The crystal and magnetic structure of LaTiO3 has been studied by x-ray and neutron diffraction techniques using nearly stoichiometric samples. We find a strong structural anomaly near the antiferromagnetic ordering, TN =146 K. In addition, the octahedra in LaTiO3 exhibit an intrinsic distortion which implies a splitting of the t2g-levels. Our results indicate that LaTiO3 should be considered as a Jahn-Teller system where the structural distortion and the resulting level splitting are enhanced by the magnetic ordering.LaTiO 3 has been studied already in the seventies and was thought to be a text book example of a Mottinsulator with antiferromagnetic order [1]. Ti is in its three-valent state with a single electron in the t 2g -orbitals of the 3d-shell. The titanate is hence an electron analog to the cuprates with a single hole in the 3d-shell. However, the t 2g -orbitals in the LaTiO 3 are less Jahn-Teller active and, therefore, the orbital moment may not be fully quenched in the titanate. The physics of the orbital degree of freedom has recently reattracted attention to this material [2,3].The ordered moment in LaTiO 3 amounts to 0.46 µ B which is much smaller than the value of 1µ B expected for a single electron with quenched orbital moment [4]. Quantum fluctuations can explain only about 15% reduction in the 3D-case. A straight-forward explanation could be given in terms of spin-orbit coupling, as an unquenched orbital moment would align antiparallel to the spin-moment in the titanate. However, in a recent neutron scattering experiment the magnon spin gap was observed at 3.3 meV, and it was argued that the strong interaction of an orbital moment with the crystal lattice implies a much larger value for the spin gap [2]. An orbital contribution to the ordered moment in LaTiO 3 was hence excluded. On the basis of standard theories, however, even the G-type antiferromagnetic ordering in LaTiO 3 may not be explained without a spinorbit coupling. Instead one expects ferromagnetism [5,6] related with the orbital degeneracy. Under the assumption of a specific structural distortion, Moshizuki and Imada recently presented a successful model for the antiferromagnetic order in LaTiO 3 [7]. However, there is no experimental evidence for such a distortion. The puzzling magnetic properties of LaTiO 3 led Khaliullin and Maekawa to suggest a novel theoretical description for RETiO 3 based on the idea of an orbital liquid. They were able to explain many of the magnetic characteristics of LaTiO 3 [3], but the presumed orbital fluctuations have not been observed [8]. Therefore, magnetism in LaTiO 3 still remains an open issue.We have reanalyzed the crystal and magnetic structure of LaTiO 3 by x-ray and by neutron diffraction samples with almost perfect stoichiometry. First, we find a clear structural anomaly at the Néel-ordering and, second, the shape of the octahedra in this compound is not ideal but distorted. From these observations we conclude that LaTiO 3 has to be considered as a soft Jahn-Teller system thereby explaining m...

We present an investigation of the influence of structural distortions in charge-carrier doped La1−xMxCoO3 by substituting La 3+ with alkaline earth metals of strongly different ionic sizes, that is M = Ca 2+ , Sr 2+ , and Ba 2+ , respectively. We find that both, the magnetic properties and the resistivity change non-monotonously as a function of the ionic size of M. Doping La1−xMxCoO3 with M = Sr 2+ yields higher transition temperatures to the ferromagnetically ordered states and lower resistivities than doping with either Ca 2+ or Ba 2+ having a smaller or larger ionic size than Sr 2+ , respectively. From this observation we conclude that the different transition temperatures and resistivities of La1−xMxCoO3 for different M (of the same concentration x) do not only depend on the varying chemical pressures. The local disorder due to the different ionic sizes of La 3+ and M 2+ play an important role, too.

A detailed analysis of the crystal structure in RETiO3 with RE = La, Nd, Sm, Gd, and Y reveals an intrinsic coupling between orbital degrees of freedom and the lattice which cannot be fully attributed to the structural deformation arising from bond-length mismatch. The TiO6 octahedra in this series are all irregular with the shape of the distortion depending on the RE ionic radius. These octahedron distortions vary more strongly with temperature than the tilt and rotation angles. Around the Ti magnetic ordering all compounds exhibit strong anomalies in the thermal-expansion coefficients, these anomalies exhibit opposite signs for the antiferromagnetic and ferromagnetic compounds. Furthermore the strongest effects are observed in the materials close to the magnetic cross-over from antiferromagnetic to ferromagnetic order.

Raman scattering is used to observe pronounced electronic excitations around 230 meV--well above the two-phonon range--in the Mott insulators LaTiO3 and YTiO3. Based on the temperature, polarization, and photon energy dependence, the modes are identified as orbital excitations. The observed profiles bear a striking resemblance to magnetic Raman modes in the insulating parent compounds of the superconducting cuprates, indicating an unanticipated universality of the electronic excitations in transition metal oxides.

Spin correlations in La2-xSrxCoO4 (0.3 < or = x < or = 0.6) have been studied by neutron scattering. The commensurate antiferromagnetic order of La2CoO4 persists in a very short range up to a Sr content of x = 0.3, whereas small amounts of Sr suppress commensurate antiferromagnetism in cuprates and in nickelates. La2-xSrxCoO4 with x > 0.3 exhibits incommensurate spin ordering with the modulation closely following the amount of doping. These incommensurate phases strongly resemble the stripe phases observed in cuprates and nickelates, but incommensurate magnetic ordering appears only at larger Sr content in the cobaltates due to a reduced charge mobility.