CRYSTAL-CHEMISTRY OF NONSTOICHIOMETRIC MG-AL SYNTHETIC SPINELS

Twenty-seven single-crystal structural refinements were performed on synthetic spinels along the MgAl2O4 - Al8/3square1/3O4 join. The number of vacancies (from 0.09 to 0.22) was determined according to microprobe analysis. Results indicate that vacancies are limited only to octahedral sites, with a vacancy-to-oxygen distance refined to 2.113(3) angstrom. Mg is ordered in the tetrahedral site, and only for a0 > 7.99 angstrom are the amounts of octahedral Mg significant. The decrease in a0 shown by Al8/3square1/3O4-rich terms is essentially related to the strong decrease in the T - O distance, due to large amounts of tetrahedral Al. A linear relation between a0 and the vacancy content V(M)'' was derived satisfactorily fitting data in the literature, including end-members.