Characterization of the high-pressure structures and phase transformations in SnO2. A density functional theory study

Theoretical investigations concerning the high-pressure polymorphs, the equations of state, and the phase transitions of SnO2 have been performed using density functional theory at the B3LYP level. Total energy calculations and geometry optimizations have been carried out for all phases involved, and the following sequence of structural transitions from the rutile-type (P42/mnm) driven by pressure has been obtained (the transition pressure is in parentheses): --> CaCl2-type, Pnnm (12 GPa) --> alpha-PbO2-type, Pbcn (17 GPa) --> pyrite-type, Pa (17 GPa) --> ZrO2-type orthorhombic phase I, Pbca (18 GPa) --> fluorite-type, Fmm (24 GPa) --> cotunnite-type orthorhombic phase II, Pnam (33 GPa). The highest bulk modulus values, calculated by fitting pressure-volume data to the second-order Birch-Murnaghan equation of state, correspond to the cubic pyrite and the fluorite-type phases with values of 293 and 322 GPa, respectively.     

Synthesis and crystal structure determination of a new pressure-induced iridium ditelluride phase, m-IrTe2, and comparison of the crystal structures and relative stabilities of various IrTe2 polymorphs

The new monoclinic IrTe2 phase m-IrTe2 was synthesized under pressure, and its structure was determined by X-ray powder diffraction. The relative stabilities of the three known and three hypothetical IrTe2 polymorphs were discussed on the basis of tight binding electronic band structure calculations. m-IrTe2 exhibits structural features of both CdI2- and pyrite-type IrTe2 phases and is expected to be nearly as stable as that of the CdI2-type IrTe2. The hypothetical IrS2- and ramsdellite-type IrTe2 phases are predicted to be more stable than the CdI2-type IrTe2.     

The high-pressure phase transition of TiS2 from first-principles calculations

The structural stability of TiS₂ under high pressures has been investigated by using first-principles plane-wave pseudopotential density functional theory within the local density approximation (LDA). The obtained results predict that TiS₂ undergoes a pressure-induced first-order phase transition from its trigonal 1T-type structure to orthorhombic cotunnite-type structure at 16.20 GPa. The calculated transition pressure agrees quite well with the experimental finding of 20.7 GPa. The equation of state determined from our calculated results yields bulk moduli of 58.91 and 118.10 GPa for the 1T-type and cotunnite-type phases, respectively. This indicates higher incompressibility of the high-pressure phase of TiS₂. In addition, the electronic structures of the two phases of TiS₂ are also calculated and discussed. The results suggest the structural phase transition of TiS₂ at high pressure is followed by a semimetal to metal electronic transition.     

Why pressure induces an abrupt structural rearrangement in PdTe2 but not in PtTe2

High-pressure X-ray diffraction measurements were carried out for polymeric CdI₂-type compounds MTe₂ (M=Pt, Pd) to investigate if they undergo a structural phase transition under pressure as does IrTe₂. Up to 27 GPa at room temperature PtTe₂ does not undergo any structural phase transition. In contrast, however, an abrupt change in the inter-atomic distances occurs in PdTe₂ above 15.7 GPa at room temperature, and above 5 GPa at 300 °C, but the volume vs. pressure curve exhibits no discontinuity. To account for the differences between the isostructural compounds PtTe₂, PdTe₂ and IrTe₂, their electronic structures and bonding were analyzed on the basis of first principles electronic band structure calculations.     

Synthesis, structural transformation, thermal stability, valence state, and magnetic and electronic properties of PbNiO3 with perovskite- and LiNbO3-type structures

We synthesized two high-pressure polymorphs PbNiO(3) with different structures, a perovskite-type and a LiNbO(3)-type structure, and investigated their formation behavior, detailed structure, structural transformation, thermal stability, valence state of cations, and magnetic and electronic properties. A perovskite-type PbNiO(3) synthesized at 800 °C under a pressure of 3 GPa crystallizes as an orthorhombic GdFeO(3)-type structure with a space group Pnma. The reaction under high pressure was monitored by an in situ energy dispersive X-ray diffraction experiment, which revealed that a perovskit-type phase was formed even at 400 °C under 3 GPa. The obtained perovskite-type phase irreversibly transforms to a LiNbO(3)-type phase with an acentric space group R3c by heat treatment at ambient pressure. The Rietveld structural refinement using synchrotron X-ray diffraction data and the XPS measurement for both the perovskite- and the LiNbO(3)-type phases reveal that both phases possess the valence state of Pb(4+)Ni(2+)O(3). Perovskite-type PbNiO(3) is the first example of the Pb(4+)M(2+)O(3) series, and the first example of the perovskite containing a tetravalent A-site cation without lone pair electrons. The magnetic susceptibility measurement shows that the perovskite- and LiNbO(3)-type PbNiO(3) undergo antiferromagnetic transition at 225 and 205 K, respectively. Both the perovskite- and LiNbO(3)-type phases exhibit semiconducting behavior.     

Compressibility and structural behavior of pure and Fe-doped SnO2 nanocrystals

We have performed high-pressure synchrotron X-ray diffraction experiments on nanoparticles of pure tin dioxide (particle size ∼30 nm) and 10 mol % Fe-doped tin dioxide (particle size ∼18 nm). The structural behavior of undoped tin dioxide nanoparticles has been studied up to 32 GPa, while the Fe-doped tin dioxide nanoparticles have been studied only up to 19 GPa. We have found that both samples present at ∼13 GPa a second-order structural phase transition from the ambient pressure tetragonal rutile-type structure (P4₂/mnm) to an orthorhombic CaCl₂-type structure (space group Pnnm). No phase coexistence was observed for this transition. Additionally, pure SnO₂ presents a phase transition to a cubic structure at ∼24 GPa. The evolution of the lattice parameters with pressure and the room-temperature equations of state are reported for the different phases. The reported results suggest that the partial substitution of Sn by Fe induces an enhancement of the bulk modulus of SnO₂. Results are compared with previous studies on bulk and nanocrystalline SnO₂. The effects of pressure on Sn-O bonds are also analyzed.     

Theoretical calculations of high-pressure phases of NiF<Subscript>2</Subscript>: An ab initio constant-pressure study

We have studied the structural properties of the antiferromagnetic NiF₂ tetragonal structure with P4₂/mnm symmetry using density functional theory (DFT) under rapid hydrostatic pressure up to 400 GPa. For the exchange correlation energy we used the local density approximation (LDA) of Ceperley and Alder (CA). Two phase transformations are successfully observed through the simulations. The structures of XF₂-type compounds crystallize in rutile-type structure. NiF₂ undergoes phase transformations from the tetragonal rutile-type structure with space group P4₂/mnm to orthorhombic CaCl₂-type structure with space group Pnnm and from this orthorhombic phase to monoclinic structure with space group C2/m at 152 GPa and 360 GPa, respectively. These phase changes are also studied by total energy and enthalpy calculations. According to these calculations, we perdict these phase transformations at about 1.85 and 30 GPa.     

Theoretical studies of solid bicyclo-HMX: effects of hydrostatic pressure and temperature

On the basis of density functional theory (DFT) and molecular dynamics (MD), the structural, electronic, and mechanical properties of the energetic material bicyclo-HMX have been studied. The crystal structure optimized by the LDA/CA-PZ method compares well with the experimental data. Band structure and density of states calculations indicate that bicyclo-HMX is an insulator with the band gap of ca. 3.4 eV and the N-NO(2) bond is the reaction center. The pressure effect on the bulk structure and properties has been investigated in the range of 0-400 GPa. The crystal structure and electronic character change slightly as the pressure increases from 0 to 10 GPa; when the pressure is over 10 GPa, further increment of the pressure determines significant changes of the structures and large broadening of the electronic bands together with the band gap decreasing sharply. There is a larger compression along the c-axis than along the a- and b-axes. To investigate the influence of temperature on the bulk structure and properties, isothermal-isobaric MD simulations are performed on bicyclo-HMX in the temperature range of 5-400 K. It is found that the increase of temperature does not significantly change the crystal structure. The thermal expansion coefficients calculated for the model indicate anisotropic behavior with slightly larger expansion along the a- and c-axes than along the b-axis.     

Pressure‐induced Oxidation State Change of Ytterbium in YbGa2

The crystal structure and the electronic properties of YbGa₂ realising a CaIn₂ type atomic arrangement were characterised at ambient conditions using single crystal X‐ray diffraction data and magnetic susceptibility measurements at ambient pressure. Pressure‐induced changes of structural and electronic properties of YbGa₂ were measured by means of angle‐dispersive X‐ray powder diffraction and XANES at the Yb L_III threshold. At pressures above 22(2) GPa, YbGa₂ undergoes a structural phase transition into a high pressure modification with a UHg₂ type crystal structure. Parallel to the pressure‐induced structural alterations, ytterbium in YbGa₂ undergoes an increase of the oxidation state from +2 at ambient conditions to +3 in the high‐pressure phase. Quantum chemical calculations of the Electron‐Localisation‐Function confirm that the phase transition is associated with a conversion of the three‐dimensional gallium network of the low‐pressure crystal structure into two‐dimensional gallium layers in the high‐pressure modification.     

Density Functional Theory Studies on the High-pressure Behavior of Crystalline 5,7-Dinitrobenzo-1,2,3,4-tetrazine-1,3-dioxide

The structural and electronic properties of the solid 5,7-dinitrobenzo-1,2,3,4-tetrazine-1,3-dioxide(DNBTDO) under the hydrostatic pressure of 0~100 GPa were investigated using density functional theory method. The predicted crystal structure with the LDA/CA-PZ functional agrees well with the experimental data at the ambient pressure. The structural results show that the b axis is the most compressible, whereas the a and c axes both have slight variation with pressure. The band gap generally decreases with the increasing pressure, which shows that the DNBTDO molecular crystal undergoes an electronic phase transition from semiconductor to metallic system. Through the analysis of band gap, the title compound is most sensitive at 70 GPa. The density of states analysis indicates that the strong peaks split into some small peaks and become wider under compression, which shows the increase of charge overlap and delocalization among the bonded atoms in the system.     

The high-pressure electronic structure of the [Ni(ptdt)2] organic molecular conductor

The electronic structure of the single component molecular crystal [Ni(ptdt)(2)] (ptdt = propylenedithiotetrathiafulvalenedithiolate) is determined at ambient and high pressure using density functional theory. The electronic structure of this crystal is found to be of the "crossing bands" type with respect to the dispersion of the HOMO and LUMO, resulting in a small, non-zero density of states at the Fermi energy at ambient pressure, indicating that this crystal is a "poor quality" metal, and is consistent with the crystal's resistivity exhibiting a semiconductor-like temperature dependence. The ambient pressure band structure is found to be predominantly one-dimensional, reflecting enhanced intermolecular interactions along the [100] stacking direction. Our calculations indicate that the band structure becomes two-dimensional at high pressures and reveals the role of shortened intermolecular contacts in this phenomenon. The integrity of the molecular structure is found to be maintained up to at least 22 GPa. The electronic structure is found to exhibit a crossing bands nature up to 22 GPa, where enhanced intermolecular interactions increase the Brillouin zone centre HOMO-LUMO gap from 0.05 eV at ambient pressure to 0.15 eV at 22 GPa; this enhanced HOMO-LUMO interaction ensures that enhancement of a metallic state in this crystal cannot be simply achieved through the application of pressure, but rather requires some rearrangement of the molecular packing. Enhanced HOMO-LUMO interactions result in a small density of states at the Fermi energy for the high pressure window 19.8-22 GPa, and our calculations show that there is no change in the nature of the electronic structure at the Fermi energy for these pressures. We correspondingly find no evidence of an electronic semiconducting-metal insulator transition for these pressures, contrary to recent experimental evidence [Cui et al., J. Am. Chem. Soc. 131, 6358 (2009)].     

New layered cobalt oxyfluoride, Sr2CoO3F

The first Ruddlesden-Popper type layered cobalt oxyfluoride, Sr(2)CoO(3)F, has been synthesized under a pressure of 6 GPa at 1700 °C and shown to adopt a K(2)NiF(4)-type structure with distorted square pyramidal coordination around Co and with O/F disorder at the apical sites.     

Pressure-induced metallization in the absence of a structural transition in the layered transition-metal dichalcogenide ZrSe2

First-principles calculations were carried out on the ZrSe₂ compound, which has been of interest owing to its technologically important physical properties. The structural, electronic and optical properties of this compound were investigated under pressure through the plane wave pseudopotential approach within the framework of density functional theory. A comparison between the computed crystal structure parameters and the corresponding experimental counterparts shows a very good agreement between them. Fitting the pressure–volume data using the third-order Birch–Murnaghan equation of state yielded a bulk modulus B₀ = 38.17 GPa and a pressure derivative of bulk modulus  = 8.2 for hexagonal ZrSe₂. The relationship between the band structure and pressure is revealed. We calculated the total density of state (TDOS) under different pressures and partial density of state (PDOS) from 0 to 10 GPa. According to our calculations, metallization of hexagonal ZrSe₂ is predicted to occur at around 10 GPa and pressure-induced band-gap engineering reveals the transformation of the indirect to direct band gap with increasing pressure. Furthermore, optical properties, such as the complex dielectric function, refractive index and reflectivity spectra of this compound, were studied for incident electromagnetic waves in an energy range up to 45 eV. The contributions to various transition peaks in the optical spectra are analyzed and discussed with the help of the energy-dependent imaginary part of the dielectric function.     

<Emphasis Type="Italic">Ab initio</Emphasis> study of the pressure effect on the structural and electronic properties of crystalline hydrogen azide

Within density functional theory with regard to the dispersion interaction the crystal structure parameters of hydrogen azide are determined. The pressure effect on its structural and electronic properties is studied in the range of 0-10 GPa. By means of the Vinet equation of state the bulk modulus of compression is found to be 9.26 GPa. It is shown that with an increase in the pressure molecules approach each other in molecular layers and this is accompanied by an increase in the total electron density contours, which means the principal possibility for polymerization. The external pressure of 10 GPa leads to the broadening of the upper valence energy bands and a decrease in the band gap from 6.14 eV to 5.51 eV.     

Pressure-induced isostructural phase transition and correlation of FeAs coordination with the superconducting properties of 111-type Na(1-x)FeAs

The effect of pressure on the crystalline structure and superconducting transition temperature (T(c)) of the 111-type Na(1-x)FeAs system using in situ high-pressure synchrotron X-ray powder diffraction and diamond anvil cell techniques is studied. A pressure-induced tetragonal to tetragonal isostructural phase transition was found. The systematic evolution of the FeAs(4) tetrahedron as a function of pressure based on Rietveld refinements on the powder X-ray diffraction patterns was obtained. The nonmonotonic T(c)(P) behavior of Na(1-x)FeAs is found to correlate with the anomalies of the distance between the anion (As) and the iron layer as well as the bond angle of As-Fe-As for the two tetragonal phases. This behavior provides the key structural information in understanding the origin of the pressure dependence of T(c) for 111-type iron pnictide superconductors. A pressure-induced structural phase transition is also observed at 20 GPa.     

Structural,elastic, electronic and optical properties of the newly synthesized monoclinic Zintl phase BaIn2P2

The present study explores the structural, elastic, electronic and optical properties of the newly synthesized monoclinic Zintl phase BaIn₂P₂ using a pseudopotential plane-wave method in the framework of density functional theory within the generalized gradient approximation. The calculated lattice constants and internal coordinates are in very good agreement with the experimental findings. Independent single-crystal elastic constants as well as numerical estimations of the bulk modulus, the shear modulus, Young's modulus, Poisson's ratio, Pugh's indicator of brittle/ductile behaviour and the Debye temperature for the corresponding polycrystalline phase were obtained. The elastic anisotropy of BaIn₂P₂ was investigated using three different indexes. The calculated electronic band structure and the total and site-projected l-decomposed densities of states reveal that this compound is a direct narrow-band-gap semiconductor. Under the influence of hydrostatic pressure, the direct D–D band gap transforms into an indirect B-D band gap at 4.08 GPa, then into a B–Γ band gap at 10.56 GPa. Optical macroscopic constants, namely, the dielectric function, refractive index, extinction coefficient, reflectivity coefficient, absorption coefficient and energy-loss function, for polarized incident radiation along the [100], [010] and [001] directions were investigated.     

Structural, electronic, and dynamical properties of methane under high pressure

The electronic structure and lattice dynamical properties of solid methane under high pressure have been studied based on density functional theory. We identify a cubic structure with space group of I43m below 14 GPa, the Pmn2(1) structure in the range of 14-21 GPa, and the P2(1)/c structure from 21 to 65 GPa. Our obtained Raman spectra of the P2(1)/c structure agree well with the typical Raman active modes in the available experimental data. At 65 GPa, methane undergoes a phase transition from P2(1)/c to Pnma. The structures with P2(1)/c and Pnma symmetries are insulating, and under any pressure studied methane always remains in molecular form. For Pnma phase, the orientational ordering of CH(4) molecules varies significantly at 79, 88, and 92 GPa, and by further increasing pressure the rotation of the molecules freezes and orientational ordering remains unchanged.     

Ab initio and molecular dynamics studies of crystalline TNAD (trans-1,4,5,8-tetranitro-1,4,5,8-tetraazadecalin)

The structural and electronic properties of the energetic crystal TNAD (trans-1,4,5,8-tetranitro-1,4,5,8- tetraazadecalin) have been studied using plane-wave ab initio calculations based on the density function theory method with the ultrasoft pseudopotentials. It is found that the predicted crystal structure is in good agreement with experimental data and there are strong inter- and intramolecular interactions in bulk TNAD. Band structure calculations indicate that TNAD is an insulator with the band gap of ca. 3.3 eV. The hydrostatic compression effect on TNAD has been studied in the pressure range of 0-600 GPa. The results show that a pressure less than 10 GPa does not significantly change the geometric parameters, charge distributions, and electronic bands. When the pressure is over 10 GPa, increasing the pressure determines significant changes of the geometrical and electronic structures and large broadening of the electronic bands together with a sharp decrease of the band gap. Isothermal-isobaric molecular dynamics simulations at atmospheric pressure were further performed on the TNAD crystal in the temperature range 5-500 K. Average equilibrium lattice parameters and elastic properties as functions of temperature were determined. The thermal expansion coefficients calculated for the crystal indicate anisotropic behavior with the largest expansion along the b axis.     

High pressure polymorphs of LiCoPO4 and LiCoAsO4

Olivine-LiCoXO₄ (X = P, As) compounds might transform to the denser spinel-type and Na₂CrO₄-type structures under pressure. In this work, the relative energetic stability of the three polymorphs and the pressure of the possible polymorphic transformations are investigated combining experiments and first principles calculations. Olivine-LiCoAsO₄ is predicted to transform to the Na₂CrO₄-like structure at 0.4 GPa and to the spinel structure at 5.8 GPa (0 K). Quenching HP/HT experiments show that olivine-LiCoAsO₄ treated at 6 GPa/1173 K transforms to the spinel-like structure. Computational results indicate that olivine-LiCoPO₄ transforms to the Na₂CrO₄-like form at around 4 GPa (0 K), the latter being the stable form till very high pressures (21.6 GPa). In good agreement with this, olivine-LiCoPO₄ when subjected to 6 GPa/1173 K and 15 GPa/1173 K is converted to the Na₂CrO₄-type polymorph. Crystallographic data of the new compound LiCoPO₄ within the Na₂CrO₄ structural type are provided.     

First Principles Investigation of Electronic Property and High Pressure Phase Stability of SmN

An investigation of electronic property and high pressure phase stability of SmN has been conducted using first principles calculations based on density functional theory. The electronic properties of SmN show a striking feature of a half metal, the majority-spin electrons are metallic and the minority-spin electrons are semiconducting. It was found that SmN undergoes a pressure-induced phase transition from NaCl-type (B1) to CsCl-type structure (B2) at 117 GPa. The elastic constants of SmN satisfy Born conditions at ambient pressure, indicating that B1 phase of SmN is mechanically stable at 0 GPa. The result of phonon spectra shows that B1 structure is dynamically stable at ambient pressure, which agrees with the conclusion derived from the elastic constants.