Synthesis and Properties of Narrow-band Materials

先前对电子性质演化的研究表明，自然界以多种方式适应交叉的一阶特征：从局部电子行为到流动电子行为，从混合金属氧化物中的主要离子到主要共价金属-氧键，从单价和混合价体系中的居里-魏斯顺磁性到泡利顺磁性。经典的相偏析、电荷密度波或铁电位移都是静态调节的研究对象。这项研究解决了更微妙的动态适应问题。最初，主要重点将放在具有钙钛矿相关结构的氧化物上。ABO3钙钛矿包含BO3阵列的角共享BO6八面体。BO6八面体的协同旋转使B-O- b键角从180度弯曲，可以缓解A- o和B-O平衡键长度之间的不匹配。根据B- o -B相互作用的强度，具有过渡金属B阳离子的钙钛矿可能包含B阳离子氧化还原偶或能量接近或重叠费米能量的窄反键带。B-O-B相互作用的强度以及BO3阵列的带宽和频带占用可以通过a位点上的等价和/或同价取代而改变。此外，ca 180 B- o -B键可以通过桥接原子向一个B原子移动而远离另一个B原子，从而在同一晶体结构内容纳不同B- o键强度的共存。这些位移通常是合作的，可以是静态的，也可以是动态的。高温超导现象似乎是动态调节的一个例子。另一个是钙钛矿体系Srl-xCaxV03中Mott-Hubbard态和Fermi-liquid态的明显共存，其中谱权从后者转移到前者。开发了几种技术来合成这些微稳定的氧化物，并测量压力下电阻、磁化率和塞贝克系数的温度依赖性。这些探针可以提供动态过程的重要信息，即使是间接的。这项研究的动机是三个基本问题：1)导致高tc超导现象的潜在物理过程是什么？2)如何利用对这一现象的理解来指导设计改进的高tc材料？3)在固体中，自然界有哪些不同的方式来适应从局部电子行为到流动电子行为的转变？最初的重点将放在具有钙钛矿相关结构的氧化物的研究上。

9528826 Goodenough Previous studies of the evolution of electronic properties have revealed that nature accommodates the first-order character of the cross-overs in a variety of ways: from localized to itinerant electronic behavior, from primarily ionic to primarily covalent metal-oxygen bonding in mixed- metal oxides, from Curie-Weiss to Pauli paramagnetism in both single-valent and mixed-valent systems. Classical phase segregation, charge-density waves, or ferroelectric displacements represent static accommodations that are well- studied. This research addresses the more subtle problem of dynamic accommodations. Initially, a major emphasis will be placed on oxides with perovskite-related structures. An ABO3 perovskite contains a BO3 array of corner-shared BO6 octahedra. A mismatch between the A-O and B-O equilibrium bond lengths may be relieved by a cooperative rotation of the BO6 octahedra that bend the B-O-B bond angle from 180 . Perovskites with transition-metal B cations may contain, depending on the strength of the B-O-B interactions, either B-cation redox couples or narrow, antibonding bands with energies close to or overlapping the Fermi energy. The strength of a B-O-B interaction and therefore the bandwidth and the band occupancy of the BO3 array may be varied by isovalent and/or aliovalent substitution on the A sites. Moreover, the ca 180 B-O-B bond may accommodate the coexistence of different B-O bond strengths within the same crystallographic structure by the displacement of a bridging atom toward one B atom and away from the other on the opposite side. These displacements are generally cooperative and may be either static or dynamic. The phenomenon of high- temperature superconductivity appears to be one example of a dynamic accommodation. Another is the apparent coexistence of Mott-Hubbard and Fermi-liquid states in the perovskite system Srl-xCaxV03 where spectral weight is transferred from the latter to the former. Several techniques are de veloped for synthesizing these marginally stable oxides, and measuring the temperature dependence of resistance, magnetic susceptibility, and Seebeck coefficient under pressure. These probes can give important, if indirect, information on dynamic processes. %%% This research is motivated by three fundamental questions: 1) What is the underlying physical process responsible for the phenomenon of high-TC superconductivity? 2) How can one use the understanding of this phenomenon as a guide in the design of improved high-TC materials? 3) What are the different ways by which nature accommodates the transition from localized to itinerant electronic behavior in solids? Initial emphasis will be placed on the study of oxides with perovskite-related structures.