Perovskite-related-structure Materials with High Pressure Synthesis and Their Physical Properties

• Main Authors: Ting-Hui Kao, 高莛蕙
• Other Authors: Hung-Duen Yang
• Format: Others
• Language: en_US
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/7rfadt

博士 === 國立中山大學 === 物理學系研究所 === 106 === The 3d and 4d transition metal (TM) oxides with perovskite-related structures are attracting research interest because they show a rich variety of interesting physical properties. The focus of this dissertation is making high-quality materials and studying their physical properties. Synthesizing high-quality materials in high-pressure and high-temperature are needed for some special compounds. This dissertation is focus on the physical properties of K2NiF4-type structure and A-site ordered perovskites compounds. The first topic is "the crystal structure and physical properties of Cr and Mn oxides with 3d3 electronic configuration and a K2NiF4-type structure". YSrCrO4 is first synthesized and found to be a hettotype of the K2NiF4 structure, although the combination of Y and Sr ions in K2NiF4-type oxides is very rare. The space group of the compound is determined to be orthorhombic Pccn by the electron diffraction and the powder X-ray diffraction. Magnetic and dielectric properties of the compound, together with LaSrCrO4, YCaCrO4 and A2MnO4 (A = Sr and Ca), are investigated. YSrCrO4 shows two-dimensional (2D) spin correlations and a canted antiferromagnetic (AF) ordering. With increasing distortion of the crystal structure, 2D spin correlations and the Antiferromagnetic (AF) transition temperature decrease, while spontaneous magnetization increases. From the multi-frequency electron spin resonance (ESR) measurements, the g-value of the paramagnetic state are estimated to be 1.976, 1.978 and 1.976 for YSrCrO4, LaSrCrO4 and YCaCrO4, respectively. Evidence of AF ordering of the Cr oxides is obtained microscopically from ESR. The dielectric measurements suggest the existence of in-gap states, while no magneto-dielectric coupling was observed in the above compounds. The second topic is "electronic phase transition between localized and itinerant states in the solid-solution system CaCu3Ti4-xRuxO12". The solid solution between CaCu3Ti4-xRuxO12 end members was first synthesized using a high-pressure synthesis technique. The residual magnetization at 2 K sharply changes at 2 x 3 and the spin-glass-like phase suddenly disappears at x = 2.5, suggesting a first-order electronic phase transition. Magnetic susceptibility shows a strong temperature dependence for x ≤ 1 and a weak one for x ≥ 3. At an intermediate x, the weak temperature dependence at high temperatures changes to a strong temperature dependence at low temperatures. The temperature at the change rapidly decreases with increasing x. The electrical resistivity shows variable-range- hopping type behavior for x ≤ 1, metallic behaviors for x ≥ 3.5, and resistivity minima at Tp ∼ 150 K, 8.5 K, and 3.5 K for x = 2.0, 2.5, and 3.0, respectively. A proposed electronic phase diagram of the solid-solution system accounts for these results. The local magnetic moments at CuO4 squares of the Ti compound become itinerant with increasing Ru content by means of a first-order phase transition. The transition can be regarded as a unique Mott transition because the atomic orbitals of the Ru cations play an important role in the increased hopping amplitude of the electrons/holes of CuO4 molecular-like orbitals. In the last topic, we studied the origin of the magnetic susceptibility maximum in CaCu3Ru4O12 and electronic states in the A-site substituted compounds. CaCu3Ru4O12 shows a broad maximum at around 200 K in temperature dependence of magnetic susceptibility, whose origin is under debate. The present study addresses this problem, using high-quality samples of Ca1−xAxCu3Ru4O12 (A = La, Na, and Sr) made by high pressure synthesis technique. Unlike in a previous report, the maximum shifts to lower temperatures for the La substitution, becomes obscure by the Na substitution, and is less influenced by the Sr substitution. This behavior strongly suggests that the susceptibility maximum is caused by a sharp peak in the density of states just above the Fermi level, which induces strong spin fluctuations. Furthermore, the nature of electronic states of LaCu3Ru4O12 and NaCu3Ru4O12 are discussed; the former likely bears a Kondo character, and the latter has spin fluctuations of different origin below approximately 150 K.