Electron paramagnetic resonance and ENDOR studies of Cr3+ - Al3+ pairs in forsterite

The electron paramagnetic resonance (EPR) spectrum of Cr³⁺ in synthetic crystals of forsterite consists primarily of lines of Cr³⁺ “isolated” at the M1 and M2 positions in a “perfect” crystal environment without local charge compensation. In addition it shows two nonequivalent superhyperfine-split sextets with different intensities which are due to strong interaction of the Cr³⁺ electron spin S (S=3/2) with an adjacent nuclear spin I(I=5/2). Electron nuclear double resonance (ENDOR) experiments revealed that the sextets result from Cr³⁺ (M1) - Al³⁺ and Cr³⁺ (M2) - Al³⁺ pairs, Al being located at adjacent tetrahedral Si sites. The g, D, A, and A′ tensor components of the Cr³⁺ - Al³⁺ pairs have been determined at room temperature. The values of the pairs are distinct although they are not very different from the corresponding data of “isolated” Cr³⁺. From the intensities of the EPR spectra the relative concentration of the Cr³⁺ - Al³⁺ pairs with respect to “isolated” Cr³⁺ has been estimated.