Alloying behavior of wide band gap alkaline-earth chalcogenides

Abstract

Alloying is a powerful tool for tuning materials that facilitates the targeted design of desirable properties for a variety of applications. In this work, we provide a comprehensive investigation of the synthetic accessibility and electronic properties of nine alkaline-earth chalcogenide anion alloys (CaS$_{1-x}$O$_x$, CaS$_{1-x}$Se$_x$, CaS$_{1-x}$Te$_x$, SrS$_{1-x}$O$_x$, SrS$_{1-x}$Se$_x$, SrS$_{1-x}$Te$_x$, MgS$_{1- x}$O$_x$, MgS$_{1-x}$Se$_x$, and MgS$_{1-x}$Te$_x$). We show that isostructural alloying within the rock salt structure is favored for all systems except MgS$_{1-x}$Te$_x$, which is predicted to be a heterostructural alloy between the rock salt and wurtzite structures. Alloys of S and Se are shown to be readily accessible for all cations with low miscibility critical temperatures, enabling continuous tuning of electronic properties across this composition space. Alloys of S and Te have higher critical temperatures but may be accessible through non-equilibrium synthesis strategies and are predicted here to have desirable electronic properties for optoelectronics with wide band gaps and lower effective masses than alloys of S and Se. Anion alloying in MgS$_{1-x}$Te$_x$ stabilizes the wurtzite structure across a significant fraction of composition space, which may make it of particular interest as a transparent conducting material due to its lower effective masses and a higher band gap than the rock salt structure. Zero-point corrected random phase approximation (RPA) energies were computed to resolve the small polymorph energy differences of the Mg compounds and are shown to be critical for accurately describing the thermodynamic properties of the corresponding alloys.