Combined synthesis methods for producing LaNbO4 ceramics and investigation of microwave dielectric properties based on complex chemical bond theory

Highly reactive LaNbO4 nanopowders with particle sizes of 20–50 nm were synthesized at 700 °C through the citric gel process. Niobium oxalate was used as raw material for the first time, which provided a more convenient and precise way for the whole process. In order to reduce the sintering temperature of the LaNbO4 ceramic without introducing other impurities, LaNbO4 nanopowders were added to densify the ceramic at a lower temperature. When different mass percentage x wt% (x = 0, 0.5, 1, 2, 4) of the nanopowders were added, the sintering temperature of LaNbO4 ceramics prepared through conventional solid-state reaction method was successfully lowered more than 100 °C. The obtained ceramics presented good microwave dielectric properties. Rietveld refinement was used to investigate the crystal structure of the samples. The relationships between bond ionicity and dielectric constant (εr), lattice energy and the Q×f0 value, bond energy and the temperature coefficient of resonant frequency (τf) were calculated and analyzed with P–V-L theory. LaNbO4 ceramics with 1 wt% LaNbO4 nanopowders additive sintered at 1275 °C for 4 h showed excellent microwave dielectric properties: εr = 20.26, Q×f0 = 59740 GHz and τf = 7.44 ppm/°C.