Enhanced proton conductivity in low-temperature sintered pristine and Ca-doped LaNbO<sub>4</sub> nanocrystals synthesized via microwave hydrothermal method

Abstract

Recently, LaNbO4-based proton-conducting materials have emerged as promising alternatives to conventional electrolytes, particularly due to their lower sintering temperatures, making them suitable for hydrogen and humidity sensing applications at temperatures below similar to 700 degrees C. However, LaNbO4 undergoes a structural phase transition from a monoclinic fergusonite to a tetragonal scheelite-type structure at elevated temperatures, which hinders its performance. Controlling this phase transition is, therefore, a critical to enhance proton conduction. The synthesis method plays a pivotal role in stabilizing the phases and optimizing the microstructure of ceramic materials, thereby improving their transport properties. This study demonstrates a novel synthesis of pristine and calcium-doped LaNbO4 nanocrystals using the microwave hydrothermal (MH) method. X-ray diffraction (XRD) analysis confirms the formation of single-phase monoclinic LaNbO4 at a significantly lower calcination temperature (800 degrees C for 3 h) than conventional methods (similar to 1000 degrees C). Calcium doping enhances phase stability and proton conductivity by introducing oxygen vacancies and reducing grain boundary resistance. Impedance analysis further reveals that La0.99Ca0.01NbO4 a proton conductivity of 5.23 x 10(-4) S.cm(-1) at 700 degrees C, markedly higher than pristine LaNbO4 (9.5 x 10(-5) S.cm(-1)). These findings position La0.99Ca0.01NbO4 as a highly promising candidate for hydrogen energy applications.