Thermochemical reduction of yttrium aluminium garnet crystals

Abstract

Transmission electron microscopy, X-ray microanalysis and optical absorption measurements have been used to characterize strained and unstrained regions in as-grown and subsequently thermochemically reduced yttrium aluminium garnet crystals. As the thermochemical reduction takes place at increasing temperature, a dark colouration emerges at the strained region. The colouration at the strained region is always darker than the surrounding unstrained region, regardless of the temperature of reduction. Optical absorption measurements reveal that the colouration is due to a very broad band peaking at about 800 nm. There is no evidence from transmission electron microscopy and X-ray microanalysis to indicate the presence of precipitates in as-grown and thermochemically reduced crystals, in contrast with oxides in which cation impurities tend to form metallic precipitates preferentially at strained regions during thermochemical reduction.