Structural and relaxor-like dielectric properties of unfilled tungsten bronzes Ba5−5xSm5xTi5xNb10−5xO30

Abstract

New unfilled tetragonal tungsten bronze (TTB) oxides, Ba5−5xSm5xTi5xNb10−5xO30 (BSTN-x), where 0.10 ≤ x ≤ 0.35, have been synthesized in this work. Their crystal structure was determined and analyzed based on Rietveld structural refinement. It is found that single TTB phase can be formed in a particular x range (i.e., 0.15 ≤ x ≤ 0.3) due to the competition interaction between tolerance factor and electronegativity difference. Furthermore, dielectric and ferroelectric results indicate that phase transitions and ferroelectric states are sensitive to x. Referring to the local chemistry, we suggest that the raise of vacancies at the A2-site compared with that of A1-site will intensely depress the normal ferroelectric phase and is in favor of relaxor ferroelectric state. Macroscopically, previous A-site size difference standpoint on fill TTB compounds cannot give a reasonable explanation about the variation of dielectric maximum temperature (Tm) for present BSTN-x compounds. Alternatively, tetragonality (c/a) is adopted which can well describe the variation of Tm in whole x range. In addition, one by one correspondence between tetragonality and electrical features can be found, and the compositions involving high c/a are usually stabilized in normal ferroelectric phase. It is believed that c/a is a more appropriate parameter to illustrate the variation of ferroelectric properties for unfilled TTB system.