Occupancy and site distribution of europium in barium magnesium aluminate by 151Eu Mossbauer spectroscopy -: art. no. 134429

BaMgAl10O17: Eu2+ (BAM) is a commercial, blue emitting lamp phosphor activated by Eu2+. It is usually assumed that the Eu ions substitute for Ba ions occupying the Beevers-Ross site in the beta-alumina structure. We have investigated this phosphor with varying europium concentrations and different synthesis conditions using Eu-151 Mossbauer spectroscopy. Analysis of the Mossbauer line shapes reveals a pentamodal (or higher) site distribution consisting of three Eu2+ sites, one (or two) Eu3+ site(s), and a mixed valent site whose isomer shift lies in between those for the divalent and trivalent ions. The observed quadrupole coupling constants for the divalent ions are larger than any other reported to date in any lattice. Based on calculations of the nuclear quadrupole coupling constants by a full potential band-structure method reported recently, the divalent sites are identified with occupancy of the Beevers-Ross, anti-Beevers-Ross, and mid-oxygen sites in the beta-alumina structure. Oxidative degradation of the divalent ions due to the thermal treatment in air was also studied, and a preliminary analysis of how various sites evolve during this process is also presented.