Structure modification on quaternary rare earth thiophosphates: NaYbP2S6, NaSmP2S6, and KSmP2S7

Abstract

Three rare earth quaternary compounds, NaYbP2S6, NaSmP2S6, and KSmP2S7, are synthesized using alkali metal halide or polychalcogenides flux. NaYbP2S6 was prepared from the stoichiometric reaction of Yb/S/P/NaCl flux at 700°C, and NaSmP2S6 and KSmP2S7 were prepared from Sm/S/P/Na2S (K2Sx) flux at 750°C. The structures were determined by the single-crystal X-ray diffraction technique. The air-sensitive dark-red crystals of NaYbP2S6 crystallize in the triclinic space group P-1 with a=6.8241(10) Å, b=7.0371(10) Å, c=9.107(2) Å, α=87.17(2)°, β=87.62(2)°, γ=88.30(2)°, and Z=2. The yellow plate crystals of NaSmP2S6 crystallize in the monoclinic space group P21/a with a=11.0717(10) Å, b=7.2999(1) Å, c=11.579(3) Å, β=111.30(2)°, and Z=4. The pale yellow plate crystals of KSmP2S7 crystallize in the monoclinic space group P21/a with a=8.981(2) Å, b=12.254(2) Å, c=9.609(1) Å, β=90.27(2)°, and Z=4. NaYbP2S6 and NaSmP2S6 are derivatives of the MIMIIIP2S6 family. NaYbP2S6 has a flat mono layered structure, and there are weak interactions between neighboring layers. In NaSmP2S6, two MPS3-type layers are condensed together. The ordering features of sodium and rare earth metal cations in two-dimensional layers are different: Na and Yb atoms are arranged in pairing fashion in NaYbP2S6, while Na and Sm atoms are arranged in a triangular pattern in NaSmP2S6. The structure of KSmP2S7 is similar to that of ABiP2S7 (A=K, Rb), where corrugated layers are formed from Sm3+ and [P2S7]4- units. The optically measured band gaps of NaYbP2S6 and NaSmP2S6 are 1.85 eV and 2.54 eV, respectively. © 2001 Academic Press.