Pressure-induced Structural Transformations Research Articles

Pressure-induced structural transformations in phosphorus oxynitride glasses

The structure-property relations of permanently densified sodium metaphosphate oxynitride glasses (NaPO3−3x/2Nx) with varying N/P ratio are investigated. Densification of the bulk samples is achieved through 1GPa isostatic compression at the glass transition temperature. The pressure-induced structural transformations are characterized by Raman spectroscopy and solid state 31P and 23Na NMR spectroscopy, whereas the glass hardness and brittleness are quantified through Vickers indentation. Nitridation of the ambient pressure sodium metaphosphate glass (NaPO3) results in a step-wise conversion of PO4 units into PO3N and PO2N2 oxynitride species. Upon hot compression, the glasses become permanently denser and harder, but also more brittle. This pressure-induced densification is not accompanied by changes in the types or fractions of the structural units for the NaPO3 glass. In contrast, densification of the oxynitride glasses (N/P>0) is accompanied by pressure-induced structural transformations between the oxynitride species. Based on the 31P NMR results, a structural mechanism for these transformations is proposed.

Molecular dynamics simulations of pressure-induced structural and mechanical property changes in amorphous Al2O3

Molecular dynamics (MD) simulations of amorphous alumina have been carried out to investigate the pressure-induced structural transformation and mechanical properties. We found that not only the fraction of units AlOx (x=4, 5, 6) but also the density of each AlOx type change upon compression. The density of sample can be expressed through the fraction and partial density of units AlOx. With increasing pressure, O atoms are more ordered than Al atoms and form fcc and hcp clusters. The same units AlOx link together to form atomic clusters (AC) of type AC4, AC5 and AC6 (AC4, AC5 and AC6 consist of units AlO4, AlO5 and AlO6, respectively). The Young's modulus can also be expressed through the fraction and partial Young's modulus of units AlOx.

Pressure-induced structural transformation of CaC2

The high pressure structural changes of calcium carbide CaC2 have been investigated with Raman spectroscopy and synchrotron X-ray diffraction (XRD) techniques in a diamond anvil cell at room temperature. At ambient conditions, two forms of CaC2 co-exist. Above 4.9 GPa, monoclinic CaC2-ii diminished indicating the structural phase transition from CaC2-ii to CaC2-i. At about 7.0 GPa, both XRD patterns and Raman spectra confirmed that CaC2-i transforms into a metallic Cmcm structure which contains polymeric carbon chains. Along with the phase transition, the isolated C2 dumbbells are polymerized into zigzag chains resulting in a large volume collapse with 22.4%. Above 30.0 GPa, the XRD patterns of CaC2 become featureless and remain featureless upon decompression, suggesting an irreversible amorphization of CaC2.

Shear-Induced Isostructural Phase Transition and Metallization of Layered Tungsten Disulfide under Nonhydrostatic Compression

Pressure-induced structural and electronic transformations of tungsten disulfide (WS2) have been studied to 60 GPa, in both hydrostatic and nonhydrostatic conditions, using four-probe electrical resistance measurements, micro-Raman spectroscopy, and synchrotron X-ray diffraction. The results show the evidence for an isostructural phase transition from hexagonal 2Hc phase to hexagonal 2Ha phase, which accompanies the metallization at ∼37 GPa. This isostructural transition occurs displacively over a large pressure range between 15 and 45 GPa and is driven by the presence of strong shear stress developed in the layer structure of WS2 under nonhydrostatic compression. Interestingly, this transition is absent in hydrostatic conditions using He pressure medium, underscoring its strong dependence on the state of stress. We attribute the absence to the incorporation of He atoms between the layers, mitigating the development of shear stress. We also conjecture a possibility of magnetic ordering in WS2 that may occ...

Study of Phase Transformation in BaTe2O6 by in Situ High-Pressure X-ray Diffraction, Raman Spectroscopy, and First-Principles Calculations.

Structural and vibrational properties of orthorhombic BaTe2O6, a mixed valence tellurium compound, have been investigated by in situ synchrotron X-ray diffraction (XRD) studies up to 16 GPa and Raman spectroscopy up to 37 GPa using a diamond-anvil cell. The structure of orthorhombic BaTe2O6 has layers of [Te2O6]2–, formed by TeO6 octahedra and TeO5 square pyramids and Ba2+ ions stacked alternately along the ⟨010⟩ direction. A reversible pressure-induced structural transformation from the ambient orthorhombic (Cmcm) to a monoclinic (P21/m) structure is observed in both XRD and Raman spectroscopic investigations around 10 GPa. Ab initio calculations using density functional theory (DFT) corroborate this phase transition as well as the transition pressure. Both XRD and DFT calculations reveal that the high-pressure monoclinic structure is closely related to the ambient pressure orthorhombic structure, and the transformation is accompanied by a slight rearrangement of the structural units. Pressure evolution ...

Evolution of crystal and electronic structures of magnesium dicarbide at high pressure.

Carbon-based compounds exhibit unexpected structures and electronic behavior at high pressure arising from various bonding features of carbon (e.g., sp, sp2 and sp3 C-C bonds). Here we report evolution of crystal structures of MgC2 in a wide pressure range of 0–200 GPa as predicted through ab-initio calculations in combination with an unbiased swarm structure search. Three pressure-induced structural transformations are unraveled, following the phase sequence of ambient-pressure P42/mnm (α-phase) → Cmcm (β-phase) → C2m (γ-phase) → EuGe2-type P-3m1 (δ-phase), where significant C-C bonding modifications from C-C dimer to quasi 1-dimensionzigzag chain, to polymerized ribbon and then to winkled quasi 2- dimension graphite sheet are evident. The predicted β- and γ-phases with sp2 C-C hybridization are metals, while the δ- phase characterized by a sp3C-C hybridization is a narrow-gap semiconductor with a band gap of 0.667 eV. Strong electron-phonon couplings in the compressed β- and γ- phases arepredicted with β-phase showing a high superconducting critical temperature of 11.2 K. The current results indicate that pressure is effective in tuning the crystal and electronic structures of MgC2, which is expected to have impact on physical properties for potential applications.

Pressure-induced structural phase transformation in cobalt(II) dicyanamide.

In situ synchrotron powder diffraction has been used to probe the pressure-dependent structural properties of the magnetic molecular framework material Co(dca)2 [dca = dicyanamide or N(CN)2(-)]. An orthorhombic (Pmnn) to monoclinic (P2₁/n) transformation to a high-pressure phase, namely γ-Co(dca)2, occurs at 1.1 GPa. Structural determination of γ-Co(dca)2 shows that the rutile-like topology of the pristine material is retained at high pressures, with the lower symmetry allowing a progression of volume-reducing structural distortions. γ-Co(dca)2 was stable at the maximum pressure measured of 4.2 GPa. Both phases were soft, with bulk moduli (B0) for α-Co(dca)2 and γ-Co(dca)2 of 13.15 (18) and 9.0 (6) GPa, respectively. Modest uniaxial negative linear compressibility (K) of the order of -4 TPa(-1) was observed over the entire measured pressure range.

Pressure induced structural transitions in CuSbS2 and CuSbSe2 thermoelectric compounds

We have investigated the structural behavior of CuSbS2 and CuSbSe2 thermoelectric materials under high pressure conditions up to 80GPa using angle dispersive X-ray diffraction in a diamond anvil cell (DAC). We have also performed high pressure Raman spectroscopy measurements up to 16GPa. We observed a pressure-induced structural transformation from the ambient orthorhombic structure with space group Pnma to a triclinic type structure with space group P1 beginning around 8GPa in both samples and completing at 13GPa and 10GPa in CuSbS2 and CuSbSe2, respectively. High pressure Raman experiments complement the transitions observed by high pressure X-ray diffraction (HPXRD). The transitions were found to be reversible on releasing the pressure to ambient in the DAC. The bulk modulus and compressibility of these materials are further discussed.

Metallization and superconductivity of BeH2 under high pressure

Pressure-induced metallization and potential superconductivity of BeH2 has been a topic of interest. In the present study, we extensively explored the crystal structures of BeH2 in a wide pressure range of 0-300 GPa using an unbiased structure searching method coupled with first-principles density functional calculations. A series of pressure-induced structural transformations are predicted for BeH2, as Ibam (α phase) → P-3m1 (phase II) → R-3m (phase III) → Cmcm (phase IV). Calculated pressures of phase transition are 25, 140, and 202 GPa, respectively. The phase II is isostructural to the well-known 1T structure of transition metal dichalcogenides, which is composed of covalent bonded BeH2 slabs stacked along the perpendicular direction by van der Waals forces. The phase III is constructed by the same BeH2 slabs, but differs from the phase II in the stacking sequence. The α phase, phase II, and phase III all have insulating electronic states while their band gaps decrease as pressure increases. We predicted that BeH2 reaches a metallic state by a III → IV phase transition, instead of a direct band gap closure in phase III. The phase IV has a three-dimensional extended Be-H network formed by edge-sharing BeH8 polyhedrons with delocalized electrons. Electron-phonon coupling calculations implemented using linear response theory on the metallic BeH2 predict a large electron-phonon coupling parameter of 0.63, leading to an estimation of superconducting transition temperature (Tc) of ∼38 K at 250 GPa.

高圧力下における SiO<sub>2</sub> ガラスの振る舞い

The high-pressure behavior of SiO2 glass has attracted considerable attention because of its importance not only in condensed-matter physics and materials science but also in geophysics. We have conducted X-ray absorption/diffraction measurements and optical-microscope observation to understand the behavior of SiO2 glass under hydrostatic and uniaxial compression. SiO2 glass undergoes pressure-induced structural transformations first in the intermediate-range order (the network structure consisting of SiO4 tetrahedra) and then in the short-range order (the coordination number). Under uniaxial compression, the atomic arrangement in intermediate-range order deforms largely and a large differential strain remains after decompression. This article attempts to review these interesting behaviors, mainly based on our recent researches.

Simulations of nanocrystals under pressure: Combining electronic enthalpy and linear-scaling density-functional theory

We present an implementation in a linear-scaling density-functional theory code of an electronic enthalpy method, which has been found to be natural and efficient for the ab initio calculation of finite systems under hydrostatic pressure. Based on a definition of the system volume as that enclosed within an electronic density isosurface [M. Cococcioni, F. Mauri, G. Ceder, and N. Marzari, Phys. Rev. Lett. 94, 145501 (2005)], it supports both geometry optimizations and molecular dynamics simulations. We introduce an approach for calibrating the parameters defining the volume in the context of geometry optimizations and discuss their significance. Results in good agreement with simulations using explicit solvents are obtained, validating our approach. Size-dependent pressure-induced structural transformations and variations in the energy gap of hydrogenated silicon nanocrystals are investigated, including one comparable in size to recent experiments. A detailed analysis of the polyamorphic transformations reveals three types of amorphous structures and their persistence on depressurization is assessed.

Pressure-induced structural transformations, orbital order and antiferromagnetism in La0.75Ca0.25MnO3

High pressure evolution of structural, vibrational and magnetic properties of La0.75Ca0.25MnO3 was studied by means of X-ray diffraction and Raman spectroscopy up to 39 GPa, and neutron diffraction up to 7.5 GPa. The stability of different magnetic ground states, orbital configurations and structural modifications were investigated by LDA + U electronic structure calculations. A change of octahedral tilts corresponding to the transformation of orthorhombic crystal structure from the Pnma symmetry to the Immaone occurs above P ~ 6 GPa. At the same time, the evolution of the orthorhombic lattice distortion evidences an appearance of the e g d x² − z² orbital polarization at high pressures. The magnetic order in La0.75Ca0.25MnO3 undergoes a continuous transition from the ferromagnetic 3D metallic (FM) ground state to the A-type antiferromagnetic (AFM) state of assumedly 2D pseudo-metallic character under pressure, that starts at about 1 GPa and extends possibly to 20–30 GPa.

Structure and properties of rare earth silicates with the apatite structure at high pressure

The pressure-induced structural transformation of rare earth, non-stoichiometric silicates, (REE9.33(SiO4)6O2, RE = La, Ce, Nd, Eu, and Gd) with the apatite structure type, were investigated by X-ray diffraction, photoluminescence, far-infrared spectroscopy, and DFT calculations. A pressure-induced degradation of symmetry from P6 3 /m to P6 3 occurs with increasing pressure. The transition is due to the tilting of SiO4 tetrahedra and reduced symmetry constraints on one of the O atoms in the tetrahedron. The critical transition pressure increased from ~13 GPa in La9.33(SiO4)6O2 to ~25 GPa in Gd9.33(SiO4)6O2 with the decrease in lanthanide cation size. The high-pressure phase shows an unexpectedly low value for the bulk modulus over a narrow pressure range (below ~30 GPa), as compared with the low-pressure phase, especially for the structure with larger rare earth elements. High-pressure studies of alkaline earth-doped samples (Nd8 A 2(SiO4)6O2 where A = Ca, Sr) showed that the pressure for the phase transition is mainly related to the size of lanthanides that occupy the large channels along the c axis of the apatite structure type.

Pressure-induced structural transformations in advanced ferroelectrics with relaxor behaviour

A series of model perovskite-type relaxor ferroelectrics (pure and A-/B-site doped PbSc1/2Ta1/2O3 and PbSc1/2Nb1/2O3 as well as of 0.9PbZn1/3Nb2/3O3−0.1PbTiO3) were studied by high pressure diffraction and inelastic light scattering in order to elucidate the mesoscopic-scale ferroic atomic arrangements responsible for the superb macroscopic properties of these materials. The combined analysis of the pressure-enhanced phonon anomalies observed by Raman spectroscopy and the pressure-induced long-range order detected by synchrotron X-ray and neutron diffraction revealed that at ambient conditions antiferrodistortive order coexists with the ferroelectric order on the mesoscopic scale. This suggests that the locally polarized spatial nanoregions known as polar nanoregions are ferrielectric in nature and their abundance and mean size depend on both the antiferrodistortive and ferroelectric coupling, which in turn can be tuned by appropriate chemical variations.

High-pressure Brillouin scattering of the single-crystal PbSc_{1/2}Ta_{1/2}O_{3} relaxor ferroelectric

The effect of pressure on the acoustic modes and adiabatic elastic constants of the perovskite-type ($AB$O${}_{3}$) relaxor compound PbSc${}_{1/2}$Ta${}_{1/2}$O${}_{3}$ have been studied by Brillouin spectroscopy at room temperature up to 9.2 GPa. It is shown that the elastic constants are very sensitive to the pressure-induced structural transformations that were established by diffraction and Raman scattering analysis. Changes in the evolution of the pseudocubic elastic constants ${c}_{11}$ and ${c}_{44}$ occur at the two phase-transition pressures ${p}_{c}$${}_{1}$ $=$ 1.9 GPa and ${p}_{c2}$ \ensuremath{\sim} 5.5 GPa. Changes in evolution of ${c}_{12}$ occur at the two characteristic pressures ${p}_{1}$${}^{*}$ $=$ 1.2 GPa and ${p}_{2}$${}^{*}$ \ensuremath{\sim} 3.0 GPa, which mark changes in the local structure that precede the corresponding phase transitions. The elastic anisotropy and the width of the transverse acoustic mode increase in the pressure range between ${p}_{c}$${}_{1}$ and ${p}_{c}$${}_{2}$ with a maximum near ${p}_{2}$${}^{*}$. A strong decrease in the Cauchy parameter is observed at pressures above ${p}_{2}$${}^{*}$, which indicates an enhancement of the covalent character of chemical bonding. Increasing pressure gradually suppresses the central peak typical of the relaxor ergodic state, and the quasielastic scattering vanishes at a pressure slightly above ${p}_{2}$${}^{*}$.

Pressure induced structural phase transformation in TiN: A first-principles study

Titanium nitride (TiN), which is widely used for hard coatings, reportedly undergoes a pressure-induced structural phase transformation, from a NaCl to a CsCl structure, at ∼7 GPa. In this paper, we use first-principles calculations based on density functional theory with a generalized gradient approximation of the exchange correlation energy to determine the structural stability of this transformation. Our results show that the stress required for this structural transformation is substantially lower (by more than an order of magnitude) when it is deviatoric in nature vis-à-vis that under hydrostatic pressure. Local stability of the structure is assessed with phonon dispersion determined at different pressures, and we find that CsCl structure of TiN is expected to distort after the transformation. From the electronic structure calculations, we estimate the electrical conductivity of TiN in the CsCl structure to be about 5 times of that in NaCl structure, which should be observable experimentally.

High pressure-induced phase transitions in CdS up to 1 Mbar

The pressure-induced structural transformations of CdS have been investigated using synchrotron x-ray diffraction in a diamond anvil cell up to 104 GPa and the density functional theory calculations. The x-ray diffraction experiments show that CdS is stable with the wurtzite-type structure at ambient conditions. The wurtzite-type phase transforms to NaCl-type structure at 3 GPa, which is followed by a second phase transition at 52.3 GPa. In the diffraction patterns, the peak-splitting is observed, indicating that the high-pressure phase appearing at 52.3 GPa is the Pmmn structure, rather than the Cmcm phase reported earlier. With increasing pressure, the lattice parameter a of the Pmmn phase increases abnormally, contrary to decrease of b and c axes. Our calculations reveal that the abnormal change of the a-axis could be related to the pressure-induced crystal structural change. The bulk modulus (B0), is 64.3(9) GPa for wurzite-type phase, 105(2) GPa for NaCl-type phase, and 54(4) GPa for the Pmmn phase, respectively.

Pressure induced increase in Tc for the organic-based magnet FeII(TCNE)2 (TCNE = tetracyanoethylene)

Pressure dependent magnetization and 57Fe Mössbauer studies were performed on Fe(TCNE)[C4(CN)8]1/2·zCH2Cl2 (TCNE=tetracyanoethylene). Pressure did not influence the Mössbauer parameters in paramagnetic state. Mössbauer data reveals the onset of magnetic ordering at 130K and significant enhancement of the magnetic ordering temperature from 100 to 150K accompanied by an increase of the spontaneous magnetization, which is higher than reported from the magnetic data, and application of pressure induces the reversible formation of a new, metastable magnetic species. These changes suggest an increase of the dimensionality of magnetic interaction, i.e., stronger interlayer coupling. Application of 9.3kbar increases Tc to 160K, and the moderate dTc/dP of ∼1K/kbar in the pressure-induced metastable state suggests a pressure-induced structural phase transformation that increases the number of next–nearest neighbors of iron.

Pressure-induced structural transformations in the low-cristobalite form of AlPO4

We have investigated the high-pressure behavior of low-cristobalite form of AlPO4 (c-AlPO4) using a combination of Raman scattering, synchrotron powder X-ray diffraction, and classical molecular dynamics simulations. Our experiments indicate that under non-hydrostatic conditions c-AlPO4 initially transforms to a monoclinic phase, which then transforms to the Cmcm phase via an intermediate, disordered structure. In contrast, X-ray diffraction measurements made under hydrostatic conditions show that the ambient structure transforms directly to the Cmcm phase. Our classical molecular dynamics simulations, carried out under hydrostatic conditions, also show that c-AlPO4 directly transforms to the Cmcm phase at ~13 GPa.

Pressure-Induced Polymorphic Transitions in Crystalline Diborane Deduced by Comparison of Simulated and Experimental Vibrational Spectra

We investigate and assign the pressure-induced structural transformations in crystalline diborane (B2H6) observed spectroscopically by Song and co-workers (J. Phys. Chem. B2009, 113, 13509; J. Chem. Phys.2009, 131, 174506) between 3.5 and 24 GPa at room temperature. The assignment is made by calculating the Raman and infrared vibrational spectra of 10 candidate structures at various pressures and comparing the results to experiment. We find that solid diborane undergoes a polymorphic transition at about 6 GPa from β-diborane (P21/n) to a P21/c diborane and possibly a second transition near 14 GPa to another P21/c diborane structure. We conclude that no cyclic oligomers or chains of the composition (BH3)n (n > 2) are formed from diborane up to at least 24 GPa under the experimental conditions employed by the Song group, even when such structures are thermodynamically favored. This suggests that pressure-induced chemical transformations of molecular crystals of diborane are kinetically hindered.