Single-crystal EPR and DFT study of a VIAl-O--VIAl center in jeremejevite: electronic structure and 27Al hyperfine constants

Single-crystal electron paramagnetic resonance (EPR) spectra of a gem-quality jeremejevite, Al6B5O15(F, OH)(3), from Cape Cross, Namibia, reveal an S = 1/2 hole center characterized by an Al-27 hyperfine structure arising from interaction with two equivalent Al nuclei. Spin-Hamiltonian parameters obtained from single-crystal EPR spectra at 295 K are as follows: g (1) = 2.02899(1), g (2) = 2.02011(2), g (3) = 2.00595(1); A (1)/g (e) beta (e) = -0.881(1) mT, A (2)/g (e) beta (e) = -0.951(1) mT, and A (3)/g (e) beta (e) = -0.972(2) mT, with the orientations of the g (3)- and A (3)-axes almost coaxial and perpendicular to the Al-O-Al plane; and those of the g (1)- and A (1)-axes approximately along the Al-Al and Al-OH directions, respectively. These results suggest that this aluminum-associated hole center represents hole trapping on a hydroxyl oxygen atom linked to two equivalent octahedral Al3+ ions, after the removal of the proton (i.e., a Al-VI-O--Al-VI center). Periodic ab initio UHF and DFT calculations confirmed the experimental Al-27 hyperfine coupling constants and directions, supporting the proposed structural model. The Al-VI-O--Al-VI center in jeremejevite undergoes the onset of thermal decay at 300 A degrees C and is completely bleached at 525 A degrees C. These data obtained from the Al-VI-O--Al-VI center in jeremejevite provide new insights into analogous centers that have been documented in several other minerals.