The Crystal Structures of the Calcium Aluminogallates CaAlGaO_4 and Ca_2AlGaO_5

摘要

The crystal structures of two calcium aluminogallates have been investigated by single-crystal X-ray diffraction methods. Ca2AIGaO5 is a quenched high-pressure phase prepared at 12500C and 2.5 GPa. The compound belongs to the brownmil-lerite structure type (space group flmb, a = 5.2823(6) A, b = 14.613(2) A, c = 5.4739(8) A, V= 422.5(9) K, Z= 4, Dcak = 4.04 g cm3, wR2 = 0.072 for 586 reflections with I> 2a(I)). Main building units are (1) layers of perovskite-type corner-connected MeO6-octahedra perpendicular to [010] and (2) zweier single chains of MeO4-tetrahedra running parallel to [100] (Me = Al, Ga). The stacking of the layers and sheets of single chains results in a three-dimensional network with Ca ions in the interstitial voids. Single crystals of CaAIGaO4 (space group P121/cl, a = 7.9817(1) A, b = 8.7415(1) A, c = 10.4430(2) A, 1