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 measurements of the magnetic susceptibility and of the thermal expansion of a LaCoO3 single crystal. Both quantities show a strongly anomalous temperature dependence. Our data are consistently described in terms of a spin-state transition of the Co 3+ ions with increasing temperature from a low-spin ground state (t ) without (100 K -500 K) and with (> 500 K) orbital degeneracy. We attribute the lack of orbital degeneracy up to 500 K to (probably local) Jahn-Teller distortions of the CoO6 octahedra. A strong reduction or disappearance of the Jahn-Teller distortions seems to arise from the insulator-to-metal transition around 500 K.Transition-metal oxides have fascinating physical properties as e.g. high-temperature superconductivity in the cuprates or colossal magnetoresistance in the manganites. Their properties are often governed by a complex interplay of charge, magnetic, structural, and orbital degrees of freedom. Moreover, for a given oxidation state some transition metals display different spin states as it is the case in various cobalt oxides. Quite recently a class of layered cobalt compounds with the chemical composition REBaCo 2 O 5+δ (RE = rare earth) has attracted considerable interest. These compounds show a broad variety of ordering phenomena and other transitions, e.g. (antiferro-and/or ferro-) magnetic order, charge and/or orbital order, metal-insulator transitions or spin-state transitions [1,2,3,4,5,6,7,8,9]. For TlSr 2 CoO 5 it has been proposed that a metal-insulator transition is driven by a spin disproportionation, which consists of an alternating ordering of Co The occurrence of Co 3+ in different spin states is known since the 1950s from LaCoO 3 [12, 13], which transforms with increasing temperature from a non-magnetic insulator to a paramagnetic insulator around 100 K and shows an insulator-to-metal transition around 500 K. But even for this rather simple pseudo-cubic perovskite the nature of these transitions is still unclear. The ground state is usually attributed to the low-spin configuration (LS: t 6 2g e 0 g ; S = 0) and the paramagnetic behavior above 100 K to the thermal population of an excited state. However, the question whether the excited state has to be identified with the HS or the IS state is subject of controversial discussions. Early publications often assume a LS/HS scenario [14,15,16]. In order to explain the insulating nature up to 500 K an ordering of LS and HS Co 3+ ions has been proposed which vanishes at the insulatorto-metal transition [17,18]. Yet the presence of a HS configuration below 400 K has been questioned on the basis of X-ray absorption and photoemission experiments [19]. Alternative descriptions of LaCoO 3 favoring a LS/IS scenario [20,21,22,23,24] are mainly based on the results of LDA+U calculations [25], which propose that due to a strong hybridization between Co-e g levels and O-2p levels the IS state is lower in energy than the HS state. Within this scenario the occurrence of orbital order and its melting have been proposed in order to e...

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.

We report on the magnetic, thermodynamic and optical properties of the quasi-one-dimensional quantum antiferromagnets TiOCl and TiOBr, which have been discussed as spin-Peierls compounds. The observed deviations from canonical spin-Peierls behavior, e.g. the existence of two distinct phase transitions, have been attributed previously to strong orbital fluctuations. This can be ruled out by our optical data of the orbital excitations. We show that the frustration of the interchain interactions in the bilayer structure gives rise to incommensurate order with a subsequent lock-in transition to a commensurate dimerized state. In this way, a single driving force, the spin-Peierls mechanism, induces two separate transitions.PACS numbers: 75.10. Jm, 75.40.Cx, 71.70.Ch Low-dimensional quantum spin systems exhibit a multitude of interesting phenomena. For instance a onedimensional (1D) S=1/2 chain coupled to the lattice may show a spin-Peierls transition to a non-magnetic, dimerized ground state. In recent years, detailed studies of the first inorganic spin-Peierls compound CuGeO 3 have deepened the understanding of spin-Peierls systems substantially [1]. Even richer physics is expected if the spins are coupled additionally to orbital or charge degrees of freedom. A prominent example is the complex behavior of NaV 2 O 5 , which arises from the interplay of spin dimerization, orbital order and charge order [1]. Recently, TiOCl and TiOBr have been discussed as spinPeierls compounds with strong orbital fluctuations [2-9], assuming a near degeneracy of the t 2g orbitals in these 3d 1 systems. Different quantities such as the magnetic susceptibility [2], the specific heat [9], ESR data [3] and NMR spectra [4] point towards the existence of two successive phase transitions, which clearly goes beyond a canonical spin-Peierls scenario in which a single secondorder phase transition is expected. The high transition temperatures of T c1 =67 K and T c2 =91 K found in TiOCl are fascinating in a spin-Peierls context.The structure of TiOX consists of 2D Ti-O bilayers within the ab plane which are well separated by X=Cl/Br ions [10]. Quasi-1D S=1/2 chains are formed due to the occupation of the d y 2 −z 2 orbital in the ground state (see below), giving rise to strong direct exchange between neighboring Ti sites along the b axis (y direction) and negligible coupling in the other directions. Accordingly, the magnetic susceptibility of TiOCl is well described at high temperatures by the 1D S=1/2 Heisenberg model with an exchange constant of J ≈ 676 K [2,3]. In the non-magnetic low-temperature phase, a doubling of the unit cell along the chain direction has been observed by x-ray measurements for both TiOCl [10] and TiOBr [11], supporting a spin-Peierls scenario. However, the following experimental facts are not expected in a canonical spin-Peierls system: (i) the existence of two successive phase transitions [2-4,9], (ii) the first-order character of the low-temperature phase transition [9][10][11], (iii) the observation of inequivalen...