Localized or itinerant TiO3 electrons in RTiO3 perovskites

A reinvestigation of nearly stoichiometric RTiO3 samples shows that (1) the transition from ferromagnetic to antiferromagnetic Ti-O-Ti interactions with increasing R3+-ion radius r(3+) occurs at r(3+) approximate to 1.11 angstrom where orthorhombic (Pbnm) RMO3 perovskites universally change structure from a cooperative rotation of undistorted to distorted MO6/2 octahedra about the orthorhombic b-axis, (2) the size of polaronic holes decreases progressively with r(3+) from about ten Ti sites in LaTiO3 to small polarons in GdTiO3, (3) the strength of the R-O-Ti interactions increases with the spin of the R3+ ion in the ferromagnetic compounds, (4) the saturation magnetization of the ferromagnetic TiO3 array at 5 K and the Curie temperature T-C both increase with decreasing r(3+) where the R3+ ions have no spin, and (5) there is no evidence of 'cluster-glass' behaviour where there is no variance of the R3+-ion radii. The cooperative Jahn-Teller orbital ordering and small-polaron behaviour of the ferrimagnetic and ferromagnetic compounds would favour localized Ti-3d electrons. However, the lack of saturation to 1 mu(B)/Ti of the ferromagnetic TiO3 array in the absence of a spin on the R3+ ion suggests a separation of an orbitally ordered ferromagnetic phase and an orbitally disordered paramagnetic phase.