Pressure-induced Structural Phase Transition Research Articles

Pressure-induced Structural Phase Transition and New Superconducting Phase in UTe2

We report on the crystal structure and electronic properties of the heavy fermion superconductor UTe2 at high pressure up to 11 GPa, as investigated by X-ray diffraction and electrical resistivity experiments. The X-ray diffraction measurements under high pressure using a synchrotron light source reveal anisotropic linear compressibility of the unit cell up to 3.5 GPa, while a pressure-induced structural phase transition is observed above PO–T ∼ 3.5–4 GPa at room temperature, where the body-centered orthorhombic crystal structure with the space group Immm changes into a body-centered tetragonal structure with the space group I4/mmm. The molar volume drops abruptly at PO–T, while the distance between the first-nearest neighbor of U atoms, dU–U, increases, implying a switch from the heavy electronic states to the weakly correlated electronic states. Surprisingly, a new superconducting phase at pressures higher than 7 GPa was detected at Tsc > 2 K with a relatively low upper-critical field, Hc2(0). The resistivity above 3.5 GPa, thus, in the high-pressure tetragonal phase, shows a large drop below 230 K, which may also be related to a considerable change from the heavy electronic states to the weakly correlated electronic states.

Structural and electronic properties of Weyl semimetal WTe2 under high pressure

The structural and electronic properties of WTe2 are studied from ambient pressure to 30.0 ​GPa. The pressure-induced structural phase transition is found from Td phase to 1T′ phase above 10.0 ​GPa by lattice offset, which is also reflected at approximately 11.2 ​GPa based on the discontinuous electrical parameters. A pair of hole wave and electron wave packets of Td phase gradually overlap revealing the reversible Td-to-1T′ phase structural transition. WTe2 is observed to undergo semimetal-to-semiconductor transition owing to the calculated energy band up to 15 ​GPa, which is also observed at 11.2 ​GPa according to the resistivity-temperature curves under compression and decompression conditions. The electronic density of states at the Fermi level is mainly contributed by the d orbital of W and the p and s orbital of Te. The photoconductivity property is shown by the theoretical and experimental methods.

Pressure-Induced Structural Phase Transition and Enhanced Interlayer Coupling in Two-Dimensional Ferromagnet CrSiTe3

The two-dimensional van der Waals ferromagnetic semiconductor CrSiTe3 has attracted growing interest as an intrinsic topological magnet. Both superconductivity and enhancement of ferromagnetism, usually competing for orders, have been observed in CrSiTe3 at high pressure. However, the high-pressure structure of CrSiTe3 is still unclear, setting obstacles in understanding pressure-induced novel physics. Here, combining the Raman spectra and first-principles calculations, the structure of CrSiTe3 at high pressure has been clarified. The interlayer breathing mode located at ∼42.1 cm-1 has been observed for the first time in CrSiTe3 by ultralow-frequency Raman spectroscopy at high pressure. Theoretical calculations confirm a phase transition from the R3̅ phase to the R3 phase accompanying noticeable enhancement of the Curie temperature. Our results highlight ultralow-frequency Raman spectroscopy combined with high pressure for detecting and modulating the structure and interlayer coupling of two-dimensional materials.

Effects of High Pressure on the Bandgap and the d–d Crystal Field Transitions in Wolframite NiWO4

The pressure effects on the optical and structural properties of NiWO4 have been studied experimentally and theoretically. The fundamental bandgap decreases with a pressure coefficient of −12.0 ± 0.2 meV/GPa. Meanwhile, the Ni2+ d–d transition energies increase at a rate of 7.4–14.8 meV/GPa. Therefore, the energy differences between the fundamental band and the Ni2+ d–d transition bands gradually decrease under pressure, which is beneficial to improve its optical performance. These optical phenomena are associated with structural variations. The shrinkage of the WO6 octahedron enhances the hybridization between the W 5d and O 2p orbitals, resulting in bandgap reduction. The pressure-induced enhancement of the NiO6 octahedral symmetry increases the crystal field splitting, thereby yielding increases in the Ni2+ d–d intraband transition energies. Besides, a pressure-induced structural phase transition is also observed around 20.0 GPa by both angle-dispersive synchrotron X-ray diffraction (ADXRD) and Raman experiments. This study provides valuable insight into the electron–lattice coupling of NiWO4 under compression and an effective way to modulate the electronic structure and optical properties of isomorphic wolframite materials.

Pressure-induced structural phase transitions in natural monazite

The behavior of natural monazite $(\mathrm{La},\phantom{\rule{0.16em}{0ex}}\mathrm{Ce},\phantom{\rule{0.16em}{0ex}}\mathrm{Nd})\mathrm{P}{\mathrm{O}}_{4}$ at high pressure is investigated by Raman spectroscopy. Changes in the Raman spectra indicate a phase transition from monazite to a post-monazite phase. Single-crystal x-ray diffraction measurements of the decompressed sample reveal that the post-monazite phase has a distorted barite-type structure with space group $P{2}_{1}/n$. This phase transition is irreversible under quasihydrostatic conditions, and is reversible under nonhydrostatic conditions. Therefore, it is expected that the post-monazite phase cannot be found as a metastable phase at ambient conditions in impact craters due to nonhydrostatic conditions during shock metamorphism, supporting recent mineralogical observations in impact craters.

Pressure-Induced Structural Phase Transitions in the Chromium Spinel LiInCr4O8 with Breathing Pyrochlore Lattice

This study reports high-pressure structural and spectroscopic studies on polycrystalline cubic chromium spinel compound LiInCr4O8. According to pressure-dependent X-ray diffraction measurements, three structural phase transitions occur at ∼14 GPa, ∼19 GPa, and ∼36 GPa. The first high-pressure phase is indexed to the low-temperature tetragonal phase of the system which coexists with the ambient phase before transforming to the second high-pressure phase at ∼19 GPa. The pressure-dependent Raman and infrared spectroscopic measurements show a blue-shift of the phonon modes and the crystal field excitations and an increase in the bandgap under compression. During pressure release, the sample reverts to its ambient cubic phase, even after undergoing multiple structural transitions at high pressures. The experimental findings are compared to the results of first principles based structural and phonon calculations.

Phase transition and bond symmetrization associated with noble gas bond in XeO3

The pressure-induced structural phase transition of XeO3 is studied by first principle calculations. The transition from P212121 to Pnma accompanied by a drastic reduction of volume is found at 2.18 GPa. The symmetrilized Xe−O2 bonds give rise to the better symmetry of high pressure phase. O-hopping between different possible local minima and the motion of Xe along the y axis may be responsible for phase transition. Results of electron localization function indicate that three ipsilateral Xe−O bonds lead to a lone-pair contour of Xe6+.

Pressure-Induced Abnormal Electrical Transport Transition from Pure Electronic to Mixed Ionic–Electronic in Multiferroic BiFeO3 Ceramics

Pressure-induced structural changes could induce changes in transport properties and lead to a better understanding of the structure–property relationship. The evolution of the carrier transport properties of BiFeO3 (BFO) ceramics under a high pressure was investigated through impedance spectroscopy measurements at room temperature combined with first-principles calculations. A pressure-induced abnormal transition from pure electronic to mixed ionic–electronic was found in the BFO ceramics Cmmm and Pnma phases at 7.67 and 11.01 GPa, respectively. The pressure-induced structural phase transition from the R3c phase to Cmmm and then to Pnma was responsible for the change in electrical transport behavior from pure electronic to mixed ionic–electronic conduction, accompanied by a decrease in ionic resistance. The calculations of electronic structures and electron localization function from 1 atm to 40 GPa indicated that the ionic conduction in the Pnma phase resulted from the weakened Coulomb screen of the localized electron background to O2– between Bi3+ and O2–. This work provides a critical insight into the understanding of the relationship between structure and conduction and facilitates the application of ferroelectric materials in photoelectric fields.

Pressure-induced structural phase transitions of zirconium: an ab initio study based on statistical ensemble theory

Recently, we put forward a direct integral approach to solve the partition function with ultrahigh efficiency and precision, which enables the rigorous ensemble theory to investigate phase behaviors of realistic condensed matters and has been successfully applied to the phase transition of vanadium metal (Ning et al 2022 J. Phys.: Condens. Matter 34 425404). In this work, the approach is applied to the structural phase transitions of zirconium metal under compressions up to 160 GPa and ultrahigh calculation precision is achieved. For the obtained equation of state with pressure over 40 GPa, the deviations from latest experiments are within and the computed transition pressure of is 6.93 GPa, which is about five times larger than previous theoretical predictions and in excellent agreement with the measured range of 5–15 GPa. Our results support the argument that there is no existence of the isostructural phase transition of Zr metal that was asserted by recent experimental observations.

Lattice instability, anharmonicity and Raman spectra of BaO under high pressure: A first principles study

Alkaline-earth metal oxides, MgO and CaO dominate Earth's lower mantle, therefore, exploring high-pressure behavior of these compounds is of significant geophysical research interest. Among the alkaline-earth metal oxides, BaO exhibits rich polymorphism in the pressure range of 0–1.5 Mbar. Static enthalpy calculations revealed that BaO undergoes a pressure-induced structural phase transition from NaCl-type (B1) → NiAs-type (B8) → distorted CsCl-type (d-B2) → CsCl-type (B2) at 5.1, 19.5 and 120 GPa, respectively. Transitions B1 → B8 and B8 → d-B2 are found to be first order in nature whereas d-B2 → B2 is a second order or weak first-order phase transition with displacive nature. Interestingly, the d-B2 phase shows stability over a wide pressure range of ∼19.5–113 GPa. Mechanical and dynamical stabilities of ambient and high-pressure phases are demonstrated through the computed second-order elastic constants and phonon dispersion curves, respectively. Under high pressure, significant phonon softening and soft phonon mode along the M-direction are observed for B8, d-B2 and B2 phases, respectively. Pressure-dependent Raman spectra suggest a phase transition from d-B2 to a Raman inactive phase under high pressure. Overall, the present study provides a comprehensive understanding of lattice dynamics and the underlying mechanisms behind pressure-induced structural phase transitions in BaO.

Pressure-induced structural phase transition of vanadium: a revisit from the perspective of ensemble theory

For realistic crystals, the free energy strictly formulated in ensemble theory can hardly be obtained because of the difficulty in solving the high-dimension integral of the partition function, the dilemma of which makes it even a doubt if the rigorous ensemble theory is applicable to phase transitions of condensed matters. In the present work, the partition function of crystal vanadium under compression up to 320 GPa at room temperature is solved by an approach developed very recently, and the derived equation of state is in a good agreement with all the experimental measurements, especially the latest one covering the widest pressure range up to 300 GPa. Furthermore, the derived Gibbs free energy proves the very argument to understand most of the experiments reported in the past decade on the pressure-induced phase transition, and, especially, a novel phase transition sequence concerning three different phases observed very recently and the measured angles of two phases agree with our theoretical results excellently.

Pressure-Induced Structural Phase Transition, Anomalous Insulator-to-Metal Transition, and n-p Conduction-Type Switching in Defective, NiAs-Type Cr1-δTe.

Cr1-δTe, as a unique series of defective compounds with a NiAs-type structure and periodic metal vacancies, has attracted intensive research interest because of its diversity in structure and property dependent on the tunable stoichiometric ratio. Another feature of these compounds is their ability to switch between NiAs- and MnP-type structures, in which there often exist composition-, temperature-, or pressure-induced phase transitions accompanied by intriguing physical property switching. Herein, we report the syntheses of a series of Cr1-δTe compounds with similar compositions but distinct crystal structures, their phase transitions, anomalous transport, and photoelectric conduction behaviors under high pressure (HP). For the three Cr1-δTe compounds with δ = 0, 0.25, 0.375, CrTe undergoes pressure-induced NiAs-to-MnP phase transition at around 15 GPa, while Cr3Te4 and Cr5Te8 undergo isostructural phase transitions at around 12 and 11 GPa, respectively. Electrical transport measurements indicate anomalous conduction behaviors: CrTe undergoes a semiconductor-to-metal transition at around 24 GPa, while Cr3Te4 and Cr5Te8 show unexpected metal-semiconductor-metal transitions sequentially upon compression. Besides, CrTe and Cr5Te8 exhibit pressure-induced n-p conduction-type switching at around 9 and 3 GPa, respectively, while Cr3Te4 shows p-type conductivity in the full pressure range. Local structure analyses based on in situ HP Raman spectra are performed to understand the structure and property evolutions of Cr1-δTe under HP. Defective transition-metal chalcogenides with pressure-induced NiAs-to-MnP phase transition and conduction-type conversion provide a potential platform for the rational design of photoelectric conversion devices.

Pressure-Dependent Structure of BaZrO3 Crystals as Determined by Raman Spectroscopy.

The structure of dielectric perovskite BaZrO3, long known to be cubic at room temperature without any structural phase transition with variation in temperature, has been recently disputed to have different ground state structures with lower symmetries involving octahedra rotation. Pressure-dependent Raman scattering measurements can identify the hierarchy of energetically-adjacent polymorphs, helping in turn to understand its ground state structure at atmospheric pressure. Here, the Raman scattering spectra of high-quality BaZrO3 single crystals grown by the optical floating zone method are investigated in a pressure range from 1 atm to 42 GPa. First, based on the analyses of the infrared and Raman spectra measured at atmospheric pressure, it was found that all the observed vibrational modes could be assigned according to the cubic structure. In addition, by applying pressure, two structural phase transitions were found at 8.4 and 19.2 GPa, one from the cubic to the rhombohedral Rc phase and the other from the rhombohedral to the tetragonal I4/mcm phase. Based on the two pressure-induced structural phase transitions, the true ground state structure of BaZrO3 at room temperature and ambient pressure was corroborated to be cubic while the rhombohedral phase was the closest second.

Pressure-Induced Intermetallic Charge Transfer and Semiconductor-Metal Transition in Two-Dimensional AgRuO 3

Pressure-Induced Intermetallic Charge Transfer and Semiconductor-Metal Transition in Two-Dimensional AgRuO ₃

Pressure-induced structural phase transition in BaHCl

We have investigated the structural evolution of BaHCl under high pressure using in-situ synchrotron X-ray diffraction up to 30 ​GPa. The in-situ observation found that the ambient pressure BaHCl phase crystallizing in the PbFCl-type structure (P4/nmm) is transformed into an orthorhombic phase at 29 ​GPa and 300 ​K. Rietveld refinements of synchrotron X-ray and neutron diffraction data for the quenched sample revealed that BaHCl adopts PbCl2-type structure (Pnma), which consists of a distorted hcp framework of Ba2+ ions, with H− and Cl− ions at tetrahedral and octahedral interstitials, respectively. The complete ordering of anions in the PbCl2-type BaHCl contrasts with the partial ordering in the PbCl2-type AHF (A ​= ​alkali earth metal). This result may promote further exploration of hydride-based mixed anion materials and understanding and control of anion order/disorder.

Pressure-Induced Structural Phase Transition in EuNi2P2.

EuNi2P2 was studied with a diamond anvil cell (DAC) and X-ray diffraction (XRD). The structural evolution of powder crystal EuNi2P2 under high pressure up to 137 GPa and its single-crystal structure up to 9 GPa were reported. The unique structural phase transition of this 122-type crystal occurred above 70 GPa in powder crystal EuNi2P2. The diffraction data from single-crystal EuNi2P2 revealed the coordinate change of the P atom, and the stability of the crystal at 9 GPa was confirmed. The crystal EuNi2P2 remained stable with a tetragonal phase without obvious symmetry changes during compression to 137 GPa.

Semiconducting BaS3 phase featuring v-shape S3 unit at high pressure

Pressure-induced structural transformation of known compounds has become an effective routine to obtain new materials. The sulfur motifs that are present in compounds have a considerable effect on their crystal structures and electronic properties. Here, we focus on exploring the evolution of S motifs in the $\mathrm{Ba}{\mathrm{S}}_{3}$ compound under high pressure. A novel $P{3}_{1}21\text{\ensuremath{-}}\mathrm{Ba}{\mathrm{S}}_{3}$, consisting of an equivalent V-shape ${\mathrm{S}}_{3}$ unit and quasi-hexagon Ba ring, is identified through the combination of first-principles swarm-intelligence search and phonon softening calculations. The bond angle in the ${\mathrm{S}}_{3}$ unit decreases, accompanied by pressure-induced structural phase transition. $P{3}_{1}21\text{\ensuremath{-}}\mathrm{Ba}{\mathrm{S}}_{3}$ shows a semiconducting character, mainly originating from the contribution of S $3p\text{\ensuremath{-}}\mathrm{electron}$ states. Along with the newly proposed $\mathrm{Ba}{\mathrm{S}}_{3}$ structure, the temperature effect on stability is estimated using the temperature-dependent effective potential method. For another S-rich $\mathrm{Ba}{\mathrm{S}}_{2}$, the disappearance of the ${\mathrm{S}}_{2}$ unit induced by pressure ($>150\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$) is mainly responsible for structural instability.

Pressure-Induced Structural and Charge-Transfer Phase Transitions for a Two-Dimensional Mixed-Valence Iron(II,III) Coordination Polymer

A two-dimensional (2D) mixed-valence iron(II,III) coordination polymer, (Ph4P)[FeIIFeIII(dto)3] (1; Ph4P = tetraphenylphosphonium, dto = 1,2-dithiooxalato), was prepared, and its crystal structure was revealed. Compound 1 exhibited a 2D honeycomb structure and a rigid layered framework, due to a hydrogen bond between the cation and anion layers. The temperature dependence of the magnetic susceptibilities for 1 under high pressure showed a thermal hysteresis loop corresponding to a charge-transfer phase transition, although there was no charge-transfer phase transition under ambient pressure.

Pressure-Induced Structural Phase Transition and Metallization of CrCl3 under Different Hydrostatic Environments up to 50.0 GPa.

High-pressure structural, vibrational, and electrical transport properties of CrCl3 were investigated by means of Raman spectroscopy, electrical conductivity, and high-resolution transmission electron microscopy under different hydrostatic environments using the diamond anvil cell in conjunction with the first-principles theoretical calculations up to 50.0 GPa. The isostructural phase transition of CrCl3 occurred at 9.9 GPa under nonhydrostatic conditions. As pressure was increased up to 29.8 GPa, CrCl3 underwent an electronic topological transition accompanied by a metallization transformation due to the discontinuities in the Raman scattering and electrical conductivity, which is possibly belonging to a typical first-order metallization phase transition as deduced from first-principles theoretical calculations. As for the hydrostatic condition, a ∼2.0 GPa pressure delay in the occurrence of two corresponding transformations of CrCl3 was observed owing to the different deviatoric stress. Upon decompression, we found that the phase transformation from the metal to semiconductor in CrCl3 is of good reversibility, and the obvious pressure hysteresis effect is observed under different hydrostatic environments. All of the obtained results on the structural, vibrational, and electrical transport characterizations of CrCl3 under high pressure can provide a new insight into the high-pressure behaviors of representative chromium trihalides CrX3 (X = Br and I) under different hydrostatic environments.

An Investigation of the Pressure-Induced Structural Phase Transition of Nanocrystalline α-CuMoO4

The structural behavior of nanocrystalline α-CuMoO4 was studied at ambient temperature up to 2 GPa using in situ synchrotron X-ray powder diffraction. We found that nanocrystalline α-CuMoO4 undergoes a structural phase transition into γ-CuMoO4 at 0.5 GPa. The structural sequence is analogous to the behavior of its bulk counterpart, but the transition pressure is doubled. A coexistence of both phases was observed till 1.2 GPa. The phase transition gives rise to a change in the copper coordination from square-pyramidal to octahedral coordination. The transition involves a volume reduction of 13% indicating a first-order nature of the phase transition. This transformation was observed to be irreversible in nature. The pressure dependence of the unit-cell parameters was obtained and is discussed, and the compressibility analyzed.