Thermal Stability and Proton Conductivity of Rare Earth Orthophosphate Hydrates

摘要

Hydrated orthophosphate powders of three rare earth metals, lanthanum, neodymium and gadolinium,were prepared and studied as potential proton conducting materials for intermediate temperatureelectrochemical applications. The phosphates undergo a transformation from the rhabdophane structureto the monazite structure upon dehydration. The thermal stability of the hydrate is studied and found tocontain water of two types, physically adsorbed and structurally bound hydrate water. The adsorbedwater is correlated to the specific surface area and can be reversibly recovered when dehydrated aslong as the rhabdophane structure is preserved. The bound hydrate water is accommodated in theo rhabdophane structure and is stable at temperatures of up to 650 C. The thermal stability of thehydrate water and the phosphate structure are of significance for the proton conductivity. TheLaPO4螄0.6H2O and NdPO40.5H2O exhibited the structure dependence of the proton conductivitywhile the GdPO40.5H2O showed a large effect of the phosphate morphology.