SYNTHESIS IN MOLTEN ALKALI-METAL POLYSELENOPHOSPHATE FLUXES - A NEW FAMILY OF TRANSITION-METAL SELENOPHOSPHATE COMPOUNDS, A(2)MP(2)SE(6) (A=K, RB, CS, M=MN, FE) AND A(2)M'2P2SE6 (A=K, CS M'=CU, AG)

Five isostructural quaternary alkali metal transition metal selenophosphate compounds, K2MnP2Se6 (I), Rb2MnP2-Se-6 (II), Cs2MnP2Se6 (III), K2FeP2Se6 (IV), and Cs2FeP2Se6 (V), were synthesized by the molten alkali polyselenophosphate flux technique. In addition, the compounds CS2CU2P2Se6 (VI), K2Ag2P2Se(6) (VII), and Cs-2-Ag2P2Se6 (VIII) were synthesized similarly but possess different structures. Crystals were grown with M/P-4-Se-10/A(2)Se/Se flux reactions at 450 degrees C while pure material was obtained by direct combination of Mn/P/A(2)Se/Se at 500 degrees C. The structures of I, III, IV, VI, Vn, and VIII have been determined by single-crystal X-ray diffraction analysis. Orange crystals of I crystallize in the monoclinic space group P2(1)/n (No. 14) with a = 6.5349(9) Angstrom, b = 12.696(3) Angstrom, c = 7.589(2) Angstrom, beta = 102.67(2)degrees, V = 614.3(4) Angstrom(3), and Z = 2. Compound III also crystallizes in the space group P2(1)/n (No. 14) with a = 6.4761(9) Angstrom, b = 13.006(2) Angstrom, c = 7.974(1) Angstrom, beta = 93.09(1)degrees, V = 670.6(2) Angstrom(3), and Z = 2. Red crystals of IV are X-ray isomorphous to I with a = 6.421(2) Angstrom, b = 12.720(5) Angstrom, c = 7.535(3) Angstrom, beta = 102.58(3)degrees, V = 600.7(7) Angstrom(3), and Z = 2. Dark green crystals of VI are also monoclinic, space group P2(1)/c (No. 14), with a = 9.958(3) Angstrom, b = 13.067(3) Angstrom, c = 10.730(2) Angstrom, beta = 102.46(2)degrees, V = 1363(1) Angstrom(3), and Z = 4. Orange crystals of VII are monoclinic, space group P2(1)/c (No. 14) with a = 8.528 Angstrom, b = 11.251(6) Angstrom, c = 20.975(4) Angstrom, beta = 93.24(3)degrees, V = 2009(3) Angstrom(3), and Z = 4. Yellow rod-like crystals of VIII crystallize in the monoclinic space group P2(1)/n (No. 14) with a = 6.807(3) Angstrom, b = 12.517(3) Angstrom, c = 8.462(3) Angstrom, beta = 95.75(3)degrees, V = 717.3(8) Angstrom(3), and Z = 2. The structure of the [MP(2)Se(6)](2n-)(n) (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 a-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 M(2+) 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 degrees C, II at 781 degrees C, III at 831 degrees C, IV at 662 degrees C, V at 769 degrees C, VI at 670 degrees C, VII at 542 degrees C, and VIII at 594 degrees C.