First principles study of electronic, optical and elastic properties of A ₃ B (A = Li, Na, K, Rb, Cs; B = Sb, Bi)

Trialkali metal pnictides exhibit diverse physical properties. This study reports the structural, electronic, optical and elastic properties of tri-alkali metal pnictides A 3 B (A = Li, Na, K, Rb, Cs; B = Sb, Bi) in the halite phase using density functional theory. Crystal optimization via the generalized gradient approximation reveals that as the alkali metal changes from Li to Cs, the lattice parameter increases while the bulk modulus decreases. Band structure analysis indicates that all of these pnictides in the halite phase are semiconductors, with mBJ-potential bandgaps aligning more closely with experimental results compared to those obtained using GGA. Additionally, alkali metal antimonides exhibit larger bandgaps than alkali metal bismuthates. The density of states analysis highlights the significant contributions of the s and p states of Sb and Bi, respectively, as well as the p and d states of the alkali metals. Optical parameters like refractive index, reflectivity and optical conductivity show a notable decrease as the alkali metal changes from Li to Cs. A narrow valence bandwidth leads to single peak-like features in the optical spectra.