EPR study of chromium-doped forsterite crystals: Cr3+(M1) with and without associated nearest-neighbor Mg2+(M2) vacancy

Electron paramagnetic resonance (EPR) study of single crystals of chromium-doped forsterite grown by the Czochralski method in two different research laboratories has revealed, apart from the known paramagnetic centers Cr3+(M1), Cr3+(M2) and Cr4+ a new center Cr3+(M1)-V-Mg(2+)(M2) formed by a Cr3+ ion substituting for Mg2+ at the M1 structural position with a nearest-neighbor Mg2+ vacancy at the M2 position. For this center, the conventional zero-field splitting parameters D and E and the principal g values and A values of the Cr-53 hyperfine splitting have been determined as follows: D = 33.95(3) GHz, E = 8.64(1) GHz, g = [1.9811(2), 1.9787(2), 1.9742(2)], A = [51(3), 52(2), 44(3)] MHz. The center has been identified by comparing EPR spectra with those of the charge-uncompensated ion Cr3+(M1) and the ion pair Cr3+(M1)-Li+ (M2) observed in forsterite crystals codoped with chromium and lithium. It has been found that the concentration of the new center decreases to zero, whereas that of the Cr3+(M1) and Cr3+(M1)-Li+(M2) centers increases with an increase of the Li content from 0 up to similar to 0.03 wt% (at the same Cr content similar to 0.07 wt%) in the melt. The known low-temperature luminescence data pertinent to the centers under consideration are also discussed.