Properties of the microwave dielectric phase Ba_6−3XNd_8+2XTi₁₈O₅₄

Abstract

Abstract The Ba6-3xRE8+2xTi18O54 (RE = rare-earth element) solid solution has been well established as a useful microwave resonator material due to its high dielectric constant (ϵr), low loss (tanδ = 1/Q), and tunable temperature coefficient of resonant frequency (τf). Ceramics of this phase (RE = Nd) have been processed in this study by a mixed-oxide route, and the variation in properties caused by divalent and tetravalent doping was investigated. Dielectric properties have been measured for single-phase pellets with the stoichiometries x = 0.25, 0.50, and 0.75. As the value of x increased, Q was found to increase significantly, and both ϵr and τf decreased. Various dopants, including Ca, Sr, Pb, Zr, and Hf, were used in the x = 0.75 composition to tune electrical properties, with varying degrees of success. Most doped samples were multiphase. Ca- and Sr- doping, while raising ϵr of the ceramic, resulted in very poor Q's and unacceptably high values of τf. Zr- and Hf-doping, on the other hand, caused a modest reduction in τf but produced drastic reductions in Q and ϵr. Only Pb-doping was found to usefully reduce τf whilst leaving both ϵr and Q at reasonable levels. The composition which exhibited the best properties was (Ba0.875Pb0.125)3.75Nd9.5Ti18O54, for which ϵr = 72, Q = 1000 (at 6. 1 GHz), and τf = 8-11 ppm/°C.