Structural and mechanical properties of sol–gel prepared pyrochlore lanthanum niobates

摘要

Lanthanum orthoniobate LaNbO4 (LN) precursors were prepared by a modified polymeric complex sol–gel method using Nb-tartrate in different solvents (ethanol or methanol) with calcination at 750 °C. The monoclinic LaNbO4 and orthorhombic LaNb5O14 phases were formed after annealing at 900 °C from precursors synthesized in ethanol (LNet) and methanol (LNmet), respectively. FTIR and Raman spectra confirmed different structures of LN at 900 °C in both solvents. The microstructures indicate the strong effect of solvents on surface morphology. TEM and SAED patterns verified the monoclinic LaNbO4 and orthorhombic LaNb5O14 phases with ferroelastic domains at 750 and 900 °C, respectively. Mechanical characterization was conducted for the first time by nanoindentation on the sol–gel derived LN after annealing at a low temperature. The hardness and elastic modulus of LNmet (LNet) were measured to be ~2.6(1.5) and ~45(30) GPa, respectively. The modulus and hardness of LNmet were higher in comparison with LNet. The strong solvent effect on mechanical properties of lanthanum niobate was confirmed.