An optical in-situ study of the re-oxidation kinetics of mixed valent Yb3Al5O12

The optical spectra of blue Yb-garnet single crystals have been studied in the range of 200 to 1200 nm. The spectra exhibit absorption bands due to Yb2+ and Yb3+ ions including 4f-4f transitions on Yb3+ as well as 4f-5d transitions on Yb2+ ions, and a band due to Yb2+-Yb3+ intervalence charge-transfer (IVCT) responsible for the blue colour of the crystal. At high temperatures, the blue colour is found to bleach in an oxidising atmosphere. The oxidation reaction was studied by means of optical spectroscopy at in-situ conditions in the temperature range between 861 and 1065 degreesC. Basically, the oxidation kinetics were found to follow a parabolic rate law. The kinetics have been analysed in the framework of a refined model of internal oxidation accounting for both the transport of the oxidising defects and the diffusion of the reduced component Yb2+.