The crystal structures of the calcium aluminogallates CaAlGaO4 and Ca2AlGaO5

The crystal structures of two calcium aluminogallates have been investigate d by single-crystal X-ray diffraction methods. Ca2AlGaO5 is a quenched high -pressure phase prepared at 1250 degreesC and 2.5 GPa, The compound belongs to the brownmillerite structure type (space group 12mb, a = 5.2823(6) Angs trom, b = 14.623(2) Angstrom, 5.4739(8) Angstrom, V = 422.5(9) Angstrom (3) , Z = 4, D-calc = 4.04 g cm(-3), wR2 = 0.072 for 586 reflections with I > 2 sigma (I)). Main building units are (1) layers of perovskite-type corner-c onnected MeO6-octahedra perpendicular to [010] and (2) zweier single chains of MeO4-tetrahedra running parallel to [100] (Me = Al, Ga). The stacking o f the layers and sheets of single chains results in a three-dimensional net work with Ca ions in the interstitial voids, Single crystals of CaAlGaO4 (s pace group P12(1)/c1, a = 7.9817(1) Angstrom, b = 8.7415(1) Angstrom, c = 1 0.4430(2) Angstrom, beta = 94,84(2)degrees, V = 726.0(3) Angstrom (3), Z = 8, D-calc = 3.67 g cm(-3), wR2 = 0.079 for 971 reflections with I > 2 sigma (I)) were synthesized at ambient pressure and a temperature of 1500 degree sC, The structure consists of layers of S6R of MeO4-tetrahedra perpendicula r 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, CaAlGaO4 is isotypic with the monoclinic modification of CaGa2O4, (C) 2001 Academic Press.