Optical and electron paramagnetic resonance studies of Fe impurities in yttrium aluminum garnet crystals

Optical-absorption and electron paramagnetic resonance (EPR) techniques were used to study iron impurities in yttrium aluminum garnet (YAG) crystals. Due to its low symmetry, YAG containing Fe³⁺ exhibited many EPR lines. Fe³⁺ ions in octahedral and tetrahedral sites were identified. A correlation study involving both EPR and optical-absorption measurements indicated that the absorption band at 255 nm, which had been previously attributed to an Fe³⁺ charge-transfer band, was a composite charge-transfer band made up of contributions from substitutional Fe³⁺ at both symmetry sites. Oxidation and reduction experiments up to 1700 K were performed. The former increased the Fe³⁺ concentration, and the latter diminished it. Conversion of Fe²⁺ to Fe³⁺ was governed by a thermal activation energy of 3.0±{}0.3 eV.