Pressure-induced Structural Phase Transition Research Articles

Structural Phase Transitions and Metallization in Zirconium Disulfide (ZrS2) under High Pressure.

The high-pressure study on zirconium disulfide (ZrS2) has been conducted up to ∼33.9 GPa by using in-situ synchrotron X-ray diffraction at room temperature and electrical transport measurements, aiming to investigate the pressure-induced structural phase transitions and the metallization of the material. The experiments indicated that a progressive structural evolution occurs as the sequence below: hexagonal (space group (SG): P3̅m1) → monoclinic (SG: P21/m) started at 2.8 GPa → orthorhombic (SG: Immm) started at 9.9 GPa → tetragonal (SG: I4/mmm) started at 30.4 GPa. The lattice parameters and bulk modulus of the phases at high pressure are calculated. The decompression results demonstrate the coexistence of ZrS2 stability in monoclinic (SG: P21/m) and orthorhombic (SG: Immm) structures. The electrical transport results demonstrate a significant enhancement in the conductivity of ZrS2 at 8 GPa, and metallization occurs at ∼29.8 GPa. Combined with XRD results, the metallization is coincident with the phase transition to the tetragonal (SG: I4/mmm) structure. Our study presents a more precise phase transition sequence, calculates the cell parameters and bulk modulus of ZrS2 under high-pressure conditions based on experimental results, and confirms that metallization is induced by the structural phase transition. These findings provide an experimental basis for the further utilization of transition metal sulfides (TMDs) under high pressure.

Pressure-Induced Structural Phase Transitions and Photoluminescence Properties of Micro/Nanocrystals HoF3.

HoF3 crystals of varying sizes and morphologies were synthesized by controlling the amount of water. As the water content gradually decreased, the size of the crystals reduced, transforming from microcrystals to approximately 100 nm nanocrystals with unique morphologies. At room temperature, the pressure-induced phase transitions, and photoluminescence (PL) properties of HoF3 micro/nanocrystals are studied using in situ high-pressure PL technique. The PL spectra show that the HoF3 micro/nanocrystals exhibit two structural phase transformations at 5 (6 GPa for NCs) and 12 GPa. Nanoparticles have higher fluorescence intensity, which initially increases and then decreases with changes in pressure. Based on first-principles calculations, HoF3 transforms from an orthorhombic structure to a hexagonal structure during the phase transition, with the coordination number of holmium atoms increasing from 9 to 11. The high-pressure Raman spectra on lattice modes also confirmed the existence of the two phase transitions. This work not only provides precise structural changes but also facilitates the understanding of two typical structures of rare-earth trifluoride (REF3), which may play an important role in the application of the RE family.

Pressure-induced multiple structural phase transitions on multiferroic CaMn7O12

CaMn7O12 (CMO) is an outstanding multiferroic material known for its significant magnetically-induced electric polarization. Its strong magnetoelectric (ME) effect, i.e., electric polarization controlled via magnetic fields or vice versa, makes this material suitable for a variety of technological applications. In this study, we employed synchrotron X-ray powder diffraction to explore polycrystalline CMO's pressure-induced structural phase transitions (SPTs). Our results indicate that CMO undergoes two distinct pressure-induced SPTs: the first transition occurs at pressures above 7.0 GPa, changing from a rhombohedral to an orthorhombic structure, and the second occurs around 13.0 GPa, transforming into a monoclinic structure. These findings differ from the pressure-induced behavior of CMO single crystals and highlight CMO as one of the rare quadruple perovskites exhibiting multiple pressure-induced non-isostructural phase transitions. This study expands the understanding of phase stability behavior in multiferroic materials under high-pressure conditions.

High-Pressure Behavior of NdF3: Structural and Photoluminescence Properties

Abstract This paper investigates the pressure-induced photoluminescence properties and structural phase transitions of functional material NdF3 at room temperature using diamond anvil cell (DAC), in-situ photoluminescence technology, and simulation calculations. Under hydrostatic experimental pressure and simulated pressure up to 20 GPa, the photoluminescence spectra, lattice parameters, and bond lengths of NdF3 were obtained, showing their variations with pressure. Experimental results show that a new emission line, corresponding to the 4F3/2→4I9/2 transition of Nd3+ ions, appears around 12 GPa and persists up to the highest pressure of the experiment. Upon decompression, the new emission line disappears, indicating a reversible phase transition. Notably, from 0 to 12 GPa, the emission intensity of NdF3 decreases significantly under pressure, which may be attributed to the increased energy of lattice phonons in the compressed crystal (shorter Nd-F bonds), leading to enhanced phonon-assisted non-radiative relaxation.

High pressure ferroelectric-like semi-metallic state in Eu-doped BaTiO3.

We have conducted a detailed high-pressure (HP) investigation on Eu-doped BaTiO3 using angle-resolved x-ray diffraction, Raman spectroscopy, and dielectric permittivity measurements. The x-ray diffraction data analysis shows a pressure-induced structural phase transition from the ambient tetragonal to the mixed cubic and tetragonal phases above 1.4GPa. The tetragonality of the sample due to the internal deformation of the TiO6 octahedra caused by the charge difference from Eu doping cannot be lifted by pressure. Softening, weakening, and disappearance of low-frequency Raman modes indicate ferroelectric tetragonal to the paraelectric cubic phase transition. However, the pressure-induced increase in the intensity of [E(LO), A1(LO)] and the octahedral breathing modes indicate that the local structural inhomogeneity remains in the crystal and is responsible for spontaneous polarization in the sample. The low-frequency electronic scattering response suggests pressure-induced carrier delocalization, leading to a semi-metallic state in the system. Our HP dielectric constant data can be explained by the presence of pressure-induced localized clusters of microscopic ferroelectric ordering. Our results suggest that the HP phase coexistence leads to a ferroelectric-like semi-metallic state in Eu-doped BaTiO3 under extreme quantum limits.

Optical tunning of high-luminescent iodine-substituted CsPb2(Br0.84I0.16)5 under pressure

Metal halide compounds hold significant interest for optoelectronics, given their substantial potential for solar cells and light-emitting applications. Low-dimensional (0D, 1D, and 2D) metal halide perovskites or perovskite-like compounds exhibit various compositions with soft lattices and modular optical properties. Thus, the finding of new strategies to further improve such properties is a current research challenge. In this scenario, the 2D perovskite-like compound CsPb2Br5 has attracted attention due to its structural stability and favorable optical properties. This study presents the pressure-induced structural phase transitions of the iodine substituted CsPb2(Br0.84I0.16)5. This substitution induces strong green photoluminescence in single crystals at ambient conditions. High-pressure optical and synchrotron X-ray diffraction experiments reveal a tetragonal-tetragonal structural phase transitions around 1 GPa, while the photoluminescence emission is quenched around 2.7 GPa. Additionally, a new high-pressure orthorhombic phase is identified in the iodine-doped compound beyond 4.5 GPa. Further analysis of structural features prove that the pressure dependent photoluminescence behavior is directly related to structural distortions observed upon pressure increase.

Pressure-induced structural, electronic, and superconducting phase transitions in TaSe3

Abstract TaSe3 has garnered significant research interests due to its unique quasi-one-dimensional crystal structure, which gives rise to distinctive properties. Using crystal structure search and first-principles calculations, we systematically investigated the pressure-induced structural and electronic phase transitions of quasi-one-dimensional TaSe3 up to 100 GPa. In addition to the ambient pressure phase (P21/m-I), we identified three high-pressure phases: P21/m-II, Pnma, and Pmma. For the P21/m-I phase, the inclusion of spin–orbit coupling (SOC) results in significant SOC splitting and changes in the band inversion characteristics. Furthermore, band structure calculations for the three high-pressure phases indicate metallic natures, and the electron localization function suggests ionic bonding between Ta and Se atoms. Our electron–phonon coupling calculations reveal a superconducting critical temperature of approximately 6.4 K for the Pmma phase at 100 GPa. This study provides valuable insights into the high-pressure electronic behavior of quasi-one-dimensional TaSe3.

Pressure-induced structural phase transitions and metallization in cuprous oxide under different hydrostatic environments up to 25.3 GPa

High-pressure vibrational and electrical transport behaviors of cuprous oxide were systematically investigated in a diamond anvil cell through in-situ Raman spectroscopy and electrical conductivity measurements under different hydrostatic environments up to 25.3 GPa. Upon compression, Cu2O undergoes a series of structural transformations from Cu2O–I to Cu2O–Ia to Cu2O–Ib to Cu2O–II to Cu2O–III phases at the respective pressures of 1.7(2), 10.3(2), 12.9(3) and 21.0(2) GPa under non-hydrostatic condition. Meanwhile, the metallization of Cu2O at 19.5(3) GPa is well characterized by the variable-temperature electrical conductivity experiments. Under hydrostatic condition, the pressure delay of 0.6–1.6 GPa is detected in the phase transitions from Cu2O–Ia to Cu2O–Ib to Cu2O–II to Cu2O–III phases. Upon decompression, one huge discrepancy is observed in the Raman spectra and electrical conductivity before and after the metallization, which indicates the irreversibility of phase transitions under different hydrostatic environments.

Pressure-induced structural and electronic phase transitions in GaGeTe

Pressure-induced structural and electronic phase transitions in GaGeTe

Artificial neural network for deciphering the structural transformation of condensed ZnO by extended x-ray absorption fine structure spectroscopy

Pressure-induced structural phase transitions play a pivotal role in unlocking novel material functionalities and facilitating innovations in materials science. Nonetheless, unveiling the mechanisms of densification, which relies heavily on precise and comprehensive structural analysis, remains a challenge. Herein, we investigated the archetypal B4 → B1 phase transition pathway in ZnO by combining x-ray absorption fine structure (XAFS) spectroscopy with machine learning. Specifically, we developed an artificial neural network (NN) to decipher the extended-XAFS spectra by reconstructing the partial radial distribution functions of Zn–O/Zn pairs. This provided us with access to the evolution of the structural statistics for all the coordination shells in condensed ZnO, enabling us to accurately track the changes in the internal structural parameter u and the anharmonic effect. We observed a clear decrease in u and an increased anharmonicity near the onset of the B4 → B1 phase transition, indicating a preference for the iT phase as the intermediate state to initiate the phase transition that can arise from the softening of shear phonon modes. This study suggests that NN-based approach can facilitate a more comprehensive and efficient interpretation of XAFS under complex in-situ conditions, which paves the way for highly automated data processing pipelines for high-throughput and real-time characterizations in next-generation synchrotron photon sources.

A pressure-induced superhard SiCN4 compound uncovered by first-principles calculations.

Silicon-carbon-nitride (Si-C-N) compounds are a family of potential superhard materials with many excellent chemical and physical properties; however, only SiCN, Si2CN4 and SiC2N4 were synthesized. Here, we theoretically report a new SiCN4 compound with P41212, Fdd2 and R3̄ structures by first-principles structural predictions based on the particle swarm optimization algorithm. Pressure-induced structural phase transitions from P41212 to Fdd2, and then to the R3̄ phase were determined at 2 GPa and 249 GPa. By comparing enthalpy differences with 1/3Si3N4 + C + 4/3N2, it was found that these structures tend to decompose at ambient pressure. However, with the increase of pressure, the enthalpy differences of Fdd2 and R3̄ structures turn to be negative and they can be stabilized at a pressure of more than 41 GPa. They are also dynamically stable as no imaginary frequencies were found in their stabilized pressure ranges. The calculated band gap is 4.37 eV for P41212, 3.72 eV for Fdd2 and 3.81 eV for the R3̄ phase by using the Heyd-Scuseria-Ernzerhof (HSE06) method and the estimated Vickers hardness values are higher than 40 GPa by adopting the elastic modulus based hardness formula, which confirmed their superhard characteristics. These results provide significant insights into Si-C-N systems and will inevitably promote the future experimental works.

Experimental observation of two-dimensional phase in compressed FeF2

Iron difluoride (FeF2) has attracted considerable attention for its physical characteristics and practical applications, and its compression behaviors usually play a key role in the in-depth understanding of this compound. Since its high-pressure crystal structure evolution determining a more profound comprehension remains disputable, we carried out extensive experiments to focus on the pressure-induced structural phase transitions of FeF2. Through in situ high-pressure synchrotron x-ray diffraction measurements, we not only confirmed a reported high-pressure orthorhombic Pbca phase at 11 GPa but also identified an interesting two-dimensional structure with hexagonal close packed symmetry (P-3m1) that appears above 25 GPa at room temperature. Furthermore, the spontaneous strain fitting and electronic transport measurements suggest that its ambient rutile-type structure (P42/mnm) evolves into an orthorhombic structure (Pnnm) through a second-order phase transition at 5 GPa. These experimental results elaborate on the pressure-induced phase transitions of FeF2 on the order of P42/mnm → Pnnm → Pbca → P-3m1, shedding light on a rare three-dimensional to two-dimensional configuration transition in difluorides.

Enhanced optical anisotropy of six-coordinated silica polymorphs via high-pressure hydrothermal treatment

Pressure-induced structural phase transition offers a promising alternative to boost the optical anisotropy of birefringent materials.

Pressure-induced structural evolution to a nearly perfect kagome lattice in β-Mn2(OH)3Cl nanosheets

The frustrated magnets exhibiting kagome geometries and the investigation of their structure-property relationships have garnered substantial attention as potential candidates for quantum spin liquid (QSL). In this study, β-Mn2(OH)3Cl nanosheets with distorted kagome geometries were synthesized via a solid-state reaction, and low-temperature magnetic measurements revealed a spin-glass-like magnetic phase transition around 40 K in the frustrated β-Mn2(OH)3Cl. A pressure-induced structural phase transition was further realized by employing in situ synchrotron X-ray diffraction, leading to a hexagonal crystal structure with a nearly perfect kagome lattice of magnetic Mn2+ ions. High-pressure Raman and UV–vis absorption measurements indicates that the topological evolution can be attributed to the synergistic effects of pressure driven internal hydrogen bonds and the intrinsic Jahn-Teller effects. This study provides a strategy for synthesizing candidate compounds of QSL and sheds light on the mechanism underlying the topological evolution of kagome lattices under high pressure.

Pressure-induced reversible structural phase transitions and metallization in GeTe under hydrostatic and non-hydrostatic environments up to 22.9 GPa

The pressure-dependent vibrational and electrical transport properties of GeTe have been investigated in a diamond anvil cell through in-situ Raman spectroscopy and electrical conductivity measurements under hydrostatic and non-hydrostatic environments up to 22.9 GPa. Upon compression, two structural transformations from rhombohedral to cubic NaCl-type to orthorhombic GeTe occurred at 3.2 GPa and 12.3 GPa under non-hydrostatic condition. Similarly, two corresponding phase transitions were detected at much higher pressures of 5.0 GPa and 15.4 GPa under hydrostatic condition. Additionally, a 3.3 GPa of electronic transition accompanying by the rhombohedral to cubic NaCl-type transition was characterized by the variable-temperature electrical conductivity experiments. Upon decompression, the recoverable Raman spectra and resumable electrical conductivity suggested that the structural and electronic transitions of GeTe were reversible. The reversibility was further confirmed by the microscopic structural observations from high-resolution transmission electron microscopy, fast Fourier transform and atomic force microscopy.

First-principles study of structural phase transitions and metallization of XPSe3 (X = Fe, Mn) under high pressure

The pressure-induced structural phase transition and metallization of FePSe3 and MnPSe3 are investigated from ambient pressure to 40 GPa by first-principles calculation. FePSe3 and MnPSe3 undergo reversible structural transitions from R3¯ structure to C2/m structure and then to P3¯1m structure. The structural phase transitions of FePSe3 are at about 6 GPa and 13 GPa, while the pressure points are at approximately 13 GPa and 25 GPa for MnPSe3, reflecting from pressure-dependent enthalpy calculation, cell volume collapses, and sudden changes of lattice parameters. The computed metallization transition is also observed at around 8 GPa and 16 GPa owing to energy-band closure for FePSe3 and MnPSe3, respectively. The metallic character of XPSe3 is strengthened with increasing pressure based on the electronic energy band and electron density of states. The layer sliding of atoms is revealed using the P2-P3 atom pair of XPSe3 during structural phase transitions. The paper is conducive to considering the electronic structure and metallization of transition metal thiophosphates XPSe3-type family under extreme conditions.

Pressure induced structural phase transitions in calcium halofluorides

Density Functional Theory (DFT) is a powerful tool to investigate the pressure-induced structural phase transitions in materials. We predicted that CaClF and CaBrF undergo a structural transition from P4/nmm to Pnma at 57 GPa and 135 GPa, respectively, while no phase transition is observed for CaIF until the maximum studied pressure of 200 GPa. The electronic structure calculations reveal that the valence band of CaXF are separated into two manifolds due to electronegativity difference between F and X (Cl, Br, I), the manifold close to Fermi level is mainly originated from p-states of X while the well below manifold is derived from the 2p-states of F. The highest electronegativity of F atom makes the difference between alkaline-earth halofluorides and dihalides from their electronic structure, which determines their application as storage phosphors rather than highlight output scintillators similar to alkaline-earth dihalides.

Pressure-Induced Reversible and Irreversible Multistate Switching in NiAs-Type Chromium Selenides

Materials capable of switching in multiple states under external stimuli have garnered extensive attention in the field of memory devices and switches. The coupling of various switching properties is of paramount significance to the creation of multifunctional devices. Herein, we report the pressure-induced multiswitching behaviors of both the structural and physical properties in two chromium selenides, CrSe and Cr2Se3. Comprehensive high-pressure characterizations reveal the collaborative occurrence of pressure-induced structural phase transitions, spin-crossover, metallization, and n–p conduction-type switching in both compounds. Upon compression, Cr2Se3 demonstrates an unexpected increase in resistivity and band gap, which is associated with the disordering of the self-intercalation structure. Of particular interest is that due to the dimensional differences in crystal structures, the photoelectric properties of the two decompressed samples are inversely regulated after pressure treatment. Notably, the band gap of Cr2Se3 is pronouncedly broadened after decompression due to the partially irreversible disorder in the structure. These results demonstrate that pressure engineering can regulate the photoelectric properties of materials effectively and flexibly, serving as a potential foundation for fabricating innovative pressure-responsive multifunctional devices.

High-pressure effect on the superconductivity of NaAlGe and pressure-induced structural phase transition

The structural, electronic, phonon, and electron–phonon properties of NaAlGe have been investigated under high pressure by means of density-functional theory simulations. The calculated electronic structure and density of states reveal that the electronic states near the Fermi level, which are responsible for electrical conductivity, are mostly composed of Ge 4p states. The largest contribution to the average electron–phonon coupling parameter is from Ge-related vibrations, and this parameter is calculated to be λ∼ 0.609. The superconducting critical temperature at ambient pressure is calculated to be Tc = 2.31 K. This temperature agrees with the previously experimentally obtained value of 1.80 K. The electronic density of states at the Fermi energy, λ, and Tc decrease as pressure is increased, and superconductivity is suppressed at 12 GPa. We also report the phonon dispersion at different pressures showing that NaAlGe develops a dynamical instability at 13 GPa favoring a structural phase transition Based on enthalpy calculations the crystal structure of the high-pressure phase has been proposed to be orthorhombic and described by space group Pnma. This phase does not exhibit superconductivity. The pressure dependence of unit-cell parameters and Raman- and infrared-active phonons of the low-pressure and high-pressure phase is also reported.

Pressure-Induced Structural Phase Transition of Co-Doped SnO2 Nanocrystals

Co-doped SnO2 nanocrystals (with a particle size of 10 nm) with a tetragonal rutile-type (space group P42/mnm) structure have been investigated for their use in in situ high-pressure synchrotron angle dispersive powder X-ray diffraction up to 20.9 GPa and at an ambient temperature. An analysis of experimental results based on Rietveld refinements suggests that rutile-type Co-doped SnO2 undergoes a structural phase transition at 14.2 GPa to an orthorhombic CaCl2-type phase (space group Pnnm), with no phase coexistence during the phase transition. No further phase transition is observed until 20.9 GPa, which is the highest pressure covered by the experiments. The low-pressure and high-pressure phases are related via a group/subgroup relationship. However, a discontinuous change in the unit-cell volume is detected at the phase transition; thus, the phase transition can be classified as a first-order type. Upon decompression, the transition has been found to be reversible. The results are compared with previous high-pressure studies on doped and un-doped SnO2. The compressibility of different phases will be discussed.