Nonequilibrium cation distribution in nanocrystalline MgAl 2O 4 spinel studied by 27Al magic-angle spinning NMR

Abstract

Nanocrystalline magnesium aluminate (MgAl 2O 4) powders were prepared by high-energy milling of the bulk material. Due to the ability of nuclear magnetic resonance (NMR) spectroscopy to discriminate between probe nuclei on inequivalent crystallographic sites, valuable insight into the mechanically induced evolution of a local cation disorder in MgAl 2O 4 can be obtained. It, thus, was revealed for the first time that the mechanical treatment of MgAl 2O 4 tends to randomize ions over the cation spinel sublattices. Quantitative microstructural information on the nonequilibrium cation distribution provided by 27Al MAS-NMR is complemented by XRD and TEM investigations revealing the nanoscale nature of the milled material. The cation inversion parameter of the nanosized MgAl 2O 4 is compared with that of the bulk material at non-ambient conditions.