Cation disorder and vacancy distribution in nonstoichiometric magnesium aluminate spinel, MgO center dot xAl(2)O(3)

Neutron diffractometry and X-ray diffractometry were used to characterize t he cation disorder and vacancy distribution in nonstoichiometric spinel, Mg O . xAl(2)O(3), where x greater than or equal to 1.0. Both synthetic and na tural samples were examined. Least-squares refinements of integrated intens ities for single crystals and Rietveld profile refinements for powders yiel ded the average scattering "power" from tetrahedral and octahedral sites wi thin the almost-perfect cubic close-packed oxygen sublattice. The cation di sorder was calculated assuming models in which the vacancies resided on tet rahedral, octahedral, or both types of sites. No degree of cation disorder was consistent with the tetrahedral vacancy model, and vacancies most likel y resided on octahedral sites.