Synthesis and microwave dielectric behavior of (Bi1−xPbx)NbO4 ceramics

Ceramic samples of (Bi1-xPbx)NbO4 (x = 0, 0.025, 0.05, 0.10, 0.15, 0.20) with 0.75 wt.% V2O5 addition sintered at 920 degrees C, 940 degrees C and 960 degrees C are investigated. Pb is selected as a substitute for Bi3+ in BiNbO4 ceramics as it exists in two stable valence states of +2 and +4 and the average valency matches to that of Bi3+. The average Shannon radius (for octahedral coordination) of Pb2+ (1.19 angstrom) and Pb4+(0.775 angstrom) cations is 0.9825 angstrom, which is similar to that of Bi3+ ion (1.03 angstrom). The dense (> 94%) polycrystalline (Bi1-xPbx)NbO4 samples fabricated mostly reveal orthorhombic (Pnna) phase (alpha-BiNbO4, Sp. Gp. 52) by powder XRD. Presence of satellite Pb2Nb2O7 phase, the amount of which is increasing with increase in Pb content, is also noticed. The microwave dielectric constant (epsilon(r)') values of the niobates are found to increase from 42 to 71, whereas the quality factor (Qu.f) values are found to decrease from 5400 to 550 GHz with increasing substitution of Pb. The compositions so synthesized are important as hardly there are any microwave dielectric ceramics available with 45 < epsilon(r)' < 75.