Three new Sr-bearing arsenates, hydrothermally synthesized in the system SrO–MO–As2O5–H2O (M2+ = Mg, Cu, Zn)

During systematic research on the mineral-related synthetic arsenates, with particular focus on their structural and spectroscopic classification, three new Sr-containing arsenates Sr₂Mg(AsO₄)₂· 2H₂O (1), Sr₂Zn₄(AsO₄)₄·H₂O (2) and Sr₂Cu₇(AsO₄)₆ (3) have been synthesized under low-temperature hydrothermal conditions in the system SrO−MO−As₂O₅−H₂O (M²⁺ = Mg, Zn, Cu). Their crystal structures were determined using room-temperature single-crystal X-ray diffraction data and refined to the following R-values: 1– R₁ = 0.025; 2– R₁ = 0.030; 3– R₁ = 0.035. Compounds 1 and 2 are structural analogues of two different arsenate minerals: 1 is the Sr analogue of talmessite, Ca₂Mg(AsO₄)₂· 2H₂O, and 2 is the Sr analogue of prosperite, Ca₂Zn₄(AsO₄)₄· H₂O. Compound 1 is the first synthetic compound incorporating kröhnkite-type chains where a divalent cation occupies the interstitial A position (A₂M(XO₄)₂· 2H₂O, A = Ca²⁺, Na⁺; M = Mg²⁺, Fe²⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺; X = P⁵⁺, As⁵⁺, S⁶⁺). Compound 3 represents the second structurally characterized phase in the ternary SrO−CuO−As₂O₅ system besides SrCuAs₂O₇. The investigation showed that it is isotypic with Pb₂Cu₇(AsO₄)₆ and Ba₂Cu₇(AsO₄)₆. The CuO_x (x = 4, 5, 6) polyhedra in 3 and in Pb and Ba analogues show a wide range of coordination geometries. This article focuses on the similarities and differences in their crystal structures and topology.