First-principles study on the electronic structure and optical properties of Tl₂Cd₂(SO₄)₃ and Rb₂Cd₂(SO₄)₄

With the help of ab initio full-potential linearized augmented plane wave (FPLAPW) method, calculating the electronic structure and linear optical properties is carried out for XCd_2(SO_4)_3 (X=Tl, Rb). The results show that Tl_2Cd_2(SO_4)_3 (TlCdS) has a larger band gap than Rb_2Cd_2(SO_4)_3 (RbCdS) and the energy bands for RbCdS are more dispersive than those of TlCdS. From their partial densities of states (PDOS), we have observed that the hybridization between S ionic 2p and O atomic 2p orbitals forms SO_4 ionic groups. The remarkable difference between RbCdS and TlCdS is, however, the degree of hybridization between cation (Tl and Rb) and its surrounding oxygen atoms. In the view of quantum chemistry, the strong p-d hybridization indicates the existence of their cation ionic bonds (Cd-O, Rb-O, and Tl-O). The calculations of TlCdS and RbCdS show their optical properties to be less anisotropic. Their anisotropies in the optical properties mainly occur in a low photon energy region of 5--16 eV.