Quaternary Chalcogenide Semiconductors with 2D Structures: RbZnBiSe and CsCdBiTe.

Two new layered compounds RbZnBiSe and CsCdBiTe are described. RbZnBiSe crystallizes in the orthorhombic space group Pnma, with lattice parameters of a = 15.6509(17) Å, b = 4.218(8) Å, and c = 18.653(3) Å. CsCdBiTe crystallizes in the monoclinic C2/ m space group, with a = 28.646(6) Å, b = 4.4634(9) Å, c = 21.164(4) Å, and β = 107.65(3)°. The two structures are different and composed of anionic layers which are formed by inter connecting of BiQ octahedra (Q = Se or Te) and MQ (M = Zn or Cd) tetrahedra. The space between the layers hosts alkali metal as counter cations. The rubidium atoms of RbZnBiSe structure can be exchanged with other cations (Cd, Pb and Zn) in aqueous solutions forming new phases. RbZnBiSe is an n-type semiconductor and exhibits an indirect band gap energy of 1.0 eV. RbZnBiSe is a congruently melting compound (mp ∼644 °C). The thermal conductivity of this semiconductor is very low with 0.38 W·m·K at 873 K. Density functional theory (DFT) calculations suggest that the low lattice thermal conductivity of RbZnBiSe is attributed to heavy Bi atom induced slow phonon velocities and large Gruneisen parameters especially in the a and c directions. The thermoelectric properties of RbZnBiSe were characterized with the highest ZT value of ∼0.25 at 839 K.