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 ${\mathrm{Fe}}^{3+}$ exhibited many EPR lines. ${\mathrm{Fe}}^{3+}$ 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 ${\mathrm{Fe}}^{3+}$ charge-transfer band, was a composite charge-transfer band made up of contributions from substitutional ${\mathrm{Fe}}^{3+}$ at both symmetry sites. Oxidation and reduction experiments up to 1700 K were performed. The former increased the ${\mathrm{Fe}}^{3+}$ concentration, and the latter diminished it. Conversion of ${\mathrm{Fe}}^{2+}$ to ${\mathrm{Fe}}^{3+}$ was governed by a thermal activation energy of 3.0±0.3 eV.