Enhanced proton conductivity in low-temperature sintered pristine and Ca-doped LaNbO4 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 ~ 700 °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 °C for 3 h) than conventional methods (~ 1000 °C). Calcium doping enhances phase stability and proton conductivity by introducing oxygen vacancies and reducing grain boundary resistance. Impedance analysis further reveals that La_0.99Ca_0.01NbO₄ a proton conductivity of 5.23 × 10^‒4 S·cm^‒1 at 700 °C, markedly higher than pristine LaNbO₄ (9.5 × 10^‒5 S·cm^‒1). These findings position La_0.99Ca_0.01NbO₄ as a highly promising candidate for hydrogen energy applications.