Ashok K. Verma

Theoretical solid and liquid state shock Hugoniots of Al, Ta, Mo and W

We present the Hugoniots of Al, Ta, Mo and W in their solid as well as liquid phases. The liquid phase calculations are carried out on the basis of the corrected rigid spheres (CRIS) model. The 0 K isotherm of the solid phases, which are the necessary inputs for our computations, have been obtained by full potential first principles electronic structure calculations with generalized gradient approximation (GGA) for the exchange–correlation terms. The melting curve as a function of pressure was obtained according to the recently published model based on dislocation mediated melting, and also compared with that using Lindemann criterion. Though the adiabatic pressure–volume curve is affected little by melting, the pressure–temperature curve shows substantial change.

Hydrogen Bond Symmetrization in Glycinium Oxalate under Pressure.

The study of hydrogen bonds near symmetrization limit at high pressures is of importance to understand proton dynamics in complex bio-geological processes. We report here the evidence of hydrogen bond symmetrization in the simplest amino acid-carboxylic acid complex, glycinium oxalate, at moderate pressures of 8 GPa using in-situ infrared and Raman spectroscopic investigations combined with first-principles simulations. The dynamic proton sharing between semioxalate units results in covalent-like infinite oxalate chains. At pressures above 12 GPa, the glycine units systematically reorient with pressure to form hydrogen-bonded supramolecular assemblies held together by these chains.

Stability of the hcp phase and temperature variation of the axial ratio of iron near Earth-core conditions

We theoretically document the stability of hcp iron for pressure‐temperature conditions of the Earth’s inner core by separately computing the electronic and phonon contributions to the free energy. These pseudopotential-based quasiharmonic calculations reveal that the hcp phase remains stable compared to bcc and that the c/a ratio of lattice parameters exhibits only a modest temperature dependence at inner-core conditions.

Structural phase transitions in vanadium under high pressure

Results of structural phase stability under high pressure using first-principles electronic-structure calculations are presented for vanadium up to few-megabar pressures. A unique structural phase transition sequence is predicted for the first time in V. Our detailed electronic-structure analysis shows that it is an example of a band Jahn-Teller–induced structural phase transition.

On the stability of rhenium up to 1 TPa pressure against transition to thebcc structure

We have carried out electronic structure total energy calculations on rhenium in the hexagonal close packed (hcp) and body centred cubic (bcc) phases, by the full potential linear muffin-tin orbital method, in order to verify the stability of the ambient pressure hep phase against transition to the bcc structure at high pressures. As per our results, no hcp to bcc structural transition can occur up to 1 TPa pressures. Moreover, our Bain path calculations show that face centred cubic and body centred tetragonal structures are also not energetically preferred over hcp in this pressure range. The axial ratio (c/a) of Re changes by less than 0–33% in the pressure range studied.

Equation of state of condensed matter in laser-induced high-pressure regime

We have simulated the shock Hugoniot of copper and uranium based on the results of first principles electronic structure calculations. The room temperature isotherm has been obtained by evaluating the accurate ground state total energies at various compressions, and the thermal and electronic excitation contributions were obtained by adopting isotropic models using the results obtained by the band structure calculations. Our calculations ensure smooth consideration of pressure ionization effects as the relevant core states are treated in the semi-core form at the ambient pressure. The pressure variation of the electronic Gruneisen parameter was estimated for copper using the band structure results, which leads to good agreement of the simulated shock Hugoniot with the measured shock data. The simulation results obtained for U are also compared with the experimental data available in literature and with our own data.

First-principles study of self-diffusion and viscous flow in diopside (CaMgSi2O6) liquid

Abstract We have carried out equilibrium molecular dynamics simulations of CaMgSi2O6 (diopside) liquid as a function of pressure (up to 150 GPa) and temperature (2200 to 6000 K) using density functional theory. Self-diffusion of Mg/Ca atoms decouples most from that of framework (Si/O) atoms at 2200 K and zero pressure, and all diffusivities become increasingly similar as temperature and pressure increase. The predicted temperature variations of all transport coefficients at zero pressure closely follow the Arrhenian law with activation energies of 107 to 161 kJ/mol. However, their pressure variations show significant deviations from the Arrhenius behavior. Along the 3000 K isotherm, the Si and O self-diffusivities show non-monotonic variations up to 20 GPa and then rapidly decrease upon further compression. The melt viscosity also shows a weak anomaly in the low-pressure regime before it starts to increase rapidly with pressure. Our results agree favorably with experimental observations of low-pressure non-uniform variations of Si and O self-diffusivities and viscosity. The predicted complex dynamical behavior requires pressure-volume dependent activation volumes and can be associated with structural changes occurring on compression.

First-principles prediction of pressure-enhanced defect segregation and migration at MgO grain boundaries

Abstract Understanding the ability of grain boundaries to accommodate point defects and enhance diffusion rates in mantle materials represents an important but challenging problem. Extant experimental studies and recent computational efforts are mainly limited to the ambient pressure. Here, we investigate this problem for MgO at the atomistic level by performing first-principles simulations, based on density functional theory, of the {310)}/[001] tilt grain boundary in MgO at pressures up to 100 GPa. Our results show that native defects and impurities (Ca, Al, and proton modeled here) favorably segregate to the boundary, with the segregation considerably increasing with pressure. They also imply that grain boundary diffusion is easier, and more anisotropic and complex than bulk (lattice) diffusion: The calculated migration enthalpies for host ions and impurities at the grain boundary are smaller than the bulk values, more so at higher pressures with their values being as low as ~1.5 eV at 100 GPa compared to the bulk values of ~4 eV. Thus demonstrated high-defect activity of grain boundaries in MgO-a major phase of Earth’s lower mantle is expected to be relevant to our understanding of mantle rheology and geochemical process.

Electronic topological transitions in Nb3X (X = Al, Ga, In, Ge, and Sn) under compression investigated by first principles calculations

First principles electronic structure calculations of A-15 type Nb3X (X = Al, Ga, In, Ge, and Sn) compounds are performed at ambient and high pressures. Mechanical stability is confirmed in all the compounds both at ambient as well as under compression from the calculated elastic constants. We have observed four holes and two electron Fermi surfaces (FS) for all the compounds studied and FS nesting feature is observed at M and along X-Γ in all the compounds. A continuous change in the FS topology is observed under pressure in all the compounds which is also reflected in the calculated elastic constants and density of states under pressure indicating the Electronic topological transitions (ETT). The ETT observed at around 21.5 GPa, 17.5 GPa in Nb3Al and Nb3Ga are in good agreement with the anomalies observed by the experiments.

Conformation and hydrogen-bond-assisted polymerization in glycine lithium sulfate at high pressures.

The conformation of glycine has been a subject of extensive research for the past several years. As glycine exists in zwitterionic form in liquids and solids, the experimental observations of its neutral conformation are very limited. The complexes of glycine are simple prototypes to study the conformational properties of glycine. We have investigated the high-pressure behavior of glycine lithium sulfate (GLS), a semiorganic complex of glycine using X-ray diffraction, Raman spectroscopy, and density functional theory (DFT)-based first principles calculations. Our Raman studies and DFT calculations suggest formation of an intramolecular hydrogen bond at higher pressures. Subsequent to a structural transformation to a new high-pressure phase at ∼9 GPa, the observed spectral changes in the Raman spectra above 14 GPa indicate possible conformational change of glycine from zwitterionic to neutral form. At pressures above 18 GPa, the characteristic features in the Raman spectra and the X-ray diffraction patterns suggest transformation to a hydrogen-bond-assisted polymeric phase with intermediate range order.