Pressure-induced Phase Transition Research Articles

Pressure-induced phase transitions in a new luminescent gold(I)-arylacetylide.

Stimulus-responsive molecular materials are highly desirable because of the wide range of their potential applications. In particular, switching of physical properties opens application pathways for molecular materials as sensors or actuators. Property switching in solids can be achieved by inducing single-crystal-to-single-crystal (SCSC) phase transitions. Elucidating the mechanisms of such transformations and identifying the factors and (supra)molecular motifs that increase their probability is thus paramount to understanding property switching and materials design. Here, we present a new compound, (p-methoxy-phenylacetylide)(triethylphosphine)gold(I) (ArPEt), combining the photoluminescent core of gold(I) acetylide with triethylphosphine that should provide a rich conformational landscape and favor SCSC phase transitions due to low-energy interconversion pathways. We demonstrate its potential to undergo multiple pressure-induced SCSC transformations at ≈0.5 GPa and ≈2 GPa, the obtainable phases being dependent on the types of applied pressure-transmitting media. We describe structures of polymorphs of ArPEt and the mechanism of pressure-induced structural reorganizations based on very high-quality single-crystal X-ray diffraction data.

Luminescence of the Cs2ZrCl6 under High Pressure.

The photoluminescence (PL) and Raman spectra of the Cs2ZrCl6 crystal over a wide range of pressures were studied in this work for the first time. PL measurements were performed up to 10 GPa, while the Raman spectra were measured up to 20 GPa. The PL data revealed a linear blue shift of the emission maximum from about 2.5 eV at ambient pressure to 3.1 eV at 5 GPa and a strong intensity quenching. The indirect-to-direct bandgap transition at about 5 GPa, a phenomenon previously predicted only theoretically, was used to explain the strong quenching of the PL. This model was confirmed by fitting PL intensity data and analysis of the PL decay kinetics, which exhibited a shortening of the pulse decay time with the increasing pressure. Raman spectra confirmed the stability of Cs2ZrCl6 up to 20 GPa and showed no evidence of pressure-induced structural phase transitions. An energetic scheme of excitonic levels, which takes into account the indirect-to-direct bandgap transition, was proposed to explain the rapid PL quenching with increasing pressure.

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.

Crystallographic symmetry increasing in the pressure-induced phase transition of the hydrogen-bonded organic crystal 1-methylhydantoin.

In situ high-pressure Raman spectroscopy and synchrotron angular dispersive x-ray diffraction techniques, combined with first-principles calculations, have been performed to investigate the 1-methylhydantoin (C4H6N2O2, 1-MH) molecular crystal. High-pressure experiments have shown that phase I (monoclinic system) begins to transform into phase II (orthorhombic system) at pressures above 4.0GPa, and the transformation range is from 4.0 to 14.2GPa. It is proposed that the mechanism of phase transition is the interlayer contraction and rearrangement of the hydrogen-bonding network due to the enhanced strong hydrogen-bonded interactions at high pressures. This study provides some theoretical basis for this rare pressure-induced phase transition from low symmetry to high symmetry in organic supramolecular polymorphism.

Pressure-induced phase transition from monomers to dimers in liquid crystals

ABSTRACT The crystal structures of liquid crystals, particularly various oligomers, have garnered significant attention, laying the groundwork for developing new materials and devices. Here, we demonstrate pressure plays a crucial role in the structure of liquid crystals. High-pressure Raman and photoluminescence spectra were on 5CB (4-cyano-4’−pentylbiphenyl) liquid crystal. Upon compression, the appearance of prominent fluorescence peaks suggests a phase transition. The Raman peaks, reflecting the C–H and C≡N bonds, exhibit splitting at a pressure of 3.2 GPa, further confirming the pressure-induced phase transition. The splitting is consistent with earlier calculations on the stretching vibration of the C≡N bond for various oligomers and indicates a phase transition from monomers to dimers in 5CB liquid crystal. The opposite shifts in Raman modes under pressure, specifically the blue-shift of C–H bonds and the red-shift of C≡N bonds, indicate that pressure enhances intermolecular interactions and leads to conjugation between the electrons on the C≡N bonds and the C–H bonds, resulting in dimer formation. Our findings not only reveal a novel strategy for producing oligomers in liquid crystals through the application of high pressure but also highlight that Raman spectroscopy is a valid, non-contact method for investigating the mechanisms of oligomerization.

Pressure-directed enhanced ionic conductivity in MgMoO4 mixed-phases

The mixed conductors have attracted much attention for potential applications in fuel cells, sensors, supercapacitors. Here, the electrical transport behavior of MgMoO4 were systematically investigated at pressures up to 30.0 GPa using impedance spectroscopy measurements and theoretical calculations. The discontinuous changes in electrical parameters reflect the pressure-induced phase transition from β-MgMoO4 to γ-MgMoO4. The transport mechanism was determined to be mixed ionic-electronic conduction, and the contributions of carriers were distinguished. In the pressure range where the low- and high- pressure phase coexist, ionic transport plays a dominant role. The optimal solution for ionic conductivity of MgMoO4 occurs in the mixed phases, and the ionic conductivity has been increased by two orders of magnitude in comparison to that observed at ambient conditions. The resistances were analyzed with band gap, transference number and the relative ionic diffusion coefficient. The pressure constrains the atomic amplitude and limits ion-phonon scattering, resulting in a significant enhancement of the ionic diffusion coefficient and facilitates the ionic conduction of γ-MgMoO4. Meanwhile, it is found that the grain boundary response was weakened under compression, and the static dielectric constant was improved by pressure. These results offer valuable insights for optimizing and applying ABO4-based mixed conductors.

Erratum: Pressure-induced phase transition and increase of oxygen-iodine coordination in magnesium iodate [Phys. Rev. B 105, 054105 (2022)

Erratum: Pressure-induced phase transition and increase of oxygen-iodine coordination in magnesium iodate [Phys. Rev. B 105, 054105 (2022)

Compositional effect on pressure-induced polymorphism in high-entropy alloys

Recently, pressure-induced polymorphic phase transitions were discovered in several fragmented high-entropy alloys (HEAs), offering a valuable opportunity to deepen our understanding of these materials. However, the chemical and physical factors that govern these transitions are still unclear. Here, we combined in situ high-pressure synchrotron X-ray diffraction, X-ray emission spectroscopy (XES), and high-resolution transmission electron microscopy (HRTEM) to systematically study the evolution of the atomic and electronic structures in the Cantor alloy and its face-centered-cubic (fcc) subset alloys (CoCrFeMnNi, CoCrFeNi, CoCrMnNi, CoFeMnNi, CoCrNi, CoFeNi, CoMnNi, CrFeNi, and FeMnNi). Surprisingly, diverse behavior was observed among these closely related alloys during compression and decompression, which includes irreversible, reversible fcc to hexagonal close-packed (hcp) phase transitions, or even no detectable phase transitions up to ∼40 GPa. HRTEM measurements confirmed that the fcc and hcp phases abided by the classic Shoji-Nishiyama orientation relationship during the transitions. XES data indicated that high-pressure suppresses the local magnetic moments in all the studied alloys, suggesting that magnetic states do not significantly influence the polymorphic transitions. By comparing the effects of the atomic size difference, entropy, valence electron concentration, and stacking fault energy across all the compositions studied, only the stacking fault energy shows a strong correlation with the phase transitions, indicating it plays a key role in inducing polymorphism in HEAs.

Density functional theory investigation of the phase transition, elastic and thermal characteristics for AuMTe2(M = Ga, In) chalcopyrite compounds.

We explored the pressure-induced structural phase transitions and elastic properties of AuMTe2 (M = Ga, In) using the full-potential linearized augmented plane wave method within the framework of density functional theory, applying both generalized gradient and local density approximations. Thermodynamic properties were further assessed through the quasi-harmonic model. We determined the transition pressures for the phase shift from the chalcopyrite structure to the NaCl rock-salt phase in both AuGaTe2 and AuInTe2. Additionally, we calculated and analyzed mechanical properties, such as bulk modulus, shear modulus, Young's modulus, Poisson's ratio, elastic anisotropy, ductility versus brittleness, and hardness for the polycrystalline forms of AuMTe2 (M = Ga, In). The study also examined how temperature and pressure affect the Debye temperature, heat capacities, thermal expansion, entropy, bulk modulus, Grüneisen parameter, and hardness, utilizing the quasi-harmonic Debye model.

The mechanism behind pressure-induced isosymmetric second-order phase transitions

Abstract Pressure-induced polymorphic phase transitions are important in fundamental physics, chemistry, materials science, and geoscience. The isosymmetric phase transition is unique in that it retains crystal symmetry through the transition and exhibits both first- and second-order characteristics. However, the underlying mechanisms of the isosymmetric second-order phase transition have not been well constrained under high pressures. Here, we report a novel case of pressure-induced isosymmetric phase transition in cerite, a rare earth element (REE) mineral, using a diamond anvil cell combined with in-situ synchrotron single-crystal X-ray diffraction. This phase transition is triggered by an increase in coordination number (CN) and exhibits characteristics of second-order. By combining our findings with previous research, we identify five distinct types of pressure-induced isosymmetric phase transitions. Furthermore, we propose an explanation for the nature (first- or second-order) of phase transitions triggered by coordination number increases under pressure. The complexity of a mineral's crystal structure plays a crucial role in determining the type of isosymmetric phase transition observed. Single-building unit crystal structures tend to exhibit first-order transitions, while those with multiple building units exhibit greater flexibility and are more prone to second-order transitions. This study provides new evidence for pressure-induced coordination number increase-triggered isosymmetric second-order phase transitions and offers valuable insights into the mechanisms governing pressure-induced isosymmetric phase transitions.

Unraveling the atomic-scale pathways driving pressure-induced phase transitions in silicon

Silicon exhibits several metastable allotropes which recently attracted attention in the quest for materials with superior (e.g. optical) properties, compatible with Si technology. In this work we shed light on the atomic-scale mechanisms leading to phase transformations in Si under pressure. To do so, we synergically exploit different state-of-the-art approaches. In particular, we use the advanced GAP interatomic potential both in NPT molecular dynamics simulations and in solid-state nudged elastic band calculations, validating our predictions with ab initio DFT calculations.We provide a link between evidence reported in experimental nanoindentation literature and simulation results. Particular attention is dedicated to the investigation of atomistic transition paths allowing for the transformation between BC8/R8 phases to the hd one under pure annealing. In this case we show a direct simulation of the local nucleation of the hexagonal phase in a BC8/R8 matrix and its corresponding atomic-scale mechanism extracted by the use of SS-NEB. We extend our study investigating the effect of pressure on the nucleation barrier, providing an argument for explaining the heterogeneous nucleation of the hd phase and unraveling its main parameters with possible applications to the design of nanostructured materials.

Polymorphism in Type-II Dirac Semimetal WSi2 under Pressure: Structural, Mechanical, and Electronic Insights.

The type-II Dirac candidate semimetal WSi2 is a promising candidate for electronic devices, quantum computing, and topological materials research, owing its distinct electronic structure and superior mechanical properties. Here, we synthesized high-quality WSi2 materials and systematically investigated their compressive behavior, and structural and electronic properties under high pressure using in-situ high pressure experiments, complemented by first-principles calculations. The results confirms that WSi2 has the properties of a type-II Dirac semimetal. Our results demonstrate that WSi2 maintains structural stability under high pressure but undergoes an electronic phase transition from a semimetal to a metal around 40 GPa. Additionally, the mechanical hardness softens discontinuously at this pressure. The structural stability of WSi2 under high pressure is attributed to the strong hybridization of Si-3p and W-5d electrons, the rigid crystal lattice, and the adaptable electronic structure. The pressure-induced electronic phase transition and softening are primarily governed by the energy band reconstruction and W-5d orbitals. This study provides valuable insights into the high-pressure behavior of type-II Dirac semimetal, highlighting their potential for advanced applications in electronic devices and topological quantum computing under extreme conditions by elucidating their structural stability and electronic phase transition mechanisms.

Pressure-induced superconductivity and phase transition in PbSe and PbTe

Abstract The IV–VI semiconducting chalcogenides are a large material family with distinct physical behavior. Here, we systematically investigate the effect of pressure on the electronic and crystal structures of PbSe and PbTe by combining high-pressure electrical transport and synchrotron x-ray diffraction (XRD) measurements. The resistivity of PbSe and PbTe changes dramatically under high pressure and a non-monotonic evolution of ρ(T) is observed. Both PbSe and PbTe are found to undergo semiconductor–metal transition upon compression and show superconductivity under higher pressure. The structural evolutions from the F m 3 ¯ m to Pnma phase and then to the P m 3 ¯ m phase in PbSe are verified by the x-ray diffraction. The present findings reveal the internal correlation between the structural evolution and the physical properties in lead chalcogenides.

Pressure-induced phase transition and indirect band gap semiconductor in ZnSnN2: First Principles Calculation

In this study, we investigate the phase transition of ZnSnN2 from Pna21 to Pmnb using Density Functional Theory (DFT) across a pressure range of 0–70 GPa. Our results show the enthalpy intersection of the Pna21 and Pmnb phases at 19.28 GPa, indicating a phase transition from Pna21 to Pmnb ZnSnN2. The decrease in Hv of the Pna21 phase under pressure before the phase transition is attributed to the reduction of the G and weakening covalent bond of Sn–N pair. The new Pmnb phase exhibits an increased Vickers hardness, Debye temperatures, and brittleness. Moreover, the band gap is an indirect band gap of 1.41 eV due to a rearrangement of lower energy levels for Sn s and p states in conduction band minimum (CMB) and N s and p states in valence band maximum (VBM) at Γ-point. These characteristics make The Pmnb phase promising candidates for applications were longer carrier lifetimes are needed. The mechanical properties, dynamical behavior, and electron localization functions (ELFs) have been investigated and discussed.

A Discovery of Pressure-Induced New Semiconductor Electronic Phase Transitions by DFT Calculations: Introducing a Glimpse of a Novel Semiconductor Family.

This study introduces a discovery of pressure-induced new semiconductor electronic phase transitions. A novel semiconductor family that exhibits pressure-induced nonmonotonic changes in band gaps was found and meets the definition of phase transitions, challenging the traditional understanding of linear and monotonic band gap modification through pressurization. Our findings suggest a complex interplay of atomic spacing and electron orbital contributions under varying pressure conditions, resulting in the variation of band gaps. This behavior, which includes three distinct steps: first, narrowing, second, broadening, and third, narrowing again and ultimately metalizing; some compounds could bypass step 1, has potential applications in piezoelectric and semiconductor technologies. We propose two new semiconductor electronic phase transitions (SEPT) associated with specific inflection points in the pressure-dependent band gap curve. Our results open avenues for further research into the electronic properties of crystals under high pressure, with the ultimate goal of uncovering the more profound physical principles governing these phenomena.

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.

Modulation of the vibrational and optical properties in β copper phthalocyanine under compression

Because copper phthalocyanine (CuPc) has significant uses in industrial devices and pigments, it is a subject of great interest in basic scientific research. Studying and adjusting its photoelectric qualities, in particular, has drawn much attention. In this work, in situ high-pressure experiments were used to investigate in detail the vibrational and optical properties of ground-state β-CuPc up to 13 GPa. Both micro-Raman and ultraviolet and visible light (UV–vis) absorption measurements revealed a phase transition occurring around 3 GPa. Notably, the macrocycle breathing vibration was found to have a remarkable characteristic of first weakening and then strengthening in response to this pressure-induced phase transition. The UV–vis absorption data revealed that the bandgaps corresponding to the two absorption edges decrease gradually with increasing pressure, with one edge disappearing entirely above 11 GPa. Furthermore, the blue β-CuPc can be adjusted to dark green at 10 GPa and ultimately entirely black due to the merging of its absorption bands and redshifts at high pressure. This work improves our understanding of CuPc's intrinsic characteristics under harsh settings and manipulates its color effectively.

Macroscopic perspective on phase transition behavior of natural single-crystal graphite under different pressure environments

Comprehensive understanding of the direct transformation pathway from graphite to diamond under high temperature and high pressure has long been one of the fundamental goals in materials science. Despite considerable experimental and theoretical progress, current experimental studies have mainly focused on the local microstructural characterizations of recovered samples, which has certain limitations for high-temperature and high-pressure products, which often exhibit diversity. Here, we report on the pressure-induced phase transition behavior of natural single-crystal graphite under three distinct pressure-transmitting media from a macroscopic perspective using in situ two-dimensional Raman spectroscopy, scanning electron microscopy, and atomic force microscopy. The surface evolution process of graphite before and after the phase transition is captured, revealing that pressure-induced surface textures can impede the continuity of the phase transition process across the entire single crystal. Our results provide a fresh perspective for studying the phase transition behavior of graphite and greatly deepen our understanding of this behavior, which will be helpful in guiding further high-temperature and high-pressure syntheses of carbon allotropes.

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.