First-principles calculations to investigate the polymorph effects of CuAlO2

Polymorphism is a very common phenomenon for optoelectronic functional materials. The in-depth study of polymorph is an important basis for understanding the structure–activity relationship, polymorph effect and multifunctional application of optoelectronic functional materials. In this work, the crystal microstructure, electronic structure and optical properties of 9 polytypes of CuAlO2 are compared and studied in detail by using density functional theory calculation. We found that the structural motif and its connection mode in the polytype determine the stability of crystal phase and the main characteristics of electronic structure. The coordination environment of Cu is the key factor to determine the stability of crystal phase: the delafossite polytype containing two-fold-coordinated Cu is the most stable; followed by chalcopyrite and wurtzite polytype containing four-fold-coordinated Cu; the polytypes containing high-fold-coordinated Cu or more complex connection mode of motif are the least stable. Moreover, the binding energy and stability of these nine crystalline polytypes are mainly determined by the coordination number of Cu atom, the degree of deformity of structural motif and the connection mode of structural motif. When CuAlO2 contains linear two-coordinated Cu and is connected by sharing vertices, the absolute value of binding energy is the largest and most stable. At the same time, the type of Cu containing structural motif and the connection mode between structural motifs are the key factors to determine the fundamental band gap of CuAlO2. These 9 polytypes of CuAlO2 have well-defined indirect bandgap semiconductor characteristics. Finally, according to the main characteristics of electronic structures and optical properties, we also predict the potential applications of these polytypes of CuAlO2 in different optoelectronic fields.