Phase diagram and superconductivity of calcium alanates under pressure

Abstract

In this paper we present a first-principles study of the high-pressure superconducting phase diagram of calcium alanates (Ca–Al–H), based on ab-initio crystal structure prediction and anisotropic Migdal–Eliashberg Theory. Calcium alanates have been intensively studied at ambient pressure for their hydrogen-storage properties, but their high-pressure behavior is largely unknown. By performing a full scan of the ternary convex hull at several pressures between 0 and 300 GPa, we identify several new structural motifs, characterized by a high Al–H coordination, where Al d orbitals participate in the bonding. Among all new phases thus identified, we focus in particular on a phase with CaAlH7 composition, which lies on the convex hull at 300 GPa, and remains dynamically stable down to 50 GPa, with a predicted superconducting T c of 82 K, which likely represents a new promising template to achieve increase chemical precompression in ternary hydrides. Our findings reveal important insights into the structure-property relationships of calcium alanates under high pressure, and highlight a possible strategy to achieve conventional superconductivity at low pressures.