Materials Science and Engineering, Nagoya Institute of Technology
Tungstenbronze-type like Ba6-3xR8+2xTi18O54 (R=rare earth) solid solution reveals simultaneously high dielectric constant εr and high quality factor Q, proposed as an application for mobile phone. The crystal data in the case of R=Sm with superlattice doubled of c-axis are as follows: orthorhombic Pbnm (No.62), a=12.131(13), b=22.271(5), c=7.639(5) Å and Z=2. The fundamental structure is composed of three types of large cation sites: ten A1 rhombic sites in 2×2 perovskite blocks, four A2 pentagonal sites and four trigonal sites. The pentagonal and trigonal sites are located among the perovskite blocks. The fundamental structure is expressed by the formula [R8+2xBa2-3xVx]A1 Ba6-3xR8+2xTi18O54 ( 0 ≦ x ≦ 2/3 ). Here, V means vacancy. In the 0 ≦ x < 2/3 composition region, the A1-sites are occupied mainly by medium-sized R-ions, and also by a small amount of large Ba-ions. For x = 2/3, the A1-sites and A2-sites are occupied by R-ions and Ba-ions, respectively. A1(1)- and A1(5)-sites are two-cap trigonal prisms with 8 coordination numbers CN, A1(2)- and A1(4)-sites are a distorted cubic dodecahedra with 8 CN, and the A1(3)-site is a three-cap trigonal prism with 9 CN, whereas A2(1)- and A2(2)-sites are a two-cap hexahedron with 10 CN. In this study, relationships between crystal structure and microwave dielectric properties are presented. There are three important properties such as dielectric properties, quality factors and temperature coefficient of resonant frequency. We would like to present guidelines for design based on the relationship.