Polarization effects in the excitation and emission of Fe(3+) in orthoclase and their relevance to the determination of lattice sites of unknown defects.

A method was recently proposed for determining possible lattice sites of an unknown defect from polarization effects in its optical transitions. In this paper the method is tested using the optically excited 1.77 eV (700 nm) fluorescence of Fe(3+) ions which predominantly occupy the T1 sites in orthoclase feldspar. The emission intensity depended 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 each of the transitions at 1.77, 2.79 and 3.26 eV, and one pair was aligned with symmetry axes in the average geometry of the four anions around the T1 sites. An analysis of EPR data for Fe(3+) ions in feldspar showed that there was a symmetry axis in the crystal field similar to one of those deduced from the polarization data. Group theory calculations were used to determine if the transitions were dipolar-a major assumption of the method. Three symmetries (S(4), C(2v) and C(2)) were found to lead to dipolar transitions consistent with the excitation results, and four (D(2), C(3), C(2) and C(s)) were consistent with a 1.77 eV dipolar emission.