Effect of lanthanide ion substitution on RF and microwave dielectric properties of BiNbO4 ceramics

Ceramic samples of BiNbO4 and (Bi(0.95)Ln(0.05))NbO4 (Ln = Dy3+, Gd3+, Nd3+ or La3+) with 0.5 wt% V2O5 addition sintered at 960 degrees C were investigated. The dense (>92%) polycrystalline niobate samples revealed clean orthorhombic (Pnna) phase by powder XRD (except La3+ substitution, showing co-existing similar to 31% triclinic phase (P (i) over bar)). The values of (C-band) microwave dielectric constant (epsilon(r)') and quality factor (Q(u).f) of the niobates were found to increase with increasing ionic radius of the substituted Ln ion. The Q(u).f values showed a strong dependence on the bond valance parameter, whereas, the s; values replicated changes in dielectric polarizability. The epsilon(')(r) values (at 4-5 GHz) were 46.5 (pure BiNbO4), 40.8 (Ln = Dy3+), 41.7 (Ln = Gd3+), 43.4 (Ln = Nd3+) and 47.5 (Ln =La3+). Dielectric measurements made on the sintered samples at RF frequencies (1 kHz-1 MHz) showed 44.7 <= epsilon(r)' <= 53.4, and also revealed almost a monotonic rise of antiferroelectric-ferroelectric transition temperature from 360 to 370 degrees C with Ln (Dy3+ to La3') substitution. The Arrhenius plot of d.c. conductivity for the Ln ion substitutions corroborated with the variations seen in Qd values. (C) 2009 Elsevier B.V. All rights reserved.