich fluids played a role in transporting heavy minerals (i.e., magnetite, apatite, pyrochlore). Consequently, the textural and chemical evidence in the Boa Vista Nb mine indicates that the origin of pyrochlore-rich phoscorites is the result of physical segregation of heavy minerals from a carbonatite magma by fractional crystallization, leading to the emplacement of pyrochlore-rich carbonatite and phoscorite dikes. The implications at Catalão II may extend to other APIP alkaline-carbonatite complexes, as they share a genetic connection, and should motivate further studies focusing on pyrochlore geochemistry in other carbonatite-related Nb deposits, which will be crucial for advancing our knowledge of global Nb metallogenesis. [ABSTRACT FROM AUTHOR]

High-entropy pyrochlore ceramics have received significant attention in the field of nuclear materials due to their ability to immobilize radioactive waste. In this study, a range of medium and high-entropy Ti-based pyrochlore powders were synthesized by a two-step method. Interestingly, a monoclinic phase was observed in some powders. An innovative experiment was conducted to adjust the cation ratio at the A site, which revealed that certain cations with larger ionic radii exhibit low solid solubility in the pyrochlore lattice, resulting in the formation of the monoclinic structure. To avoid the formation of monoclinic phase, the crystal structure was adjusted by introducing Zr4+ at the B site. Based on the selection of single-phase powders, high-density pyrochlore ceramic bulks with uniform distribution of elements were prepared by spark plasma sintering. Here (Sm 1/5 Yb 1/5 Gd 1/5 Er 1/5 Dy 1/5) 2 Ti 2 O 7 bulks exhibited the smallest grain size at approximately 2.48 µm among all sintered samples and demonstrated a hardness of around 10.14 GPa. [ABSTRACT FROM AUTHOR]

This article is devoted to the study of the processes of formation and growth of nanocrystals with the pyrochlore structure in the TiO 2 -Gd 2 O 3 system. A mixture of an aqueous solution of gadolinium nitrate and a solution of titanium isopropoxide with polyethylene glycol in isopropyl alcohol was evaporated to a gel state, and then it was heat-treated using a one- or two-stage scheme. The study showed that the material being formed has a trabecular structure with a developed system of macropores. The use of the two-stage roasting method allows a uniform distribution of titanium and gadolinium atoms in the sample. Gadolinium titanate with a pyrochlore structure, which is formed using an aggregation mechanism at a temperature T = 700 °C from an X-ray amorphous precursor, is a variable composition compound. It has been shown that in terms of obtaining a monophasic product, the possibility of varying the size of nanocrystals, and defects in the pyrochlore structure, the most promising heat treatment mode is synthesis in the temperature range of 750–900 °C. In this case, the possibility of controlling the above characteristics of Gd 2 Ti 2 O 7 nanoparticles can be implemented by varying the ratio of titanium- and gadolinium-containing precursors in the initial mixture, the duration of the isothermal exposure process. [Display omitted] [ABSTRACT FROM AUTHOR]