Analysis of the spin-Hamiltonian parameters for Cr3+ in mirror and inversion symmetry sites of alexandrite (Al2−xCrxBeO4). Determination of the relative site occupancy by EPR

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Abstract
The EPR spectra of Cr3+ in single crystals of alexandrite (Al2−xCrxBeO4, x2BeO4 has two distinct Al3+ sites in the unit cell corresponding to Cs and Ci symmetry. The EPR spectra at 35 GHz of Cr3+ in the Cs 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%.

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