Exploration of Dopant Species for Lanthanum Polyphosphate.

Doped lanthanum polyphosphate (LaP₃O₉) exhibits relatively high proton conductivity. For the practical applications such as the electrolyte of fuel cells, however, its conductivity must be improved by 2 orders of magnitude. Protons are introduced into matrix by lower-valent cation doping, and proton conductivity depends on dopant species. To date, LaP₃O₉ has been doped with only Ca, Sr and Ba. In this work, we tried to dope LaP₃O₉ with Na⁺, K⁺, Mg²⁺ and Pb²⁺, as the new dopant species, due to their close ionic radii to La³⁺. Among them, only Pb could substitute for La at a comparable concentration to those of alkaline earth metals and its highest doping level was 6.4 mol% (Doping level is defined as the concentration ratio of dopant (M) to host cation (La) site in matrix (≡M/(La + M) x 100 (mol%))). Though Pb can exist as either divalent or tetravalent state, Pb in LaP₃O₉ was identified to be divalent state by XPS analysis. Proton conduction was demonstrated by H/D isotope effect. The electrical conductivity of Pb-doped LaP₃O₉ increased with Pb-doping level, owing to the increase in proton concentration. The conductivity of 4.5 mol% Pb-doped LaP₃O₉ was about one order of magnitude lower than that of 7.9 mol% Sr-doped LaP₃O₉. [ABSTRACT FROM AUTHOR]