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...

The crystal and magnetic structure of La1−xSr1+xMnO4 (0 ≤ x ≤ 0.7) has been studied by diffraction techniques and high resolution capacitance dilatometry. There is no evidence for a structural phase transition like those found in isostructural cuprates or nickelates, but there are significant structural changes induced by the variation of temperature and doping which we attribute to a rearrangement of the orbital occupation.

LaTiO3 is known as a Mott insulator which orders antiferromagnetically at T(N)=146 K. We report on results of thermal expansion and temperature dependent x-ray diffraction together with measurements of the heat capacity, electrical transport measurements, and optical spectroscopy in untwinned single crystals. At T(N) significant structural changes appear, which are volume conserving. Concomitant anomalies are also observed in the dc resistivity, in bulk modulus, and optical reflectivity spectra. We interpret these experimental observations as evidence of orbital order.

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