Electron paramagnetic resonance of Cr3+-Li+ centers in (Cr,Li): Mg2SiO4 synthetic forsterite

Synthetic single crystals of chromium-and lithium-doped forsterite, namely, (Cr,Li): Mg 2 SiO 4 , are studied using electron paramagnetic resonance spectroscopy. It is revealed that, apart from the known centers Cr 3+ ( M 1) and Cr 3+ ( M 2) (with local symmetries C i and C s , respectively), these crystals involve two new types of centers with C 1 symmetry, namely, Cr 3+ ( M 1)′ and Cr 3+ ( M 2)′ centers. The standard parameters D and E in a zero magnetic field [zero-field splitting (ZFS) parameters expressed in GHz] and principal components of the g tensor are determined as follows: D =31.35, E =8.28, and g =(1.9797, 1.9801, 1.9759) for Cr 3+ ( M 1)′ centers and D =15.171, E =2.283, and g =(1.9747, 1.9769, 1.9710) for Cr 3+ ( M 2)′ centers. It is found that the lowsymmetric effect of misalignment of the principal axes of the ZFS and g tensors most clearly manifests itself (i.e., its magnitude reaches 19°) in the case of Cr 3+ ( M 2)′ centers. The structural models Cr 3+ ( M 1)-Li + ( M 2) and Cr 3+ ( M 2)-Li + ( M 1) are proposed for the Cr 3+ ( M 1)′ and Cr 3+ ( M 2)′ centers, respectively. The concentrations of both centers are determined. It is demonstrated that, upon the formation of Cr 3+ -Li + ion pairs, the M 1 position for chromium appears to be two times more preferable than the M 2 position. Reasoning from the results obtained, the R 1 line (the 2 E → 4 A 2 transition) observed in the luminescence spectra of (Cr,Li): Mg 2 SiO 4 crystals in the vicinity of 699.6 nm is assigned to the Cr 3+ ( M 1)′ center.