Electronic structure and optical properties of rhombohedral CsGeI₃ crystal

We analyse the electronic structure of the perovskite crystal CsGeI₃ by means of first-principles calculations and compare our findings to experimental results. Our calculation indicates that CsGeI₃ has a direct-transition gap of 0.74 eV at = (π/{a})(111). The top of the valence bands was found to mainly comprise the 5p orbitals of iodine, while the bottom of the conduction bands is dominated by the 4p orbital of germanium. Photoluminescence (PL) measurements on a single crystal of CsGeI₃ indicate two peaks, one at 0.82 µm (1.51 eV) and the other at 1.15 µm (1.08 eV). The shorter-wavelength PL peak is assigned as arising from an interband transition at = (π/{a})(111) and the longer-wavelength PL is presumably ascribable as originating from a transition involving an energy level within the fundamental band gap. Fourier-transformed infrared spectroscopy reveals that the transparent range of CsGeI₃ could extend from ~2 µm to >12 µm. The short-wavelength cut-off is mainly limited by the energy band gap, while the long-wavelength limit possibly originates from lattice phonon absorption. Raman spectra of the crystal exhibit two major peaks at 105 cm^-1 and 151 cm^-1 and the corresponding overtones at 220 cm^-1 and 293 cm^-1.