Structural and defect properties of the LaPO4 and LaP5O14-based proton conductors

Atomistic simulation techniques are used to perform a comparative study of intrinsic defects, dopant incorporation and protonic groups in two lanthanum phosphate compounds, namely, the orthophosphate (LaPO4) and the ultraphosphate (LaP5O14). The suitability of dopant incorporation predicted from the dopant solution energies (with Ca and Sr the most favorable) is in excellent agreement with trends in ionic conductivity from recent experimental investigations. The defect chemistry of the phosphates related to protonic defects and oxygen vacancies created from extrinsic doping is investigated. The results indicate favorable orientations for the protonic defect within the structures. The binding energies for proton-dopant interactions indicate that defect association may occur. In LaPO4 it was observed that the relaxed local atomic structure around an oxygen vacancy is analogous to the formation of a P2O7 pyrophosphate anion.