Structural Phase Transitions in PtIn2 at High Pressure: A Theoretical Investigation

Novel ground state structures of N-doped LuH3

Ab-initiocrystal structure searches have played a pivotal role in recent discoveries of high-Tc hydride superconductors under high pressure. Using evolutionary crystal searches, we predict novel ground state structures of N-doped LuH3at ambient conditions. We find an insulating ground state structure for LuN0.125H2.875(∼1.0 wt.% N), contrary to earlier studies where assumed structures were all metallic. This insulating behavior of ground state was found to persist up to ∼45 GPa. However our crystal structure searches revealed a metallic state for an H-deficient variant of LuN0.125H2.875. We study bonding characteristics of important structures by calculating electronic density of states, electronic-localization functions and Bader charges. Our Bader charge analysis shows that insulators have both H+and H-ions whereas metals have only H-ions. We find that H+ions are bonded to N atomsviaa very short covalent bond. Thus we identify a clear relationship between formation of N-H bonds and insulating behavior of materials. Besides this, we perform crystal structure searches for three more compositions with higher N-content (>1.0 wt.%). Analysis of electronic properties shows that the ground states of these compositions are insulator.

First principles investigations on the controversy of structural phase transitions in thorium dialuminide under pressure

We study the high pressure structural aspects of thorium dialuminide, ThAl2, by performing evolutionary crystal structure searches and first principles calculations. We predict a phase transition from the ambient AlB2-type hexagonal structure to a P63/mmc hexagonal structure above 2.4 GPa. We also note that the new structure remains stable up to 50 GPa. This differs from the finding of an earlier study in which three phase transitions were reported close to 5.5, 12 and 25 GPa. An analysis of the electronic structures, Bader charge and electron localization function (ELF) indicates a complex chemical bonding comprising partial metallic, ionic (Th-Al) and covalent (Al-Al) characters in the ambient phase. Comparison of crystal parameters and ELFs for ambient and high pressure phases indicates that a change of Al-Al bonding under pressure is responsible for the structural phase transition. Furthermore, we computed the high P-T phase diagram by including the temperature effects via free energies for the most relevant structures, namely, AlB2-type, MgCu2-type and new hexagonal. We found that a MgCu2-type structure becomes stable only under high P-T conditions. We also provide a plausible explanation for the observed isostructural transition in the ambient AlB2-type structure at 5.5 GPa.

Pressure induced multi-centre bonding and metal–insulator transition in PtAl2

An electron localization function showing interstitial charge localization and multi-centre bonding in an insulating Cmcm phase is studied.

A unique metallic phase of H3S at high-pressure: sulfur in three different local environments

Structural behaviour of compressed H₃S.