New dielectric material systems of Sr_xNd₂₍₁–_x)/3TiO₃ perovskites-like at microwave frequencies

Perovskite compounds, Sr_xNd_2(1−x)/3TiO₃ (SNTx) with x ranging from 0.05 to 0.2, were prepared by the conventional solid–state reaction method. X-ray diffraction patterns illustrated that high density and single phase products were obtained for all samples, indicating that a small level of Sr addition could substantially stabilise the structure and maintain a single phase for Nd_2/3TiO₃ ceramics. But there was also an orthorhombic space groups transition from Pnma to P4/mmm when x was above 0.18. In addition, the dielectric constant (ε_r) of SNTx ceramics had improved from ∼85.6 to ∼112 with increasing x value due to the larger ionic polarizability of Sr²⁺ than that of Nd³⁺. Notably, the product of dielectric Q value and resonant frequency (Q × f) of ∼295–650 at 3.07–3.32 GHz was strongly dependent on the grain size and the cation deficient in the present systems. Either increasing grain size or decreasing A-site vacancies led to an increase in the Q × f value. Besides, a temperature coefficient of resonant frequency (τ_f) of the SNTx ceramics sintered at their optimum densification temperature gradually increases from 191.7 ppm/°C (x = 0.05) to 221.3 ppm/°C (x = 0.2), which results from the lower rattling effect and higher symmetry in BO₆ octahedra of perovskite structure according to bond valence theory and tolerance factor (t). - Highlights: • A small level of Sr addition could stabilise the structure for Nd_2/3TiO₃ ceramics. • The rattling effect of BO₆ octahedra increases with decreasing B-site bond valence. • There was an orthorhombic space groups transition from Pnma to P4/mmm. • Homogeneous grain size and decreasing A-site vacancies led to an increase in Q × f.