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

Ceramic samples of BiNbO4 and (Bi0.95Ln0.05)NbO4 (Ln = Dy3+, Gd3+, Nd3+ or La3+) with 0.5 wt% V2O5 addition sintered at 960 °C were investigated. The dense (>92%) polycrystalline niobate samples revealed clean orthorhombic (Pnna) phase by powder XRD (except La3+ substitution, showing co-existing ∼31% triclinic phase (Pī)). The values of (C-band) microwave dielectric constant (ε′rε′r) and quality factor (Qu.f) of the niobates were found to increase with increasing ionic radius of the substituted Ln ion. The Qu.f   values showed a strong dependence on the bond valance parameter, whereas, the ε′rε′r values replicated changes in dielectric polarizability. The ε′rε′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≤ε′r≤53.444.7≤ε′r≤53.4, and also revealed almost a monotonic rise of antiferroelectric–ferroelectric transition temperature from ∼360 to ∼370 °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 Qu.f values.