Ab Initio Discovery of Novel Crystal Structure Stability in Barium and Sodium-Calcium Compounds under Pressure using DFT

TL;DR

This study uses density functional theory and genetic algorithms to discover and analyze high-pressure crystal structures of barium and sodium-calcium compounds, revealing new metastable phases and the potential for novel stable structures.

Contribution

It introduces an ab initio computational approach combining DFT and genetic algorithms to predict unknown high-pressure phases in elemental and binary systems.

Findings

Identified all expected barium phases and new metastable structures.

Predicted a new stable -Sm structure in barium at 30-42 GPa.

Demonstrated the feasibility of the search method in Na-Ca system.

Abstract

Group I/II materials exhibit unexpected structural phase transitions at high pressures, providing potential insight into the origins of elemental superconductivity. We present here a computational study of elemental barium and binary sodium-calcium alloys to identify both known and unknown phases of barium under pressure, as well as stable high-pressure compounds in the immiscible Na-Ca system. To predict stability, we performed density functional theory calculations on randomly generated structures and evolved them using a genetic algorithm. For barium, we observed all of the expected phases and a number of new metastable structures, excluding the incommensurate Ba-IV structure. We also observed a heretofore unreported structure (\alpha-Sm) predicted to be the ground state from 30-42 GPa. In the Na-Ca system, we demonstrate feasibility of our search method, but have been unable to predict any stable compounds. These results have improved the efficacy of the genetic algorithm, and should provide many promising directions for future work.