New dielectric material systems of SrxNd2(1–x)/3TiO3 perovskites-like at microwave frequencies

Abstract

Perovskite compounds, SrxNd2(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 Nd2/3TiO3 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 (εr) 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 (τf) 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).