Structural And Electrical Properties Of Superconductors BaBi_1-xPb_xO₃ And Magnetic And Transport Properties Of Layered Cobalt Oxides

The contents in this dissertation are mainly focus on two parts, one is about the structural and transport properties of high-critical temperature (high-T_C) superconductor BaBi_1-xPb_xO₃, the other is about the structural, magnetic and transport properties of Ln_2-xSr_xCoO₄ with Ln = Nd and Pr. They are presented as follows:In chapter 1, we make a brief introduction to the fundamental properties of superconductors such as superconductivity, crystal structure and mechanism models; the structure and superconducting mechanism of high-T_C superconductor BaBi_1-xPb_xO₃; and the structural, magnetic and transport properties of layered cobalt oxides, including Ruddlesden-Popper structure, Jahn-Teller effect, spin state of cobalt ions in a tetragonal structure, variation of electrical and magnetic properties with different number of layers.In chapter 2, the relationship between transport property and crystal structure was studied for the series of BaBi_1-xPb_xO₃ in the doping rang, 0.0≤x≤1.0. With increasing doping, the crystal structure of BaBi_1-xPb_xO₃ was confirmed by X-ray diffraction (XRD) to undergo from monoclinic through orthorhombic and tetragonal to orthorhombic. The lattice parameters keep decreasing tendency in the semiconducting range, whereas vibrate like a sine wave in the superconducting range. The variation was ascribed to the breathing mode of Bi(Pb)O₆ in the specimen, which was presented in the superconducting range and was frozen out in the semiconducting range. Raman spectra indicate that T_C depends not only on the deformation potential of the soft A_1g mode, but also on the mode's energy shift. The electrical property of the specimen is connected sensitively with the crystal structure, and its metal-insulating transition and superconductivity may be understood by overlapping bands.In chapter 3, structural property was studied for the series of Nd_2-xSr_xCoO₄ in the doping range 1.0≤x≤1.6 as well as at different temperatures. The room-temperature XRD patterns indicate that the lattice parameters and bond lengths increase monotonously with increasing Sr content in the doping range, 1.00≤x≤1.50, but present a downturn when x = 1.60. Relative to the small-radius ions, Nd²⁺ (99.5 pm), the increase is consistent with the introduction of the bigger ones, Sr²⁺ (112 pm), but the uptrend might break down for the mismatch among the crystal lattices. The frequency shift of the phonon bands in Raman and infrared spectra is found to be consistent very well with the variation of the bond length, which suggests that the frequency shift mainly comes from the change of Coulomb force between ions. For the specimen with x = 1.25, XRD and Raman scattering indicate that the specimen remains in K₂NiF₄ structure from 83 K to 373 K. All the lattice parameters, bond lengths, and frequency shift of phonon bands present a discontinuity at T_C, suggesting the phonon bands of the material might track the structural and magnetic temperature evolutions.In chapter 4, magnetic and transport properties were closely studied for the series of Nd_2-xSr_xCoO₄ (x = 1.25, 1.33, 1.60) by measurements of resistivity, thermopower, susceptibility, hysteresis loop, electron spin resonance, and magnetoresistance. The specimen shows semiconducting properties over all the temperature range measured and follows the Arrhenius law at high temperatures. Small polaron model may be applied to explain the transport mechanism of the material. With decreasing temperature the specimen undergoes from paramagnetic state through Griffiths state and ferromagnetic state to spin-glass state reentrance. Negative magnetoresistance is observed for the specimen. The cobalt ions with different valences, Co³⁺ and Co⁴⁺, are both in middle-spin state configuration. Referring to the magnetic property of similar-structural cobalt oxides with different rare-earth ions doping, the plenty of magnetic features for the specimen are ascribed to the introduction of Nd ions, and the appearance of them is under the condition of a limited range of disorder and/or aberrant change in crystal domain.In chapter 5, structure and transport properties of Nd_0.75Sr_1.25CoO₄ thin filmswere studied. Thin films were grown on five different substrates. Only diffraction peaks along the c direction were observed in the pattern, indicating single phase of the films with K₂NiF₄ structure that epitaxially grow along the c axis. As in bulk specimen, semiconducting behavior (dρ/dT < 0) is observed in the ab-plane resistivity for the whole temperature region measured without any abnormality. However, a step-like increase in thermopower presents in Griffiths phase, which indicates that thermopower may be a good method for exploring the spin state feature. The transport property of thin films is mostly governed by the 2-dimentional variable range hopping (2D-VRH) model. The reason may be the mismatch in crystal lattice, and the disorder in energy band.In chapter 6, magnetic and transport properties of Pr_2-xSr_xCoO₄ (x = 1.2, 1.3, 1.5) were studied. Semiconducting behavior (dρ/dT < 0) is also observed for the specimen, but with increasing doping it changes from thermally activated model to 2D-VRH model. Similarly, the specimen undergoes from paramagnetic state through Griffiths state and ferromagnetic state to spin-glass state. The substitution of Sr ions can enhance the ferromagnetic property, but weaken and eliminate the spin-glass state and Griffiths singularity. For the specimen the appearance or disappearance of spin-glass state and Griffiths singularity may occur at the same time, and the magnetic and transport properties can be understood by double-electromagnetic phase separation.