Search

[EN] High pressure X-ray diffraction, Raman scattering, and electrical measurements, together with theoretical calculations, which include the analysis of the topological electron density and electronic localization function, evidence the presence of an isostructural phase transition around 2 GPa, a Fermi resonance around 3.5 GPa, and a pressure-induced decomposition of SnSb2Te4 into the high-pressure phases of its parent binary compounds (alpha-Sb2Te3 and SnTe) above 7 GPa. The internal polyhedral compressibility, the behavior of the Raman-active modes, the electrical behavior, and the nature of its different bonds under compression have been discussed and compared with their parent binary compounds and with related ternary materials. In this context, the Raman spectrum of SnSb2Te4 exhibits vibrational modes that are associated but forbidden in rocksalt-type SnTe; thus showing a novel way to experimentally observe the forbidden vibrational modes of some compounds. Here, some of the bonds are identified with metavalent bonding, which were already observed in their parent binary compounds. The behavior of SnSb2Te4 is framed within the extended orbital radii map of BA(2)Te(4) compounds, so our results pave the way to understand the pressure behavior and stability ranges of other "natural van der Waals" compounds with similar stoichiometry., This work has been performed under financial support from the Spanish MINECO under Project MALTA-CONSOLIDER TEAM network (RED2018-102612-T) and Project FIS2017-83295-P, from Generalitat Valenciana under Project PROMETEO/2018/123. This publication is a product of the "Programa de Valoracion y Recursos Conjuntos de I+D+i VLC/CAMPUS and has been financed by the Spanish Ministerio de Educacion, Cultura y Deporte, as part of "Programa Campus de Excelencia Internacional". Supercomputer time has been provided by the Red Espanola de Supercomputacion (RES) and the MALTA cluster. J.A.S. acknowledges a "Ramon y Cajal" fellowship (RYC-2015-17482) for financial support, and E.L.D.S. acknowledges Marie Sklodowska-Curie Grant No. 785789-COMEX from the European Union's Horizon 2020 research and innovation program. We also thank ALBA synchrotron and DIAMOND light source for funded experiments.

Zinc and nickel ferrites were prepared through a solvothermal method with different microstructures: as assemblies of particles and not forming assemblies. By means of Raman spectroscopy, the catio...

ESA and NASA will launch two missions to explore the main Galilean moons of Jupiter in the coming years, JUpiter ICy moons Explorer-JUICE and Europa Clipper, respectively. Both missions will be able to determine the distribution of the potential habitable zone in the upper dozens of kilometers of the ice bodies by using onboard radar instruments [Bruzzone et al. 2013, Grasset et al., 2013, Phillips and Pappalardo 2014, Aglyamov et al. 2017]. Considering the possible presence of mixtures of water with salts, volatiles, and clays in the ice and liquid layers, we performed laboratory experiments to obtain the electrical properties of these chemical systems at solid and liquid state, and at different pressure conditions (Table 1). The results that we present at the conference will facilitate the interpretation of the future data received from the radar sensors. We measured the dielectric properties of these samples with a BDS80 Broadband Dielectric Spectroscopy system (Novocontrol), which allows working in a frequency range from 1 Hz to 10 MHz and temperatures from 143 to 323 K. Both, real permittivity (ε´) and electric conductivity (σ) were measured at 0.1 MPa while cooling the samples in temperature steps of 10 K. From these data, we calculated the activation energy and the attenuation of the radar wave depending on the chemical composition, the temperature of the sample, and the frequency of the applied electric field [Petrenko and Whitworth 1999, Pettinelli et al. 2015]. Conductivity measurements at high pressure were carried out using a modification of a chamber used previously for planetary simulation experiments (Muñoz-Iglesias et al. 2019). It is based on a stainless steel cylinder, which has different access on the bases and along the main body. Pressurization of the sample is carried out by water or gas from one access at the chamber base, while a sapphire window is at the opposite side for visual control and spectral analysis. Connections throughout the body of the cylinder are for the thermocouple, the pressure gauge and a special one to plug in the electrical sensor. The key feature is the plug for in-situ conductivity measurements with feed through wires connected to the probe, made of two Pt electrodes placed in a PTFE structure in order to maintain the configuration and to ensure the distance between electrodes (around 500 micrometers). Both electrodes have connections to isolated copper wires, which pass through the cell body to be connected to the signal transducer and the computer to record the data. Acknowledgements This work was supported by the ESA contract number 4000126441/19/ES/CM. We thank Anezina Solomonidou for assistance in the project proposal. References Aglyamov et al. (2017) Bright prospects for radar detection of Europa’s ocean, Icarus, 281, 334-337. Bruzzone et al. (2013) RIME: Radar for Icy moon Exploration, IEEE International Geoscience and Remote Sensing Symposium - IGARSS, Melbourne, 3907-3910. Grasset et al. (2013). Jupiter Icy moons explorer (JUICE): an ESA mission to orbit Ganymede and to characterise the Jupiter system. Planet. Space Sci. 78, 1-21 Muñoz-Iglesias et al. (2019). Experimental Petrology to Understand Europa's Crust. JGR-Planets 124, 2660-2678 Phillips and Pappalardo (2014). Europa Clipper mission concept: exploring Jupiter’s ocean moon. Eos Trans. AGU 95, 165-167. Pettinelli et al. (2015) Dielectric properties of Jovian satellite ice analogs for subsurface radar exploration: A review. Reviews of Geophysics, 53, 593-641.

Thermal properties of frozen salt solutions are crucial to interpret the JUpiter ICy moons Explorer (JUICE) (ESA) and Europa Clipper (NASA) missions, which will be launched in the upcoming years to make detailed observations of the giant gaseous planet Jupiter and three of its largest moons (Ganymede, Callisto, and Europa) due to the scarcity of experimental measurements. Therefore, we have conducted a set of experiments to measure and study the thermal conductivity and calorimetry of macroscopic frozen salt solutions of particular interest in these regions, including Na-chloride (NaCl), Mg-sulphate (MgSO4), sodium sulphate (Na2SO4), and Magnesium chloride (MgCl2). A climatic chamber has been used to mimic the cryogenic conditions in the Jovian Icy Moons. Measurements were performed at atmospheric pressure and temperatures from 0 to -70ºC. Temperature and thermal conductivity were measured during the course of the experiments. A side effect of these measurements is that they served to spot phase changes in the ice mixtures. A small sample of the liquid salt-water solution was set aside for the calorimetry measurements. These experiments and the measurements of thermal conductivity and calorimetry will be valuable to constrain the chemical composition, physical state, and temperature of the upper layers of the icy crusts of Ganymede, Callisto, and Europa (please see abstracts EPSC Muñoz Iglesias et al. 2020 and EPSC Solomonidou et al. 2020).

In the coming years The JUpiter ICy moons Explorer (JUICE) (ESA) and Europa Clipper (NASA) missions will study the icy crusts of the main Galilean moons of Jupiter. They will use the penetrating radars RIME and REASON, which will work at wave frequency ranges able to penetrate up to 9 and 30 Km depth respectively, in combination with other instruments [Bruzzone et al. 2013, Aglyamov et al. 2017]. In this regard, we have started a set of experiments to study the electrical properties of materials at low temperatures with the aim to help with the interpretation obtained from the level of attenuation of the radar waves. Ultimately, they will be useful to constrain the chemical composition, physical state and temperature of the upper layers of the icy crusts of Ganymede, Callisto and Europa (please see abstracts EPSC González Díaz et al. 2020 and EPSC Solomonidou et al. 2020). The first set of experiments have been done in a high-pressure chamber equipped with pressure and temperature sensors in direct contact with the sample and a large sapphire window which allows textural and spectroscopic analyses. We have characterized aqueous solutions with salts (MgSO4, NaCl, MgCl2, Mg(ClO4)2, Na2CO3), volatiles (CO2) and clays (nontronite, montmorillonite) at temperatures down to 223 K and pressures up to 60 MPa. Samples were studied by pressure-temperature (P-T) cycles in two ways: (a) first freezing the solution and pressurizing it (TPPT method) and (b) first pressurizing the solution and then freezing it (PTTP method), in order to examine textural and grain size heterogeneities and fracture formation depending on the method of formation. The cooling of the samples led to the final formation of water ice, hydrated salts and clathrate hydrates. Raman spectroscopy was used to control the mineral assemblages and understand better the crust environments and processes that can explain the resulting values, like the appearance of supercooled brines, amorphous phases and recrystallizations during the P-T cycles. We measured the dielectric properties of these samples with a BDS80 Broadband Dielectric Spectroscopy system (Novocontrol) which allows to work in a frequency range from 1 Hz to 10 MHz and temperatures from 143 to 323 K. Both permittivity and electric conductivity were measured at 0.1 MPa while cooling the samples in temperature steps of 10 K. From these data we estimated, on the one hand, the activation energy for motion of the electric charges of each solution, and on the other hand, the attenuation of the radar wave depending on the chemical composition and the temperature of the sample, and the frequency of the electric field applied [Pettinelli et al. 2015]. The already obtained novel data will be used as reference for a second set of experiments, consisting on the same dielectric properties’ characterization but, in this set, samples will be also subjected to high pressure conditions. References Aglyamov et al. (2017) Bright prospects for radar detection of Europa’s ocean, Icarus, 281, 334-337. Bruzzone et al. (2013) RIME: Radar for Icy moon Exploration, IEEE International Geoscience and Remote Sensing Symposium - IGARSS, Melbourne, 3907-3910. Pettinelli et al. (2015) Dielectric properties of Jovian satellite ice analogs for subsurface radar exploration: A review, Reviews of Geophysics, 53, 593-641.

Using in situ high pressure Raman spectroscopy, two structural changes were observed in a sample of the composition LiLa5O5(VO4)2. Taking this into account and by combining different conditions, three new compounds were further obtained from high pressure–high temperature synthesis. Their crystal structure description was done using the antiphase approach, which implies the presence of oxygen-centered [OLn4] building units, where Ln is La for (1) β-LiLa5O5(VO4)2 and (2) β-LiLa2O2(VO4) or Nd for (3) LiNd5O5(VO4)2 compounds. (1) crystallizes in the triclinic space group P[1 with combining macron] with unit cell parameters of a = 5.8167(15) Å, b = 12.2954(28) Å, c = 18.7221(69) Å, α = 102.03(2)°, β = 98.76(2)°, and γ = 103.54(2)°; a 3D structure was deduced from the ambient pressure polymorph. (2) also crystallizes in P[1 with combining macron] with a = 5.8144(7) Å, b = 5.8167(7) Å, c = 8.5272(1) Å, α = 98.184(7)°, β = 100.662(7)° and γ = 92.579(7)°. It shows a 2D structure with [La2O2]2+ layers surrounded by [LiO4] and [VO4] tetrahedra sharing corners and edges. (3) exhibits a 3D architecture isotypic with AP-LiLa5O5(VO4)2. The crucial role of high pressure in such types of synthesis and materials is also discussed. 49

In this work, we study the structural changes of the polar [NH4][Cd(HCOO)3] hybrid perovskite under external hydrostatic pressure. We report a reversible framework rearrangement as a function of pressure characterized by: (i) a gradual modification of one formate ligand, which changes its coordination mode from a bridging syn–anti mode at atmospheric pressure (LP-phase) to a chelating-anti mode at high-pressure (HP-phase) and (ii) a change in the coordination of the Cd2+ cations from six-coordinated (LP-phase) to hepta-coordinated (HP-phase). Very interestingly, this unprecedented framework arrangement displays a large electrical polarization. For instance, the polarization value observed at p = 17.7 GPa is about four times the polarization at atmospheric pressure. Therefore, we report that the external pressure induces a novel framework rearrangement in the polar [NH4][Cd(HCOO)3] hybrid perovskite with enhanced electrical polarization. This structure–property relationship offers new insights for designing novel ferroelectric materials based on pressure-responsive hybrid perovskite materials.

Combination of Raman spectroscopy and electrical resistance measurements carried out under non-hydrostatic stress reveals a direct correlation between the transport properties and the concentration of structural defects in graphite, as measured by the relative intensity of the D and D′ bands. We have found that the piezoelectric behavior of highly defected specimens can be modulated through changes in the microstructure, essentially by affecting the crystallite size. The observed correlation suggests some interesting applications based on the use of Raman spectroscopy as a technique for estimating the relative electrical behavior of microstructurally controlled graphite samples under quasi-uniaxial strain conditions.

Single domain superparamagnetic ferrite nanoparticles with the composition MFe2O4 (M = Fe, Co, Zn) have been prepared by thermal decomposition of metal acetylacetonates in diphenyl ether or dibenzyl ether, using oleic acid in the presence of oleylamine as a stabilizing agent. The Fe, Co and Zn ferrite nanoparticles are monodisperse with diameters of 4.9, 4.4 and 4.7 nm, respectively. The TG and IR results indicate that four or six carboxylate groups per nm2 are bonded at the surface of the particles acting as chelating and/or bridging bidentate ligands depending on the composition. The oleate groups minimize the interparticle interactions in Fe and Zn ferrite samples, while in the Co ferrite sample dipolar interactions produce broad maxima in the ZFC and energy barriers distribution curves. The inversion degree has been estimated from the Raman spectra and the obtained x values have been used to calculate the saturation magnetization and compare them with the experimental MS values. Compared to bulk materials, the magnetization value is higher for the Zn ferrite sample due to its mixed spinel cation distribution. For the Co ferrite sample, and probably for the Fe one, the low value of saturation magnetization seems to be due to the surface disordered layer of canted spins. Compared to non-coated nanoparticles with the same composition and similar size, the oleate groups, covalently bonded to the superficial cations, increase the anisotropy field and decrease the magnetization.

Cationic rearrangement is a compelling strategy for producing desirable physical properties by atomic-scale manipulation. However, activating ionic diffusion typically requires high temperature, and in some cases also high pressure in bulk oxide materials. Herein, we present the cationic rearrangement in bulk Mn2 FeMoO6 at unparalleled low temperatures of 150-300 (o) C. The irreversible ionic motion at ambient pressure, as evidenced by real-time powder synchrotron X-ray and neutron diffraction, and second harmonic generation, leads to a transition from a Ni3 TeO6 -type to an ordered-ilmenite structure, and dramatic changes of the electrical and magnetic properties. This work demonstrates a remarkable cationic rearrangement, with corresponding large changes in the physical properties in a bulk oxide at unprecedented low temperatures.

The first transition-metal-only double perovskite compound, Mn2+ Fe-2(3+) Re5+ O-6, with 17 unpaired d electrons displays ferrimagnetic ordering up to 520K and a giant positive magnetoresistance of up to 220% at 5K and 8 T. These properties result from the ferrimagnetically coupled Fe and Re sublattice and are affected by a two-to-one magnetic-structure transition of the Mn sublattice when a magnetic field is applied. Theoretical calculations indicate that the half-metallic state can be mainly attributed to the spin polarization of the Fe and Re sites.

Pb2MReO6 (M = Co, Cr) perovskites were prepared by high pressure–high temperature method. Rietveld refinements of synchrotron powder X-ray diffraction show that the crystal structure of Pb2CoReO6 is trigonal (space group R-3) with almost complete ordering of Co and Re cations, while Pb2CrReO6 (space group Pm-3m) is a cubic perovskite with one single site for Cr and Re atoms. The difference between the symmetry and the degree of order was further clarified by X-ray absorption spectroscopy that establishes formal oxidation states in these phases as Pb2Co2+Re6+O6 and Pb2Cr3+Re5+O6. Pb2CrReO6 is a simple perovskite with a high magnetic ordering temperature of 643 K. Pb2CoReO6 is a double perovskite with −23% high field negative magnetoresistance at 10 K and 9 T. First-principles calculations of Pb2CoReO6 indicate a half metallic electronic structure.

In Spain there are at least 156 ice caves inventoried in the Pyrenees, Cantabrian Mountains, and Teide. In the Pyrenees there are ice caves in at least seven different massifs and in the Cantabrian Mountains are concentrated mainly in the Picos de Europa. In eight ice caves there have been applied techniques such as endoclimatic survey, geochronology, CCC analysis, mapping, ice structure, mass balances, TLS surveys, orthothermography and GPR surveys from the last 8 years. The ice volume has a significant interannual variation together with a regressive trend during the last decades, and the ice has ages ranging from centuries to a few millennia. In Spain Ice there are all kinds of possible ice caves, from the static to the dynamic, with metamorphic and congelation ice. Due to their risk of disappearance in the short medium term, it is very important to understand the ice cave systems and their past dynamics.

In this work we report on the microwave assisted synthesis of nano-sized Gd2Ti2O7 (GTO) and Ho2Ti2O7 (HTO) powders from the RE2Ti2O7 pyrochlore family (RE = rare earth). Synchrotron X-ray powder diffraction was used to study RE–Ti cationic anti-site defects with concentrations that decrease in both samples with increasing temperature starting from 1100 °C, and the defects disappear at 1400 °C. SQUID magnetometry measurements revealed that GTO shows a predominantly anti-ferromagnetic structure, whereas HTO exhibits magnetic saturation and a ferromagnetic component at low temperature. Impedance spectroscopy data revealed strongly increased ionic oxygen vacancy conduction in HTO ceramic pellets as compared to GTO, which may be associated with a higher degree of oxygen vacancy disorder. This argument was supported by Raman spectroscopy data.

Multiferroic materials exhibit two or more ferroic orders and have potential applications as multifunctional materials in the electronics industry. A coupling of ferroelectricity and ferromagnetism is hereby particularly promising. We show that the synthetic melanostibite mineral MnFeSbO (R (Formula presented.) space group) with ilmenite-type structure exhibits cation off-centering that results in alternating modulated displacements, thus allowing antiferroelectricity to occur. Massive magnetoelectric coupling (MEC) and magnetocapacitance effect of up to 4000 % was detected at a record high temperature of 260 K. The multiferroic behavior is based on the imbalance of cationic displacements caused by a magnetostrictive mechanism, which sets up an unprecedented example to pave the way for the development of highly effective MEC devices operational at or near room temperature., The authors thank MINECO for funding through projects MAT2013‐44964‐R, MAT2013‐41099‐R, MAT2015‐71070‐REDC, MAT2014‐52405‐C02‐02, CTQ2015‐67755‐C2‐1‐R (MINECO/FEDER) and FPI (BES‐2013‐066112) and Ramon y Cajal (RyC‐2010–06276) fellowships, and Comunidad de Madrid through S‐2013/MIT‐2753 grant.

The AA′3B4O12-type quadruple perovskite LaCu3Ir4O12 prepared at high pressure (9 GPa) and temperature (1523 K) crystallizes in cubic symmetry (Im3, a = 7.52418(3) A) with square planar CuO4 and octahedral IrO6 coordination as established from synchrotron powder X-ray diffraction studies. Both crystal structure and X-ray absorption near edge spectroscopy analyses indicate formal oxidation states of LaCu2+3Ir3.75+4O12. The temperature dependence of resistivity of LaCu3Ir4O12 is metallic down to 10 K, with Femi-liquid behavior above T* ∼ 155 K, and non-Fermi-liquid behavior below T*. The two-fluid behavior of magnetic susceptibility and the dramatic downturn of the resistivity below T* indicate strong Cu2+ 3d and Ir3.75+ 5d orbital hybridization below T*, also supported by an enhanced electronic specific heat coefficient at low temperature. Theoretical calculations are in good agreement with the experimental results and show that the electronic structure of LaCu3Ir4O12 is different from that of CaCu2+3Ir4+4...

The A-site ordered perovskite oxide CdCu3Mn4O 12 has been synthesized for the first time in polycrystalline form under high pressure (7 GPa) and high temperature (1000 °C) conditions, required to stabilize the small Cd and Cu cations at the A positions of the perovskite. The crystal structure has been studied by X-ray powder diffraction at room temperature. This oxide crystallizes in the cubic space group Im3̄ (no. 204) with the unit-cell parameter a = 7.2179(5) Å at 300 K. The MnO6 network is extremely tilted, giving rise to a square planar coordination for Cu2+ cations. The magnetic characterization shows that this compound is ferrimagnetic with an ordering temperature TC = 347 K, well above room temperature. A metallic behavior is displayed between 10 and 300 K. Negative magnetoresistance (MR) of 15% is achieved at 10 K for H = 9 T; MR is still significant at room temperature, displaying values above 7% for H = 9 T. Density functional theory calculations carried out on the density of states lead to electronic and magnetic features in good agreement with the obtained experimental results. © 2014 American Chemical Society., We thank the financial support of the Spanish Ministry of Science and Innovation to project MAT2010-16404, of the Comunidad de Madrid to project S2009PPQ-1551 (QUIMAPRES), and of the Spanish Consolider Ingenio 2010 Program (project CDS2007-00045). J.S.-B. acknowledges receipt of a Ramón y Cajal Fellowship (RyC-2010-06276) from the Ministerio de Economía y Competitividad.

The structural and magnetic properties of the Al-doped La 0.75 Sr 0.25 Mn 0.5 Cr 0.5− x Al x O 3− δ ( x = 0.0, 0.1, 0.2, 0.3) were investigated by X-ray powder diffraction, neutron powder diffraction and magnetization measurements. Rietveld refinement of the diffraction data confirms that the compounds crystallize in rhombohedral symmetry (space group, R-3C ). Unit cell volume decreases with increasing concentration of Al at the B-site. Upon cooling from room temperature, we have observed multiple magnetic phase transitions, i.e. paramagnetic (PM), ferromagnetic (FM), antiferromagnetic (AFM) and spin–glass (SG), in the samples. A low temperature magnetic hysteresis study demonstrates the presence of ferromagnetic domains for all compositions. The antiferromagnetic transition temperature decreases with the Al-doping AC susceptibility measurements at 97 Hz and 1 Oe show SG behaviors with a spin-freezing temperature close to 50 K for all samples. The in-phase ac susceptibility ( χ / ) decreases in magnitude and spin–glass transition ( T SG ) increase toward higher temperature with increasing frequency. The spin–glass behavior accompanied by the anomalous magnetic transitions is due to the competing interactions between FM and AFM. The results also shows that a part of the samples lose magnetic order to form a SG state accompanied by an AFM state at low temperature.

Perovskite hydrides containing light elements are attractive for hydrogen storage. We describe a new series of composition NaKMgH (0 ≤ x ≤ 1) prepared by an original synthesis method based on the direct reaction of simple hydrides under high-pressure and high-temperature conditions. Mixtures of MgH, NaH and KH were enclosed in gold capsules and treated in a piston-cylinder hydrostatic press at moderate pressures of 2 GPa and temperatures around 775 °C. Well-crystallized hydride samples were analyzed by x-ray and neutron powder diffraction, collecting excellent patterns even in non-deuterated samples that provided an accurate description of the crystal structure features. For x ≤ 0.75 the perovskites are isostructural with GdFeO-type oxides, characterized by the same tilting scheme of the MgH octahedra. For the end-member KMgH the structure is cubic, adopting the canonical aristotype of the perovskite structure. The analysis of the tolerance factor across the series accounts for the stability of those samples in the studied compositional range., This research was supported by the MICINN and MINECO under projects MALTA-Consolider Team (CSD2007-00045, MAT2015-71070-REDC), MAT2013-41099-R and CTQ2015-67755-C2-1-R (MINECO/FEDER). A. A-C. acknowledges support from a MINECO-FPI grant (BES-2013-066112).

New complex perovskites of the series CaCu(MnRe)O have been prepared from citrate precursors under moderate pressure conditions of 2 GPa and 1000 °C, in the presence of KClO as oxidizing agent to stabilize Mn/Mn mixed valence. The polycrystalline samples have been characterized by x-ray diffraction, neutron powder diffraction (NPD), magnetic, and magnetotransport measurements. The crystal structures are cubic, space group Im-3. The unit-cell parameters increase from a = 7.2379(2) Å for the parent (x = 0) compound to a = 7.2420(4) Å for CaCu(MnRe)O. Both oxides adopt a superstructure of the perovskites ABO with long-range 1:3 ordering for Ca and Cu ions at the A sublattice. For the compound doped with Re (x = 0.1), the result of a NPD study shows that Re ions are randomly located at the octahedral positions, being the (Mn,Re)O octahedra strongly tilted, with superexchange (Mn,Re)-O-(Mn,Re) angles of 142.03°. Neutron diffraction data clearly show that some Mn ions are located together with Cu at the square-planar 6b positions. The magnetic structure determined from low-temperature NPD data unveils a ferromagnetic coupling between (Mn,Re) spins at octahedral positions and weak antiferromagnetism with (Cu,Mn) spins. Interestingly, an enhancement of the magnetoresistance effect is observed for the Re-doped compound, well beyond that found for the parent perovskite., We thank the financial support of the Spanish Ministry of Economy and Competitiveness to the projects MAT2013-41099-R and RyC-2010-06276.

Whether $\mathrm{CaCr}{\mathrm{O}}_{3}$ is a Mott insulator or a correlated metal is still controversial. We have performed measurements of magnetization, specific heat, and thermal conductivity on $\mathrm{CaCr}{\mathrm{O}}_{3}$ samples selected from many batches of high-pressure synthesis. The single-crystal $\mathrm{CaCr}{\mathrm{O}}_{3}$ sample exhibits an unprecedentedly sharp transition at a N\'eel temperature ${T}_{N}\ensuremath{\approx}90\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. The critical behavior of specific heat cannot be rationalized by the renormalization group theory for a second-order magnetic transition. More surprisingly, the thermal conductivity $\ensuremath{\kappa}$ exhibits an anomalous drop on cooling through ${T}_{N}$, which is opposite to all known influence on $\ensuremath{\kappa}$ from either spin or orbital ordering. We have argued, on the basis of anomalies found in all three measurements and structural data, for the coexistence of itinerant \ensuremath{\pi}-bonding electrons in a $c$-axis band and localized $xy$ electrons in $xy$ orbitals responsible for type-C antiferromagnetic order below ${T}_{N}$ and the occupation of a pure, localized $xy$ orbital undergoing a Bose-Einstein condensate at ${T}_{N}$.

Magnesium base alloys are very attractive for hydrogen storage due to their large hydrogen capacity, small weight and low-cost. We have designed a new synthesis method for the ternary metal hydride perovskite system Na1−xLixMgH3, based on the direct reaction of simple hydrides under high-pressure and moderate-temperature conditions. Well-crystallized samples were obtained in a piston-cylinder hydrostatic press at moderate pressures of 2 GPa and temperatures around 750 °C from mixtures of MgH2, NaH and LiH enclosed in gold capsules. X-ray and neutron powder diffraction analysis were used to identify the purity of the samples and provide an accurate description of the crystal structure features (GdFeO3 type). Na1−xLixMgH3 hydrides series (0 ≤ x ≤ 0.18) show an orthorhombic symmetry with space group Pnma (No. 62). Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) have been carried out to determine the hydrogen desorption temperatures.

Many transition-metal oxides in elevated valence states [e.g. Mn(V), Co(IV), Ni(III), Cu(III) ] present a metastable character and, given the difficulty of their synthesis, have been relatively little studied. However, they are very interesting materials presenting strong electronic correlations that are bound to exotic properties such as superconductivity, metal behavior, metal–insulator transitions or colossal magnetoresistance. The metastability of these compounds requires special synthesis conditions such as the application of high pressure. In the last years, we have prepared and investigated a good number of materials belonging to several families such as RNiO3 (R = rare earths), Ba3Mn2O8 , (Ba,Sr)CoO3 , La2(Ni,Co)O4+δ , etc. In the study and correct characterization of these oxides it has been decisive the use of elastic neutron diffraction, most of the times in powder samples. This technique has allowed us to access the structural details typically related to the octahedral tilting in perovskite structures, the oxygen stoichiometry and order–disorder of the oxygen sublattice, the distinction between close elements in the Periodic Table, the resolution of magnetic structures and, in general, the establishment of a correlation between the structure and the properties of interest. This letter is organized around the binomial "high-pressure synthesis" and "characterization by neutron diffraction" and illustrated with some selected examples among the metastable materials above mentioned.

We have designed a new synthesis method for the ternary metal hydride Mg2FeH6 based on the direct reaction of simple hydrides under high-pressure conditions. Well-crystallized samples were prepared in a piston-cylinder hydrostatic press at 2 GPa and temperatures around 750 °C from mixtures of MgH2 and Fe enclosed in gold or platinum capsules. Seven different samples have been prepared under different conditions. X-ray powder diffraction analysis was used to identify and assess the purity of the samples, through Rietveld analyses of the crystal structure (K2PtCl6-type). Mg2FeH6 shows a cubic symmetry with space group Fm-3m. SEM images show an average particle size of 1–2 μm for Mg2FeH6; the microcrystals present well-grown faces and display a high homogeneity of shapes and sizes. Thermogravimetric analysis has been carried out to determine not only the hydrogen desorption temperature but also the hydrogen contents.

The crystal structure of CaH2 has been studied from neutron powder diffraction (NPD) at 295 K in a non-deuterated sample; a good quality NPD pattern was obtained in spite of the hydrogen incoherent scattering. The structure was refined by the Rietveld method in the Pnma space group (No. 62), Z = 4, with unit-cell parameters a = 5.9600(1), b = 3.6006(7) and c = 6.8167(1) Å. The two kinds of crystallographically independent H atoms, H1 and H2, are located in tetrahedral and square-pyramidal cavities, respectively, while Ca ions are nine-fold coordinated to hydrogen atoms. The average 〈Ca—H1〉 and 〈Ca—H2〉 bond lengths are 2.279 and 2.544 Å, respectively. Bond valence calculations show that Ca—H1 bonds are under compressive stress, whereas Ca—H2 bonds undergo tensile stress in a structure with a relatively high global instability index. It is also remarkable that the displacement factors for H2 are significantly larger than for H1, suggesting an increased lability for the Ca—H2 bonds. We provide with an analysis of the isotope effect, by comparing the present results on CaH2 with literature data on CaD2; we indeed observe a higher distortion of the H1 and H2 coordination polyhedra with respect to the deuteride, as observed in other isostructural dihydrides.

We have studied the series of nominal composition LaCu3(Mn4–x Fe x )O12 (x = 0.5, 1) where Mn is replaced by Fe cations in the ferrimagnetic perovskite LaCu3Mn4O12. These materials have been synthesized in polycrystalline form under moderate pressure conditions of 2 GPa, in the presence of KClO4 as oxidizing agent. All the samples have been studied by neutron powder diffraction (NPD) at room temperature (RT) in order to study the crystal structure. The sample with nominal x = 1 has also been studied by NPD at low temperature to determine its magnetic structure. These oxides crystallize in the cubic space group Im-3 (No. 204) with general formula A'A3B4O12 and contain La3+ and (Cu2+, Mn3+) at the A sublattice of the perovskite, in a 1 : 3 ordered arrangement. NPD suggests that Mn4+/Mn3+ and Fe3+ occupy at random the octahedral B positions of the perovskite structure. The materials have also been characterized by magnetic and magnetotransport measurements. All the samples are ferrimagnetic and show a progressive decrease of T C as the Fe content increases. The replacement of Mn3+ cations by Fe3+ at the octahedral sublattice give rise to a decrease of T C since the Fe ions disturb the ferromagnetic interactions within this magnetic sublattice. The introduction of Fe changes the resistivity response from metallic to a semiconductor behavior. However, the magnetoresistance remains unchanged upon Fe doping and a sharp low-field response is observed below T C.

A combination of high-pressure single crystal X-ray diffraction and high-pressure SQUID magnetometry has been used to study two oxo-bridged mixed-valent MnIII/MnIV dimers. [Mn2O2(bpy)4](ClO4)3·3CH3CN, (1·3CH3CN; bpy = 2,2ʹ-bipyridine) has been compressed to 2.0 GPa whilst [Mn2O2(bpy)4](PF6)3·2CH3CN·1H2O, (2·2CH3CN·1H2O) could be measured crystallographically up to 4.55 GPa. The PF6 salt of [Mn2O2(bpy)4]3+ has never been reported before while 1 has been reported as a hydrate and in a different crystallographic space group. The application of hydrostatic pressure imposes significant distortions and modifications in the structures of both complexes. In particular, in complex 1·3CH3CN the Mn-Mn separation is reduced by the contraction of some of the Mn-O bond lengths, whilst in 2·2CH3CN·1H2O the Mn-O-Mn bridging angles and the Mn-O bond lengths are substantially unchanged. Interestingly 2·2CH3CN·1H2O also shows a constant contraction in nearly all the Mn-N bonds. The magnetic behaviour of the complexes has been measured up to 0.87 GPa for 1·3CH3CN and 0.84 GPa for 2·2CH3CN·1H2O.

A combination of high pressure single crystal X-ray diffraction and high pressure SQUID magnetometry has been used to study three hydroxo-bridged copper(II) dimers. [Cu2(OH)2(H2O)2(tmen)2](ClO4)2 (1; tmen = tetramethylethylenediamine), [Cu2(OH)2(tben)2](ClO4)2 (2; tben = di-tbutylethylenediamine) and [Cu2(OH)2(bpy)2](BF4)2 (3; bpy = 2,2'-bipyridine) have been structurally determined to 2.5, 0.9 and 4.7 GPa, respectively. The application of hydrostatic pressure imposes significant distortions and modifications in the structures of all three complexes. This is particularly true of the bond distances and angles between the metal centres and the bridging hydroxo groups. Compound 1 undergoes a phase transition between 1.2 and 2.5 GPa caused by the loss of a coordinated water molecule. This leads to a loss of symmetry and dramatic changes in the molecular structure of the complex. The structural changes are manifested in changes in the magnetic behaviour of the complexes as seen in dc susceptibility measurements up to approximately 0.9 GPa for 1, 2 and 3: the exchange becomes less antiferromagnetic in 1 and 2 and more ferromagnetic in 3.

The complex perovskite ThCu3Mn4O12 has been prepared at moderate pressures of 2 GPa. With respect to the parent compound CaCu3Mn4O12, the replacement of Ca2+ by Th4+ involves a double electronic injection that leads to a substantial increment of TC, up to 370 K. The crystal structure was refined in the space group Im3̄ from NPD data collected with λ = 1.33 Å at r. t. An additional NPD pattern recorded at 1.8 K with λ = 2.42 Å allowed to refine the magnetic structure, which displays a ferrimagnetic coupling between Mn3+/Mn4+ and Cu2+ spins, aligned along the c direction. The refined magnetic moments at the Mn and Cu substructures of 2.5 and −0.5 μB, respectively, account for the observed saturation magnetisation at 2 K, of 7 μB/f. u. A semiconducting behaviour is observed between 10 and 350 K which can be correlated with the appearance of a gap in the conduction band for the ~50% Mn3+/50% Mn4+ mixed valence observed in the B substructure of this perovskite.

Folding under pressure: Crystallographic studies on a Mn6 single-molecule magnet under high pressure conditions show the drastic structural changes of the magnetic core (see picture, Mn purple, O red, N blue), which impact on the magnetic properties of ferromagnetic exchange between the metal atoms will be in booster weaker, and under extremely high pressure, a transition to antiferromagnetic behavior.

Light scattering experiments have been performed under hydrostatic pressures up to 90 kbar in NH4[Eu(SO4)2]. This material behaves as an active catalyst and presents a layered structure with two kinds of sulfate groups bonded to Eu3+ ions. Raman phonons related to the SO4 groups as well as Eu3+ photoluminescence spectra evidence the induction by pressure of two discontinuous structural transformations at around 30 and 70 kbar. No signs of pressure induced disorder is detected, on the contrary, the narrower phonons correspond to the 65 kbar phase. At low pressures the structure accommodates mainly through an anisotropic compression along the a-axis perpendicular to the Eu(SO4)2 layers.

We report high-pressure X-ray diffraction and electrical measurements of the topological insulator SnBi2Te4 at room temperature. The pressure dependence of the structural properties of the most stable phase of SnBi2Te4 at ambient conditions (trigonal phase) have been experimentally determined and compared with results of our ab initio calculations. Furthermore, a comparison of SnBi2Te4 with the parent compound Bi2Te3 shows that the central TeSnTe trilayer, which substitutes the Te layer at the center of the TeBiTeBiTe layers of Bi2Te3, plays a minor role in the compression of SnBi2Te4. Similar to Bi2Te3, our resistance measurements and electronic band structure simulations in SnBi2Te4 at high pressure suggest that this compound exhibits a pressure-induced electronic topological transition or Lifshitz transition between 3.5 and 5.0 GPa. (C) 2016 Published by Elsevier B.V., We thank Dr. Philipp Urban for preparing the sample. This work has been performed under financial support from Spanish MINECO under projects MAT2013-46649-C4-2-P, MAT2015-71070-REDC and CTQ2015-67755-C2-1-R and from Spanish Ministerio de Educacion, Cultura y Deporte as part of "Programa Campus de Excelencia Internacional/Programa de Valoracion y Recursos Conjuntos de I + D + i VLC/CAMPUS" through projects SP20140701 and SP20140871. One of the experiments were performed at MSPD-BL04 beamline at ALBA Synchrotron with the collaboration of ALBA staff. J.A.S. thanks "Juan de la Cierva" fellowship program for funding. A. A.-C. and J.S.-B. are also grateful to Spanish MINECO for the FPI (BES-2013-066112) and Ramon y Cajal (RyC-2010-06276) fellowships. We acknowledge Diamond Light Source for time on beamline I15 under Proposal EE9102.

The title perovskites (obtained from citrate precursors at 900 °C for 12 h) are characterized by XRD, neutron powder diffraction, and magnetic measurements.

We report on the design of a miniature high-pressure cell designed to work at sub-Kelvin temperatures with 3He insert into a commercially available physical property measurement system (PPMS) which is popular among researchers. The issue of severe space limitation around the 3He sample platform has been overcome by using a new design for an opposed-anvil pressure cell, which we called a split-cylinder design. To our knowledge, this is the first pressure cell created for 3He insert into PPMS and probably one of the smallest pressure cells ever made for resistivity measurements. The pressure cell itself has been built from non-magnetic Cu–Be alloy and used with diamond anvils generating pressures of 10 GPa. The cell has been used for electrical resistivity measurements using standard PPMS data acquisition system. In addition to the pressure cell, a special holder has been built to maintain the alignment of the cell during pressure applications.

RNiMnO (R = Tb, Ho, Er, Tm) perovskites have been prepared by soft-chemistry techniques followed by high oxygen-pressure treatments; they have been investigated by X-ray diffraction, neutron powder diffraction (NPD), and magnetic measurements. In all cases the crystal structure is defined in the monoclinic P2/n space group, with an almost complete order between Ni and Mn cations in the octahedral perovskite sublattice. The low temperature NPD data and the macroscopic magnetic measurements indicate that all the compounds are ferrimagnetic, with a net magnetic moment different from zero and a distinct alignment of Ni and Mn spins depending on the nature of the rare-earth cation. The magnetic structures are different from the one previously reported for LaNiMnO, with a ferromagnetic structure involving Mn and Ni moments. This spin alignment can be rationalized taking into account the Goodenough-Kanamori rules. The magnetic ordering temperature (T) decreases abruptly as the size of the rare earth decreases, since T is mainly influenced by the superexchange interaction between Ni and Mn (Ni-O-Mn angle) and this angle decreases with the rare-earth size. The rare-earth magnetic moments participate in the magnetic structures immediately below T., We acknowledge the financial support of the Spanish Ministry of Economy and Competitiveness through the projects MAT2013-41099-R and CSD2007-00045 and funding of the Universidad Complutense de Madrid and Banco Santander through the project UCM2014-971703. J.S.-B. acknowledges receipt of a Ramón y Cajal Fellowship (RyC-2010-06276) from the Spanish Ministry of Economy and Competitiveness.

This work was supported by the National Science Foundation (DMR MIRT 1122603) and Robert A. Welch Foundation (F-1066) in the U.S. J.A.A., R.M.C., J.S.B., and M.T.F. thank the Spanish MINECO Projects No. MAT2013-41099-R and No. RyC-2010-06276 for financial support.

Pb2MReO6 (M: Co (I), Cr (II)) perovskites are prepared by hot pressing of stoichiometric amounts of PbO, Cr2O3 (CoO), Re, and ReO3 (Pt capsule in multianvil press, 1423 (I) and 1473 K (II), 8 GPa, 30 min).

We have synthesized two new series of metastable oxides, namely REMn2O5 and RECu3Mn4O12 (RE = rare earths) under moderate pressure conditions. A novel series of ferrimagnetic oxides has been obtained by replacing Mn3+ by Fe3+ in the parent REMn2O5 compounds (RE = Y, Dy, Ho, Er, Tm, Yb). The crystal structure has been studied by neutron powder diffraction (NPD); it contains chains of edge-linked Mn4+O6 octahedra connected via dimeric groups of Fe3+O5 square pyramids. The magnetic susceptibility and the thermal evolution of the NPD patterns reveal the onset of a ferrimagnetic structure below TC ≈ 165 K, characterized by the propagation vector k = 0. Immediately below TC, the Fe3+ and Mn4+ moments lie in an antiparallel arrangement along the c-axis direction. At lower temperatures, the magnetic moment of the rare-earth cations also participates in the magnetic structure, adopting a parallel arrangement with the Fe3+ spins. Some new derivatives of CaCu3Mn4O12 have been prepared at moderate pressures of 2 GPa by replacing Ca2+ by RE3+ cations in the series RECu3Mn4O12 (RE = Pr, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Yb); the concomitant electronic injection leads to a substantial contribution to TC. The crystal structures of the new materials were refined in the space group Im3̅ from NPD data for the non-absorbing RE cations. The unit cell parameters are considerably expanded with respect to CaCu3Mn4O12, as a result of the electronic injection. The r. t. magnetic structure displays a ferrimagnetic coupling between Mn3+/4+ and Cu2+ spins; at low temperatures there is an antiferromagnetic coupling of the RE3+ moments with the Mn substructure, which substantially reduces the susceptibility and the saturation magnetization.

The effect of substituting Ti for Mn in the ferrimagnetic perovskite CaCu3Mn4O12 has been studied in the series CaCu3Mn4−xTixO12 (x = 0.3,0.5,1.0,1.5,2.0,3.0). These materials have been prepared in polycrystalline form under moderate pressure conditions of 2 GPa and 1000 °C in the presence of KClO4 as an oxidizing agent. The crystal structure is cubic, space group (No. 204); the unit cell parameters vary linearly from a = 7.2361(4) A (x = 0.3) to a = 7.3489(5) A (x = 3.0) at room temperature (RT). A neutron powder diffraction study has been performed for a selected sample of nominal composition CaCu3Mn3TiO12. In the ABO3 perovskite superstructure, the A positions are occupied by Ca2+ and (Cu2.52+Mn0.53+), ordered in a 1:3 arrangement, giving rise to the body-centring of the unit cell. At the B positions, Mn and Ti are randomly distributed over the octahedral sites; (Mn,Ti)O6 octahedra are considerably tilted by 19°, due to the relatively small size of the A-type cations. The Curie temperatures decrease from 331 K (x = 0.3) to 310 K (x = 3.0). The saturation magnetization at 5 K is strongly reduced upon Ti introduction, from Ms = 10.4 μB fu−1 (x = 0.3) to 1.0 μB fu−1 (x = 3.0). All the samples exhibit negative magnetoresistance (MR), reaching a maximum value of 41% for the x = 0.5 sample at 5 K for H = 9 T; the MR(9 T) at RT is as high as 7% for x = 0.5, and shows an appreciable temperature stability.

Neutron powder diffraction, magnetic and magnetotransport studies were carried out on new derivatives of the CaCu3Mn4O12 (A′A3B4O12) complex perovskite. The samples were prepared in polycrystalline form under moderate pressure conditions. Substitutions at A and A′ sites of CaCu3Mn4O12, with only Mn4+ and insulating behavior, imply electron doping that affects the magnetic and transport properties. X-ray Absorption Spectroscopy showed that Mn3+/Mn4+ valence mixing occurs only at B site, progressively filling the eg band and providing the metallic character in these compounds, as we observe in most of these samples. A semiconducting behavior is observed in samples with 50% Mn3+ at B site. This can be understood by the opening of a gap in the conduction band corresponding to the half filling of the eg states. This is the case of the tetravalent rare earth doped samples (Ce and Th at A’ site) and of the appropriate A site doped Ca(CuMn2)Mn4O12 sample. At the strongly distorted A positions, Mn3+, with localized eg electrons, act as magnetic impurities at very low temperatures (

We report on the study of some new derivatives of the complex perovskite CaCu 3 Mn 4 O 12 . Ca 2+ cations can be replaced by rare earths in the RCu 3 Mn 4 O 12 (R = rare earths) family, implying an electron doping effect that dramatically affects the magnetic properties. These compounds must be prepared under high pressure conditions. We have synthesized for the first time, at 2 GPa in a piston-cylinder press, some new members of the family, with R = Pr, Sm, Eu, Gd, Dy, Ho, Tm and Yb, for which we describe structural and magnetization data. The materials with non-absorbing rare earths have been studied by means of neutron powder diffraction, allowing us to determine the subtle structural features which are related to the observed physical properties. In fact, for most of the R cations, the ferromagnetic T C increases remarkably up to 400 K.

A stoichiometric calcium vanadium (IV) oxide with formula CaVO3 has been prepared by soft-chemistry procedures, followed by annealing under reducing conditions (H2/N2 flow). This material has been studied by X-ray and neutron powder diffraction (NPD), thermal analysis, magnetic and magnetotransport measurements. CaVO3.0 perovskite crystallizes in the orthorhombic Pbnm (No. 57) space group, with the GdFeO3-type structure. The unit-cell parameters are a=5.3219(1) A , b=5.3427(1) A and c=7.5472(1) A . In this distorted perovskite the VO6 octahedra are tilted by 10.1° in order to optimize the Ca–O bond-lengths. A bond valence study from NPD data confirms the tetravalent oxidation state for V cations. The perovskite is fully oxygen stoichiometric, as demonstrated from thermal analysis and the refinement of the oxygen occupancy factors. The magnetic susceptibility is predominantly Pauli paramagnetic-like, although a non-negligible temperature-dependent component due to isolated V4+ spins is patent at low temperatures. The transport measurements show a metallic behavior between 2 and 300 K; at low temperatures a positive magnetoresistance as large as 14% for H=9 T is interpreted as the result of quantum interference effects.

CaCu2.5Mn4.5O12 perovskite has been prepared in polycrystalline form under moderate pressure conditions of 20 kbar, in the presence of KClO4 as oxidizing agent. This material has been studied by X-ray and neutron powder diffraction (NPD), magnetic, magnetotransport, and thermopower measurements. The crystal structure is cubic, space group Im3 (No. 204), with a = 7.2279(1) A at room temperature (RT). In the ABO3 perovskite superstructure, the A positions are occupied by Ca2+ and (Cu2.52+Mn0.53+), ordered in a 1:3 arrangement giving rise to the body-centering of the unit cell. At the B positions, Mn adopts a mixed oxidation state of 3.875+; MnO6 octahedra are considerably tilted by 19°, due to the relatively small size of the A-type cations. The Curie temperature is 345 K. Low temperature (2 K) NPD data show evidence for a ferrimagnetic coupling between Mn3.875+ and (Cu2.52+Mn0.53+) spins, with ordered magnetic moments of 2.32(5) and −0.54(3) μB, respectively. An additional canting effect of the Mn moments...

The insulating character of undoped manganites RMnO 3 is related to the Jahn-Teller (JT) distortion of the MnO 6 octahedra, which can be reduced by applying hydrostatic pressure. We analyze the dependence of the Raman phonons with pressure up to 10 GPa, for PrMnO 3 and TbMnO 3 . The variation of the stretching and bending modes indicates that the compressibility of Mn-O bonds is higher than R-O ones in PrMnO 3 , but smaller in TbMnO 3 . The variation of the tilt mode frequency, in TbMnO 3 , can be explained by an increase of the octahedra tilt angle, which is consistent with a larger compressibility of the Tb-O bonds. Therefore, while PrMnO 3 evolves towards the structure of the metallic-ferromagnetic doped perovskites, TbMnO 3 does not.

We have studied and compared the optical properties of SrAl1.7B0.3O4:Eu, R (R=Nd, Dy) pigments that present long-lasting phosphorescence obtained by different synthesis techniques. Samples obtained by ceramic methods, in our laboratories and by an industrial process, present better phosphorescent properties than those obtained by sol–gel technique. Raman spectra show that grinding produces severe damage of the lattice. We have obtained and analyzed the Eu3+ crystal field luminescence indicating that Eu3+ is found in quite different sites comparing ceramic and sol–gel samples. Codoping, with Nd or Dy is necessary in order to reduce the Eu3+ content, in all cases. The green luminescence band, obtained under UV illumination, can be fitted to two and three components in ceramic and sol–gel samples, respectively, due to different Eu2+ sites. Eu–Dy samples present the longest and the most efficient phosphorescence. The time evolution of the afterglow is well described by a t−1 law, up to about 2 h, indicating that the recombination process is achieved by electron–hole tunneling.