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Preparation and single crystal structure refinement of mixed orthophosphates M 3 Cr 4 (PO 4 ) 6 (M = Mg, Zn, Cu) – copper(II) in compressed octahedral coordination 1

1997; De Gruyter; Volume: 212; Issue: 7 Linguagem: Inglês

10.1524/zkri.1997.212.7.510

2196-7105

Michael Gruß, Robert Glaum,

Crystal Structures and Properties

Abstract Crystals of M 3 Cr 4 (PO 4 ) 6 (M = Mg, Zn, Cu) suitable for X-ray single crystal investigations were grown by chemical vapour transport experiments using chlorine as transport agent. The mixed metal orthophosphates belong to the Fe 3 Fe 4 (PO 4 ) 6 -structure type ( P [unk]; Z = 1; Mg 3 Cr 4 (PO 4 ) 6 : a = 9.212(3) Å, b = 7.840(2) Å, c = 6.265(2) Å, α = 101.63(2)°, β = 108.20(2)°, γ = 105.15(2)°, wR 2 = 0.068 (weighted residual based on F 2 ); 3459 unique reflections, 173 free parameters; Zn 3 Cr 4 (PO 4 ) 6 : a = 9.236(2) Å, b = 7.830(2) Å, c = 6.274(2) Å, α = 101.52(1)°, β = 108.12(1)°, γ = 105.33(1)°, wR 2 = 0.081; 3476, 173; Cu 3 Cr 4 (PO 4 ) 6 : a = 9.2718(6) Å, b = 7.8360(5) Å, c = 6.2297(4) Å, α = 100.950(6)°, β = 107.238(6)°, γ = 106.321(5)°, wR 2 = 0.053, 3493, 173). The structure contains Cr 3+ on two different sites with slightly distorted octahedral coordination (1.87 Å ≤ d Cr–O ≤ 2.09 Å [unk] Cr–O ≈ 1.98 Å). For the divalent cations Mg, Zn and Cu 6-fold (compressed-octahedral; [M 2+ O 6 ]) and 5-fold (distorted trigonal-bipyramidal; [M 2+ O 5 ]) coordination is observed. Infinite chains built by edge-sharing of [Cr 3+ O 6 ] and [M 2+ O 5 ] extent along [[unk]01]. These chains are interconnected by [PO 4 ] and [M 2+ O 6 ] via common vertices. Comparison of several isotypic compounds shows the compression of the [M 2+ O 6 ] to be enhanced by ions such as Cu 2+ and Cr 2+ which are subject to Jahn-Teller distortion. Furthermore, a significant amount of statistical disorder of the divalent and trivalent ions among the various metal sites is observed.