Studies on some MG(6)center dot LR(6) compounds .2. Crystal structure of zinc fluotitanate hexahydrate and its low temperature phase

A previous determination of the structure of zinc fluotitanate hexahydrate was considered unsatisfactory because an abnormal anisotropic thermal parameter was attributed to the F atom. The structure (space group R (3) over bar, hexagonal cell parameters a = 9.585(4) Angstrom, c = 9.925(5) Angstrom) has been redetermined and refined, using 243 reflections with I > 3 sigma(I) measured diffractometrically with MoKalpha radiation, reaching R = 0.047 and wR = 0.059. The structure obtained is disordered, with two equally occupied sites F1 and F2 for the F-atom, and is isostructural with ZnSiF6 . 6H(2)O, except for the occupancy ratio F1:F2 which is 1:2 in the latter case. Low temperature study below the reported transition point revealed a three-component twin with monoclinic cells appearing in three orientations with a pseudo-trigonal symmetry about the former three-fold axis. Angles of rotation of different octahedra in neighboring columns, that reduce the symmetry from rhombohedral to monoclinic, have been calculated. The monoclinic phase is found to be isostructural with the lower temperature phase of MgSiF6 . 6H(2)O.