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

Single-crystal electron paramagnetic resonance (EPR) spectra of a gem-quality jeremejevite, Al B O (F, OH) , from Cape Cross, Namibia, reveal an S = 1/2 hole center characterized by an Al 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 = 2.02899(1), g = 2.02011(2), g = 2.00595(1); A /g β = -0.881(1) mT, A /g β = -0.951(1) mT, and A /g β = -0.972(2) mT, with the orientations of the g - and A -axes almost coaxial and perpendicular to the Al-O-Al plane; and those of the g - and A -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 Al ions, after the removal of the proton (i.e., a Al-O - Al center). Periodic ab initio UHF and DFT calculations confirmed the experimental Al hyperfine coupling constants and directions, supporting the proposed structural model. The Al-O - Al center in jeremejevite undergoes the onset of thermal decay at 300 °C and is completely bleached at 525 °C. These data obtained from the Al-O - Al center in jeremejevite provide new insights into analogous centers that have been documented in several other minerals. © 2012 Springer-Verlag.