Preparation, Crystal Structure Characterization and Prediction, and Study of the Bandgap of Cu4tise4

In this study, we present a solid state synthesis of tetracopper tetraselenotitanate Cu4TiSe4. Obtained single crystals are characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Tauc method. The existence of single crystals of the title compound is confirmed by X-ray diffraction studies and by EDX investigation which confirmed the elemental composition of Cu4TiSe4. The resolved crystal structure proves the one recently reported for this compound. Additionally, SEM image recorded using backscattered electrons shows the compositional homogeneity of synthesized crystals. Moreover, we used Uspex (evolutionary algorithm) for crystal structure prediction of the title compound to check for the existence of polymorphs. It turns out that Cu4TiSe4 may theoretically occur in three different crystal structures (space groups: I-42m (no. 121), R3m (no. 160), and P-43m (no. 215)), from which the rhombohedral phase has the lowest energy, however, it wasn't observed experimentally so far. The ab initio study of the bandgap of Cu4TiSe4 shows that it is indirect for each polymorphic structure and varies in the range 1.23-1.26 eV, while experimental investigation reveals direct transition of energy 1.35 eV thus showing the potential of this compound for applications in solar cells. Theoretical calculations suggest that the rhombohedral phase of Cu4TiSe4 should exhibit a negative or relatively low (0.64 eV) bandgap.