Temperature-Dependent Structural and Optical Properties of CsGeI ₃

Three-dimensional all-inorganic perovskites with low optical bandgaps, with the possibility of hosting spontaneous polarization, as realized in CsGeX3 (X = Cl–, Br–, and I–), hold significant promise in the field of ferroelectric photovoltaics, with the possibility of charting a different path from their more celebrated Pb-based analogues. Depending on the octahedral tilting within the [MI6]4– framework, CsPbI3 exhibits temperature-driven phase transitions involving cubic, tetragonal, and orthorhombic phases, which influence its optoelectronic properties, such as optical bandgaps, photoluminescence, and charge carrier dynamics. The temperature-dependent studies of CsGeI3 below room temperature are limited, and the role of the 4s2 lone pair of Ge in stabilizing various polymorphs has not been explored. In this work, we investigate CsGeI3, establishing a distinct structural evolution arising from the interplay of the stereochemical activity of 4s2 lone-pair electrons on the Ge2+ cation and octahedral tilting. Temperature-dependent powder and single-crystal X-ray diffraction measurements reveal a series of polar–polar structural phase transitions arising from subtle changes in the Ge2+ off-centering and lattice distortions. We also present nearly identical thermal evolutions of photoluminescence and absorption spectra of CsGeI3 over a wide temperature range, suggesting the existence of a low exciton binding energy. In addition, temperature-dependent second-harmonic generation measurements confirm the polar nature of all phases of CsGeI3 at room temperature and below.