Occupancy and site distribution of europium in barium magnesium aluminate by151EuMössbauer spectroscopy

Abstract

${\mathrm{BaMgAl}}_{10}{\mathrm{O}}_{17}:$ ${\mathrm{Eu}}^{2+}$ (BAM) is a commercial, blue emitting lamp phosphor activated by ${\mathrm{Eu}}^{2+}.$ It is usually assumed that the Eu ions substitute for Ba ions occupying the Beevers-Ross site in the β-alumina structure. We have investigated this phosphor with varying europium concentrations and different synthesis conditions using ${}^{151}\mathrm{Eu}$ Mössbauer spectroscopy. Analysis of the Mössbauer line shapes reveals a pentamodal (or higher) site distribution consisting of three ${\mathrm{Eu}}^{2+}$ sites, one (or two) ${\mathrm{Eu}}^{3+}$ 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 β-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.