Single-crystal X-ray structure analysis of Mn-substituted barium hexaaluminates as-grown and after reduction

Abstract

An X-ray single-crystal structure analysis was performed on manganese-substituted barium hexaaluminate (Ba_0.780Mn_0.254Al_10.706O_17.153) with the β-alumina structure, which was grown by the floating zone method and reduced by subsequent evacuation at 1150 °C. Both as-grown and reduced crystals have a hexagonal structure (P6₃/mmc, Z= 2) and their unit-cell dimensions are a= 0.5591(1), c= 2.2659(2) nm and a= 0.5587(2), c= 2.2656(3) nm respectively. The structure refinement from the X-ray reflection data collected by using the ω-2θ scan technique was carried out by the least-squares method to give a final R value of 0.03. The resultant structure was identical with that of β-alumina (Ba_0.75Al_11.0O_17.25) reported by Iyi et al.¹ Di- and tri-valent manganese partially and preferentially replaced Al(2) sites in a tetrahedral environment in the spinel block. UV–VIS spectra showed that the evacuation treatment brought about the reduction of some of the Mn³⁺ to Mn²⁺. The charge compensation for the reduction was found to be effected by O^2– defect formation in the mirror plane whereas the occupancies of oxygen sites inside the spinel block remained unchanged.