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°C, and NaSmP₂S₆ and KSmP₂S₇ were prepared from Sm/S/P/Na₂S (K₂S_x) 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 M^IM^IIIP₂S₆ 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.