Order vs. disorder in puninite morphotropic series: synthesis and crystal structures of Cs4Cu7+xO3+x(SO4)6 with x = 0 and x = 0.2

Abstract

Two new cesium copper oxosulfates, Cs₄Cu₇O₃(SO₄)₆ (1) and Cs₄Cu₇O₃(SO₄)₆[Cu_0.2O_0.2] (2) which can be considered as representatives of the Cs₄Cu_7+xO_3+x(SO₄)₆ series with x = 0 and x = 0.2, were prepared in evacuated silica tubes. Both compounds are triclinic, \(P\overline{1 }\) and contain complex polynuclear ensembles of edge-sharing OCu₄ tetrahedra decorated by sulfate anions; these building units are arranged into pseudo-layers. The partially disordered Cs⁺ cations fill the interstices in the structure. The overall topology of the two structures is very similar; however, in 2, the copper-oxide ensembles are additionally stitched by weakly occupied Cu–O fragments into pseudo-chains; this also results in essential disorder in the Cs sublattice. Both structures have very much in common with those of the puninite-euchlorine-fedotovite morphotropic series. We analyze the crystal chemical trends in this morphotropic series.