Phase evolution and microwave dielectric properties of A₅M₅O₁₇-type ceramics

A number of A₅M₅O₁₇ (A = Na, Ca, Sr, La, Nd, Sm, Gd, Dy, Yb; B = Ti, Nb, Ta) type compounds were prepared by a solid-state sintering route and characterized in terms of structure, microstructure and microwave dielectric properties. The compatibility of rare earths with mixed niobate/tantalate and titanate phases was investigated. The larger ionic radii mismatch resulted in the formation of pyrochlore and/or mixed phases while in other cases, pure A₅M₅O₁₇ phase was formed. The samples exhibited relative permittivity in the range of 35 to 82, quality factor (Q × fo) = 897 GHz to 11946 GHz and temperature coefficient of resonance frequency (τ_f) = -120 ppm/°C to 318 ppm/°C.