Structure and microwave dielectric property relations in (Ba_1-xSr_x)₅Nb₄O₁₅ system

The effects of strontium substitution for barium on the structure and microwave dielectric properties of Ba₅Nb₄O₁₅ compounds is investigated using X-ray diffraction, Raman spectroscopy, and microwave dielectric properties measurement. The symmetric stretching vibration of Ba-rich compounds splits into two narrow Raman-active modes and indicates the existence of a strong anharmonic lattice. Nevertheless, the symmetry changes from P over(3, ̄) m 1 to P over(3, ̄) c 1 for the Sr-rich compounds. The relative permittivity (ε_r) and temperature coefficient of resonant frequency (τ_f) increases linearly with an increase of strontium content in Ba-rich compounds, but decreases in Sr-rich compounds. The transition of symmetry influences the variation of ε_r and τ_f. The behavior of ε_r and τ_f can also be correlated to the variation of symmetric stretching vibrations of NbO₆ octahedra and (c/a) ratio of lattice parameter in the unit cell.