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

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 degreesC, and NaSmP2S6 and KSmP2S7 were prepared from Sm/S/P/Na2S (K2Sx) flux at 750 degreesC. 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) Angstrom, b 7.0371 (10) Angstrom, c = 9.107(2) Angstrom, alpha = 87.17(2)degrees, beta = 87.62(2)degrees, gamma = 88.30(2)degrees, and Z = 2. The yellow plate crystals of NaSmP2S6 crystallize in the monoclinic space group P2(1)/a with a = 11.0717(10) Angstrom, b = 7.2999(1) Angstrom, c = 11.579(3) Angstrom, beta = 111.30(2)degrees, and Z = 4. The pale yellow plate crystals of KSmP2S7 crystallize in the monoclinic space group P2(1)/a with a = 8.981(2) Angstrom, b = 12.254(2) Angstrom, c = 9.609(1) Angstrom, beta = 90.27(2)degrees, and Z = 4. NaYbP2S6 and NaSmP2S6 are derivatives of the (MMP2S6)-M-I-P-III family. NaYbP2S6 has a flat monolayered 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 ABiP(2)S(7) (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. (C) 2001 Academic Press.