Effect of solvent on phase composition and particle morphology of lanthanum niobates prepared by polymeric complex sol–gel method

Lanthanum niobates were prepared by a new polymeric complex sol–gel method using Nb-citrate or -tartrate complexes in different solvent (ethanol or methanol) and calcination at 750–1,050 °C. The perovskite La1/3NbO3 and pyrochlore LaNb5O14 phases were formed after calcination at 900 and 1,050 °C from gels synthesized from ethanol and methanol solvents respectively. The very similar xerogel thermal decomposition processes were observed independently on applied solvents, where the pyrochlore monoclinic LaNbO4 and Nb2O5 phases were intermediate products at lower calcination temperatures during transformation. The particle morphologies changed from spherical 20–50 nm particles at 750 °C to granular LN particles (ethanol) or rectangular (methanol) at 1,050 °C. HRTEM images and SAED verified the coexistence of minority monoclinic LaNbO4 phase with majority phases in individual LN particles after annealing. The strong effect of alcohol solvent on phase formation was shown, while the effect of chelating agent was insignificant.