Phase transitions and the doping effect in

The electronic states of the system, where , have been explored through measurements of the x-ray diffraction, magnetization, electrical resistivity, and thermoelectric power. Three structural phases appear as the composition and temperature are varied: the high-temperature orthorhombic (HO) phase with a pseudobrookite-type structure and the slightly deformed monoclinic (HM) one, and the low-temperature monoclinic (LM) one which is completely different from the former two phases. For , the HO-HM transition with a small magnetic anomaly and the HM-LM transition with significant spin-singlet Ti-Ti pairs successively take place, but for , no transition to the LM phase exists there. While the HO-phase is like a highly correlated metal, the HO-phase compounds with have a strongly localized state due to the disorder and the polaron formation, and exhibit a spin-glass-like phase at low temperatures.