PREPARATION AND SINGLE-CRYSTAL STRUCTURE REFINEMENT OF MIXED ORTHOPHOSPHATES M3CR4(PO4)(6) (M=MG, ZN, CU) - COPPER(II) IN COMPRESSED OCTAHEDRAL COORDINATION

Crystals of M3Cr4(PO4)(6) (M = Mg, Zn, Cu) suitable for X-ray single c rystal investigations were grown by chemical vapour transport experime nts using chlorine as transport agent. The mixed metal orthophosphates belong to the Fe3Fe4(PO4)(6)-structure type (P (1) over bar; Z = 1; M g3Cr4(PO4)(6): a = 9.212(3) Angstrom, b = 7.840(2) Angstrom, c = 6.265 (2) Angstrom, alpha = 101.63(2)degrees, beta = 108.20(2)degrees, gamma 105.15(2)degrees, wR2 = 0.068 (weighted residual based on F-2); 3459 unique reflections, 173 free parameters; Zn3Cr4(PO4)(6): a = 9.236(2) Angstrom, b = 7.830(2) Angstrom, c = 6.274(2) Angstrom, alpha = 101.52 (1)degrees, beta = 108.12(1)degrees, gamma = 105.33(1)degrees, wR2 = 0 .081; 3476, 173; Cu3Cr4(PO4)(6): a = 9.2718(6) Angstrom, b = 7.8360(5) Angstrom, c = 6.2297(4) Angstrom, alpha = 100.950(6)degrees, beta = 1 07.238(6)degrees, gamma = 106.321(5)degrees, wR2 = 0.053, 3493, 173). The structure contains Cr3+ On two different sites with slightly disto rted octahedral coordination (1.87 Angstrom less than or equal to d(Cr -o) less than or equal to 2.09 Angstrom; (d) over bar(Cr-o) approximat e to 1.98 Angstrom). For the divalent cations Mg, Zn and Cu 6-fold (co mpressed-octahedral; [M2+O6]) and 5-fold (distorted trigonal-bipyramid al; [M2+O5) coordination is observed. Infinite chains built by edge-sh aring of [Cr3+O6] and [M2+O5] extent along [(1) over bar 01]. These ch ains are interconnected by [PO4] and [M2+O6] via common vertices. Comp arison of several isotypic compounds shows the compression of the [M2O6] to be enhanced by ions such as Cu2+ and Cr2+ which are subject to Jahn-Teller distortion. Furthermore, a significant amount of statistic al disorder of the divalent and trivalent ions among the various metal sites is observed.