An atomistic simulation method has been used to study the equilibrated geometry, defect structure, and phase mixing of barium magnesium aluminate (BAM) and related compounds. The calculated lattice energies are used to study the stability of defect structures in the β-alumina lattice, particularly in the presence of excess alumina in the starting materials. It is shown that excess alumina in the starting compounds could lead to the formation of Reidinger defects in BAM. The calculated structural modification of the lattice due to a Reidinger defect is in excellent agreement with the experimental results. These defects are responsible for generation of F+ and F-type color centers by (vacuum) ultraviolet radiation and bombardment of energetic particles from the discharge. BAM could be made resistant to the formation of color centers by adhering to a strictly stoichiometric formulation of this phosphor. © 2005 The Electrochemical Society. All rights reserved.