Synthesis and structure of calumetite-like SrCu4(OH)8Cl2⋅3.5H2O

Abstract

The synthesis and structure of the title compound, 1, is presented, refined using Rietveld against powder X-ray diffraction data. 1 crystal-lises dominantly in a pseudotetragonal C-centred orthorhombic lattice with dimensions a = 6.6791(6) angstrom, b = 15.5006(6) angstrom, c = 6.6811(6) angstrom and V = 691.70(10) angstrom(3). The structural model proposed here refined by Rietveld is Sr0.928(8)Cu4(OH)(8)Cl-2 center dot 3.60(21)H2O in space group Cmcm (63), with Z = 2. The chemistry and diffraction pattern of 1 are similar to that for the known Ca analogue, calumetite. The copper sites are arranged with square planar coordination at 1/4 and 3/4 height and are bonded to four (protonated) oxygens at an average of 1.966 angstrom (effective coordination of 3.82 angstrom). The more distant Cl sites (at Cu-Cl = 3.190(6) angstrom) complete the heavily Jahn-Teller distorted Cu[(OH)(4),Cl-2] polyhedra. The 1/2-occupied Sr sites are 8 coordinated to four protonated oxygens shared with the Cu-layer (at 2 x 2.438(8) angstrom, 2 x 2.566(15) angstrom) and by 4 bonds to the proposed water sites (Sr-Ow = 2.760(9) angstrom). The structure of 1 is predisposed towards defects, based on a notional tetragonal, P4/nmm, substructure with a(sub) approximate to a(1), c(sub) = b(1/2) dimensions. Average diffraction models have been further elaborated in order to resolve additional peaks (and peak-shapes) using DIFFaX+.