Relative stability of perovskite and non-perovskite phases of inorganic trihalogeno germanates.

One of the main problems of halide perovskites is the existence of competing non-perovskite phases. Here we focus on (Cs,Rb)GeX₃ compounds which exist either in a rhombohedrally distorted ferroelectric perovskite phase exhibiting cornersharing octahedra or a monoclinic and various orthorhombic phases with edge-sharing octahedra. We show that the GeX₃ tetrahedral building blocks are arranged antiferroelectrically (AFE) in he monoclinic phase and ferroelectrically (FE) in the perovskite phase. The electrostic dipole interaction favors the AFE arrangement but the additional bond formation favors the perovskite. A spin model and first-principles calculations explain why the Rb based compounds prefer the monoclinic and Cs compounds prefer the perovskite form. Similar phases exist for CsSnX₃ .The different structural arrangement is accompanied by a large change in band structure with a much larger gap and flat bands in the monoclinic form, which is unsuitable for photovoltaic applications. This new insight in the stability of these phases suggests the use of an electric field to control the phase transition.