Synthesis in Molten Alkali Metal Polyselenophosphate Fluxes: A New Family of Transition Metal Selenophosphate Compounds, A₂MP₂Se₆ (A = K, Rb, Cs; M = Mn, Fe) and A₂M′₂P₂Se₆ (A = K, Cs; M′ = Cu, Ag)

Five isostructural quaternary alkali metal transition metal selenophosphate compounds, K₂MnP₂Se₆ (I), Rb₂MnP₂-Se₆ (II), Cs₂MnP₂Se₆ (III), K₂FeP₂Se₆ (IV), and Cs₂FeP₂Se₆ (V), were synthesized by the molten alkali polyselenophosphate flux technique. In addition, the compounds Cs₂Cu₂P₂Se₆ (VI), K₂Ag₂P₂Se₆ (VII), and Cs₂-Ag₂P₂Se₆ (VIII) were synthesized similarly but possess different structures. Crystals were grown with M/P₄-Se₁₀/A₂Se/Se flux reactions at 450 °C while pure material was obtained by direct combination of Mn/P/A₂Se/Se at 500 °C. The structures of I, UI, IV, VI, VII, and VIII have been determined by single-crystal X-ray diffraction analysis. Orange crystals of I crystallize in the monoclinic space group P2₁/n (No. 14) with a = 6.5349(9) Å, b = 12.696(3) Å, c = 7.589(2) Å, ß = 102.67(2)°, V = 614.3(4) ų, and Z = 2. Compound III also crystallizes in the space group P2₁/n (No. 14) with a = 6.4761(9) Å, b = 13.006(2) Å, c = 7.974(1) Å, β = 93.09(1)°, V = 670.6(2) ų, and Z = 2. Red crystals of IV are X-ray isomorphous to I with a = 6.421(2) Å, b = 12.720(5) Å, c = 7.535(3) Å, ß = 102.58(3)°, V = 600.7(7) ų, and Z = 2. Dark green crystals of VI are also monoclinic, space group P2₁/c (No. 14), with a = 9.958(3) Å, b = 13.067(3) Å, c = 10.730(2) Å, ß = 102.46(2)°, V = 1363(1) ų, and Z = 4. Orange crystals of VII are monoclinic, space group P2₁/c (No. 14) with a = 8.528 Å, b = 11.251(6) Å, c = 20.975(4) Å,ß = 93.24(3)°, V= 2009(3) ų, and Z = 4. Yellow rod-like crystals of VIII crystallize in the monoclinic space group P2₁/n (No. 14) with a = 6.807(3) Å, b = 12.517(3) Å, c = 8.462(3) Å, ß = 95.75(3)°, V = 717.3(8) ų, and Z = 2. The structure of the [MP₂Se₆]_n²ⁿ⁻ (M = Mn, Fe) anion (I-V) is closely related to the TiI₃ structure type. The transition metal ion and the P-P pairs reside in octahedra that share faces in the α-direction. These chains are well separated by alkali metal ions. The structures of VI and VIII are closely related to those of I-V. Cu⋯Cu and Ag⋯Ag dimers are found in the octahedral metal sites to form the chain structure. The structure of VII is unrelated to the chain structure but forms a three-dimensional tunnel framework consisting of tetrahedral AgSe₄ and [P₂Se₆]⁴⁻ units. Magnetic susceptibility measurements indicate that I-IV couple antiferromagnetically between 10 and 20 K and the d electrons of M²⁺ are found in a high-spin state. The band gaps were determined by optical spectroscopy to be 2.33, 2.41, 2.19, 1.72, and 2.02 eV respectively for I-V. The band-gaps of VI-VIII can be assessed at 2.44, 2.39, and 2.55 eV respectively. All compounds melt congruently with I melting at 717 °C, II at 781 °C, III at 831 °C, IV at 662 °C, V at 769 °C, VI at 670 °C, VII at 542 °C, and VIII at 594 °C.