Computer modeling of the local structure and mixing properties of a solid solution of MgAl2O4-MgCr2O4

An original method of the atomistic computer modeling of substitutional solid solutions is applied to the noble spinel MgAl2O4-magnesia chromite MgCr2O4 binary system. The parameters of the interatomic potentials in the partially ionic approximation are optimized using the experimental values of the structural, elastic, and thermodynamic characteristics of these minerals. The point defect formation energy is estimated. The modeling of Mg(Al (x) Cr1 - x )(2)O-4 solid solutions is carried out in a supercell of the 4 x 4 x 4 structural type of spinel that contains 3584 atoms; the GULP 3.4.9 program, which is implemented in the SKIF Chebyshev supercomputer (Moscow State University), is used. The mixing properties of the solid solution such as the mixing enthalpy, the interaction parameters, the mixing volumes, the deviations of the compression moduli from additivity, and the oscillatory entropy were determined. Calculations of the Gibbs energy have yielded the assessments of the stability fields of the MgAl2O4-MgCr2O4 solid solution and the comparisons of these assessments with the experimental data. Histograms of interatomic spacings are plotted and the values of the relaxation parameters of the Cr-O and Al-O bonds are estimated; these values agree well with the experimental values obtained by measuring the optical absorption spectra on a Cr3+ ion.