Synthesis, Crystal Structure, and Dielectric Properties of Ba_xY₂₆Si₁₆O_71+x (x ≈ 10.2)

Abstract

To reexamine the crystal structure of the tetragonal oxide previously reported as Ba 5.2 Y 13 Si 8 O 41 , single crystals were prepared by the self-flux method at 1600 °C and X-ray crystal structure analysis was performed. In addition to the fundamental reflections indexed by the cell parameters of the tetragonal system (a = 18.9229(3) Å, c = 5.34880(10) Å), the X-ray diffraction pattern showed faint satellite peaks, suggesting that the oxide is an incommensurate composite crystal. Crystal structure analysis with the fundamental reflection data was performed using an average structure model with split sites of Ba and Y atoms. The space group of the model was I̅ 4̅ 2m, the same as previously reported, and the composition formula determined by refining the occupancies of the split Ba sites was Ba 10.22 Y 26 Si 16 O 81.22 . Ceramics with the composition Ba x Y 26 Si 16 O 71+x (9 ≤ x ≤ 11), which consisted of the tetragonal oxide as the main phase, were synthesized, and their dielectric properties were evaluated. The relative dielectric constants (ε r s) and dielectric losses (tan δs) measured at room temperature for the x = 10.4 ceramics sintered at 1690 °C were 1400 and 0.02 at 10 Hz and 220 and 0.48 at 1 MHz, respectively. The dielectric dispersion could be expressed with the Cole−Cole model. A bulk sample containing the tetragonal oxide with the c axis aligned in one direction was fabricated via unidirectional solidification. The ε r s of the bulk measured in the directions parallel to the c-axis direction (720 at 10 Hz and 350 at 1 MHz) were more than 10 times larger than those in the direction normal to the c-axis direction (49 at 10 Hz and 32 at 1 MHz).