Physical properties of perovskite-type lithium ionic conductor

The purpose of this chapter is to understand the ionic conduction of perovskite-type oxides. It is based on the fundamental theories of perovskite structure, ionic conductivity, conductivity measurement, X-ray diffraction and Rietveld analysis, and nuclear magnetic resonance (NMR). Typical examples of lithium ionic conductor are introduced. Introduction Perovskite-type oxides have been considered to be important materials due to their potential applications, such as lithium ion batteries, solid oxide fuel cells, and oxygen sensors, etc [1-6]. To understand the Correspondence/Reprint request: Dr. Naoki Inoue, Department of Physics, Faculty of Science, Ehime University, Matsuyama, Ehime 790-8577, Japan. E-mail: [email protected] Naoki Inoue & Yanhui Zou 248 physical properties of ionic conductor, the crystal structure of perovskite-type oxides, basic theories of ionic conduction, conductivity measurement [6-7], Xray diffraction [8], Rietveld analysis [9,10] and NMR [11-17] are discussed. Section 2 deals with the crystal structure of perovskite-type oxides. Section 3 deals with the subjects of ionic conduction and conductivity measurement, and an example of lithium ionic conductor. Section 4 deals with the principles of X-ray diffraction and Rietveld analysis. Section 5 deals with the basic theory of NMR. Basically, there are some quantities including high resolution NMR spectrum, those are the chemical shifts dependent on the chemical environment, the line-widths dependent on the ionic motion, spinlattice and spin–spin relaxation times, quadrupole interaction etc. Quadrupole effect reflects the information about local symmetry. Section 6 deals with the summary. 1. Perovskite-type oxides structure Figure 1 shows the structure of perovskite-type oxide with the general formula ABO3. These oxides can tolerate different ions in Aand B-sites. The coordination numbers of Aand B-sites are 12 and 6, respectively. In the idealized perovskite-type oxide, the structurally related parameters have the relation ) ( 2 O B O A r r r r + = + (1) Figure 1. Perovskite-type oxides structure. where rA and rB are the ionic radii for cations of Aand B-sites, respectively, and rO is the radius of oxygen ion. But, in order to characterize the real perovskite-type oxide, we use the parameter of tolerance factor as Physical properties of perovskite-type lithium ionic conductor 249 ). ( 2 / ) ( O B O A r r r r t + + = (2) In many compounds of perovskite-type oxide, the parameter of t is in the range of 0.8 to 1.0. When the host ions A and B are replaced by allo-valent ions, the charge neutral compensation is needed. The defect concentration in the perovskite-type oxide is either increased or decreased depending on different types of allo-ion and their concentrations. Materials of perovskite-type structure have a large variety of properties: CaZrO3, SrTiO3 and SrZrO3 are proton conducting materials, and the solid solutions La4/3-yLi3y□2/3-2yTi2O6 (LLTO) are lithium conductors with vacant defects □2/3-2y in which the conductivity is about 10 S/cm at room temperature [4]. In this compound, La ions are located in A-site and Ti ions in B-site. 2. Ionic conduction 2.1. Conductivity The stationary electric current J induced by the electric field x ∂ ∂ / φ is