The crystal structures of two calcium aluminogallates have been investigated by single-crystal X-ray di4raction methods. Ca 2 AlGaO 5 is a quenched high-pressure phase prepared at 12503C and 2.5 GPa. The compound belongs to the brownmillerite structure type (space group I2mb, a โซุโฌ 5.2823(6) A s , b โซุโฌ 14.613(2) A s , c โซุโฌ 5.4739(8) A s , V โซุโฌ 422.5(9) A s 3 , Z โซุโฌ 4, D calc โซุโฌ 4.04 g cm ุ3 , wR2 โซุโฌ 0.072 for 586 re6ections with I > 2 (I)). Main building units are (1) layers of perovskite-type corner-connected MeO 6 -octahedra perpendicular to [010] and (2) zweier single chains of MeO 4 -tetrahedra running parallel to [100] (Me โซุโฌ 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 CaAlGaO 4 (space group P12 1 /c1, a โซุโฌ 7.9817(1) A s , b โซุโฌ 8.7415(1) A s , c โซุโฌ 10.4430(2) A s , โซุโฌ 94.84(2)3, V โซุโฌ 726.0(3) A s 3 , Z โซุโฌ 8, D calc โซุโฌ 3.67 g cm ุ3 , wR2 โซุโฌ 0.079 for 971 re6ections with I > 2 (I)) were synthesized at ambient pressure and a temperature of 15003C. The structure consists of layers of S6R of MeO 4 -tetrahedra perpendicular to [100]. The stacking of the layers parallel to the a-axis results in a framework with channels occupied by the Ca cations. The sequence of up (U) and down (D) pointing apices of adjacent tetrahedra within a single ring is UUDUDD. CaAlGaO 4 is isotypic with the monoclinic modi5cation of CaGa 2 O 4 . 2001 Academic Press EXPERIMENTAL DETAILS
Sample Preparation
The starting material for the high-pressure phase Ca AlGaO was prepared by solid-state reactions, using CaCO (Fluka, '98%), Ga O (Fluka, 99.99%), and Al O (Aldrich, 99.8%) as reagents. The mixed powders