REFINEMENT OF THE STRUCTURES OF SR3AL2O6 AND THE HYDROGARNET SR3AL2(O4D4)3 BY RIETVELD ANALYSIS OF NEUTRON POWDER DIFFRACTION DATA

Sr3Al2O6, M(r) = 412.81, cubic, Pa3, a = 15.8556 (4) angstrom, V = 3986.0 (5) angstrom 3, Z = 24, D(x) = 4.126 g cm-3, lambda = 1.400 angstrom, mu-R = 0.020, T = 295 K, R(exp) = 0.0295, R(wp) = 0.0447 for 2415 step intensities, R(Bragg) = 0.0168 for 1606 reflections. The noteworthy feature of the strontium aluminate structure is a puckered six-membered AlO4 tetrahedral ring, with the average bridging Al-O bond length, 1.768 (8) angstrom, slightly greater than the average non-bridging Al-O bond length, 1.746 (5) angstrom. Sr3Al2(O4D4)3, M(r) = 532.98, cubic, Ia3d, a = 13.0319 (1) angstrom, V = 2213.2 (2) angstrom 3, Z = 8, D(x) = 3.128 g cm-3, lambda = 1.400 angstrom, mu-R = 0.012, T = 295 K, R(exp) = 0.0316, R(wp) = 0.0464 for 2400 step intensities, R(Bragg) = 0.0148 for 224 reflections. Sr3Al2(O4D4)3 has the hydrogarnet structure, which in contrast to the silicate garnets, has the D4O4 unit in place of the SiO4 unit. The O-D distance is 0.914 (4) angstrom, and the refined D content of the H position is 90%. Both the Sr hydrogarnet and the aluminate are isostructural with the Ca analogs. The enhanced reactivity of Sr3Al2O6 to form the hydrogarnet as compared to that of Ca3Al2O6 is ascribed to the increased cell size and strain caused by the substitution of the larger sized Sr for Ca.