Structural relaxation and crystal field stabilization in Cr3+-containing oxides and silicates

Abstract

The effect of crystal structure relaxation in oxygen-based Cr3+-containing minerals on the crystal field stabilization energy (CFSE) is considered. It is shown that the dependence of \( {\text{CFSE}}_{{{\text{Cr}}^{ 3+ } }} \), which is found from optical absorption spectra, on the average interatomic distances is described by the power function with a negative exponent \( {c \mathord{\left/ {\vphantom {c {\bar{R}^{n} }}} \right. \kern-\nulldelimiterspace} {\bar{R}^{n} }} \), where n approaches 5, as predicted theoretically, for pure Cr3+ compounds, but decreases to 1.0–1.5 for Cr3+-containing oxide and silicate solid solutions. The deviation of the experimental dependence for solid solutions from the theoretical curve is due to structure relaxation, which tends to bring the local structure of Cr3+ ions closer to the structure in the pure Cr compound, thus producing changes in interatomic distances between the nearest neighbors with respect to those in the average structure determined by X-ray diffraction. As a consequence, the mixing enthalpy of Cr3+-bearing solid solutions can be represented by the sum of contributions from lattice strain and CFSE. The latter contribution is most often negative in sign and, therefore, brings the Al–Cr solid solutions close to an ideal solid solution. It is supposed that the increased Cr content in minerals from deep-seated mantle xenoliths and mineral inclusions in diamonds results from the effect of \( {\text{CFSE}}_{{{\text{Cr}}^{ 3+ } }} \) enhanced by high pressure.