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The effects of magnesium and niobium substitution for titanium on the microwave dielectric properties of Ba 3.75 Nd 9.5 Ti 18Àz (Mg 1/3 Nb 2/3 ) z O 54 (0 £ z £ 3) ceramics were studied. The temperature coefficient of resonant frequency (s f ) decreased from about +60 ppm/°C to +17 ppm/°C when z £ 1. Excellent quality factor (Qf = 7300 GHz) as well as high dielectric constant (e r = 80.96) were obtained. For z ‡ 1.5, Nd 2 (Ti,Mg,Nb) 2 O 7 secondary phase appeared which would obviously influence the microwave dielectric properties. As z varied from 0 to 3, matrix grain size degraded which would obviously deteriorate the microwave dielectric properties by conducting more pores, especially the Qf value. The s f value was found to be related to b-site bond valence (V B-O ) and unit cell volume (V m ). Average ionic polarizability (a D ) and relative density evidently influenced the dielectric constant.

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