Structural distortions and orbital ordering in LaTiO3 and YTiO3

– Theoretical investigations of the electronic, magnetic and structural properties of LaTiO3 and YTiO3 have been made. In the framework of GGA and GGA+U scheme we analyzed the effect of the local Coulomb interaction (U) value on the atomic forces acting in the experimental structure. The optimal parameters of the electron-electron on-site interactions as well as the orbital configurations and magnetic properties are determined. Introduction. – Transition-metal perovskite oxides LaTiO3 and YTiO3 are classical MottHubbard insulators. In spite of the fact that they are formally isoelectronic with one 3d electron in the t2g shell LaTiO3 has a G-type antiferromagnetic (AFM) structure whereas YTiO3 has a ferromagnetic (FM) one. The most unusual feature of these compounds relates with a nearly isotropic magnon spectrum in both titanates despite of their distorted crystal structures [1, 2]. There are two theoretical models aimed to explain these effects in LaTiO3. One of them originates from the work of Kugel and Khomskii [3] and operates in terms of the lattice distortions and an orbital ordering (OO). Another one is proposed by Khalliulin and Maekawa [4] and describes the above effects by the model of orbital liquid (OL) where the fluctuations of orbital degrees of freedom play an important role. According to [4] the energy of the orbital fluctuations is estimated to be Worb ∼ 160 meV. Thus OL model is applicable only if the splitting of the t2g orbital in the crystal field ∆ is significantly smaller than Worb. Otherwise the nearly isotropic exchange is likely to be due to an orbital ordering with a peculiar orbital configuration [9]. The OL model was supported by neutron experiments of Keimer et al [1] where no OO in LaTiO3 was found. Fritsch with co-workers [5] made heat capacity and magnetic measurements. Having supposed the nearly cubic structure of LaTiO3 they concluded that the energy of the spin-orbit coupling for Ti-sites was ESO = 30 meV and the crystal field effect was small. The theoretical estimations made by Solovyev in Ref. [6] in the framework of the local spin density approximation (LSDA) and LSDA+U theory showed that ∆ in LaTiO3 was as small