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 CsGe X 3 ( 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 [ M I 6 ] 4– framework, CsPbI 3 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 CsGeI 3 below room temperature are limited, and the role of the 4s 2 lone pair of Ge in stabilizing various polymorphs has not been explored. In this work, we investigate CsGeI 3, establishing a distinct structural evolution arising from the interplay of the stereochemical activity of 4s 2 lone-pair electrons on the Ge 2+ 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 Ge 2+ off-centering and lattice distortions. We also present nearly identical thermal evolutions of photoluminescence and absorption spectra of CsGeI 3 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 CsGeI 3 at room temperature and below.