Electron spin resonance of trivalent chromium in forsterite, Mg₂SiO₄

Abstract

The electron spin resonance (ESR) spectrum of Cr³⁺ in a synthetic single crystal of forsterite doped with Cr₂O₃ was studied at room temperature in the X-band frequency range. The dependence of the observed spectra on the crystal orientation with respect to the applied magnetic field was investigated. The ESR spectra are described by the spin Hamiltonian $H = \beta HgS + D(S_Z^{\text{2}} - {\text{1/3}}S{\text{(}}S{\text{ + 1)) + }}E{\text{(}}S_x^{\text{2}} - S_y^{\text{2}} {\text{)}}$ with S=3/2. The spin resonance reveals that the chromium ions are located at both the M1 and M2 positions. Other possible substitutional or interstitial Cr³⁺ positions may be possible, but were not observed. The site occupancy numbers of Cr³⁺ at M1 and M2 are roughly 1.2×10⁻⁴ and 0.8×10⁻⁴, respectively, assuming that chromium is oxidized completely. The preference of the chromium ions for M1 was interpreted qualitatively in terms of crystal field criteria. The rhombic and axial spin Hamiltonian parameters, D and E, and the directions of the magnetic axes obtained for M1 and M2 are consistent with the respective oxygen coordination polyhedra.