The rare earth-titanium (III) perovskite oxides—An isostructural series with a remarkable variation in physical properties

The rare earth-titanium(III) perovskites, RTiO3 (R ≡ rare earth), form an isostructural series for R  La to Tm, including Y. Discontinuous changes in electrical and magnetic properties occur as the radius of R decreases. For R  La and Ce, metallic conductivity is observed with metal-semiconductor transitions at low temperatures. All other R are semiconducting with activation energies which increase from 10−2 eV (Pr, Nd) to 10−1 eV (Gd, Y). For R  La, Ce, and Pr, antiferromagnetic order occurs below temperatures near 100 K, for R  Nd only paramagnetism is seen and for R  Gd to Tm, including Y, ferromagnetic (R  Y) or ferrimagnetic (R  Gd-Tm) behavior is observed. The above changes can be correlated with the variation of the Ti-O-Ti angle and understood, qualitatively, using the Hubbard model and super-exchange theory.