Syntheses, crystal structures, and band gaps of Cs₂Cd₃Te₄ and Rb₂Cd₃Te₄

Single crystals of Cs₂Cd₃Te₄ grow as yellow plates from the reaction of Cs₂Te₃ with Cd and Te at 873 K; orange plates of Rb₂Cd₃Te₄ grow from the reaction of Rb₂Te₃ with Cd and Te at 773 K. The structures of both compounds have been determined from single crystal X-ray diffraction data. Cs₂Cd₃Te₄ crystallizes in the orthorhombic space group Ibam with unit cell parameters a = 6.667(1) angstrom, b = 13.010(3) angstrom, c = 15.390(3) angstrom, Z = 4, T = 113 K. Rb₂Cd₃Te₄ crystallizes in the monoclinic space group C2/c with unit cell parameters a = 14.424(2) angstrom, b = 15.350(2) angstrom, c = 12.864(2) angstrom, β = 116.249(2), Z = 8, T = 153 K. Both structures comprise layers of edge-shared CdTe₄ tetrahedra separated by alkali metal cations; however, the connectivity of the CdTe₄ tetrahedra and the stacking of the layers are different in each compound. Bandgaps of 2.48 eV for Cs₂Cd₃Te₄ and 2.30 eV for Rb₂Cd₃Te₄ were determined from UV/Vis diffuse reflectance measurements made at 20°C.