Structure determination, electronic and optical properties of NaGe2P3O12 and Cs2GeP4O13

Two alkali–metal germanium phosphates, namely, NaGe2P3O12 and Cs2GeP4O13, have been synthesized by the solid-state reactions and their structures determined by single-crystal X-ray diffraction. The structure of NaGe2P3O12 features a Nasicon-type structure, crystallizing in space group R3¯ with a = 8.109(2), c = 21.536(8) (Å), V = 1226.3(6) Å3, Z = 6, R1 = 0.0383 and ωR = 0.0960; Cs2GeP4O13 crystallizes in triclinic space group P1¯ with a = 9.420(3), b = 10.209(4), c = 12.407(4) (Å), α = 104.136(6), β = 108.132(5), γ = 95.338(6)°, V = 1081.0(7) Å3, Z = 2, R1 = 0.0445 and ωR = 0.1117. Additionally, the calculations of band structure and density of states were performed with the density functional theory method for the title compounds. The results show that the solid-state compounds of NaGe2P3O12 and Cs2GeP4O13 are insulator with indirect band-gap of 3.529 and 3.888 eV, respectively.