Preparation and single crystal structure refinement of mixed orthophosphates M3Cr4(PO4)6 (M = Mg, Zn, Cu) – copper(II) in compressed octahedral coordination1
Abstract
Crystals of M3Cr4(PO4)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 Fe3Fe4(PO4)6-structure type (P[unk]; Z = 1; Mg3Cr4(PO4)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)°, wR2 = 0.068 (weighted residual based on F2); 3459 unique reflections, 173 free parameters; Zn3Cr4(PO4)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)°, wR2 = 0.081; 3476, 173; Cu3Cr4(PO4)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)°, wR2 = 0.053, 3493, 173).
The structure contains Cr3+ on two different sites with slightly distorted octahedral coordination (1.87 Å ≤ dCr–O ≤ 2.09 Å [unk]Cr–O ≈ 1.98 Å). For the divalent cations Mg, Zn and Cu 6-fold (compressed-octahedral; [M2+O6]) and 5-fold (distorted trigonal-bipyramidal; [M2+O5]) coordination is observed. Infinite chains built by edge-sharing of [Cr3+O6] and [M2+O5] extent along [[unk]01]. These chains are interconnected by [PO4] and [M2+O6] via common vertices.
Comparison of several isotypic compounds shows the compression of the [M2+O6] to be enhanced by ions such as Cu2+ and Cr2+ 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.
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- Finite periodicity and crystal structures
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