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Structural phase transitions are accompanied by a movement of one nucleus (or a few) in the crystallographic unit cell. If the nucleus movement is continuous, a second‐order phase transition without latent heat results, whereas an abrupt nucleus displacement indicates a first‐order phase transition with accompanying latent heat. Herein, a Hamiltonian including electron–phonon coupling (EPC) as proposed by Kristoffel and Konsel is taken. Contrary to their treatment, both the kinetic energy of the nucleus and its position are treated. The interaction of the many‐electron system with the single nucleus is taken into account by the Born–Oppenheimer approximation and perturbative expressions for the free energies are derived. The nuclei corrections due to the entangled electrons are found to be minor, but highlight the importance of the symmetry breaking at low temperature. Furthermore the free energy for a canonical ensemble is computed, whereas Kristoffel and Konsel use a grand canonical ensemble, which allows to derive more stringent bounds on the free energy. For the zero‐order nucleus correction, the shift of the phase transition temperature by evaluating the free energy is deduced. [ABSTRACT FROM AUTHOR]

Understanding the origin of low thermal conductivities in ionic conductors is essential for improving their thermoelectric efficiency, although accompanying high ionic conduction may present challenges for maintaining thermoelectric device integrity. This study investigates the thermal and ionic transport in Cu7PSe6, aiming to elucidate their fundamental origins and correlation with the structural and dynamic properties. Through a comprehensive approach including various characterization techniques and computational analyses, it is demonstrated that the low thermal conductivity in Cu7PSe6 arises from structural complexity, variations in bond strengths, and high lattice anharmonicity, leading to pronounced diffuson transport of heat and fast ionic conduction. It is found that upon increasing the temperature, the ionic conductivity increases significantly in Cu7PSe6, whereas the thermal conductivity remains nearly constant, revealing no direct correlation between ionic and thermal transport. This absence of direct influence suggests innovative design strategies in thermoelectric applications to enhance stability by diminishing ionic conduction, while maintaining low thermal conductivity, thereby linking the domains of solid‐state ionics and thermoelectrics. Thus, this study attempts to clarify the fundamental principles governing thermal and ionic transport in Cu+‐superionic conductors, similar to recent findings in Ag+ argyrodites. [ABSTRACT FROM AUTHOR]

New data on phase states and structural phase transitions in alloys Fe73Ga27 doped with Dy, Er, Tb, and Yb in an amount of about 0.5 at % are presented. Structural data were obtained in neutron diffraction experiments performed with high resolution and in continuous temperature scanning mode during heating to 850°C and subsequent cooling at a rate of ±2°C/min. It has been established that both the sequence of forming and disappearing structural phases and the final state of the alloy depend on the type of rare earth element. Phase transitions in the alloy with Dy are similar to those in the initial Fe73Ga27 alloy, excluding the final state. The procedure of doping with Er and Tb leads to the formation of disordered A1 and A3 phases instead of the L12 and D019 ordered close packed phases, respectively. In the case of doping with Yb, neither of the above phases is observed. The formation of the L60 (m-D03) and D022 tetragonal structural phases previously discovered in similar alloys by the electron diffraction method is not confirmed. [ABSTRACT FROM AUTHOR]

The crystal structure of the solid solutions Bi0.65Ba0.35-zSrzFe0.65Ti0.35O3 within the dopant concentration range 0 ≤ z ≤ 0.35 was investigated based on the results of X-ray diffraction, infrared and Raman spectroscopy. The morphology and chemical composition of the solid solutions were studied using scanning electron microscopy and energy-dispersive X-ray spectroscopy. It is shown that all the studied compounds are characterized by high chemical homogeneity; an increase in the concentration of strontium ions leads to a decrease in the average crystallite size, while the morphology of crystallites remains practically unchanged. The obtained structural data allowed to estimate the changes in the crystal structure occurred within the region of the “rhombohedral-cubic” morphotropic phase boundary. The initial compound Bi0.65Ba0.35Fe0.65Ti0.35O3 is characterized by a two-phase structure, assuming a coexistence of the polar rhombohedral and cubic phases. Wherein the cubic phase is dominant in the initial compounds, and an increase in the concentration of Sr ions leads to the structural transition to the single-phase cubic state. An inhomogeneous structural state in the compounds under study has been determined to consist of the main matrix with paraelectric cubic structure, along with polar active nanometer-sized clusters having rhombohedral type of the unit cell. The presence of the nanosize polar clusters is confirmed by the results of dielectric measurements. The obtained data allowed to establish the evolution of the structural parameters during the phase transition from the rhombohedral to the cubic structure, as well as to determine the concentration region of the two-phase structural state and to specify the features of dielectric properties in the region of the morphotropic “rhombohedral-cubic” phase boundary.

Abstract—New data on the phase compositions and structural transformations in a number of Fe81Ga19 alloys doped with trace amounts (≤0.2 at %) of rare earth elements are presented. The data were obtained in neutron diffraction experiments performed with high resolution and in a continuous temperature scanning mode under heating to 900°C and subsequent cooling. It has been established that structural rearrangements generally proceed in an identical manner both in the original Fe81Ga19 alloy and in its doped analogues. Slow heating and subsequent cooling of the alloys (at a rate of ±2°C/min) leads to the formation of clusters of the D03 phase with sizes in the range of 200–300 Å in the matrix of the disordered A2 phase. The sizes and volume fraction of clusters (~0.3 of the sample volume) weakly depend on a specific composition. The degree of ordering of the atomic structure of clusters changes with temperature according to a phase transition of the second kind and is close to unity at room temperature. The search for structural ordering corresponding to the modified D03 phase, which was discovered in a number of electron-diffraction studies, did not lead to a positive result. [ABSTRACT FROM AUTHOR]

Vibrational and structural properties have an enormous influence on optoelectronic properties of hybrid halide perovskites CH3NH3PbX3 (X = I, Br, Cl), which are perspective materials for different photovoltaic applications. Here, we report results of the first high-resolution optical spectroscopy measurements of large CH3NH3PbBr3 single crystals in wide frequency (20–20 000 cm−1) and temperature (5–300 K) ranges. Fit of polarized far-infrared reflection spectra using Lorentz model of damped oscillators revealed parameters of three phonons at room temperature and 38 phonons at 10 K. An analysis of mid-infrared transmission spectra gave evidence of a strong anharmonicity in this hybrid perovskite. A splitting of some lines observed in the orthorhombic phase was presumably assigned to the tunneling of the CH3NH3+ cation molecule between several potential energy minima. The temperature behavior of the transmission spectra near the fundamental absorption edge was tentatively explained by the competition between two different exciton transitions in CH3NH3PbBr3. Positions of spectral lines and of the absorption edge measured at cooling and heating the sample demonstrate hysteresis loops in the vicinities of phase transitions around 237, 155, and 149 K, thus revealing the first-order nature of all three phase transitions in CH3NH3PbBr3.

A recent experimental study of the metallization of hydrogen tracked the direct band gap and vibron frequency via infrared measurements up to ∼425 GPa (Loubeyreet al(2020Nature577631). Above this pressure, the direct gap has a discontinuous drop to below the minimum experimentally accessible energy (∼0.1 eV). The authors suggested that this observation is caused by a structural phase transition between theC2/c-24 molecular phase to another molecular phase such asCmca-12. Here, throughab initiocalculations of pressure dependent vibron frequency and direct band gap, we find that the experimental data is consistent with theC2/c-24 phase up to 425 GPa, and suggest that this consistency extends beyond that pressure. Specifically, we find that qualitative changes in the band structure of theC2/c-24 phase lead to a discontinuous drop of the direct band gap, which can explain the observed drop without a structural transition. This alternative scenario, which naturally explains the absence of hysteresis in the measurements, will hopefully motivate further experimental studies to ascertain the structure of the phase above the high pressure 'phase transition'.

A recent experimental study of the metallization of hydrogen tracked the direct band gap and vibron frequency via infrared measurements up to ∼425 GPa (Loubeyre et al (2020 Nature 577 631). Above this pressure, the direct gap has a discontinuous drop to below the minimum experimentally accessible energy (∼0.1 eV). The authors suggested that this observation is caused by a structural phase transition between the C2/c-24 molecular phase to another molecular phase such as Cmca-12. Here, through ab initio calculations of pressure dependent vibron frequency and direct band gap, we find that the experimental data is consistent with the C2/c-24 phase up to 425 GPa, and suggest that this consistency extends beyond that pressure. Specifically, we find that qualitative changes in the band structure of the C2/c-24 phase lead to a discontinuous drop of the direct band gap, which can explain the observed drop without a structural transition. This alternative scenario, which naturally explains the absence of hysteresis in the measurements, will hopefully motivate further experimental studies to ascertain the structure of the phase above the high pressure 'phase transition'.

The excitonic insulator is an electronically driven phase of matter that emerges upon the spontaneous formation and Bose condensation of excitons. Detecting this exotic order in candidate materials is a subject of paramount importance, as the size of the excitonic gap in the band structure establishes the potential of this collective state for superfluid energy transport. However, the identification of this phase in real solids is hindered by the coexistence of a structural order parameter with the same symmetry as the excitonic order. Only a few materials are currently believed to host a dominant excitonic phase, Ta2NiSe5 being the most promising. Here, we test this scenario by using an ultrashort laser pulse to quench the broken-symmetry phase of this transition metal chalcogenide. Tracking the dynamics of the material's electronic and crystal structure after light excitation reveals spectroscopic fingerprints that are compatible only with a primary order parameter of phononic nature. We rationalize our findings through state-of-the-art calculations, confirming that the structural order accounts for most of the gap opening. Our results suggest that the spontaneous symmetry breaking in Ta2NiSe5 is mostly of structural character, hampering the possibility to realize quasi-dissipationless energy transport. [ABSTRACT FROM AUTHOR]

In the present study, the structural, electronic, and mechanical properties of the sodium nitride (Na3N) were determined under up to 150 GPa hydrostatic pressure with the generalized gradient approach (GGA) based on the density functional theory (DFT). The Na3N compound was transformed from the hexagonal structure with the space group P6/mmm to the cubic structure with the space group Fm 3 ¯ m with gradually increasing pressure. Enthalpy and total energy were calculated to observe the consistency of the study findings with experimental results. The calculations revealed that the phase transition of the Na3N compound was observed at about 9.5 GPa from hexagonal structure to cubic structure. Furthermore, the electronic properties of the Na3N compound were investigated in the P6/mmm and Fm 3 ¯ m phases. The elastic properties of Na3N were determined, and it was concluded that both phases were mechanically stable. Cauchy pressure, Poisson's ratio, Kleinman parameter, and Pugh's ratio were calculated to better understand the elasticity of the material. [ABSTRACT FROM AUTHOR]

In this study; the structural, electronic and mechanical properties of sodium borohydride (NaBH4) compound were studied under high hydrostatic pressure from 0 to 300 GPa using the generalized gradient approach (GGA) within the framework of density functional theory (DFT). In the study, the NaBH4 compound transformed from the tetragonal structure with the space group P42/nmc to the triclinic structure with the space group P1 with the effect of gradually increasing pressure. Enthalpy and total energy calculations were performed to observe the agreement of the study with the experimental results. As a result of the calculations, it was concluded that a phase transition of the NaBH4 compound occurred at approximately 22.6 GPa from the tetragonal structure to the triclinic structure. In addition, the electronic properties of the NaBH4 compound were investigated and 6.22 and 4.95 eV band gaps were obtained for the P42/nmc and P1 phases, respectively. [ABSTRACT FROM AUTHOR]

The sub-THz polarized Raman response was measured along the [100] direction in the tetragonal BaTiO3 single crystal. The soft phonon was detected as a resonance peak for the first time by Raman spectroscopy, thanks to the use of notch filters that suppressed elastic scattering. No relaxation peak was observed and we demonstrated that, contrary to the situation along the ferroelectric c-axis, the soft phonon is sufficient to describe the dielectric response of BaTiO3 along the a-axis, except in close vicinity of the tetragonal-cubic phase transition. [ABSTRACT FROM AUTHOR]

The sub-THz polarized Raman response was measured along the [100] direction in the tetragonal BaTiO3 single crystal. The soft phonon was detected as a resonance peak for the first time by Raman spectroscopy, thanks to the use of notch filters that suppressed elastic scattering. No relaxation peak was observed and we demonstrated that, contrary to the situation along the ferroelectric c-axis, the soft phonon is sufficient to describe the dielectric response of BaTiO3 along the a-axis, except in close vicinity of the tetragonal-cubic phase transition. [ABSTRACT FROM AUTHOR]

[Display omitted] • Biocompatible water / AOT / isopropyl myristate microemulsion and liquid crystals drug delivery systems were obtained. • Structural phase transitions microemulsion-liquid crystals were studied by SAXS, NMR, and DLS methods. • The obtained results open the possibility of managing the morphology of the drug delivery systems. This work reports preparation and characterization of biocompatible sodium bis(2-ethylhexyl) sulfosuccinate (AOT) / water/ isopropyl myristate (IPM) systems. We studied their ability to form various self-organized structures such as microemulsions and liquid crystal templates for producing nanoparticles such as drug delivery vehicles, which play an important role in delivery of various substances. We determined concentration and temperature conditions that allow for forming various structures in this system and characterized the structural phase transition from the microemulsion to liquid crystalline phase, which is important for delivery of bioactive compounds. The parameters of the liquid crystalline phase of the water/AOT/isopropyl myristate system were calculated for the first time in this work. The sizes of microemulsion droplets were determined. To describe molecular motions in the studied microemulsion, self-diffusion coefficients of each component were measured. The values of relative diffusion coefficients demonstrated that reverse microemulsions were observed at AOT concentrations up to 60 % wt. Further increase in surfactant concentration slows down their molecular mobility and results in formation of aggregates with a bimodal size distribution and a subsequent formation of the hexagonal liquid crystalline phase. Geometry of the mesophase and its structure were investigated by X-ray diffraction. The respective phase diagram was plotted and analyzed using the data of polarized light microscopy, dynamic light scattering, and NMR self-diffusion. In this diagram, the areas corresponding to the microemulsion, the liquid crystal phase, and the structural phase transition were identified. The study of the structural phase transition from the microemulsion to liquid crystal will offer a predictive power for characterizing incorporation of drugs and controlling their delivery to target cells. A deeper understanding of the structural changes is necessary to clearly determine the conditions of drug delivery. [ABSTRACT FROM AUTHOR]

Exploring structural phase transitions and luminescence mechanisms in Zero-dimensional (0D) metal halides poses significant challenges, that are crucial for unlocking the full potential of tunable optical properties and diversifying their functional capabilities. Herein, we have designed two inter-transformable 0D Cu(I)-based metal halides, namely (C19H18P)2CuI3 and (C19H18P)2Cu4I6, through distinct synthesis conditions utilizing identical reactants. Their optical properties and luminescence mechanisms were systematically elucidated by experiments combined with density functional theory calculations. The bright cyan-fluorescent (C19H18P)2CuI3 with high photoluminescence quantum yield (PLQY) of 77% is attributed to the self-trapped exciton emission. Differently, the broad yellow-orange fluorescence of (C19H18P)2Cu4I6 displays a remarkable PLQY of 83%. Its luminescence mechanism is mainly attributed to the combined effects of metal/halide-to-ligand charge transfer and cluster-centered charge transfer, which effects stem from the strong Cu-Cu bonding interactions in the (Cu4I6)2- clusters. Moreover, (C19H18P)2CuI3 and (C19H18P)2Cu4I6 exhibit reversible structural phase transitions. The elucidation of the phase transitions mechanism has paved the way for an unforgeable anti-counterfeiting system. This system dynamically shifts between cyan and yellow-orange fluorescence, triggered by the phase transitions, bolstering security and authenticity. This work enriches the luminescence theory of 0D metal halides, offering novel strategies for optical property modulation and fostering optical applications., (© 2024 Wiley‐VCH GmbH.)

Discovery of superconductivity in electride materials has been a topic of interest as their intrinsic electron-rich properties might suggest a considerable electron-phonon interaction. LayeredY2Cis a ferromagnetic quasi-two-dimensional electride with polarized anionic electrons confined in the interlayer space. In this theoretical study, we reportY2Cundergoes a series of structural phase transitions into two superconducting phases with estimatedTcof 9.2 and 21.0 K at 19 and 80 GPa, respectively, via the suppression of magnetism. Our extensive first-principles swarm structure searches identify that these two high-pressure superconducting phases possess an orthorhombic Pnma and a tetragonal I 4 /m structures, respectively, where the Pnma phase is found to be a one-dimensional electride characterized by electron confinements in channel spaces of the crystal lattice, while the electride property in I 4 /m phase has been completely destroyed. We attribute the development of an unprecedentedly highTcsuperconductivity in Y-C system to the destructions of magnetism and the delocalization of interlayered anionic electrons under pressures. This work provides a unique example of pressure-induced collapse of magnetism at the onset of superconductivity in electride materials, along with the dramatic changes of electron-confinement topology in crystal lattices., (© 2024 IOP Publishing Ltd.)

The effect of Molybdenum (Mo) doping in pure barium titanate at Ti site has been investigated using electron microscopy, x-ray diffraction and dielectric and impedance spectroscopy. The room temperature XRD data confirms the formation of Mo substituted BaTi 0.94 Mo 0.06 O 3 in tetragonal symmetry. Even with the minimal doping of Mo, the first two structural transition shifts to the lower temperature by 60 and 30 K, respectively, as compared to pure barium titanate, confirmed via dielectric, polarization and x-ray diffraction measurements. In Mo doped BaTiO 3 , the cubic to tetragonal phase transition has been observed at ∼335 K, tetragonal to orthorhombic at ∼250 K. The orthorhombic to rhombohedral transition remains almost unaffected with Mo doping. Furthermore, transmission electron microscopy (TEM) has been employed to see the changes in microstructure/domain structure across various ferroelectric phases below 335 K which confirms the emergence of multiple twins in different ferroelectric phases. • Significant shifts in different transition temperature are observed with Mo doping. • Changes in domain structure across various ferroelectric phases are presented. • Ferroelectricity is confirmed using dielectric and pyroelectric measurements. • A high temperature relaxation in cubic phase is observed. [ABSTRACT FROM AUTHOR]

In this paper I will present a personal recollection of the development of molecular‐dynamics‐based atomistic simulations in the last forty years. The paper is not meant as a historical account but rather a personal and certainly biased view of the ideas that have underpinned the author's research, allowing ever more complex systems to be studied in an ever more realistic way. [ABSTRACT FROM AUTHOR]

Structural and magnetic properties of NiCr1.7Fe0.3O4 nanoparticles, synthesized by a facile coprecipitation method, are examined using temperature‐dependent X‐ray diffraction, neutron scattering with XYZ polarizations, and magnetic measurements. The structural analysis demonstrates the stabilization of the cubic phase down to 200 K, followed by a low‐temperature tetragonal phase at 100 K and an orthorhombic phase below 50 K. The refined occupancies of cations demonstrate that the NiCr1.7Fe0.3O4 is a mixed spinel compound, wherein B sites are populated by ≈85% Cr3+ and ≈15% Ni2+ cations and A sites are populated by ≈70% Ni2+ and ≈30% Fe3+ cations. As a consequence, the reduction in the occupancy of Jahn–Teller active Ni2+ (triply degenerate: e4 t4) at A site is attributed to the stabilization of the cubic phase below room temperature. Temperature‐dependent magnetization and neutron scattering with XYZ polarizations measurements reveal the ferrimagnetic nature of the magnetic phase and the absence of spin‐spiral ordering in NiCr1.7Fe0.3O4 nanoparticles. Furthermore, field‐dependent magnetization measurement demonstrates an exchange bias field of ≈134.5 kA m−1 at 5 K. The training effect measurement is performed to discuss the origin of exchange bias based on different phenomenological models such as power‐law and multiexponent models. [ABSTRACT FROM AUTHOR]

A comprehensive study on the effects of structural phase transitions (from −190 to 80 °C) on hysteresis in air‐processed methylammonium lead iodide (MAPbI3) thin‐film transistors (TFTs) is presented. The polarization effect of ordered MA+ ions and the migration of iodine ions are found to be the main causes of hysteresis in the orthorhombic (<−100 °C) and tetragonal (from −100 to 50 °C) phases, respectively. Large carrier concentration and lattice vibrations suppress the effects of ion migration in the cubic phase (>50 °C), resulting in negligible hysteresis. Hole current is found to vary marginally with temperature throughout the three phases. Electron current, however, is slightly smaller than hole current in the orthorhombic phase but rapidly increases with temperature until it exceeds hole current by three orders of magnitude in the tetragonal phase, and then decreases by an order of magnitude in the cubic phase due to scattering. Dedicating hole and electron transport, respectively to cold and hot conditions, may thus increase the operating temperature range for MAPbI3 devices. These findings provide fundamental insight into the origin of hysteresis in halide perovskite devices and guidelines for broadening their operating temperature window. [ABSTRACT FROM AUTHOR]

The pressure‐induced structural phase transitions in bismuth tungstate Bi2WO6 have been studied using neutron diffraction and Raman spectroscopy at high pressures up to 7 and 30 GPa, respectively. A rich structural polymorphism was revealed. At P ≃ 3.5 GPa a phase transition from the initial orthorhombic phase of P21ab symmetry to an orthorhombic phase of B2cb symmetry was observed. This transition is caused by the complex spatial rotation of the WO6 octahedra. A subsequent isostructural phase transition to another orthorhombic phase of B2cb symmetry was detected at P ≃ 5.9 GPa, accompanied by changes in both the mutual rotation and tilting of the oxygen octahedra with respect to the crystal b axis. Two more pressure‐induced phase transitions in Bi2WO6 at high pressures of 11.5 and 20 GPa were observed in the Raman spectra. These pressure‐driven phase transitions in bismuth tungstate are accompanied by anomalies in the pressure dependences of the unit‐cell parameters, bond lengths and angles, and in the vibrational modes. [ABSTRACT FROM AUTHOR]

The pressure‐induced structural phase transitions in bismuth tungstate Bi2WO6 have been studied using neutron diffraction and Raman spectroscopy at high pressures up to 7 and 30 GPa, respectively. A rich structural polymorphism was revealed. At P ≃ 3.5 GPa a phase transition from the initial orthorhombic phase of P21ab symmetry to an orthorhombic phase of B2cb symmetry was observed. This transition is caused by the complex spatial rotation of the WO6 octahedra. A subsequent isostructural phase transition to another orthorhombic phase of B2cb symmetry was detected at P ≃ 5.9 GPa, accompanied by changes in both the mutual rotation and tilting of the oxygen octahedra with respect to the crystal b axis. Two more pressure‐induced phase transitions in Bi2WO6 at high pressures of 11.5 and 20 GPa were observed in the Raman spectra. These pressure‐driven phase transitions in bismuth tungstate are accompanied by anomalies in the pressure dependences of the unit‐cell parameters, bond lengths and angles, and in the vibrational modes. [ABSTRACT FROM AUTHOR]

The aluminum-sodalite-type oxide, Ca8[AlO2]12(MoO4)2, shows an improper ferroelectricity below a ferroelectric phase transition temperature around 630 ​K. Here we present effects of a Ga-substitution for Al on ferroelectric properties of Ca8[AlO2]12(MoO4)2. Systematic investigations with synchrotron x-ray powder diffractions and dielectric/pyroelectric measurements have clarified that the Ga-substitution suppresses the ferroelectricity and markedly enhances a thermal hysteresis of the ferroelectric phase transition. In contrast to the case of Sr-substitution for Ca, with which the ground state results in a relaxor-like state, the polar ordering of macroscopic scale has been found to survive under the Ga-substitution. The present results shed light on intuitive designing of ferroelectricity and the phase transition properties in the aluminum-sodalite-type oxide.

Strain engineering is a well‐known method often used to tune material properties in thin films. The most studied sources of strain are lattice mismatch and differential thermal contraction between the substrate and film. However, in materials which undergo a structural phase transition (SPT), a third and often overlooked source of strain may play a very significant role. If the substrate confines the area of the film, the SPT may induce stress which changes the evolution of the transition. This is a 2D analog of the isochoric phase transition between water and ice, where the freezing point drops below 0 °C. To illustrate this, the prototypical Mott insulator V2O3 which has an SPT coupled to a metal–insulator transition is used to show how self‐induced strain can drastically alter structural and electronic properties. This effect provides an elegant approach for mapping the phase diagram of the SPT and the transitions coupled to it. Moreover, the magnitude of self‐straining is tunable by modifying the substrate morphology. This effect may be important for numerous materials which exhibit an SPT and are subjected to geometrical constraints. [ABSTRACT FROM AUTHOR]

Vanadium dioxide (VO2) is a promising material in the development of thermal and electrically sensitive devices due to its first order reversible metal‐insulator transition (MIT) at 68 °C. Such high MIT temperature (TC) largely restricts its widespread application which could be enabled if a straightforward tuning mechanism were present. Here this need is addressed through a facile approach that uses the combined effects of temperature induced strain and oxygen vacancies in bulk VO2 colloidal particles. A simple thermal annealing process under varying vacuum is used to achieve phase transformation of metastable VO2(A) into VO2(M2), (M2+M3), (M1) and higher valence V6O13 phases. During this process, distinct multiple phase transitions including increased as well as suppressed TC are observed with respect to the annealing temperature and varied amount of oxygen vacancies respectively. The latent heat of phase transition is also significantly improved upon thermal annealing by increasing the crystallinity of the samples. This work not only offers a facile route for selective phase transformation of VO2 as well as to manipulate the phase transition temperature, but also contributes significantly to the understanding of the role played by oxygen vacancies and temperature induced stress on MIT which is essential for VO2 based applications. [ABSTRACT FROM AUTHOR]

Comparative in situ X‐ray and neutron diffraction studies of the phase transitions in a high magnetostriction Fe–27Ga alloy sample have been performed, with heating up to 850°C and subsequent cooling to room temperature. Different structural information (phase content, phase‐transition temperatures, unit‐cell behaviour) is obtained for the bulk sample and its surface, and this fact is discussed here in terms of the difference between the scattering properties of X‐rays and neutrons. In particular, a gallium‐deficient phase with A2‐type structure was found on the surface of the sample. The results demonstrate that conventional X‐ray diffraction methods are insufficient for precise structural analysis of bulk samples of galfenols and cannot be used alone to build their phase diagrams. [ABSTRACT FROM AUTHOR]

The microstructure of several Fe–xGa alloys with phase-separated structure have been studied by neutron diffraction with high Δ d/d resolution. Analysis of diffraction data shows that the microstructure of these alloys is organized as nano-sized clusters with a better-ordered atomic structure coherently embedded in a disordered or less-ordered matrix. The characteristic size of the clusters depends on the Ga content and ranges from 100–2000 Å. [ABSTRACT FROM AUTHOR]

Abstract Bonding geometry engineering of metal–oxygen octahedra is a facile way of tailoring various functional properties of transition metal oxides. Several approaches, including epitaxial strain, thickness, and stoichiometry control, have been proposed to efficiently tune the rotation and tilt of the octahedra, but these approaches are inevitably accompanied by unnecessary structural modifications such as changes in thin‐film lattice parameters. In this study, a method to selectively engineer the octahedral bonding geometries is proposed, while maintaining other parameters that might implicitly influence the functional properties. A concept of octahedral tilt propagation engineering is developed using atomically designed SrRuO3/SrTiO3 (SRO/STO) superlattices. In particular, the propagation of RuO6 octahedral tilt within the SRO layers having identical thicknesses is systematically controlled by varying the thickness of adjacent STO layers. This leads to a substantial modification in the electromagnetic properties of the SRO layer, significantly enhancing the magnetic moment of Ru. This approach provides a method to selectively manipulate the bonding geometry of strongly correlated oxides, thereby enabling a better understanding and greater controllability of their functional properties.

Bonding geometry engineering of metal–oxygen octahedra is a facile way of tailoring various functional properties of transition metal oxides. Several approaches, including epitaxial strain, thickness, and stoichiometry control, have been proposed to efficiently tune the rotation and tilt of the octahedra, but these approaches are inevitably accompanied by unnecessary structural modifications such as changes in thin‐film lattice parameters. In this study, a method to selectively engineer the octahedral bonding geometries is proposed, while maintaining other parameters that might implicitly influence the functional properties. A concept of octahedral tilt propagation engineering is developed using atomically designed SrRuO3/SrTiO3 (SRO/STO) superlattices. In particular, the propagation of RuO6 octahedral tilt within the SRO layers having identical thicknesses is systematically controlled by varying the thickness of adjacent STO layers. This leads to a substantial modification in the electromagnetic properties of the SRO layer, significantly enhancing the magnetic moment of Ru. This approach provides a method to selectively manipulate the bonding geometry of strongly correlated oxides, thereby enabling a better understanding and greater controllability of their functional properties. [ABSTRACT FROM AUTHOR]

Structural features and kinetics of the transition between ordered metastable b.c.c.‐derived D03 and equilibrium f.c.c.‐derived L12 phases of Fe–xGa alloys (x = 27.2% and 28.0%) have been analyzed by in situ real‐time neutron diffraction during isothermal annealing in the temperature range 405–470°C. It has been revealed that the transition proceeds with alternation of the first‐ and second‐order phase transformations according to a D03 → A2 → A1 → L12 scheme, where A2 and A1 are disordered b.c.c. and f.c.c. structures. Deformations of the crystal lattice that arise due to these transitions are determined. The kinetics of the L12 phase nucleation and growth were analyzed in the frame of the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model; however, only the early stage of the D03 → L12 transition is well described by the JMAK equation. The value of the Avrami exponent corresponds to the constant growth rate of the new L12 phase and decreasing nucleation rate in the Fe–27.2Ga alloy and indicates the presence of pre‐existing nucleation centres of the L12 phase in the Fe–28.0Ga alloy. [ABSTRACT FROM AUTHOR]

The exciton spectra of Cs1–xRbxPbCl3 solid solution thin films are studied in a range of 2–6 eV. The formation of solid solutions stable at room temperature was detected in range of concentrations 0 ≤ х ≤ 0.7. A linear dependence of the exciton band and bandgap width parameters on the concentration was found. Kinks are observed along the temperature dependences of the spectral position Em(T) (x > 0) of the long-wave exciton band at 310 and 320 K that are characteristic of second-order phase transitions. The exciton excitations in CsPbCl3 are found to have a three-dimensional nature, while in Cs1–xRbxPbCl3 (x > 0) solid solutions they are found to be two-dimensional. [ABSTRACT FROM AUTHOR]

The results of complex studies of structural-phase transformations and magneto-optical properties of nanocomposites (CoFeZr)x (MgF2)100−x depending on the metal alloy content in the dielectric matrix are presented. Nanocomposites were deposited by ion-beam sputtering onto glass and glass-ceramic substrate. By studying the spectral and field dependences of the transversal Kerr effect (TKE), it was found that the transition of nanocomposites from superparamagnetic to the ferromagnetic state occurs in the region of xfm~30 at%, that corresponds to the onset the formation of ferromagnetic nanocrystals CoFeZr with hexagonal syngony in amorphous dielectric matrix of MgF2. With an increase of concentrations of the metal alloy for x > xfm, the features associated with structural transitions in magnetic granules are revealed in the TKE spectra. Comparison of the spectral and concentration dependences of TKE for nanocomposites on the glass and glass-ceramics substrates showed that the strongest differences occur in the region of the phase structural transition of CoFeZr nanocrystals from a hexagonal to a body-centered cubic structure at x = 38 at.% on the glass substrates and at x = 46 at.% on glass-ceramics substrates, due to different diffusion rates and different size of metal nanocrystals on amorphous glass substrates and more rough polycrystalline glass-ceramics substrates.

We have discovered a structural transition for the SrZnO alloy films from a wurtzite to a rock-salt structure, leading to a reduction in the (112̲0)/(0001) surface energy ratio. The films were grown by pulsed laser deposition using different SrO ratios, x. We have revealed that growth at a higher temperature, 750°C, resulted in a sharp 0002 peak at a low SrO content (5%), whereas growth at a higher SrO content (10%) resulted in a non-crystalline film with minute crystallites with a (112̲0) orientation. Generally the crystallinity decreased as the SrO content increased. No results obtained for the crystalline films showed any orientation of significant peaks besides the peak attributed to the (0001) plane, suggesting epitaxial growth. Optical measurements showed difference in transmission widows of alloys with different SrO percentage, and this was correlated to SrO influence on growth mode as indicated by scanning electron imaging. The studied SrZnO films, with SrO/(SrO + ZnO) ≤ 0.25, were grown by pulsed laser deposition using different SrO ratios, x. The effects of temperature and oxygen pressure during growth on the films’ structural properties were investigated. XRD results indicate that the film crystallinity was improved as the temperature and O2 pressure increased up to 650°C and 0.5 Torr, respectively.

A direct band gap 2D corrugated layer lead chloride hybrid, [(CH3)4N]4Pb3Cl10 (1), shows analogous topology to the {Mg3F104−}∞ layer in Cs4Mg3F10, and with the (CH3)4N+ cations locating in the inorganic layer voids and between the interlayers. Two reversible structural phase transitions occur in 1 at 225/210 K and 328/325 K upon heating/cooling, respectively. On going from the low‐ to intermediate‐temperature phase, the space group changes from P21/c to Cmca, and the crystallographic axis perpendicular to the layers is doubled with the order–disorder transformation of (CH3)4N+ cations between the interlayers. The intermediate‐ and high‐temperature phases are isomorphic with similar cell parameters and packing structure; their main difference concerns the disorder degree of the (CH3)4N+ cations between the interlayers. The two‐step structural phase transitions lead to dielectric anomalies around the corresponding Tc. Interestingly, 1 shows multiband emission, originating from the recombination of exciton and emission of defects. Moreover, 1 exhibits divergent thermochromic luminescent features around the Tc on the intermediate to low temperature transition. Thermochromic luminescence: A direct band gap 2D corrugated layer lead chloride hybrid, [(CH3)4N]4Pb3Cl10 (1), shows analogous topology to the {Mg3F104−}∞ layer in Cs4Mg3F10, and with the (CH3)4N+ cations locating in the inorganic layer voids and between the interlayers. Two reversible structural phase transitions occur in 1 at 225/210 K and 328/325 K upon heating/cooling, leading to dielectric anomalies around the corresponding Tc and divergent thermochromic luminescent features. [ABSTRACT FROM AUTHOR]

A direct band gap 2D corrugated layer lead chloride hybrid, [(CH3)4N]4Pb3Cl10 (1), shows analogous topology to the {Mg3F104−}∞ layer in Cs4Mg3F10, and with the (CH3)4N+ cations locating in the inorganic layer voids and between the interlayers. Two reversible structural phase transitions occur in 1 at 225/210 K and 328/325 K upon heating/cooling, respectively. On going from the low‐ to intermediate‐temperature phase, the space group changes from P21/c to Cmca, and the crystallographic axis perpendicular to the layers is doubled with the order–disorder transformation of (CH3)4N+ cations between the interlayers. The intermediate‐ and high‐temperature phases are isomorphic with similar cell parameters and packing structure; their main difference concerns the disorder degree of the (CH3)4N+ cations between the interlayers. The two‐step structural phase transitions lead to dielectric anomalies around the corresponding Tc. Interestingly, 1 shows multiband emission, originating from the recombination of exciton and emission of defects. Moreover, 1 exhibits divergent thermochromic luminescent features around the Tc on the intermediate to low temperature transition. Thermochromic luminescence: A direct band gap 2D corrugated layer lead chloride hybrid, [(CH3)4N]4Pb3Cl10 (1), shows analogous topology to the {Mg3F104−}∞ layer in Cs4Mg3F10, and with the (CH3)4N+ cations locating in the inorganic layer voids and between the interlayers. Two reversible structural phase transitions occur in 1 at 225/210 K and 328/325 K upon heating/cooling, leading to dielectric anomalies around the corresponding Tc and divergent thermochromic luminescent features. [ABSTRACT FROM AUTHOR]

A method for determining the crystal phase and estimating the crystal lattice deformations in HxLi1-xNbO3 proton-exchanged optical waveguides from an analysis of the Raman light scattering spectra measured with high spatial resolution (the so-called method of micro-RS spectroscopy) is suggested. [ABSTRACT FROM AUTHOR]

Abstract In this paper, we investigate the effect of thermally induced structural phase transitions on the photoluminescence (PL) and optical absorption behaviour of Zn 0.45 Cd 0.55 S nanoparticles (NPs). Analysis of X-ray diffraction (XRD) patterns and high-resolution electron microscope (HRTEM) images reveal that the as-synthesized sample possesses zinc-blende-type cubic structure. In addition, at annealing temperature (T a ) 400 °C, the cubic structure transforms completely into the wurtzite-type hexagonal structure. Furthermore, the second phase transition of the as-synthesized sample has observed at 700 °C, where the cubic structure has transformed into mixed polycrystalline phases of hexagonal ZnO, cubic CdO, monoclinic CdSO 3 , and orthorhombic ZnSO 4 structures. These new phases have also confirmed from the analysis of Raman and FTIR spectra. Analysis of UV–visible optical absorption spectra demonstrates that Increasing T a results in the decrease of optical band gap due the improvement in crystallinity accompanied by the increase in the particle size. The PL emission bands at an excitation energy of 3.818 eV exhibit redshift and a decrease in the intensity with increasing T a up to 500 °C. Meanwhile, further increase in T a up to 700 °C results in the enhancement of green emission intensity. On the other hand, PL emission spectra at 3.354 eV and T a 700 °C, reveal a dramatic increase in the emission intensity nearly by one-order of magnitude with respect to its value of the as-synthesized sample. This behaviour is ascribed to the incorporation of oxygen-related defects via thermal annealing in air, which act as additive radiative centers. Also, we have interpreted the observed spectral blue shift of PL emission spectrum with increasing excitation energy. Highlights • Thermal Induced Structural Phase transition in nanostructured Zn 0.45 Cd 0.55 S. • Low temperature structural phase transition from cubic to hexagonal structure. • Thermal annealing effect on FTIR and Raman Spectra. • Thermal annealing effect on optical absorption of Zn 0.45 Cd 0.55 S nanoparticles. • Excitation-wavelength dependent photoluminescence emission spectra. [ABSTRACT FROM AUTHOR]

We report a detailed Rietveld analysis of sintering temperature (T s) induced structural phase transitions in Ba(Fe 0.7 Ta 0.3)O 3-δ ; a high-temperature oxygen sensing material. The compound was synthesized through the conventional solid-state reaction method, and sintered in the temperature range of 1200 °C ≤ T s ≤ 1350 °C at an interval of 50 °C. The misfit regions found in the characteristic Bragg reflections (200), and (211) in refined patterns of single-phase structural models discount the stabilization of the compound in single-phase. Refinement using two-phase plausible structural models confirms the compound sintered at 1200 °C stabilizes in orthorhombic and rhombohedral mixed phase, whereas the compound sintered at and beyond 1250 °C stabilizes in tetragonal and rhombohedral. The phase fraction of underlined phases is quantified and reported. The sensing performance of material depends on structural and microstructural features, hence the derived understanding could be useful to develop Ba(Fe 0.7 Ta 0.3)O 3-δ materials in appropriate conditions. [ABSTRACT FROM AUTHOR]

The temperature dependence of the crystal structure and thermoelastic properties of RbCaF 3 perovskite are reported from a high-resolution neutron powder diffraction investigation in the temperature interval 2–300 K. Two phase transitions have been identified from these data, the first from space group P m 3 ¯ m to I 4/ mcm , at 200 K, has been well characterized in earlier studies, the second, from space group to I 4/ mcm to Pbnm has previously been less well understood. New results from the former transition show the order parameter evolves with a critical exponent of 0.284(5), and the phase is characterized by an Einstein temperature of 192(8) K, and a Debye temperature of 270(10) K. The temperature variation of the tetragonal spontaneous strain in this phase is consistent with an additional fourth order invariant in the Landau potential. The second phase transition is found to be strongly first order and a temperature dependent, and heating rate/cooling rate dependent phase coexistence is found at all temperatures below ~ 80 K on heating, and ~ 65 K on cooling. The evolution of the orthorhombic unit cell is atypical in the region of the phase transition, with a and b exhibiting strong negative linear thermal expansion, whilst the c axis behaves in the expected manner. The crystal structure of the Pbnm phase at 2 K is reported from an unconstrained Rietveld refinement for the first time. [ABSTRACT FROM AUTHOR]

A combination of high-resolution and in situ real-time neutron diffraction was used to collect the experimental data concerning coherent atomic ordering in Fe 3 Al-type alloys in a wide temperature range. The analysis of the obtained data was carried out within the framework of two models: antiphase domains and dispersed clusters embedded in matrix. The second model, for which the atomic ordering is organized in the form of clusters of mesoscopic sizes (from ∼200 to ∼400 Å) randomly distributed inside a less ordered matrix, provides a better fit of the data. For two chemically identical Fe 3 Al samples - the first was in the as-cast polycrystalline bulk state, and the second was grown as a single crystal - the initial state can be described as a partially ordered B2 structure (matrix) with dispersed clusters of the D0 3 ordered phase. The initial (quenched) state of the (Fe 0.88 Cr 0.12 ) 3 Al polycrystalline sample is the disordered A2 phase with clusters of the partially ordered B2 phase. After heating and subsequent slow cooling, the structure of both binary and ternary alloys is B2 matrix with dispersed D0 3 clusters, whose dimensions are increased up to ∼900 Å. [ABSTRACT FROM AUTHOR]

This paper presents a review of the experimental and theoretical investigations of halogen interaction with metal surfaces. The emphasis was placed on the recent measurements performed with a scanning tunneling microscope in combination with density functional theory calculations. The surface structures formed on metal surface after halogen interaction are classified into three groups: chemisorbed monolayer, surface halide, bulk-like halide. Formation of monolayer structures is described in terms of surface phase transitions. Surface halide phases are considered to be intermediates between chemisorbed halogen and bulk halide. The modern theoretical approaches in studying the dynamics of metal halogenation reactions are also presented. [ABSTRACT FROM AUTHOR]

Abstract: The bimetallic molecular compound Dimolybdenum tetraacetate (MoMo‐Methyl) is grown on a Cu(111) surface with submonolayer coverage. Scanning tunneling microscopy experiments reveal that the compound forms two different structural phases on the Cu surface, whose ratio can be reversibly controlled by changing the sample temperature. The so‐called chain‐phase is characterized by tilted MoMo dimers bonded to the Cu surface via the methyl groups. In the so‐called mesh‐phase, on the other hand, the molecules adsorb in a flat lying adsorption configuration with one of the Mo‐atoms in direct contact with the Cu surface. Crucially, the different structural properties of the two phases reflect the different inter‐ and intramolecular interactions between the Mo metal centers, as well as the different interactions between Mo and the Cu surface atoms. In this way, the structural changes result in a modification of the cooperative effects in the system. Therefore, it is proposed that the observed reversible structural phase transition could be used to control the strength of cooperative effects in MoMo‐Methyl on Cu(111). [ABSTRACT FROM AUTHOR]

The history of the research of anomalous diffuse scattering by perfect single crystals has been briefly described. Some traditional techniques for recording the diffuse scattering have been considered. The mono-Laue experiment has been considered in detail. A series of mono-Laue measurements on single-crystal cobalt (hcp structure) has been carried out at room temperature. The results are in a good agreement with theoretical calculations. The principal possibility of these measurements under laboratory conditions has been shown. The possible use of diffuse scattering measurements in the study of structural phase transitions in metals has been substantiated. [ABSTRACT FROM AUTHOR]

Real and imaginary parts of the dielectric function of TlIn(S 1- x Se x ) 2 ( x = 0.05, 0.08, 0.25) single crystals were determined in the spectral range from 1 to 5 eV within a temperature interval 140–293 K from spectroscopic ellipsometry measurements. The energies of interband transitions (critical points) of the TlIn(S 1− x Se x ) 2 crystals were obtained from the second derivative of the real and imaginary parts of dielectric function. Structural phase transitions are behind the observed change of electronic band structure. [ABSTRACT FROM AUTHOR]

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