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

Abstract

AbstractThe synthesis and structure of the title compound, 1, is presented, refined using Rietveld against powder X-ray diffraction data. 1 crystallises dominantly in a pseudotetragonal C-centred orthorhombic lattice with dimensions a = 6.6791(6) Å, b = 15.5006(6) Å, c = 6.6811(6) Å and V = 691.70(10) Å3. The structural model proposed here refined by Rietveld is Sr0.928(8)Cu4(OH)8Cl2⋅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 ¼ and ¾ height and are bonded to four (protonated) oxygens at an average of 1.966 Å (effective coordination of 3.82 Å). The more distant Cl sites (at Cu−Cl = 3.190(6) Å) complete the heavily Jahn–Teller distorted Cu[(OH)4,Cl2] polyhedra. The ½-occupied Sr sites are 8 coordinated to four protonated oxygens shared with the Cu-layer (at 2 × 2.438(8) Å, 2 × 2.566(15) Å) and by 4 bonds to the proposed water sites (Sr−Ow = 2.760(9) Å). The structure of 1 is predisposed towards defects, based on a notional tetragonal, P4/nmm, substructure with asub ≈ a1, csub = b½ dimensions. Average diffraction models have been further elaborated in order to resolve additional peaks (and peak-shapes) using DIFFaX+.