The origin of the antiferromagnetic order and puzzling properties of LaTiO 3 as well as the magnetic phase diagram of the perovskite titanates are studied theoretically. We show that in LaTiO 3, the t 2g degeneracy is eventually lifted by the La cations in the GdFeO 3-type structure, which generates a crystal field. This allows the description of the low-energy structure of LaTiO 3 by a single-band Hubbard model as a good starting point. The lowest-orbital occupation in this crystal field stabilizes the AFM(G) state, and well explains the spin-wave spectrum of LaTiO 3 obtained by the neutron scattering experiment. The orbital-spin structures for RTiO 3 where R = Pr, Nd and Sm are also accounted for by the same mechanism. We point out that through generating the R crystal field, the GdFeO 3-type distortion has a universal relevance in determining the orbital-spin structure of the perovskite compounds in competition with the Jahn-Teller mechanism, which has been overlooked in the literature. Since the GdFeO 3-type distortion is a universal phenomenon as is seen in a large number of perovskite compounds, this mechanism may also play important roles in other compounds of this type.
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