EPR study of chromium-doped forsterite crystals: Cr3+(M1) with associated trivalent ions Al3+ and Sc3+

Abstract

Electron paramagnetic resonance (EPR) study of single crystals of forsterite co-doped with chromium and scandium has revealed, apart from the known paramagnetic centers Cr3+(M1) and Cr3+(M1)–\( V_{{{\text{Mg}}^{2 + } }} \)(M2) (Ryabov in Phys Chem Miner 38:177–184, 2011), a new center Cr3+(M1)–\( V_{{{\text{Mg}}^{2 + } }} \)(M2)–Sc3+ formed by a Cr3+ ion substituting for Mg2+ at the M1 structural position with a nearest-neighbor Mg2+ vacancy at the M2 position and a Sc3+ ion presumably at the nearest-neighbor M1 position. For this center, the conventional zero-field splitting parameters D and E and the principal g values have been determined as follows: D = 33,172(29) MHz, E = 8,482(13) MHz, g = [1.9808(2), 1.9778(2), 1.9739(2)]. The center has been compared with the known ion pair Cr3+(M1)–Al3+ (Bershov et al. in Phys Chem Miner 9:95–101, 1983), for which the refined EPR data have been obtained. Based on these data, the known sharp M1″ line at 13,967 cm−1 (with the splitting of 1.8 cm−1), observed in low-temperature luminescence spectra of chromium-doped forsterite crystals (Glynn et al. in J Lumin 48, 49:541–544, 1991), has been ascribed to the Cr3+(M1)–Al3+ center. It has been found that the concentration of the new center increases from 0 up to 4.4 × 1015 mg−1, whereas that of the Cr3+(M1) and Cr3+(M1)–\( V_{{{\text{Mg}}^{2 + } }} \)(M2) centers quickly decreases from 7.4 × 1015 mg−1 down to 3 × 1015 mg−1 and from 2.7 × 1015 mg−1 down to 0.5 × 1015 mg−1, i.e., by a factor of 2.5 and 5.4, respectively, with an increase of the Sc content from 0 up to 0.22 wt % (at the same Cr content 0.25 wt %) in the melt. When the Sc content exceeds that of Cr, the concentration of the new center decreases most likely due to the formation of the Sc3+(M1)–\( V_{{{\text{Mg}}^{2 + } }} \)(M2)–Sc3+ complex instead of the Cr3+(M1)–\( V_{{{\text{Mg}}^{2 + } }} \)(M2)–Sc3+ center. The formation of such ordered neutral complex is in agreement with the experimental results, concerning the incorporation of Sc into olivine, recently obtained by Grant and Wood (Geochim Cosmochim Acta 74:2412–2428, 2010).