Microstructure development of proton-conducting La-based metaphosphate during heat-treatment

We report a TEM microstructural and compositional study of proton-conducting (La,Ca)P3O9 (Al-free) and (La,Ca,Al)P3O9 (Al-containing) glass-ceramics, prepared by the crystallization of quenched glass melts. Bright-field Transmission Electron Microscopy imaging, annular dark-field Scanning Transmission Electron Microscopy, and energy dispersive x-ray compositional analysis were used to characterize the structure and chemistry of the glass and crystalline phases of the samples that developed following the treatments at various temperatures, heating-rate and dwell-time. TEM chemical characterization revealed that the crystallized domains were mainly Ca-doped LaP3O9, while the remaining phase was a Ca-rich phosphate glass. Crystallization improved conductivity first (up to a conductivity of nearly 10− 4.5 S/cm at 550 °C in air). However, the conductivity dropped when the samples were treated at higher temperature or for longer dwell-times, due to a continued Ca-departure from the La-rich crystalline into glass phase. Possible proton transfer mechanisms and strategy to improve conductivity in this doped rare earth metaphosphate glass-ceramic are discussed.