Quaternary Chalcogenide Semiconductors with 2D Structures: Rb₂ZnBi₂Se₅ and Cs₆Cd₂Bi₈Te₁₇

Two new layered compounds Rb₂ZnBi₂Se₅ and Cs₆Cd₂Bi₈Te₁₇ are described. Rb₂ZnBi₂Se₅ 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. Cs₆Cd₂Bi₈Te₁₇ 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 BiQ₆ octahedra (Q = Se or Te) and MQ₄ (M = Zn or Cd) tetrahedra. The space between the layers hosts alkali metal as counter cations. The rubidium atoms of Rb₂ZnBi₂Se₅ structure can be exchanged with other cations (Cd²⁺, Pb²⁺ and Zn²⁺) in aqueous solutions forming new phases. Rb₂ZnBi₂Se₅ is an n-type semiconductor and exhibits an indirect band gap energy of 1.0 eV. Rb₂ZnBi₂Se₅ is a congruently melting compound (mp ∼644 °C). The thermal conductivity of this semiconductor is very low with 0.38 W·m^-1·K^-1 at 873 K. Density functional theory (DFT) calculations suggest that the low lattice thermal conductivity of Rb₂ZnBi₂Se₅ 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 Rb₂ZnBi₂Se₅ were characterized with the highest ZT value of ∼0.25 at 839 K.