| Recently layered transition metal oxides have attracted great attentions by the communities of materials science and physics due to their rich physical properties and potential applications. These rich physical properties come from the electron-electron correlation of transition metal elements, variable types and intensities of the crystalline field and spin-orbit coupling. While the layered structure leads to a strong anisotropy in the crystal structure, the electronic energy band structure and phonon energy band structure, it in turns affects their electrical, thermal and magnetoelectric transport properties. Moreover, in the triangular lattice system, it generates the geometrical frustration effect if the degree of electron spin is considered. These synergetically combined effects lead to rich physical phenomena observed in layered transition metal oxides. For example, the coexistence of metallic conductivity and a huge Seebeck coefficient in NaxCoO2, extremely large magnetoresistivity in PdCoC^, and unconventional superconductor Sr2RuO4, etc.Up to now, the studies of AxB02 type layered transition metal oxides have been focused on 3d transition metal element compounds, but relatively less researches are done on 4d transition element compounds. Many problems are still unsolved. For example,1. In 4d transition metal elements, electron-electron correlation is decreased, but spin-orbit coupling is increased. How does this change influence the electric, magnetic, magnetoelectric and thermoelectric properties? 2. Theoretical work predicts that adjusting the doping concentrations and strength of the magnetic coupling in the triangular lattice would produce rich magnetic structures. It may lead to a significant anomalous Hall effect, and even a quantum anomalous Hall effect. Can we observe these phenomena in the experiments? 3. In 3d transition element AxBO2 type compounds, the rich electronic phase diagrams are observed with x varied. Will it also happen in 4d transition element compounds?In this dissertation, we choose triangular lattice 4d transition metal oxide KxRhO2 as the material system. Using high temperature solution method and chemical method, we prepared a series of high quality KxRhO2(x=0.24-0.72) single crystals. The structures, compositions, morphologies, and electric and magnetic transport properties at low temperatures of these materials were systematically characterized and analyzed by theoretical calculations. The main research work and conclusions are summarized as follows:1. We have grown the KxRhO2(x=0.55,0.58,0.63 and 0.72) single crystals successfully using the high temperature solution method. The powder XRD refinements prove their space groups as P63mmc. The growth mechanism of KxRhO2 crystals have been studied carefully. SEM,TEM and DSC results substantiate that the surface of the crystals is a smooth one. Many crystal nucleuses were formed simultaneously, these nucleuses grew along the ab-plane and finally coalesed to form crystals with a thin-foil morphology.2. We have devopled the chemical depotassiation method to prepare a series of low-K content KxRhO2 crystals. The results of the XRD, Raman and inelastic neutron scattering substantiate the inhomogeneity of K ions in the two dimensional interlayers. Theoretical calculation proves that K ions migrate along the one dimensional channel during the depotassiation process. And there exist several metastable sites for K ions in KxRhO2. The inhomogeneity of K ions in the crystals is related to these states. We have proved that combining chemical depotassiation with the low temperature annealing one can obtain high quality crystals.3. We have systematically studied the transport properties of the KxRhO2(x=0.24～0.72) crystals. Exprimetal data substantiate:(1) KxRhO2(x=0.24～0.72) crystals except x=0.50 show the bad metal behaviors, and the carrier concentrations and mobilities are around 1022 cm-3 and 1～10 cm2 V-1 s-1, respectively. The first principle calculation for KxRh02 shows the metallic behavior comes from the d bands of Rh element that are across the Fermi surface. (2) K0.5RhO2 also shows the metallic behaviors similar to others at high temperatures. While the metal-nonmetal transion occurs at low temperatures, it is accompanied by a negative magnetoresisitance and an anomalous Hall effect. According to the results of the magnetic properties and theoretical calculations, the anomalous transport properties are related to the non-coplanar magnetic structure. And this magnetic structure is sensitive to the doping concentrations. When the K content deviates from x=0.50, the anomalous transport properties disappeared. (3) KxRhO2(x=0.24～0.72) crystals show the strong anisotropies in the electric properties between ab plane and c direction, and usually demonstrate the multi-band Fermi liquid behaviors. (4) Single crystal with x=0.63 has a relatively large Seebeck coefficient 46.3 μV/K and power factor 2.2 W·cm-1 K-2 at room temperature.4. Undoped and rare-earth elements (La, Sm, Ho and Dy) doped Ko.58Rh02 crystals were grown. Below 10 K, Sm, Ho and Dy doped samples show a significant magnetoresistance (SMR) effect and an anomalous Hall effect (AHE). The SMR effect comes from the ferromagnetic clusters in the crystals. AHE is not only related to ferromagnetic clusters, but also correlated with the electrons in the f shells of magnetic ions Sm, Ho and Dy. In Sm doped samples, the ferromagnetic coupling enhances the normal Hall resistivity caused by Lorentz force. While in Ho and Dy doped samples, the antiferromagnetic coupling reduces the Hall resistivity caused by Lorentz force.Based on these studies, we comprehend the microstructures, growth mechanisms and electrical and magnetic transports of KxRhO2(x=0.24～0.72) in depth. The developed growth method and chemical depotassiation method can be applied to the synthesis of similar layered compounds. The study of physical properties in KxRhO2 provides an insight to understand the physical properties of other 4d transition metal oxides. |
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