Synthesis and Properties of Narrow-band Materials

9528826古德福先前对电子性质演化的研究表明，自然界以各种方式适应交叉的一阶特征：从定域电子行为到巡回电子行为，从混合金属氧化物中的主要离子到主要共价的金属-氧键，从单价和混价体系中的居里-魏斯顺磁性到泡利顺磁性。经典的相分离、电荷密度波或铁电位移代表了已经研究得很好的静态迁移量。这项研究解决了动态住宿这个更微妙的问题。最初，重点将放在具有钙钛矿相关结构的氧化物上。ABO3型钙钛矿中含有一个角共用的BO6八面体的BO3阵列。A-O和B-O平衡键长之间的不匹配可以通过BO6八面体的协同旋转来缓解，该八面体使B-O-B键角从180弯曲。根据B-O-B相互作用的强度，具有过渡金属B离子的钙钛矿可能包含B-阳离子氧化还原对或窄的反键带，其能量接近或重叠费米能量。B-O-B相互作用的强度以及BO_3阵列的带宽和带占有率可以通过A位上的等价和/或等价取代而改变。此外，Ca180B-O-B键可以通过桥联原子朝向一个B原子而远离另一个B原子而在同一晶体结构中容纳不同B-O键强度的共存。这些位移通常是协同的，可以是静态的，也可以是动态的。高温超导现象似乎是动态适应的一个例子。另一种是钙钛矿体系Srl-xCaxV03中Mott-Hubbard态和费米-液态相的明显共存，光谱权重从后者转移到前者。发展了几种技术来合成这些边缘稳定的氧化物，并测量压力下电阻、磁化率和塞贝克系数的温度依赖性。这些探头可以提供有关动态过程的重要信息(如果是间接的)。%这项研究是由三个基本问题推动的：1)导致高温超导现象的潜在物理过程是什么？2)如何将对这一现象的理解作为改进的高TC材料设计的指导？3)自然通过哪些不同的方式适应固体中从局域电子行为到巡回电子行为的转变？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.