The post-transition metal cations are widely used in rechargeable batteries, solar-cells, and birefringent compounds. In this paper, the electronic structures and birefringence of ternary CsSnX3 (X = Cl, I), CsPbX3 (X = Cl, Br, I), and binary MX2 (M = Sn, Pb; X = Cl, Br) compounds are investigated using the first-principles methods. The results show that these metal halides own different mechanism in stereochemical activity and birefringence, i.e., the stereochemical activity of lone-pair electrons are determined by energy difference between cation s states and halogen p states, and the stereochemical activity decreases from Cl to I. While the cation p states play an important role in determining the birefringence, and the birefringence increased from Cl to I.
