Structural phase transitions of LaScO3 from first principles

Abstract

We report a density functional theory study of LaScO3 perovskite using the SCAN meta-GGA potential. LaScO3 is of interest for applications such as hydrogen gas sensors, protonic ceramic fuel cells, and inorganic phosphors because of the useful electrical and optical properties it displays when doped, but little is known about its structural phase stability or phase transition behavior. We thus examine the relative stabilities of various candidate structures of LaScO3 and identify the likely transition sequence as a function of temperature. The low-temperature phase is confirmed to have an orthorhombic crystal structure with Pnma symmetry, with cooperative tilting of ScO6 octahedra about three pseudo-cubic axes, corresponding to Glazer tilt system a−b+a−. With increasing temperature, LaScO3 is predicted to undergo a second-order transition to a pseudo-tetragonal Imma structure (a−b0a−), followed by a first-order transition to a tetragonal I4/mcm structure (a0a0c−), and finally another second-order transition to the aristotype (cubic Pm3¯m; a0a0a0) structure below the melting point. The large difference in lattice enthalpy between the orthorhombic and cubic structures is consistent with the experimental difficulty in stabilizing the cubic structure at room temperature, although the results suggest other polymorphs may be accessible through synthesis at non-standard pressures or application of strain.