Ba-Cu ordering in bariopharmacoalumite-Q2a2b2c from Cap Garonne, France

摘要

Bariopharmacoalumite-Q2a2b2c, Ba_(0.5)(Cu,ZnO)_(0.1)H_(0.6)[Al_4(OH)_4(As_(0.9)Al_(0.1)O_4)_3]?5.5H_2O, from the south mine of the old copper mine at Cap Garonne, France, has a 2x2x2 /-centred tetragonal superstructure of the basic pharmacosiderite-type structure. Cell parameters are a = 15.405(2) A and c = 15.553(3) A. The structure was determined and refined in /42m to Rt= 0.057 for 2697 reflections with I/ > 2σ(I), using synchrotron X-ray data on a twinned crystal. The origin of the superlattice cell doubling was determined to be due predominantly to the ordering of Ba atoms in half of the [0 0 1] channels, centred at (0, 0, 0) and (14, lA, 0). The other channels, centred at 1/2, 0, 0) and (0,14, 0), were found to be occupied by corner-connected chains of Cu/Zn-centred square planar units.