Hybrid Functional Study Rationalizes the Simple Cubic Phase of Calcium at High Pressures

Simple cubic (SC) phase has been long experimentally determined as the high-pressure phase III of elemental calcium (Ca) since 1984. However, recent density functional calculations within semi-local approximation showed that this SC phase is structurally unstable by exhibiting severely imaginary phonons, and is energetically unstable with respect to a theoretical body-centered tetragonal I41/amd structure over the pressure range of phase III. These calculations generated extensive debates on the validity of SC phase. Here we have re-examined the SC structure by performing more precise density functional calculations within hybrid functionals of Heyd-Scuseria-Erhzerhof (HSE) and PBE0. Our calculations were able to rationalize fundamentally the phase stability of SC structure over all other known phases by evidence of its actual energetic stability above 33 GPa and its intrinsically dynamical stability without showing any imaginary phonons in the entire pressure range studied. We further established that the long-thought theoretical I41/amd structure remains stable in a narrow pressure range before entering SC phase and is actually the structure of experimental Ca-III' synthesized recently at low temperature 14 K as supported by the excellent agreement between our simulated X-ray diffraction patterns and the experimental data. Our results shed strong light on the crucial role played by the precise electron exchange energy in a proper description of the potential energy of Ca.Comment: submitted to Physical Review

Tuning Optical Properties of Transparent Conducting Barium Stannate by Dimensional Reduction

We report calculations of the electronic structure and optical properties of doped $n$-type perovskite BaSnO3 and layered perovskites. While doped BaSnO$_3$ retains its transparency for energies below the valence to conduction band onset, the doped layered compounds exhibit below band edge optical conductivity due to transitions from the lowest conduction band. This gives absorption in the visible for Ba2SnO4. Thus it is important to minimize this phase in transparent conducting oxide (TCO) films. Ba3Sn2O7 and Ba4Sn3O10 have strong transitions only in the red and infrared, respectively. Thus there may be opportunities for using these as wavelength filtering TCO

Anomalous melting behavior of solid hydrogen at high pressures

Hydrogen is the most abundant element in the universe, and its properties under conditions of high temperature and pressure are crucial to understand the interior of of large gaseous planets and other astrophysical bodies. At ultra high pressures solid hydrogen has been predicted to transform into a quantum fluid, because of its high zero point motion. Here we report first principles two phase coexistence and Z method determinations of the melting line of solid hydrogen in a pressure range spanning from 30 to 600 GPa. Our results suggest that the melting line of solid hydrogen, as derived from classical molecular dynamics simulations, reaches a minimum of 367 K at about 430 GPa, at higher pressures the melting line of the atomics Cs IV phase regain a positive slope. In view of the possible importance of quantum effects in hydrogen at such low temperatures, we also determined the melting temperature of the atomic CsIV phase at pressures of 400, 500, 600 GPa, employing Feynman path integral simulations. These result in a downward shift of the classical melting line by about 100 K, and hint at a possible secondary maximum in the melting line in the region between 500 and 600 GPa, testifying to the importance of quantum effects in this system. Combined, our results imply that the stability field of the zero temperature quantum liquid phase, if it exists at all, would only occur at higher pressures than previously thought.Comment: Submitted to JC

Smooth Flow in Diamond: Atomistic Ductility and Electronic Conductivity

Diamond is the quintessential superhard material widely known for its stiff and brittle nature and large electronic band gap. In stark contrast to these established benchmarks, our first-principles studies unveil surprising intrinsic structural ductility and electronic conductivity in diamond under coexisting large shear and compressive strains. These complex loading conditions impede brittle fracture modes and promote atomistic ductility, triggering rare smooth plastic flow in the normally rigid diamond crystal. This extraordinary structural change induces a concomitant band gap closure, enabling smooth charge flow in deformation created conducting channels. These startling soft-and-conducting modes reveal unprecedented fundamental characteristics of diamond, with profound implications for elucidating and predicting diamond’s anomalous behaviors at extreme conditions