High-pressure Structural Transition Research Articles

Pressure-Induced Reverse Structural Transition of Calcite at Temperatures up to 873 K and Pressures up to 19.7 GPa

In situ Raman scattering and electrical conductivity experiments have been performed to investigate the structural phase transitions of calcite during the compressed and decompressed processes in a diamond anvil cell at temperatures of 298–873 K and pressures up to 19.7 GPa. Upon compression, calcite (CaCO3-I phase) underwent three structural phase transitions from CaCO3-I to CaCO3-II phases at 1.6 GPa, from CaCO3-II to CaCO3-III phases at 2.2 GPa, and from CaCO3-III to CaCO3-VI phases at 16.8 GPa under room temperature conditions, which were evidenced by the evolution of Raman peaks, as well as the discontinuities in the pressure-dependent Raman shifts and electrical conductivity. Upon decompression, the structural phase transitions from CaCO3-VI to CaCO3-III to CaCO3-II to CaCO3-I phases took place at the respective pressures of 5.4, 1.5, and 0.4 GPa, indicating the reversibility of calcite. Furthermore, an obvious ~11 GPa of pressure hysteresis was detected in the CaCO3-VI to CaCO3-III phase transition, whereas other reverse phase transition pressures were very close to those of compressed results. At three given representative pressure conditions (i.e., 10.5, 12.5, and 13.8 GPa), a series of electrical conductivity experiments were performed at temperature ranges of 323–873 K to explore the temperature-dependent relation of CaCO3-III to CaCO3-VI structural phase transition. With increasing pressure, the transition temperature between CaCO3-III and CaCO3-VI phases gradually decreases, which reveals an obviously negative temperature-pressure relation, i.e., P (GPa) = 19.219 (±1.105) − 0.011 (±0.002) T (K). Our acquired phase diagram of calcite can be employed to understand the high-pressure structural transitions and phase stability for carbonate minerals along various subducting slabs in the deep Earth’s interior.

Pressure-driven structural and electronic transitions in a quasimolecular layered compound of antimony triiodide

High-pressure structural and electronic transitions of SbI3 under different hydrostatic environments were investigated using synchrotron X-ray diffraction, Raman spectroscopy, electrical conductivity and first-principles theoretical calculations.

A high-pressure structural transition of norsethite-type BaFe(CO3)2: Comparison with BaMg(CO3)2 and BaMn(CO3)2

Abstract Investigations on the phase stability of the norsethite-type family [BaMg(CO3)2, BaMn(CO3)2, BaFe(CO3)2] under high-pressure conditions are of great significance for understanding the structure and metal cationic (Mg2+, Fe2+, Mn2+) substitution mechanism in double divalent metal carbonates. The structural evolution and equation of state of BaFe(CO3)2 were studied at high pressure up to ~7.3 GPa by synchrotron X-ray diffraction (XRD) in diamond-anvil cell (DAC) in this study. BaFe(CO3)2 undergoes a reversible phase transition from R3m (α-phase) to C2/c (γ-phase) space groups at ~3.0 GPa. The fitted elastic parameters are V0 = 377.79(2) Å3 and K0 = 40.3(7) GPa for α-BaFe(CO3)2, V0 = 483.24(5) Å3 and K0 = 91.2(24) GPa for γ-BaFe(CO3)2 using second-order Birch-Murnaghan equation of state (BM2-EoS). Besides, the vibrational properties and structural stability of complete norsethite-type minerals were also investigated first by Raman spectroscopy combined with DAC up to 11.1 GPa. Similar structural phase transitions occur in BaMg(CO3)2, BaFe(CO3)2, BaMn(CO3)2 at 2.2–2.6, 2.6–3.7, and 3.7–4.1 GPa, respectively. The onset phase transition pressures of the norsethite-type family are much lower than that of dolomite-type Ca(Mg,Fe,Mn)(CO3)2 and calcite-type (Mg,Fe,Mn)CO3 carbonates. These results provide new insights into the divalent cation substitution effects on the stability and structural evolution of carbonates under high-pressure conditions.

A machine-learned interatomic potential for silica and its relation to empirical models

Silica (SiO2) is an abundant material with a wide range of applications. Despite much progress, the atomistic modelling of the different forms of silica has remained a challenge. Here we show that by combining density-functional theory at the SCAN functional level with machine-learning-based interatomic potential fitting, a range of condensed phases of silica can be accurately described. We present a Gaussian approximation potential model that achieves high accuracy for the thermodynamic properties of the crystalline phases, and we compare its performance (and performance–cost trade-off) with that of multiple empirically fitted interatomic potentials for silica. We also include amorphous phases, assessing the ability of the potentials to describe structures of melt-quenched glassy silica, their energetic stability, and the high-pressure structural transition to a mainly sixfold-coordinated phase. We suggest that rather than standing on their own, machine-learned potentials for silica may be used in conjunction with suitable empirical models, each having a distinct role and complementing the other, by combining the advantages of the long simulation times afforded by empirical potentials and the near-quantum-mechanical accuracy of machine-learned potentials. This way, our work is expected to advance atomistic simulations of this key material and to benefit further computational studies in the field.

Systematic structural study in praseodymium compressed in a neon pressure medium up to 185 GPa

Angle-dispersive x-ray powder diffraction experiments have been performed on praseodymium metal compressed in a soft pressure-transmitting medium at ambient temperature up to 185 GPa. We observe the previously reported high-pressure structural transition sequence up to 20 GPa and the coexistence of body-centered orthorhombic (bco) Pr and \ensuremath{\alpha}-U Pr from $\ensuremath{\sim}20$ up to $\ensuremath{\sim}38\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. The \ensuremath{\alpha}-U structure of Pr is stable from 20 to 185 GPa, and no evidence of the proposed transition to a primitive orthorhombic phase $>147\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ was observed. With density functional theory (DFT), we calculated the lattice parameters and $y$ coordinate for \ensuremath{\alpha}-U Pr and found good agreement between our calculations and experimental measurements. The obtained DFT energies of the proposed primitive orthorhombic $(P{2}_{1}{2}_{1}{2}_{1})$ and the \ensuremath{\alpha}-uranium phases at $\ensuremath{\sim}150\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ show that the \ensuremath{\alpha}-uranium phase is lower in energy. Hence, neither our experimental data nor our DFT results support the transition to a primitive orthorhombic phase $>150\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. DFT suggests, however, that Pr may transform to the $P{2}_{1}{2}_{1}{2}_{1}$ phase above $\ensuremath{\sim}220\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. We also compare the axial ratios and lattice parameters of praseodymium to \ensuremath{\alpha}-uranium structured Nd, Ce, and U.

Phase transition and electronic properties of barium fluoride at high pressure

Pressure is a useful tool to profoundly modify the volume and electronic structure of materials, resulting in the formation of new structures with exotic physical and chemical properties. The high-symmetry cubic barium fluoride (BaF2) with a space group of Fm-3m (Z = 4) is the prototypical fluorite-type compounds at ambient condition, which is shared with many alkaline-earth fluorides. The study of high-pressure evolution of the BaF2 phase is of fundamental importance in helping to understand the structural sequence and principles of crystallography. In this work, we here have systematically investigated the high-pressure structural transition of BaF2 up to 200 GPa using an effective CALYPSO methodology. Strikingly, two thermodynamically favored phases with orthorhombic Pnma and hexagonal P63/mmc symmetry are found at 3.6 and 19.2 GPa, respectively. Distinguishingly, P63/mmc phase remains stable up to 90.5 GPa, and then transform to Pnma structure. Further electronic calculations indicate that BaF2 maintains insulating feature until 200 GPa. Our current results have broad implications for other AB2-type compounds that may harbor similar novel structured evolution behavior at high pressure.

Dislocation-mediated shear amorphization in boron carbide.

The failure of superhard materials is often associated with stress-induced amorphization. However, the underlying mechanisms of the structural evolution remain largely unknown. Here, we report the experimental measurements of the onset of shear amorphization in single-crystal boron carbide by nanoindentation and transmission electron microscopy. We verified that rate-dependent loading discontinuity, i.e., pop-in, in nanoindentation load-displacement curves results from the formation of nanosized amorphous bands via shear amorphization. Stochastic analysis of the pop-in events reveals an exceptionally small activation volume, slow nucleation rate, and lower activation energy of the shear amorphization, suggesting that the high-pressure structural transition is activated and initiated by dislocation nucleation. This dislocation-mediated amorphization has important implications in understanding the failure mechanisms of superhard materials at stresses far below their theoretical strengths.

Exploration of high-pressure structural transition and electronic properties of BaFe2S3

The iron-based compound BaFe2S3 has received considerable attention in recent years due to its unique electronic properties with important applications. But, there has been relatively little attention devoted to the structure evolution of BaFe2S3 under high pressure. Here, we report a detailed theoretical study of the structural, electronic and sound velocity properties of BaFe2S3 under high pressure by CALYPSO structure search method in combination with the first-principles calculations. We have uncovered three novel structures of BaFe2S3 under high pressure with space group P2221, Imm2, and C2/m symmetries. Theoretical calculations reveal that the structures of BaFe2S3 under high pressure satisfy the phase transition sequence of Cmcm → P2221 → Imm2 → C2/m, and the corresponding transition pressures are 31.6, 47.4 and 57.0 GPa, respectively. Our results enrich the high pressure phases of BaFe2S3 and will stimulate future experiments to synthesize these new phases.

Phase Transitions and Resistivity Anomaly of Layered MoO3 at High Pressure

MoO3 has large electronic and mechanical anisotropy arising from diverse chemical bonding in its layered structure. Here, we report high-pressure structural and electronic transitions in MoO3 to 10...

High pressure magnetic, structural, and electronic transitions in multiferroic Ba3NbFe3Si2O14

The magnetic, structural, and electronic properties of multiferroic Ba3NbFe3Si2O14 from the langasite family were investigated by several methods including synchrotron X-ray diffraction (XRD) and synchrotron Mössbauer source (SMS) technique at high hydrostatic pressures (up to 60 GPa) created in diamond anvil cells. A cascade of structural phase transitions at pressures of about 3.5, 17.5, and 41 GPa was revealed by structural XRD, magnetic SMS, and electronic methods, including Raman, Mössbauer, and optical absorption. The transition at 17.5 GPa leads to strong changes in unit cell parameters and a sharp drop in the cell volume by about 7%. This drastically changes the magnetic properties of the crystal, which separates into two magnetic phases. The Néel temperature TN in one of the phases is increased up to 100 K, which is more than three times higher than the TN value of the parent compound (∼27 K). Above 41 GPa, the optical gap decreases to about 0.7 eV, implying a transition from dielectric to a semiconducting state, while the TN value of the magnetic phase decreases from 100 to 58 K. The magnetic transitions are explained by the redistribution of Fe ions between tetrahedral and possibly octahedral (or/and interstitial) sites resulting in enhancement of exchange interactions. The appearance of a low spin state of Fe3+ ions in octahedral sites is also discussed.

High-pressure insulator-to-metal transition in Sr3Ir2O7 studied by x-ray absorption spectroscopy

High-pressure x-ray absorption spectroscopy was performed at the Ir $L_3$ and $L_2$ absorption edges of Sr$_3$Ir$_2$O$_7$. The branching ratio of white line intensities continuously decreases with pressure, reflecting a reduction in the angular part of the expectation value of the spin-orbit coupling operator, $\left\langle {\bf L} \cdot {\bf S} \right\rangle$. Up to the high-pressure structural transition at 53 GPa, this behavior can be explained within a single-ion model, where pressure increases the strength of the cubic crystal field, which suppresses the spin-orbit induced hybridization of $J_{\text{eff}} = 3/2$ and $e_g$ levels. We observe a further reduction of the branching ratio above the structural transition, which cannot be explained within a single-ion model of spin-orbit coupling and cubic crystal fields. This change in $\left\langle {\bf L} \cdot {\bf S} \right\rangle$ in the high-pressure, metallic phase of Sr$_3$Ir$_2$O$_7$ could arise from non-cubic crystal fields or a bandwidth-driven hybridization of $J_{\text{eff}}=1/2,\,3/2$ states, and suggests that the electronic ground state significantly deviates from the $J_{\text{eff}}=1/2$ limit.

High-pressure structural and electronic transitions in lithium ferrites

High-pressure structural and electronic transitions in lithium ferrites

High-pressure phase diagram, structural transitions, and persistent nonmetallicity of BaBiO3 : Theory and experiment

${\mathrm{BaBiO}}_{3}$ is a mixed-valence perovskite, which escapes the metallic state through a Bi valence (and Bi-O bond) disproportionation or CDW distortion, resulting in a semiconductor with a gap of 0.8 eV at zero pressure. The evolution of structural and electronic properties at high pressure is, however, largely unknown. Pressure, one might have hoped, could reduce the disproportionation, making the two Bi ions equivalent and bringing the system closer to metallicity or even to superconductivity, such as is attained at ambient pressure upon metal doping. We address the high-pressure phase diagram of pristine ${\mathrm{BaBiO}}_{3}$ by ab initio DFT calculations based on GGA and hybrid functionals in combination with crystal structure prediction methods based on evolutionary algorithms, molecular dynamics, and metadynamics. The calculated phase diagram from 0 to 50 GPa indicates that pristine ${\mathrm{BaBiO}}_{3}$ resists metallization under pressure, undergoing instead at room temperature structural phase transitions from monoclinic I2/m to nearly tetragonal P-1 at 7 GPa, possibly to monoclinic C2/m at 27 GPa, and to triclinic P1 at 43 GPa. Remarkably, all these phases sustain and in fact increase the inequivalence of two Bi neighboring sites and of their Bi-O bonds and, in all cases except semimetallic C2/m, the associated insulating character. We then present high-pressure resistivity data, which generally corroborate these results, and show that the insulating character persists at least up to 80 GPa, suggesting that the C2/m phase is probably an artifact of the small computational cell.

High Pressure Structural Transitions in Kr Clathrate-Like Clusters

Classical thermodynamic properties of a size-specific Kr clathrate-like cluster, modeled by an ab initio and a semiempirical Kr–water potential, were computed from parallel-tempering Monte Carlo simulations. The temperature and pressure dependence of the cluster’s heat capacity was studied, and phase diagrams were constructed using a multiple-histogram method. By associating the heat capacity (maxima) and the Pearson correlation coefficient (minima/maxima) values with the phase transitions, attempts were made to identify such changes to particular cluster structures. Various isomers were computed by local optimizations of the interaction enthalpy at a set of randomly selected configurations at each temperature–pressure grid point. Their energy distributions and relative abundances were then employed to assign the observed phase transitions of the cluster. It was found that the structural changes at high pressures are related to Kr clathrate-like cages, such as those of the sI, sII, and sH hydrates, as wel...

Effects of vacancy point defects and phase transitions on optical properties of shocked Al2O3

The velocity interferometer system for any reflector (VISAR) and pyrometric measurements in dynamic highpressure experiments require the use of an optical window, and Alumina (Al2O3) or sapphires is often considered as a window material due to its high shock impedance and excellent transparency. Consequently, understanding the characteristics of its transparency and refractive index change under shock loading is crucial for explaining such experimental data. Experimental studies indicate optical transparency loss in shocked Al2O3. The mechanisms for the phenomenon are some interesting issues. A first-principles study suggests that shock-induced VO+2 (the +2 charged O vacancy) defects in Al2O3 could be an important factor causing the transparency loss. Recently, the red shift of the extinction curve (i.e., the wavelength dependence of the extinction coefficient) with increasing shock pressure has been observed. It is needed to ascertain whether this behavior is also related to shock-induced vacancy point defects. In addition, up to now, information about Al2O3 refractive index at a wavelength of 532 nm under strong shock compression (the optical source wavelength in VISAR measurement is usually set at 532 nm) has been unknown, and neither the effects of structural transitions nor vacancy point defects on the refractive index of shocked Al2O3 are determined. Here, to investigate the above-mentioned questions, we perform first principles calculations of optical absorption and refractive index properties of Al2O3 crystal without and with VO+2 and VAl3 (the -3 charged Al vacancy) defects in a pressure range of 180 GPa (the calculations in CASTEP are carried out by the plane-wave pseudo potential method in the framework of the density functional theory). Our absorption data show that the observed optical extinction in shocked Al2O3 cannot be explained by only considering pressure and temperature factors, but shock-induced VO+2 should be an important source for this behavior. On the basis of these results, we may judge that 1) the transparency loss explanation for shocked Al2O3 in the view of vacancy point defects is reasonable; 2) the absorption extinction should dominate the extinction phenomenon observed in shocked Al2O3. Our calculations find that high-pressure structural transition in Al2O3 causes an obvious enhancement of its refractive index. The refractive index decreases with increasing shock pressure in corundum and Rh2O3 regions, and decreases slightly below 172 GPa and increases slowly above 172 GPa with increasing shock pressure in CalrO3 region. The VO+2 and VAl3 defects in Al2O3 have apparent influences on the shock pressure dependence of its refractive index. These results mean that the information about Al2O3 refractive index under strong shock loading cannot be obtained simply by extrapolating its low pressure data. Our prediction could be of importance for future experimental study and new window-material development.

Elastic and thermodynamical properties of cubic (3C) silicon carbide under high pressure and high temperature

Pressure-dependent first-order phase transition, mechanical, elastic, and thermodynamical properties of cubic zinc blende to rock-salt structures in 3C silicon carbide (SiC) are presented. An effective interatomic interaction potential for SiC is formulated. The potential for SiC incorporates long-range Coulomb, charge transfer interactions, covalency effect, Hafemeister and Flygare type short-range overlap repulsion extended up to the second-neighbour ions, van der Waals interactions and zero point energy effects. The developed potential including many body non-central forces validates the Cauchy discrepancy successfully to explain the high-pressure structural transition, and associated volume collapse. The 3C SiC ceramics lattice infers mechanical stiffening, thermal softening, and ductile (brittle) nature from the pressure (temperature) dependent elastic constants behaviour. To our knowledge, these are the first quantitative theoretical predictions of the pressure and temperature dependence of mechanical and thermodynamical properties explicitly the mechanical stiffening, thermally softening, and brittle/ductile nature of 3C SiC and still awaits experimental confirmations.

High-pressure structural anomalies and electronic transitions in the topological Kondo insulator SmB6

We have scrupulously investigated the high-pressure structural behaviour of the topological Kondo insulator SmB6 through synchrotron powder x-ray diffraction. Lattice modifications are observed at a critical pressure of for three separate compression experiments at ambient and low temperatures. The initially subtle compression of the Sm-B bond length shows a sharp kink downward at the critical pressure. These anomalies coincide with the phase diagram boundaries for the pressure-induced metallization and the concurrent appearance of long-range magnetic ordering in the compound. We propose a scenario of delocalization of 4f electrons and the gradual filling of 5d conduction bands, which can also provide some clarification on the valence dependence of Sm with pressure.

High pressure structural behavior of YGa2: A combined experimental and theoretical study

High pressure structural stability studies were carried out on YGa2 (AlB2 type structure at NTP, space group P6/mmm) up to a pressure of ~35GPa using both laboratory based rotating anode and synchrotron X-ray sources. An isostructural transition with reduced c/a ratio, was observed at ~6GPa and above ~17.5GPa, the compound transformed to orthorhombic structure. Bulk modulus B0 for the parent and high pressure phases were estimated using Birch–Murnaghan and modified Birch–Murnaghan equation of state. Electronic structure calculations based on projector augmented wave method confirms the experimentally observed two high pressure structural transitions. The calculations also reveal that the ‘Ga’ networks remains as two dimensional in the high pressure isostructural phase, whereas the orthorhombic phase involves three dimensional networks of ‘Ga’ atoms interconnected by strong covalent bonds.

Structural and magnetic phase transitions in gadolinium under high pressures and low temperatures

High pressure structural transition studies have been carried out on rare earth metal gadolinium in a diamond anvil cell at room temperature to 169 GPa. Gadolinium has been compressed to 38% of its initial volume at this pressure. With increasing pressure, a crystal structure sequence of hcp → Sm-type → dhcp → fcc → dfcc → monoclinic has been observed in our studies on gadolinium. The measured equation of state of gadolinium is presented to 169 GPa at ambient temperature. Magnetic ordering temperature of gadolinium has been studied using designer diamond anvils to a pressure of 25 GPa and a temperature of 10 K. The magnetic ordering temperature has been determined from the four-point electrical resistivity measurements carried out on gadolinium. Our experiments show that the magnetic transition temperature decreases with increasing pressure to 19 GPa and then increases when gadolinium is subjected to higher pressures.

High-pressure polymorphism and structural transitions of norsethite, BaMg(CO3)2

In situ high-pressure investigations on norsethite, BaMg(CO3)2, have been performed in sequence of diamond-anvil cell experiments by means of single-crystal X-ray and synchrotron diffraction and Raman spectroscopy. Isothermal hydrostatic compression at room temperature yields a high-pressure phase transition at P c ≈ 2.32 ± 0.04 GPa, which is weakly first order in character and reveals significant elastic softening of the high-pressure form of norsethite. X-ray structure determination reveals C2/c symmetry (Z = 4; a = 8.6522(14) A, b = 4.9774(13) A, c = 11.1542(9) A, β = 104.928(8)°, V = 464.20(12) A3 at 3.00 GPa), and the structure refinement (R 1 = 0.0763) confirms a distorted, but topologically similar crystal structure of the so-called γ-norsethite, with Ba in 12-fold and Mg in octahedral coordination. The CO3 groups were found to get tilted off the ab-plane direction by ~16.5°. Positional shifts, in particular of the Ba atoms and the three crystallographically independent oxygen sites, give a higher flexibility for atomic displacements, from which both the relatively higher compressibility and the remarkable softening originate. The corresponding bulk moduli are K 0 = 66.2 ± 2.3 GPa and dK/dP = 2.0 ± 1.8 for α-norsethite and K 0 = 41.9 ± 0.4 GPa and dK/dP = 6.1 ± 0.3 for γ-norsethite, displaying a pronounced directional anisotropy (α: β −1 = 444(53) GPa, β −1 = 76(2) GPa; γ: β −1 = 5.1(1.3) × 103 GPa, β −1 = 193(6) GPa β −1 = 53.4(0.4) GPa). High-pressure Raman spectra show a significant splitting of several modes, which were used to identify the transformation in high-pressure high-temperature experiments in the range up to 4 GPa and 542 K. Based on the experimental series of data points determined by XRD and Raman measurements, the phase boundary of the α-to-γ-transition was determined with a Clausius–Clapeyron slope of 9.8(7) × 10−3 GPa K−1. An in situ measurement of the X-ray intensities was taken at 1.5 GPa and 411 K in order to identify the nature of the structural variation on increased temperatures corresponding to the previously reported transformation from α- to β-norsethite at 343 K and 1 bar. The investigations revealed, in contrast to all X-ray diffraction data recorded at 298 K, the disappearance of the superstructure reflections and the observed reflection conditions confirm the anticipated $$R\bar{3}m$$ space-group symmetry. The same superstructure reflections, which disappear as temperature increases, were found to gain in intensity due to the positional shift of the Ba atoms in the γ-phase.