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.