Effect of A-Site Ionic Radius on the Structure and Microwave Dielectric Characteristics of Sr1+xSm1-xAl1-xTixO4 Ceramics

SrSmAlO4 microwave dielectric ceramics were modified by Sr/Ti cosubstitution for Sm/Al. The effects of radius difference of A-site ions on the microwave dielectric characteristics were investigated together with the structure. Sr1+xSm1-xAl1-xTixO4 (x = 0, 0.05, 0.10, and 0.15) ceramics were prepared by a solid-state reaction approach. X-ray diffraction studies revealed a single-phase K2NiF4-type solid solution with corresponding peaks shifting to lower 2 theta as x increased. Minor inhomogeneous grain morphology for x = 0.05 and a trace amount of second phases for x = 0.10, 0.15 were detected by backscattered-electron microscopy and energy-dispersive X-ray analysis. With increasing Sr/Ti cosubstitution, the dielectric constant er increased from 18.4 to 20.4, and the temperature coefficient of resonant frequency tau(f) was adjusted from -1.8 to 7.4 ppm/degrees C almost linearly. However, the Q x f value decreased from 74,500 GHz at x = 0-53,000 GHz at x = 0.15. The internal stresses caused by the decreased tolerance factor and the large ionic radii difference between Sr2+ and Sm3+ should be the predominant reasons for such a decrease in the Q x f value. The high-resolution transmission electron microscopic results revealed an increase in the lattice distortion with increasing Sr/Ti cosubstitution, and subsequently supported the above conclusion upon the increased internal stresses.