Analysis of the spin-Hamiltonian parameters for Cr3+ in mirror and inversion symmetry sites of alexandrite (Al2-xCr xBeO4). Determination of the relative site occupancy by EPR

Abstract

The EPR spectra of Cr3+ in single crystals of alexandrite (Al2-xCrxBeO4, x<0.01) have been obtained at temperatures between RT and 4.2 K in the frequency range 9.5-9.1 GHz. The orthorhombic structure of Al2BeO4 has two distinct Al 3+ sites in the unit cell corresponding to Cs and C i symmetry. The EPR spectra at 35 GHz of Cr3+ in the Ci site have been previously analyzed by other investigators. This study reports the observation of Cr3+ in the Ci site and the refined spin-Hamiltonian parameters for both magnetic centers. The absolute sign of the fine structure tensor elements for the Ci site has been determined to be positive. The observed orientations of both the chromium mirror and inversion magnetic axes are consistent with the crystallographic symmetry relationships of pure Al2BeO4. A comparison of the measured Cr3+ EPR intensities for resonances from both sites indicates that for two magnetically dilute crystals, the mirror site is preferentially occupied with a relative site occupancy of 78%±3%. © 1983 American Institute of Physics.