Structure and microwave dielectric properties of Sr2+nCe2Ti5+nO15+3n (n e 10) homologous series

Abstract

Nominally Sr2+nCe2Ti5+nO16+3n dielectric ceramics were prepared by conventional solid-state ceramic route. The structure and microstructural features of the ceramics were investigated by X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. At optimized sintering conditions,it is shown that Ce4+ ions reduce to Ce3+ ions, which leads to the correct formula Sr2+nCe2Ti5+nO15+3n for the materials. The ceramics present a cubic SrTiO3-like structure, where Ce3+ ions and their associated vacancies randomly share the A-sites with the Sr2+ cations. Therefore, this solid solution could be alternatively described by Sr1-3x/2CexTiO3, where x e 0.40 is not necessarily an integer. For 0 e n e 10(0.40 > x > 0.133), these materials have dielectric constant and Quxf in the range of 113-185 and 6000-11000 GHz (at 2 GHz), respectively. However, the relatively high positive temperature coefficients of resonance frequencies decrease substantially to values five times lower than those commonly observed for SrTiO3.