Next-nearest neighbor effects in the Mössbauer spectra of (Cr,Al) spinels

Abstract

Fe 2+ T-site quadrupole splitting observed in the Mössbauer spectra of synthetic chromite-hercynite spinels (Fe(Cr 1− x Al x ) 2O 4, with x = 0 to 1.5) is systematically related to the substitution of octahedrally coordinated cations in the next-nearest neighbor coordination sphere. Conditions of synthesis include dry reagents contained in a stainless-steel crucible and enclosed in an evacuated silica-glass tube, held at 1250°C for up to 4 days. A simple singlet spectrum for FeCr 2O 4 changes with progressive Al substitution into a broadened doublet spectrum, with quadrupole splitting of 1.28 mm/sec and line half-width of 0.82 mm/sec, for FeCrAlO 4. Partial quadrupole splitting theory is used to calculate the quadrupole splittings for individual next-nearest neighbor configurations. Bulk quadrupole splitting is calculated relative to the statistical distribution of possible next-nearest neighbor configurations. Calculated quadrupole splittings are in good agreement with those obtained for unconstrained two-peak fits, at room temperature and at low and high temperature. Also, predicted spectral details are resolved in low temperature spectra. The local site distortion responsible for the quadrupole splitting appears to be associated with spatial accommodation of substituent Al 3+ cations.