Origin of ferroelectricity in germanium-based inorganic halide perovskites

Compared to 𝐴⁢𝐵⁢O3 oxide perovskites, intrinsic ferroelectricity is less studied in 𝐴⁢𝐵⁢𝑋3 (𝑋=Cl, Br, I) halide perovskites. Nonetheless, it presents unique opportunities, such as achieving ferroelectric photovoltaics with band gaps suitable for the solar spectrum. In this work, we focus on the CsGe⁢𝑋3 halide perovskites as prototypes for investigating the origin of their ferroelectricity and its relationship with other properties. We find that the presence of stereochemically lone pairs on the perovskite 𝐵-site is the primary driving force for ferroelectricity, causing off-center displacements of the Ge ions and enhanced covalency with the halide ligands. This contrasts with the behavior of typical ferroelectric oxide perovskites. Our calculations confirm that halide perovskites are intrinsic modest band gap semiconductors. In particular, CsGeI3 exhibits a very favorable band gap of 1.6 eV for photovoltaic applications. Our findings provide valuable insights into the mechanism underlying high-temperature ferroelectricity in halide perovskites with potential optoelectronic applications.