High-pressure Phase Transition Research Articles

Universal Phase Transitions of AlB2-Type Transition-Metal Diborides.

High-pressure phase transitions of AlB2-type transition-metal diborides (TMB2; TM = Zr, Sc, Ti, Nb, and Y) were systematically investigated using first-principles calculations. Upon subjecting to pressure, these TMB2 compounds underwent universal phase transitions from an AlB2-type to a new high-pressure phase tP6 structure. The analysis of the atomistic mechanism suggests that the tP6 phases result from atomic layer folds of the AlB2-type parent phases under pressure. Stability studies indicate that the tP6-structured ZrB2, ScB2, and NbB2 are stable and may be observed under high pressure and the tP6-structured TiB2 phase may be recovered at ambient pressure.

Effect of Alkali and Trivalent Metal Ions on the High-Pressure Phase Transition of [C2H5NH3]MI 0.5MIII 0.5(HCOO)3 (MI = Na, K and MIII = Cr, Al) Heterometallic Perovskites.

We report the high-pressure behavior of two perovskite-like metal formate frameworks with the ethylammonium cation (EtAKCr and EtANaAl) and compare them to previously reported data for EtANaCr. High-pressure single-crystal X-ray diffraction and Raman data for EtAKCr show the occurrence of two high-pressure phase transitions observed at 0.75(16) and 2.4(2) GPa. The first phase transition involves strong compression and distortion of the KO6 subnetwork followed by rearrangement of the −CH2CH3 groups from the ethylammonium cations, while the second involves octahedral tilting to further reduce pore volume, accompanied by further configurational changes of the alkyl chains. Both transitions retain the ambient P21/n symmetry. We also correlate and discuss the influence of structural properties (distortion parameters, bulk modulus, tolerance factors, and compressibility) and parameters calculated by using density functional theory (vibrational entropy, site-projected phonon density of states, and hydrogen bonding energy) on the occurrence and properties of structural phase transitions observed in this class of metal formates.

Symmetry-Adapted Finite Strain Landau Theory Applied to KMnF3

In recent years, finite strain Landau theory has been gradually developed as both a conceptual as well as a quantitative framework to study high pressure phase transitions of the group-subgroup type. In the current paper, we introduce a new version of this approach which is based on symmetry-adapted finite strains. This results in a substantial simplification of the original formulation. Moreover, it allows for replacing the clumsy use of truncated Taylor expansions by a convenient functional parametrization. Both the weaknesses of the traditional Landau approach based on infinitesimal strains as well as the major improvements made possible by our new parametrization are illustrated in great detail in an application to the ambient temperature high pressure transition of the perovskite KMnF 3 .

A Short Review of Current Computational Concepts for High-Pressure Phase Transition Studies in Molecular Crystals

High-pressure chemistry of organic compounds is a hot topic of modern chemistry. In this work, basic computational concepts for high-pressure phase transition studies in molecular crystals are described, showing their advantages and disadvantages. The interconnection of experimental and computational methods is highlighted, showing the importance of energy calculations in this field. Based on our deep understanding of methods’ limitations, we suggested the most convenient scheme for the computational study of high-pressure crystal structure changes. Finally, challenges and possible ways for progress in high-pressure phase transitions research of organic compounds are briefly discussed.

High-Pressure Phase Transitions of Zinc Difluoride up to 55 GPa.

Studying the effect of high pressure (exceeding 10 kbar) on the structure of solids allows us to gain deeper insight into the mechanism governing crystal structure stability. Here, we report a study on the high-pressure behavior of zinc difluoride (ZnF2)-an archetypical ionic compound which at ambient pressure adopts the rutile (TiO2) structure. Previous investigations, limited to a pressure of 15 GPa, revealed that this compound undergoes two pressure-induced phase transitions, i.e., TiO2 → CaCl2 at 4.5 GPa and CaCl2 → HP-PdF2 at 10 GPa. Within this joint experimental-theoretical study, we extend the room-temperature phase diagram of ZnF2 up to 55 GPa. By means of Raman spectroscopy measurements we identify two new phase transitions, HP-PdF2 → HP1-AgF2 at 30 GPa and HP1-AgF2 → PbCl2 at 44 GPa. These results are confirmed by density functional theory calculations which indicate that in the HP1-AgF2 polymorph the coordination sphere of Zn2+ undergoes drastic changes upon compression. Our results point to important differences in the high-pressure behavior of ZnF2 and MgF2, despite the fact that both compounds contain cations of similar size. We also argue that the HP1-AgF2 structure, previously observed only for AgF2, might be observed at large compression in other AB2 compounds.

Judging the phase transition pressure of the unknown parent phase if the resulting state is known.

In high-pressure phase transition experiments, the crystal structure of the intermediate phase in some phase transitions is difficult to successfully measure due to the limitations of the experimental conditions. The absence of crystal structure data for the intermediate phase also makes it difficult to calculate the pressure point from the intermediate phase to the new phase by the traditional thermodynamic criterion in theoretical simulations. The Conch Criterion is employed by us to successfully verify the phase transition points by observing the reverse shifts of the DOS (electron density of states) curves for the new phase of Cu2S, PbS, PbSe and PbTe, which breaks through the constraints of the traditional criterion and realizes tracing the source of the phase transition in theoretical calculations.

High pressure phase transitions and physical properties of Li2MgH4; implications for hydrogen storage

A systematic calculation has been performed in order to study phase transitions and hydrogen storage properties of ternary hydride Li2MgH4 under pressure. The structural, elastic, electronic and vibrational properties of Li2MgH4 are collected by means of density functional theory. There are three phases identified; Pbam at 0 GPa,Pnma at 5 GPa and Pna21 at 65 GPa Pbam and Pnma phases of Li2MgH4 are found to be mechanically and dynamically stable. Ductility of the phases are determined based on Pugh's criteria. It is found that Li2MgH4 becomes ductile at 5 GPa, otherwise it is a brittle material. Electronic band structures and corresponding partial density of states of phases are also obtained. All phases at 0 GPa, 5 GPa and 65 GPa have wide band gaps, indicating that Li2MgH4 is an insulator at all pressures. The phonon dispersion curves of Pbam and Pnma phases have no imaginary frequency indicating that both phases of Li2MgH4 are dynamically stable. The gravimetric hydrogen density of Li2MgH4 is calculated as 10.52 wt %, which is a great rate along with the hydrogen desorption temperature of 670 K.

Phase-pure bulk BC8 silicon prepared through secondary phase transition method under high pressure

Phase-pure bulk samples of silicon in the body-centered cubic structure (BC8-Si) were prepared via secondary phase transition method under high pressure and room temperature. The bulk samples of BC-Si, with diameter of 3 mm and thickness of 0.5 mm, were synthesized via high-pressure phase transitions of single crystal silicon using the large volume cubic press. The formation mechanism of phase-pure BC8-Si via phase transitions of normal single crystal silicon was investigated using in situ synchrotron X-ray diffraction. The phase-pure BC8-Si was observed through primary phase transition under hydrostatic high-pressure condition. However, the secondary phase transformation was needed to obtain the phase-pure BC8-Si sample at non-hydrostatic compression using the large volume cubic press. The structure and purity of bulk BC8-Si were verified using powder X-ray diffraction, Raman spectroscopy, and energy dispersive spectra. Our experimental results show that the secondary phase transformation method under high pressure is an effective way for phase-pure bulk sample preparation of BC8-Si, which provides an opportunity for further studies of its intrinsic physical properties.

High-Pressure Phase Transitions in Densely Packed Nanocrystallites of TiO2-II

Phase behaviors of nanocrystalline TiO2-II have not been well studied due to the difficulty in its preparations at ambient pressure. In this work, we prepared nanocrystallites of TiO2-II with average sizes of ∼10–20 nm via high-pressure processing of anatase particles in a large volume press. X-ray diffraction (XRD) and electron microscopy examinations show that the TiO2-II nanocrystallites formed from cracking and phase transformation of large anatase particles under compression. The high-pressure phase behaviors of the TiO2-II nanocrystallites were investigated using in situ synchrotron XRD and Raman spectroscopy. Results show that, under compression up to ∼35–40 GPa, the phase transitions in nano TiO2-II proceed via two parallel routes of TiO2-II (∼9–15 GPa) → baddeleyite (∼25 GPa) → TiO2-OI and TiO2-II (∼19 GPa) → TiO2-OI. At different TiO2-II sizes, the TiO2-II-to-baddeleyite transition occurs at pressures from ∼9 to 15 GPa, while the TiO2-II-to-TiO2-OI transition occurs at ∼19 GPa. This difference originates from the different interfacial energies of the involved titania phases, which control their relative thermodynamic phase stabilities at nano sizes. This work enriches our understanding of the complex phase behaviors of titania and provides fundamental knowledge for developing applications of the less-explored titania nanophase of TiO2-II.

An Isosymmetric High-Pressure Phase Transition in α-Glycylglycine: A Combined Experimental and Theoretical Study.

We investigated the effects of hydrostatic pressure on α-glycylglycine (α-digly) using a combined experimental and theoretical approach. The results of powder X-ray diffraction show a change in compressibility of the axes above 6.7 GPa, but also indicate that the structure remains in the same monoclinic space group, suggesting an isosymmetric phase transition. A noticeable change in the Raman spectra between 6 and 7.5 GPa further supports the observed phase transition. First-principles-based calculations combined with the crystal structure prediction code USPEX predict a number of possible polymorphs at high pressure. An orthorhombic structure with a bent peptide backbone is the lowest enthalpy polymorph above 6.4 GPa; however, it is not consistent with experimental observations. A second monoclinic structure isosymmetric to α-digly, α'-digly, is predicted to become more stable above 11.4 GPa. The partial atomic charges in α'-digly differ from α-digly, and the molecule is bent, possibly indicating different reactivity of α'-digly. The similarity in the lattice parameters predicted from calculations and the axial changes observed experimentally support that the α'-digly phase is likely observed at high pressure. A possible explanation for the isosymmetric phase transition is discussed in terms of relaxing strained hydrogen bonding interactions. Such combined experimental and modeling efforts provide atomic-level insight into how pressure-driven conformational changes alter hydrogen-bonding networks in complicated molecular crystals.

Ab initio study of pressure-driven phase transition in FePS3 and FePSe3

In spite of recent findings about the pressure-driven insulator-to-metal phase transition, and emerging superconductivity of ${\mathrm{FePS}}_{3}$ and ${\mathrm{FePSe}}_{3}$, the knowledge about their atomic structures is still vague. Here, we investigate the pressure-driven structural phase transitions of ${\mathrm{FePS}}_{3}$ and ${\mathrm{FePSe}}_{3}$ from 0 to 35 GPa by using ab initio calculations. We find that the ${\mathrm{FePS}}_{3}\ensuremath{-}C2/m$ B-I structure transforms to the ${\mathrm{FePS}}_{3}\ensuremath{-}C2/m$ B-II phase at about 5 GPa. Then above 17 GPa, the ${\mathrm{FePS}}_{3}\ensuremath{-}P\overline{3}1m$ B-III phase becomes energetically favored. For ${\mathrm{FePSe}}_{3}$, with increasing pressure, the ${\mathrm{FePSe}}_{3}\ensuremath{-}R\overline{3}$ T-I phase transforms to the B-II phase at around 6 GPa and further to the B-III phase at about 15 GPa. Our calculation results are consistent with experimentally observed high-pressure induced cell volume collapse, spin crossovers, and insulator-metal transition in ${\mathrm{FePS}}_{3}$ and ${\mathrm{FePSe}}_{3}$, which shed light on understanding the high-pressure physics and phase transitions of ${\mathrm{FePS}}_{3}$ and ${\mathrm{FePSe}}_{3}$.

Ultrahigh-Pressure Behavior of AO2 (A = Sn, Pb, Hf) Compounds

The high-pressure behavior of metal dioxides, AO2, is of wide interest due to their extensive polymorphism. In this study, high-pressure phase transitions in dioxides of selected group 4 and 14 ele...

High-Pressure and High-Temperature Phase Transitions in Fe2TiO4 and Mg2TiO4 with Implications for Titanomagnetite Inclusions in Superdeep Diamonds

Phase transitions of Mg2TiO4 and Fe2TiO4 were examined up to 28 GPa and 1600 °C using a multianvil apparatus. The quenched samples were examined by powder X-ray diffraction. With increasing pressure at high temperature, spinel-type Mg2TiO4 decomposes into MgO and ilmenite-type MgTiO3 which further transforms to perovskite-type MgTiO3. At 21 GPa, the assemblage of MgTiO3 perovskite + MgO changes to 2MgO + TiO2 with baddeleyite (or orthorhombic I)-type structure. Fe2TiO4 undergoes transitions similar to Mg2TiO4 with pressure: spinel-type Fe2TiO4 dissociates into FeO and ilmenite-type FeTiO3 which transforms to perovskite-type FeTiO3. Both of MgTiO3 and FeTiO3 perovskites change to LiNbO3-type phases on release of pressure. In Fe2TiO4, however, perovskite-type FeTiO3 and FeO combine into calcium titanate-type Fe2TiO4 at 15 GPa. The formation of calcium titanate-type Fe2TiO4 at high pressure may be explained by effects of crystal field stabilization and high spin–low spin transition in Fe2+ in the octahedral sites of calcium titanate-type Fe2TiO4. It is inferred from the determined phase relations that some of Fe2TiO4-rich titanomagnetite inclusions in diamonds recently found in São Luiz, Juina, Brazil, may be originally calcium titanate-type Fe2TiO4 at pressure above 15 GPa in the transition zone or lower mantle and transformed to spinel-type in the upper mantle conditions.

High-pressure phase transitions of forsterite from first-principles

From ab initio evolutionary algorithm, we predict a new high pressure phase transition of Mg2SiO4 (Forsterite) above 30 GPa. At this pressure, Forsterite changes into post-spinel structure with Pbam space group. This structure has attracted the interest of many previous works, but which have not been definitively conclusive. Forsterite is stabilized under further phase transition at 64.5 GPa. Above this pressure, the structure with Cmc21 space group becomes the most stable.

Phonon peculiarities at the high-pressure phase transition of B4.3C boron carbide

The change from opacity to transparency marks the reversible high-pressure phase transition of boron carbide between 30 and 40 GPa. After related features of the distorted structure and their effect on the electronic properties are at least roughly clarified, object of the present study is the process of structural changes within this phase transition based on the exchange of C atoms between different polar sites of icosahedra. Such minor structural modifications evade detection by X-ray and neutron diffraction, but are accessible in phonon spectra. Peculiarities in connection with the phase transition, suggested at 33(2) GPa, occur in Raman- and IR-active phonons. Interpretation is based on the close relation between the phonon spectra of α-rhombohedral boron and boron carbide and on the shift of B4.3C phonons in differently isotope-enriched samples.

High-pressure structural phase transitions, electronic properties, and intermediate states of CaSe

In this study, ab initio calculations have been carried out to understand the effect of extreme external pressure on the crystal structure of CaSe. The crystal structure of CaSe, a calcium chalcogen, is studied using density functional theory (DFT) with the generalized gradient approximation (GGA) up to 250 GPa under high hydrostatic pressure. Structurally CaSe crystallizes in cubic NaCl-type (B1) structure (space group: [Formula: see text]) at ambient conditions. The results indicated that CaSe undergoes a structural phase transition from this cubic structure to another cubic CsCl-type (B2) structure (space group: [Formula: see text]) at high pressure. This transformation is based on two intermediate states with space group [Formula: see text] and C2/m. Additionally, the electronic band structures and density of states for the obtained B1 and B2 structures of CaSe have been calculated. According to these calculations, obtained band gap values are in good agreement with the values reported in the literature.

High-Pressure Phase Transitions of Morphologically Distinct Zn2SnO4 Nanostructures.

Many aspects of nanostructured materials at high pressures are still unexplored. We present here, high-pressure structural behavior of two Zn2SnO4 nanomaterials with inverse spinel type, one a particle with size of ∼7 nm [zero dimensional (0-D)] and the other with a chain-like [one dimensional (1-D)] morphology. We performed in situ micro-Raman and synchrotron X-ray diffraction measurements and observed that the cation disordering of the 0-D nanoparticle is preserved up to ∼40 GPa, suppressing the reported martensitic phase transformation. On the other hand, an irreversible phase transition is observed from the 1-D nanomaterial into a new and dense high-pressure orthorhombic CaFe2O4-type structure at ∼40 GPa. The pressure-treated 0-D and 1-D nanomaterials have distinct diffuse reflectance and emission properties. In particular, a heterojunction between the inverse spinel and quenchable orthorhombic phases allows the use of 1-D Zn2SnO4 nanomaterials as efficient photocatalysts as shown by the degradation of the textile pollutant methylene blue.

Characterization of metallization and amorphization for GaP under different hydrostatic environments in diamond anvil cell up to 40.0 GPa.

High-pressure phase stability of gallium phosphide was explored under different hydrostatic environments up to 40.0 GPa in a diamond anvil cell. Two irreversible phase transitions from the semiconductor to metal to an amorphous state appear at 19.8 and 31.5 GPa and as well as 22.6 and 35.3 GPa under nonhydrostatic and hydrostatic environments, respectively. Furthermore, the hysteresis effect of the high-pressure phase transition of a sphalerite-structure compound under a hydrostatic environment was disclosed. All of the obtained results can provide new insight into the underlying structural evolution and electrical transport characteristics for the semiconducting compound at different hydrostatic environments.

High-pressure phase transitions of clinoenstatite

High-pressure phase transitions of clinoenstatite

Controlling the Negative Thermal Expansion and Response to Pressure in ReO3-type Fluorides by the Deliberate Introduction of Excess Fluoride: Mg1–xZr1+xF6+2x, x = 0.15, 0.30, 0.40, and 0.50

Cubic ReO3-type fluorides often display negative or very low thermal expansion. However, they also typically undergo phase transitions upon cooling and/or modest compression, which are undesirable from the perspective of potential applications. Density measurements and total scattering data for Mg1–xZr1+xF6+2x, x = 0.15, 0.30, 0.40, and 0.50, indicate that the introduction of excess fluoride into cubic MgZrF6 is accompanied by the population of interstitial fluoride sites and the conversion of corner to edge shared coordination polyhedra. Unlike MgZrF6, no phase transitions are seen upon cooling these materials to 10 K, and the first high pressure phase transition in these compositions occurs at pressures much higher than that previously reported for MgZrF6 (0.37 GPa). The introduction of excess fluoride also provides for control of thermal expansion. For all of the compositions studied, negative thermal expansion is seen at the lowest temperature examined, and positive thermal expansion is observed at th...