First principles study of pyrophosphate defects and dopant–defect interactions in stronium-doped lanthanum orthophosphate

LaPO sub(4) has been actively studied for proton conductor applications, due to its stability and proton uptake in humid atmospheres over intermediate temperature ranges. An important process underlying the application of this and related materials for proton-conductor applications is the hydrolysis of pyrophosphate defects. In this work we undertake density-functional-theory (DFT) calculations of the relative energetics of pyrophosphate defects and protons in LaPO sub(4), including their binding with divalent dopant cations. Due to the low symmetry of the monazite crystal structure for LaPO sub(4), there exists four symmetry-distinct pyrophosphate defect configurations; DFT calculations are used to identify the most stable configuration, which is 0.24 eV lower in energy than all others. Further, from supercell calculations with 1.85 mol% Sr doping, we investigate the dopant-binding energies for pyrophosphate defects to be 0.37 eV, which is comparable to the value of 0.34 eV calculated for proton-dopant binding energies in the same system. These results establish that dopant-defect interactions further stabilize proton incorporation, with the hydration enthalpies when the dopants are nearest and furthest from the protons and pyrophosphate defects being -1.66 eV and -1.37 eV, respectively. Even though our calculations show that dopant binding enhances the enthalpic favorability of proton incorporation, they also suggest that such binding is likely to substantially lower the kinetic rate of hydrolysis of pyrophosphate defects.