High-pressure Structural Sequence Research Articles

On synthesis, stability and superconductivity of ThNb2H12under pressure: ab-inito calculations

Systematic study employing first-principle electronic structure method along with the evolutionary structure search algorithm as implemented in the universal structure predictor evolutionary Xtallography (USPEX) code has been carried out to look for the possibility of synthesising the ternary hydride (ThxNbyHz) from constituent's thorium, niobium and hydrogen. Our study suggests that the ternary hydride with formula ThNb2H12 will be globally stable above 12.5 GPa indicating the possibility of its formation from constituents Th, Nb, H and binary compounds of Th-H and Nb-H. Further, though the global stability of this compound is predicted to be beyond 12.5 GPa, it is metastable at 0 GPa. We have theoretically explored different formation routes to synthesize this compound. The zero-pressure monoclinic structure (Cm) of metastable ThNb2H12 undergoes a transformation to hexagonal structure (P6/mmm) at 3.5 GPa and this hexagonal structure (P6/mmm) transforms to other hexagonal type structure (P6/m) at 50 GPa. The high pressure structural sequence so predicted through static lattice calculations has been further substantiated by confirming the elastic and lattice dynamic stability of each structure in the pressure regime of their stability. Employing Allen–Dynes modified McMillan [P. B. Allen and R. C. Dynes, Phys. Rev. B 12, 905 (1975)] formula along with the fixed Coulomb pseudopotential of 0.1 and calculated electron-phonon coupling parameter, the superconductive temperature has also been calculated for this compound. Our analysis indicates that this material will behave as superconductor with transition temperature (Tc) ∼ 25 K at 15 GPa, which, will reduce to 4 K upon further compression to 100 GPa. Our analysis on lattice thermal conductivity of this compound in P6/mmm phase, at 15 GPa, shows a significantly large anisotropy in conductivity tensor, especially at lower temperatures.

Thorium dicarbide under high pressure and high temperature: Ab initio investigation

A systematic study on the structural stability of thorium dicarbide (ThC2) under hydrostatic compression has been carried out by exploiting the evolutionary structure search algorithm as implemented in the universal structure predictor: evolutionary Xtallography (USPEX) code in conjunction with the ab initio electronic band structure calculation method. At ambient conditions, ThC2 exists in a monoclinic crystallographic phase with space group (SG) C2/c. Our calculations under generalized gradient approximation (GGA) predict the high-pressure structural sequence of monoclinic-I (SG C2/c) → monoclinic-II (SG C2/m) → orthorhombic-I (SG Pmma) → orthorhombic-II (SG Immm) → hexagonal (SG P6/mmm) for this material with transition pressures of ∼3.3, 58.3, 191.6, and 255 GPa, respectively. Out of this theoretically predicted high-pressure structural phase transition sequence, only the first transition, i.e., monoclinic-I → monoclinic-II, could be compared with the available high-pressure experimental study by Guo et al. [Sci. Rep. 7, 45872 (2017)]. The theoretically determined phase transition qualitatively agrees with the experimental results [Y. Guo et al. Sci. Rep. 7, 45872 (2017)]. Interestingly, our predicted intermediate orthorhombic-I (SG Pmma) phase has an enthalpy lower than that of the previously predicted orthorhombic Cmmm phase by Guo et al. [Sci. Rep. 7, 45872 (2017)]. The high-pressure structural sequence so predicted through static lattice calculations has been further substantiated by confirming the elastic and lattice dynamic stability of each structure in the pressure regime of its structural stability. Additionally, the superconducting transition temperature for all these structures has been determined and it is found that the monoclinic-II (C2/m) phase has the highest transition temperature of 17 K at 5 GPa. Furthermore, the thermo-physical properties along with the temperature-induced phase transitions in ThC2 have also been investigated through both the lattice dynamic simulations (within quasi-harmonic approximation) and ab initio molecular dynamics simulations.

High pressure structural stability of ThN: ab-initio study

The high pressure structural stability of thorium nitride (ThN) has been examined through first principle total energy calculations. The structural sequence predicted through present theoretical simulation is NaCl type structure (B1) to tetragonal phase (B10) to CsCl type (B2) phase with corresponding transition pressures ∼75 GPa and 120 GPa, respectively. The B1 → B10 transition with volume discontinuity of ∼10% is found to be of first order type, whereas, the B10 → B2 displays second order nature. The predicted high pressure structural sequence in ThN is different from those reported in earlier works [Gupta and Singh, Sol. State Sci., 2010 and Modak and Verma, Phys. Rev. B 2011]. The pressure induced phase transition sequence so obtained is further supported by examining the mechanical and phonon stability of the predicted structures in their structural stability regime. Also, the activation barrier for B1 to B10 transition determined from the calculated Bain path at the transition pressure turns out to be ∼0.75eV/f.u.(72.36 kJ/mol).

High pressure behaviour of uranium dicarbide (UC2): Ab-initio study

The structural stability of uranium dicarbide has been examined under hydrostatic compression employing evolutionary structure search algorithm implemented in the universal structure predictor: evolutionary Xtallography (USPEX) code in conjunction with ab-initio electronic band structure calculation method. The ab-initio total energy calculations involved for this purpose have been carried out within both generalized gradient approximations (GGA) and GGA + U approximations. Our calculations under GGA approximation predict the high pressure structural sequence of tetragonal → monoclinic → orthorhombic for this material with transition pressures of ∼8 GPa and 42 GPa, respectively. The same transition sequence is predicted by calculations within GGA + U also with transition pressures placed at ∼24 GPa and ∼50 GPa, respectively. Further, on the basis of comparison of zero pressure equilibrium volume and equation of state with available experimental data, we find that GGA + U approximation with U = 2.5 eV describes this material better than the simple GGA approximation. The theoretically predicted high pressure structural phase transitions are in disagreement with the only high experimental study by Dancausse et al. [J. Alloys. Compd. 191, 309 (1993)] on this compound which reports a tetragonal to hexagonal phase transition at a pressure of ∼17.6 GPa. Interestingly, during lowest enthalpy structure search using USPEX, we do not see any hexagonal phase to be closer to the predicted monoclinic phase even within 0.2 eV/f. unit. More experiments with varying carbon contents in UC2 sample are required to resolve this discrepancy. The existence of these high pressure phases predicted by static lattice calculations has been further substantiated by analyzing the elastic and lattice dynamic stability of these structures in the pressure regimes of their structural stability. Additionally, various thermo-physical quantities such as equilibrium volume, bulk modulus, Debye temperature, thermal expansion coefficient, Gruneisen parameter, and heat capacity at ambient conditions have been determined from these calculations and compared with the available experimental data.

Prediction of new high pressure structural sequence in thorium carbide: A first principles study

In the present work, we report the detailed electronic band structure calculations on thorium monocarbide. The comparison of enthalpies, derived for various phases using evolutionary structure search method in conjunction with first principles total energy calculations at several hydrostatic compressions, yielded a high pressure structural sequence of NaCl type (B1) → Pnma → Cmcm → CsCl type (B2) at hydrostatic pressures of ∼19 GPa, 36 GPa, and 200 GPa, respectively. However, the two high pressure experimental studies by Gerward et al. [J. Appl. Crystallogr. 19, 308 (1986); J. Less-Common Met. 161, L11 (1990)] one up to 36 GPa and other up to 50 GPa, on substoichiometric thorium carbide samples with carbon deficiency of ∼20%, do not report any structural transition. The discrepancy between theory and experiment could be due to the non-stoichiometry of thorium carbide samples used in the experiment. Further, in order to substantiate the results of our static lattice calculations, we have determined the phonon dispersion relations for these structures from lattice dynamic calculations. The theoretically calculated phonon spectrum reveal that the B1 phase fails dynamically at ∼33.8 GPa whereas the Pnma phase appears as dynamically stable structure around the B1 to Pnma transition pressure. Similarly, the Cmcm structure also displays dynamic stability in the regime of its structural stability. The B2 phase becomes dynamically stable much below the Cmcm to B2 transition pressure. Additionally, we have derived various thermophysical properties such as zero pressure equilibrium volume, bulk modulus, its pressure derivative, Debye temperature, thermal expansion coefficient and Gruneisen parameter at 300 K and compared these with available experimental data. Further, the behavior of zero pressure bulk modulus, heat capacity and Helmholtz free energy has been examined as a function temperature and compared with the experimental data of Danan [J. Nucl. Mater. 57, 280 (1975)].

Compression of scheelite-type SrMoO4 under quasi-hydrostatic conditions: Redefining the high-pressure structural sequence

The high-pressure behavior of tetragonal SrMoO4 was analyzed by Raman and optical-absorption measurements. Pressures up to 46.1 GPa were generated using diamond-anvil cells and Ne or N2 as quasi-hydrostatic pressure-transmitting media. A reversible phase transition is observed at 17.7 GPa. A second transition is found at 28.8 GPa and the onset of a third one at 44.2 GPa. The pressure dependence of Raman-active modes is reported for the different phases and the pressure evolution of the fundamental band-gap reported for the low-pressure phase. The observed changes in the Raman spectra contradict the structural sequence determined from previous experiments performed under higher non-hydrostaticity. This fact suggests that deviatoric stresses can influence pressure-driven transitions in scheelite-type oxides. We also report total-energy, lattice-dynamics, and band-structure calculations. They reproduce accurately the behavior of the physical properties of the low-pressure phase and predict the occurrence of phase transitions at pressures similar to experimental transition pressures. According to theory, the high-pressure phases have monoclinic and orthorhombic structures, which are much more compact than tetragonal scheelite. Theoretical results and experiments are compared with previous studies.

Compression of scheelite-type SrMoO4 under quasi-hydrostatic conditions: Redefining the high-pressure structural sequence

Compression of scheelite-type SrMoO4 under quasi-hydrostatic conditions: Redefining the high-pressure structural sequence

High-pressure behavior of phosphorus from first principles calculations

High-pressure structural behavior, electronic structure, and vibrational properties of P have been investigated by means of first principles calculations. We perform an ab initio search for the P-IV phase by analyzing the phonon dispersions of the simple cubic (sc) structure as a function of pressure. In particular, we use the dynamical instability of the sc structure above the transition into the simple hexagonal structure to extract a possible candidate structure for P-IV. Additionally, in contrast to general expectations, we show that the body-centered cubic (bcc) P-V phase cannot be the end member of the high-pressure structural sequence of P because it becomes dynamically unstable at high compression. We propose that at ultrahigh pressure above $280\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, the bcc structure transforms first to the $IM7$ structure and then to the hexagonal close-packed structure.

Pressure induced structural phase transition inAgSbTe2

We report a high-pressure structural sequence with a degree of reversibility for the functionally graded thermoelectric, narrow band-gap semiconductor $\mathrm{Ag}\mathrm{Sb}{\mathrm{Te}}_{2}$, discovered using angle dispersive x-ray diffraction in a diamond anvil cell with synchrotron radiation at room temperature. The compound undergoes a $B1$ to $B2$ transition between 17 and $26\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and proceeds through an intermediate amorphous phase that can be quenched to ambient conditions. The pressure induced structural transition observed in this compound belongs in this ternary cubic family. Density functional theory calculations performed for the $B1$ and $B2$ phases are in good agreement with the experimental results.

Comparative study of the high-pressure behavior of As, Sb, and Bi.

The high-pressure behavior of the heavier group 15 elements As, Sb, and Bi was investigated by means of ab initio density functional calculations employing pseudopotentials and a plane wave basis set. The high-pressure structural sequence of these elements is distinguished by the occurrence of the Bi-III structure, which is a complex, incommensurately modulated, host-guest structure. We approximated this structure by a supercell which reproduced the experimentally established pressure stability ranges of the host-guest structure for the different elements extremely well. With pressure we find an increasing admixture of d states (s-d hybridization) in the occupied levels of the electronic structure of As, Sb, and Bi. However, the s-d mixing remains at a low level. Thus, the emergence of a complex intermediate pressure structure cannot be explained by a pressure-induced altered valence state for these elements. Instead, it is argued that the Bi-III structure is a consequence of a delicate interplay between the electrostatic and the band energy contribution to the total energy. In the intermediate pressure range of heavier group 15 elements, both important parts of the total energy account equally for structural stability.