Syntheses and Crystal Structures of RE2As4O9 (RE = Nd, Sm): Oxo-Arsenates(III) according to RE4(As2O5)2(As4O8) Exhibiting the Cyclic As4O8 4– Anion

Abstract

Single crystals of light violet Nd2As4O9 and yellow Sm2As4O9 were obtained from the reactions of the respective lanthanide oxides with As2O3 in a NaCl flux at 850 °C in sealed silica ampoules. According to the structure determination, the triclinic compounds (P1̅, Z = 2, Nd/Sm: a = 686.32(9)/680.92(9), b = 763.06(8)/756.4(1), c = 954.8(1)/951.2(1) pm, α = 96.83(1)°/96.66(2)°, β = 103.78(2)°/103.67(2)°, γ = 104.40(1)°/104.35(2)°, R1 = 0.0184/0.0282 (all data)) have to be formulated as RE4(As2O5)2(As4O8) emphasizing the presence of both As2O5 4− and cyclic As4O8 4− anions. The As4O8 4− anions show Ci symmetry and connect layers of the composition {2 ∞[RE2(As2O5)]2}4+ along the [001] direction. The lanthanide ions are in nine- and eightfold coordination, respectively.