Structural, electronic, and optical properties of inhomogeneous Ca1−xMgxO alloys

Abstract

The structural, electronic, and optical properties of homogeneous and inhomogeneous Ca1−xMgxO alloys are studied with the density functional theory (DFT) under the assumption of polymorphism. Large supercells (2×2×2 and above) with different lattice symmetries and varying Mg molar fraction x were constructed, representing distinct solid phases that may coexist in micro/nanodomains of inhomogeneous alloys. We demonstrate that these polymorphs exhibit rich phenomenology like similar formation enthalpies for a given concentration x, but different electronic and optical properties. For example, Ca0.5Mg0.5O may have crystallites with four possible lattice symmetries using a 2×2×2 supercell for its description, with bandgaps varying between 3.26 eV (direct) and 4.46 eV (indirect). The DFT-simulated X-ray diffraction shows that polymorphism causes broadening and shift of the diffraction peaks. We also performed a detailed calculation of the bandgaps, optical absorption, and dielectric constants as a function of x for each polymorph, and they exhibit a structured bandgap behavior with maxima and minima in the 0.0<x<1.0 range, which is in marked contrast to Vegard’s law rule of mixtures. We also report a direct-to-indirect bandgap transition occurring between 0.072≤x≤0.1094 for the FM3¯M symmetry.