Optical band gap and disordered structure in Li8GeN4

The band gap and disordered structure in Li8GeN4 (lattice constant: 9.622 Å) are studied by optical absorption, photoacoustic spectroscopy, X-ray diffraction, and Raman scattering spectroscopy. Li8GeN4 is a semiconductor with the band gap of 2.61 eV, suggesting the transition from the N-2p valence band to the conduction band mainly consisting of Ge-4s and/or -4p orbitals. A broad Raman peak is observed at 530 cm−1, indicating the homogenously random distribution of Li and Ge atoms. A possible distribution of Li and Ge is proposed. Li8GeN4 crystallizes in a superstructure of eight face-centered N sublattices. In each sublattice, one Li and one Ge atoms are randomly and diagonally occupied at two tetrahedral sites next to N, while six Li atoms reside at the other six tetrahedral sites next to N. The possible locations of one remaining Li atom are also discussed.