Magnetic and transport properties of the series (x = 0, 0.01, 0.03, 0.05, 0.1, 0.3, 0.5, 1.0) have been investigated. It was found that with increasing Co content, the Curie temperature decreases, while the spin glass transition temperature decreases first then increases. As for the sample with x = 0.3, the relation of and the merging point (for the zero-field-cooled and field-cooled magnetization curves starting to merge) with the applied magnetic field H follows a relation , ), where and are constants. The resistivity increases drastically with Co doping and a semiconductor-like behaviour is displayed in the sample x = 0.3. These facts strongly suggest that the double-exchange interaction in the perovskite materials is very fragile since a small Co content can change the magnetic and relevant transport properties of the system.

The Ti3+-Ti4+ ion pair is proposed experimentally to be responsible for the infrared residual absorption of Ti:Al2O3. However, no theoretical verification has been presented to date. In this paper, the DV-Xalpha calculation is performed upon clusters of (Ti3+O62-)9-, (Ti3+Al3+O92-)12- and (Ti3+Ti4+O92-)11-. Our work not only has interpreted the infrared residual absorption but also has clarified the confusion about the origin of the ultraviolet absorptions. Therefore the Ti3+-Ti4+ pair postulation is strongly supported by our present work.