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

An X-ray single-crystal structure analysis was performed on manganese-substituted barium hexaaluminate (Ba(0.780)Mn(0.254)AL(10.706)O(17.153)) with the beta-alumina structure, which was grown by the floating zone method and reduced by subsequent evacuation at 1150 degrees C. Both as-grown and reduced crystals have a hexagonal structure (P6(3)/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 omega-2 theta scab 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 beta-alumina (Ba0.75Al11.0O17.25) 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. W-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 O2- defect formation in the mirror plane whereas the occupancies of oxygen sites inside the spinel block remained unchanged.