Atomistic modeling of the mixing properties and local structure of Be(Al,Cr,FeIII)2O4 solid solutions

Abstract

The thermodynamic and elastic properties of mixing have been calculated and the local structure of Be(Al,Cr,FeIII)2O4 solid solutions has been analyzed using a previously elaborated set of interatomic potentials for atomistic modeling of simple and complex beryllium oxides, which almost exactly reproduced their structural, elastic, and thermodynamic properties. Calculations have been performed for a (4a × 2b × 2c) supercell of the olivine structural type with the relieved nontranslational symmetry (space group P1), which has made it possible to specify the distribution of trivalent atoms over nonequivalent positions and to provide relaxation of the local structure. The mixing properties have been calculated over the entire range of compositions, the changes in the Gibbs free energy with variations in temperature and the regions of stability of the solid solutions have been estimated, and the critical values of the temperature and composition have been determined. The histograms of the distributions of M-M, M-O, and O-O interatomic distances, as well as MO6 octahedron volumes, have been constructed; the compliances of cation positions have been evaluated; and the groups of atoms that are most readily shiftable from their ideal positions have been established. The performed analysis has demonstrated that two different octahedral positions M1 and M2 in the olivine-type structure, namely, chrysoberyl, are extremely sensitive to their own atomic environment. This leads to a preferable incorporation of Al3+ cations into the M1 octahedral position, whereas larger-sized Cr3+ (Fe3+) cations preferentially occupy the M2 octahedral position.