B. D. Sahoo

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 pho...

On equation of state, elastic, and lattice dynamic stability of bcc bismuth under high pressure: Ab-initio calculations

First principles calculations have been carried out using density functional theory based Vienna Ab-initio Simulation Package to analyze the elastic and lattice dynamic stability and determine the equation of state of bismuth in bcc phase. The 0 K isotherm has been determined from total energy calculations. The 300 K isotherm obtained after adding thermal corrections to 0 K isotherm compares well with experimental data. The elastic stability of the bcc phase examined from 0 GPa to 220 GPa suggests that this phase is elastically stable throughout this pressure range. The calculated phonon spectra of bcc phase suggest that this phase will be unstable lattice dynamically at ambient pressure but it will attain lattice dynamic stability at ∼8 GPa (the pressure around which this phase gets stabilized energetically). Further, from theoretically calculated elastic moduli, we have derived the volume dependent Gruneisen parameter and used this in Lindemann melting rule to determine the pressure effect on the meltin...

Pressure effect on elastic, lattice dynamic and superconducting behaviour of yttrium sulfide: A first principle study

First principles calculations have been carried out to analyze structural, elastic, and dynamic stability of yttrium sulphide (YS) under hydrostatic compression. The comparison of enthalpies of rocksalt type (B1) and CsCl type cubic (B2) structures determined as a function of compression suggests the B1 → B2 transition at ∼49 GPa (the same transition occurs at ∼48 GPa at 300 K). Various physical quantities such as zero pressure equilibrium volume, bulk modulus, and pressure derivative of bulk modulus have been derived from the theoretically determined equation of state. The single crystal elastic constants derived from the energy strain method agree well with the experimental values. The activation barrier between B1 and B2 phases calculated at transition point is ∼17/mRy/f.u. Our lattice dynamic calculations show that at ambient condition, the B1 phase is lattice dynamically stable, and frequencies of phonon modes in different high symmetry directions of Brillouin zone agrees well with experimental value...

Thermo-physical properties of LiH at high pressures by ab initio calculations

First principles calculations have been carried out to analyze structural stability and to determine the equation of state and elastic constants of LiH as a function of pressure. The comparison of total energies of B1 and B2 structures determined as a function of compression suggests the B1 → B2 transition at ∼ 327 GPa. Various physical quantities including zero pressure equilibrium volume, bulk modulus, pressure derivative of bulk modulus, Debye temperature, bulk sound speed, Hugoniot parameter ‘s’ and Gruneisen parameter have been derived. All these physical quantities compare well with the available experimental data. The single crystal elastic constants have been evaluated up to the B1→B2 transition pressure.

Theoretical prediction of high pressure phase transition in ScC and YC: Ab initio calculations

The structural stability of ScC and YC has been analyzed under hydrostatic compression employing the first-principles calculations using the plane-wave pseudopotential method. The comparison of theoretically calculated enthalpies of rocksalt type (B1), primitive orthorhombic (Pmmn), and CsCl type (B2) structures as a function of pressure suggests that the B1 structure transforms to Pmmn phase instead of transforming to B2 phase that predicted by Soni et al. [J. Phys. Chem. Solids 72, 810 (2011)]. The pressure for B1 to Pmmn transition predicted for ScC and YC are ∼80 GPa and ∼30 GPa, respectively. To further substantiate the outcomes of our static lattice calculations, we have performed lattice dynamic calculations also. Our lattice dynamic calculations correctly demonstrate that the B1 phase is dynamically stable structure at ambient condition. Further, for both the carbides, we find that the Pmmn structure becomes dynamically stable around the transition pressure whereas the B2 structure remains unstabl...

On the structural stability of CeN at high pressures: Ab initio calculations

The structural stability of CeN under hydrostatic compression has been analyzed theoretically. The comparison of enthalpies calculated as a function of hydrostatic compression for rocksalt type (B1), tetragonal (B10), and CsCl type (B2) structures suggests that the B1 phase will transform to B10 structure at ∼53 GPa, which upon further compression will transform to B2 phase at ∼200 GPa. However, the static high pressure energy dispersive x-ray diffraction measurements on CeN by Olsen et al. [J. Alloys Compd. 533, 29 (2012)] report that the B1 phase transforms directly to B2 phase at ∼65 GPa. To resolve the discrepancy between our calculations and experimental results, we have performed lattice dynamic calculations on these structures. The phonon spectra calculated at zero pressure correctly show B1 phase to be dynamically stable, and B10 and B2 to be unstable. At 60 GPa, the B1 phase becomes dynamically unstable and the B10 structure emerges as a dynamically stable phase whereas B2 still remains unstable....

Stabilization of tetragonal phase in LaN under high pressure via Peierls distortion

First-principles calculations have been carried out using the full potential linearized augmented plane wave method to analyze the structural stability of LaN under hydrostatic compression. Our calculations suggest that the rocksalt-type (B1) phase will transform to a primitive tetragonal structure (HP-LaN) having space group symmetry P4/nmm at a pressure of ∼25.8 GPa as compared to the experimental value of 22.8 GPa [Schneider SB, Baumann D, Salamat A, Schnick W, J Appl Phys. 2012;111:093503-1–6]. Additionally, we predict that the HP-LaN structure will further transform to CsCl type (B2) structure at ∼169 GPa. Analysis of band structures of HP-LaN and B2 phases suggests that the low symmetry HP-LaN phase, which can be viewed as a distortion of the B2 structure also, could be stabilized at lower pressure due to total energy lowering caused by Peierls distortion. The elastic moduli of B1 phase as a function of hydrostatic compression have also been calculated. The examination of behavior of elastic moduli as a function of pressure indicates that though the C44 modulus decreases monotonically with increasing pressure, it softens completely at a pressure much beyond the B1 to HP-LaN transition pressure.

Ab initio calculations on structural, elastic and dynamic stability of CdO at high pressures

First principles calculations have been carried out to analyze structural, elastic, and dynamic stability, of CdO under hydrostatic compression. The comparison of enthalpies of rocksalt type (B1) and CsCl type (B2) structures determined as a function of compression suggests the B1 → B2 transition at ∼87 GPa, in good agreement with experimental value of 90.6 GPa [Liu et al. Phys. Rev. B 70, 0941141 (2004)]. Various physical quantities, such as zero pressure equilibrium volume, bulk modulus, pressure derivative of bulk modulus, Gruneisen parameter, and Debye temperature have been derived from the theoretically determined equation of state. All these physical quantities show a reasonably good agreement with the available experimental data. Additionally, employing the theoretically determined thermal equation of state in conjunction with Rankine Hugoniot relation, we have predicted the Hugoniot of B1 phase of this material. The single crystal elastic constants of B1 phase calculated up to the pressure of 166 ...

Structural, elastic, and lattice dynamic stability of yttrium selenide (YSe) under pressure: A first principle study

Structural, elastic, and lattice dynamical stability of YSe has been investigated as a function of pressure through first principles electronic band structure calculations. The comparison of enthalpies of rocksalt type (B1) and CsCl type cubic (B2) structures determined as a function of pressure suggests that the B1 phase will transform to B2 structure at ∼32 (30 GPa at 300 K obtained from comparison of Gibbs free energy at 300 K). The transition is identified to be of first order in nature with a volume discontinuity of ∼6.2% at the transition pressure. Furthermore, the theoretically determined equation of state has been utilized to derive various physical quantities, such as zero pressure equilibrium volume, bulk modulus, and pressure derivative of bulk modulus. The single crystal elastic constants have been predicted at various pressures for both the B1 and B2 structures using the energy strain method. The activation barrier between B1 and B2 phases calculated at transition point is ∼19.7mRy/formula un...

Pressure induced phase transition and thermo-physical properties in LuX (X = N, P)

Detailed total energy calculations have been performed in lutetium pnictides (LuX, where X = N, P) to understand their high pressure structural stability. In LuN, the ambient rocksalt type structure (B1 phase) transforms to a tetragonal structure (B10 phase) at ~240 GPa; whereas in LuP the orthorhombic structure (B33, space group Cmcm) emerges as a high pressure structure above 48 GPa. Both the transitions are found to be of first-order type with volume discontinuities of ~6% and 8.2%, respectively. The high pressure phases B10 and B33 are found to be stable up to 400 GPa, respectively. Further, the structural stability predicted from static lattice calculations has been supported by lattice dynamical stability analysis. The present calculations rule out the B1 to B2 (CsCl type) structural phase transitions predicted to occur at 241 GPa in LuN and at 98 GPa in LuP by previous all-electron calculations (Gupta and Bhat 2013 J. Mol. Model 19 5343–54). The temperature dependence of several thermo-physical properties such as volume, bulk modulus, specific heat and thermal expansion coefficient of the rocksalt structure of these compounds calculated in the present study, using quasi-harmonic approximation, awaits confirmation by experimental studies.