Polarization effects in the excitation and emission ofFe3+ in orthoclase and their relevance to the determination of lattice sites of unknowndefects

Abstract

A method was recently proposed for determining possible lattice sites of anunknown defect from polarization effects in its optical transitions. In this paperthe method is tested using the optically excited 1.77 eV (700 nm) fluorescence ofFe3+ ions which predominantly occupy the T1 sites in orthoclase feldspar. The emission intensitydepended on the polarization of the exciting photons and the emission was itself polarized.Two pairs of crystal field symmetry directions were deduced from polarization data for eachof the transitions at 1.77, 2.79 and 3.26 eV, and one pair was aligned with symmetry axes inthe average geometry of the four anions around the T1 sites. An analysis of EPR data forFe3+ ions in feldspar showed that there was a symmetry axis in the crystal field similar to one ofthose deduced from the polarization data. Group theory calculations were used to determine ifthe transitions were dipolar—a major assumption of the method. Three symmetries(S4,C2v and C2) were found to lead to dipolar transitions consistent with the excitation results, and four(D2,C3,C2 and Cs) were consistent with a 1.77 eV dipolar emission.