Calculated Transition Pressure Research Articles

Stability of high-pressure phases in II-VI semiconductors by a density functional lattice dynamics approach

Stability of the rocksalt structure with respect to the high-pressure $Cmcm$ structure was investigated for several II-VI semiconductors (ZnS, ZnSe, CdS, and CdSe). The lattice dynamics calculations reveal that the rocksalt structure is unstable with respect to the transversal acoustic mode at the Brillouin zone boundary. The phase transition is of the second order and is associated with cell doubling in the high-pressure $Cmcm$ phase with frozen deformation. The present calculation gives the transition pressure more accurately than the classical method, which uses the common tangent. Since the volume change at the transition is very small or zero in all studied crystals, transitions were estimated from the pressure at which the frequency of the zone boundary transversal acoustic mode goes to zero. The observed pressures of the phase transitions in structural measurements were reproduced more accurately than in previous calculations and the difference between observed and calculated transition pressure is of the order of 1% to 8%.

High-pressure structural transitions in Cm and Am0.5Cm0.5 binary alloy

The high-pressure behaviour of Cm and Am0.5Cm0.5 binary alloy is investigated theoretically using ab initio electronic structure methods. Our calculations reproduce the structural phase transitions, which are observed in recent experiment performed by Heathman et al. [S. Heathman, R.G. Haire, T. Le Bihan et al., Science 309 110 (2005)] and Lindbaum et al. [A. Lindbaum, S. Heathman, T. Le Bihan et al., J. Phys: Condens. Matter 15 S2297 (2003)]. Calculated transition pressures are in reasonable agreement with the experimental values. Calculations performed for an antiferromagnetic state are essential to reproduce the stability of Cm-III phase.

First-principles electronic structure study of Sc-II

Sc crystallizes in the hcp structure at ambient pressure. The structure of the high-pressure phase Sc-II has been debated for a long time. Most recently, two different solutions for the Sc-II phase were proposed, both involving a composite incommensurate structure consisting of a host and a guest substructure. To explore the Sc-II crystal structure we perform first-principles total-energy calculations and find that the approximant modeling of the structure suggested by McMahon et al. [Phys. Rev. B 73, 134102 (2006)] is the stable structure for pressures above the calculated transition pressure of $20\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. This theoretical value is in perfect agreement with the experimentally measured value of $23\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. Analyses of the band structures provide an explanation why the structure proposed by Fujihisa et al. [Phys. Rev. B 72, 132103 (2005)] is not the correct Sc-II structure.

Structural and electronic properties of carbon in hybrid diamond-graphite structures

In this paper we report on ab initio pseudopotential density-functional calculations of some possible high-pressure phases of carbon. The total energies of several hybrid diamond-graphite structures were calculated as a function of volume using density-functional theory and the local density approximation. The lowest calculated transition pressures between hexagonal-graphite and hybrid structures were 17 and $20\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, which compare well with the experimental value of $14\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ for the transition at low temperatures between graphite and a still unidentified hard transparent phase. The electronic densities of states for the different structures are presented. Also, the x-ray powder diffraction patterns for a few structures were simulated and qualitatively compared to published experimental diffraction patterns.

Pressure-induced structural phase transition inNaBH4

We present a combined experimental and theoretical study of the technologically important ${\mathrm{NaBH}}_{4}$ compound under high pressure. Using Raman spectroscopy at room temperature, we have found that ${\mathrm{NaBH}}_{4}$ undergoes a structural phase transformation starting at 10.0 GPa with the pure high-pressure phase being established above 15.0 GPa. In order to compare the Raman data recorded under high pressure with the low-temperature tetragonal phase of ${\mathrm{NaBH}}_{4}$, we have also performed a cooling experiment. The known order-disorder transition from the fcc to the tetragonal structure was then observed. However, the new high pressure phase does not correspond to this low-temperature structure. Using first-principle calculations based on the density functional theory, we show that the high-pressure phase corresponds to the $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{LiAlH}}_{4}$--type structure. We have found a good agreement between the measured and calculated transition pressures. Additionally, we present the electronic structure of both the fcc and the high-pressure phases.

High Pressure Structural transitions in Cm metal

AbstractThe high pressure behaviour of Cm metal is investigated theoretically using ab initio electronic structure methods. Our calculations reproduce the structural phase transitions which are obsereved in recent experiment performed by Heatman et al. [Science 309, 110 (2005)]. Calculated transition pressures are in reasonable agreement with the experiment. Calculations performed in antiferromagnetic state is essential to reproduce the stability of Cm-III phase. Thus stability of Cm-III phase is related to the spin polarization of 5f electrons.

Structural Phase Transitions in Heavy Alkali Metals Under Pressure

We performed a theoretical study of the crystal structures of cesium and rubidium under high compressions. Our results confirm the recent high-pressure experimental observations of new complex crystal structures for the Cs III and Rb III phases. The calculated transition pressures agree extremely well with the measured data. Thus, it is now certain that the famous isostructural phase transition in cesium is actually a new crystallographic phase transition. A d-orbital occupation number of about 0.52 is crucial for the occurrence of these complex structures.

Pressure induced phase transitions in Ga1 − x In x P*

We have theoretically investigated the effect of pressure on the structural stability of GaP : InP mixed system. The three-body-potential (TBP) model has been used. The TBP model consists of long-range as well as short-range interactions; the long-range part includes the modified Coulomb force as well as a three-body term; the short-range part in TBP defines the van der Waals and overlap repulsive interactions. We observe a pressure-induced structural phase transformation from ZnS (B3) to NaCl (B1) type phase in Ga 1−x In x P. Our calculated transition pressures for the initial GaP and final InP compound semiconductors are in good agreement with other reported data.

Vibrational and quasiharmonic thermal properties of CaO under pressure

We have investigated by first principles the phonon dispersions of CaO across its pressure induced phase transformation from the B1 (NaCl structure) to the B2 (CsCl structure) phase. Beyond their stability fields we have uncovered phonon instabilities that should be associated with this phase transition. We have also used these dispersions in conjunction with the quasiharmonic approximation (QHA) to calculate several thermodynamic properties in both phases. Comparisons between the predicted properties with experimental measurements and with similar properties of MgO indicate that the QHA is quite unsatisfactory for CaO. Despite this, the calculated transition pressure at room temperature falls within the range of measurements. This transition is shown to have a negative Clapeyron slope.

Pressure Induced Structural Phase Transitions and Metallization of BaF2

AbstractFor Abstract see ChemInform Abstract in Full Text.

High‐Pressure Structural Phase Transitions in Magnesium Fluoride Studied by Electronic Structure Calculations

AbstractFor Abstract see ChemInform Abstract in Full Text.

First-principles study of the structural phase transformation of hafnia under pressure

We investigate the phase transformation of ${\mathrm{HfO}}_{2}$ under hydrostatic pressure through first-principles pseudopotential calculations within the local-density-functional approximation (LDA) and the generalized gradient approximation (GGA). We find that with increasing of pressure, ${\mathrm{HfO}}_{2}$ undergoes a series of structural transformations from monoclinic to orthorhombic I and then to orthorhombic II, consistent with experiments. The calculated transition pressures within the GGA are in good agreement with the measured values, while they are severely underestimated by the LDA. Analyzing the distribution of electron densities for the high-pressure phases, we find that the electron densities of the orthorhombic-II phase are more homogeneous than for the orthorhombic-I phase. Due to this distinct difference in the homogeneity of electron densities, the energy difference between the orthorhombic-I and orthorhombic-II phases is enhanced in the GGA; thus, the transition pressure between the two phases increases significantly.

Pressure induced structural phase transitions and metallization of BaF 2

The electronic band structure of BaF 2 was calculated in the fluorite, orthorhombic and the hexagonal phases. The calculations were performed using the tight binding linear muffin tin orbital method (TB-LMTO). The total energies calculated within the atomic sphere approximation (ASA) were used to determine the ground state properties of the system. The calculated equilibrium lattice constants and the bulk modulus are in good agreement with the experimental results. The calculated transition pressure for fluorite→orthorhombic and orthorhombic→hexagonal phases are 2.84 and 12.8 GPa, respectively, which are in good agreement with the experimental results. In addition, BaF 2 is predicted to undergo metallization around 33 GPa. The band structures are plotted in all the three phases and at metallization. The band gap in the three phases are 7.03, 7.16, 3.34 eV, respectively. The calculated density of states at the metallization is around 10.57 states/Ry/F.U. This system is found to be an indirect band gap insulator with the band gap initially increasing with pressure which decreases upon further compression leading to band overlap metallization.

High-pressure structural phase transitions in magnesium fluoride studied by electronic structure calculations

The present work employs the self-consistent tight binding linear muffin tin orbital method (TB-LMTO) to calculate the electronic structure and the relative stability of MgF 2. MgF 2 is found to undergo a series of structural phase transitions under high pressure. The sequence of the structural phase transition is rutile→CaCl 2→PdF 2→ cotunnite that occurs around 10, 14.1 and 36.8 GPa, respectively. The calculated transition pressures are in fairly good agreement with the experimental results. The ground state properties of this compound are calculated in all the phases. The band structures are plotted at ambient as well as at high pressures. This compound is a wide band gap insulator with a gap of 6.45 eV at ambient conditions. The band gap tends to increase continuously even in the final high-pressure phase in contrast to other alkaline earth fluorides [45,46] wherein metallisation has been predicted for CaF 2, SrF 2 and BaF 2. The possible reason for this behaviour in MgF 2 is also discussed.

High pressure structural phase transitions in IV–VI semiconductors

AbstractWe have studied high pressure structural phase transitions in lead chalcogenides by means of first principles total energy calculations within the generalized gradient approximation (GGA) using the full potential linear‐muffin‐tin‐orbital (FPLMTO) method. These compounds are stable in the NaCl‐type structure at ambient conditions and transform to the CsCl‐type of structure under high pressure through an intermediate phase. The experimental data suggests that this intermediate phase could be either the TlI‐type phase or the GeS‐type phase. However this data was not enough to solve the full structural refinement. So we have performed fully relaxed total energy calculations to get the full structure of this intermediate phase. Our calculated transition pressures are in very good agreement with experiment.

Structural phase stability of CaF 2 and SrF 2 under pressure

The electronic band structure of CaF 2 and SrF 2 was calculated in the fluorite and orthorhombic phases. The calculations were performed using tight binding linear muffin tin orbital method. The calculated ground state properties are in fairly good agreement with the experimental results. The transition pressure of CaF 2 and SrF 2 was found to be 9.1 and 7 GPa, respectively. The calculated transition pressures of CaF 2 are in good agreement with the experimental results. These systems were found to be wide band gap insulators with the band gap initially increasing with pressure, which decreases upon further compression. CaF 2 and SrF 2 are predicted to undergo metallization around 210 and 92 GPa, respectively. The possible reason for the metallization was also discussed.

High pressure phase transition and elastic properties of cerium chalcogenides and pnictides

The structural properties of Cerium mono-chalcogenides and mono-pnictides have been investigated for the first time by using a modified interionic potential theory. The calculated transition pressures are in good agreement with the experimental results. The ionic bonding is found to be more pronounced in Ce-mono-chalcogenides than mono-pnictides. The calculated values of elastic constants are also predicted for the first time.

Ab initio study of the high-pressure phase transition from the cubic-diamond to the β-tin structure of Si

We have studied the high-pressure phase transition from the cubic-diamond (cd) to the β-tin structure of silicon. This work has been performed using the plane-wave pseudopotential approach to density-functional theory (DFT) and density-functional perturbation theory (DFPT) within the local-density approximation (LDA). We present results on the pressure dependence of ground-state properties, such as lattice-parameters, and electronic band structure. Furthermore, the pressure dependence of the phonon-dispersion curves is shown. The calculated transition pressure and volumes are in good agreement with experimental values. In particular, our results show no softening of phonon modes near transition. Finally, our calculation of pressure-dependent phonon-dispersion curves allows the calculation of the pressure vs. temperature phase diagram within the quasi-harmonic approximation.

Structural and superconducting transition in selenium at high pressure

First-principles calculations are performed for electronic structures of two high pressure phases of solid selenium, $\beta$-Po and bcc. Our calculation reproduces well the pressure-induced phase transition from $\beta$-Po to bcc observed in selenium. The calculated transition pressure is 30 GPa lower than the observed one, but the calculated pressure dependence of the lattice parameters agrees fairly well with the observations in a wide range of pressure. We estimate the superconducting transition temperature $T_{\rm c}$ of both the $\beta$-Po and the bcc phases by calculating the phonon dispersion and the electron-phonon interaction on the basis of density-functional perturbation theory. The calculated $T_{\rm c}$ shows a characteristic pressure dependence, i.e. it is rather pressure independent in the $\beta$-Po phase, shows a discontinuous jump at the transition from $\beta$-Po to bcc, and then decreases rapidly with increasing pressure in the bcc phase.

Electronic Structure of Phosphorus under High Pressure

We have performed band structure calculations of high pressure phases of phosphorus within the local-density-functional formalism and the norm-conserving pseudopotential method. The structural stability of the simple hexagonal phase of phosphorus has been studied, and the calculated transition pressure from the simple cubic phase to the simple hexagonal phase, the equation of states, and the c/a ratio are in good agreement with the measured results.