Spin-State Transitions and Lattice Anomalies in oxides near the Mott transition boundary

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The purpose of the present project is to investigate the spin-state transitions in oxides which locate near the metal-insulater (Mott) transition boundary. Not only the magnetic but also the electronic transport and the lattice properties have been investigated since the interactions among spin, charge, and the lattice play an important role in the "spin-state transition" in these oxides. The followings are clarified.1. Nuclear magnetic relaxations of ィイD159ィエD1Co and μィイD1+ィエD1 in LaCoOィイD23ィエD2 and LaィイD20.9ィエD2SrィイD20.1ィエD2CoOィイD23ィエD2The nuclear relaxation rates (TィイD21ィエD2ィイD1-1ィエD1 and TィイD22ィエD2ィイD1-1ィエD1) of ィイD159ィエD1Co measured by NMR for single crystalline LaCoOィイD23ィエD2 show a thermal-activation-type temperature dependence with an activation energy of 180 K. The relaxations are proved to be due to hyperfine interactions with interacting paramagnetic electrons in thermally populated magnetic Co ions. The activation energy agrees with the energy difference between the low-spi … More n (LS ; S=0) and the intermediate-spin (IS ; S=1) states previously proposed in the "two-stage spin-transition" model. In contrast to ィイD159ィエD1Co, the relaxation rate of μィイD1+ィエD1 increases on lowering temperature below 100 K, meaning that the rate increases as the population of nonmagnetic LS Co ions increases. At present, we do not understand the mechanism.2. Lattice anomalies associated with the spin-state transitions in LaCoOィイD23ィエD2Elastic anomalous (lattice softenings) associated with the spin-state transitions are found near 100 K and 500 K by ultrasonic measurements. The results are interpreted based on the "two-stage spin-transition" model successfully. An increase of the absorption of the sound is observed around 100 K, which may suggest a short-range orbital order of IS state Co ions.3. Magnetic and transport properties of LaCoィイD21-xィエD2NiィイD2xィエD2OィイD23ィエD2 (x < 0.1)Co atoms are in IS state down to the lowest temperature and their magnetic moments freeze into a spin-glass state. The resistivity below 400 K decreases by the substitution, although its temperature dependence remains to be an activation type. The Seebeck effect suggests that the carriers are positive polarons although the Hall coefficient is negative. The magnetic-electronic transition near 500 K survives in the substituted samples.4. Resonance PhotoemissionConstant initial state (ClS) spectra as well as energy distribution curve (EDC) are measured for LaCoOィイD23ィエD2 and LaィイD20.9ィエD2SrィイD20.1ィエD2CoOィイD23ィエD2. The Co(3p->3d) resonance energy depends on the spin-state of Co ions. The temperature dependences of CIS and EDC are interpreted with the "two-stage spin-transition" model.5. Crystal growth of LaSrCoOィイD24ィエD2 with the KィイD22ィエD2NiFィイD24ィエD2 structureCrystal growth of a single crystal of LaSrCoOィイD24ィエD2 using floating-zone lamp-image furnace is tested. However, Crystals sufficiently large for neutron scattering experiments have not been obtained. Less