CRYSTAL STRUCTURE OF EU4AL2O9

The phase 2(RE)2O3·Al2O3 occurs in rare earth aluminates for the series Sm-Lu. All compounds are isostructural, and Eu4Al2O9 was selected for detailed structural analysis. The compound is monoclinic, a = 7.608 (4) Å, b = 10.616 (5) Å, c = 11.101 (8) Å, β = 108.50 (9)°, space group P21/c, and there are 4 formula weights in the unit cell with a calculated ρ = 6.29 g/cm3. The integrated intensities of 3184 independent reflections were measured using a scintillation counter and monochromatic Mo Kα radiation. The structure was refined by leastsquares calculations with isotropic temperature factors to a conventional R of 0.053. The Al3+ is in tetrahedral coordination, and Al-O bond distances are approximately 1.73 Å as expected for tetrahedral aluminum. The europium ions are in sixfold and sevenfold coordination. The sixfold-coordinated ion has oxygen ions at about 2.32 Å at the corners of a distorted octahedron; the sevenfold ion has five bonds at an average distance of 2.33 Å and two at about 2.5-2.6 Å. The articulation of the polyhedra forms a cavity in the structure. The summation of ionic bond strengths shows that one oxygen ion has a local positive charge imbalance of 1/2 and two other ones have a negative imbalance of 1/4. This structure is representative for the isostructural series 2(RE)2O3·Al2O3, RE = Sm-Lu, and 2(RE)2O· Ga2O3, RE = La-Gd. © 1969, American Chemical Society. All rights reserved.