Hydroxyl impurity effects in YAG (Y3Al5O12)

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Abstract
Evidence is reported of 1020 cm−3 hydroxyl impurity density affecting crystal fields and color center formations in YAG and Nd:YAG. Also reported is a high‐temperature reactive atmosphere process to control H bonding and to bleach coloration. Absorption by O–H⋅⋅⋅O and O–D⋅⋅⋅O transitions in the 3–4 μm spectral region was observed in different crystals. Two independent O–H⋅⋅⋅O lines, both with weak satellites, were detected at 3340 and 3374 cm−1. H‐bonding sites were assigned by (i) frequency dependence on O2−–O2− distance and (ii) Nd:YAG nonequivalent crystal field effects seen in Nd3+ fluorescence. H+ concentrations 1020 cm−3 were derived from these results. Bonding site relative densities also varied with the different crystals observed. Reactive atmosphere processing (RAP) could cause H+ or D+ to be coordinated into or out of crystals, concomitant with color center bleaching. Color center formations were dissociated by surface reaction with I2 + H2O or I2 + O2 reactive atmosphere at temperatures in the range of 1350–1880 °C. The RAP effect was to produce bulk crystal electronic transfers related to hydrogen in the crystal metal–oxide system. However, photoinduced coloration was again possible. Intense coloration also resulted from vacuum heating, causing an H2O effluent. The evidence indicated YAG and Nd:YAG contained a high concentration of largely mobile H+ affecting O− ‐center coloration and influencing crystal fields by bound‐small polaron formations.