Nicole Adelstein

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

LaPO4 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 LaPO4, including their binding with divalent dopant cations. Due to the low symmetry of the monazite crystal structure for LaPO4, 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.

Magnetic stability of oxygen defects on the SiO2 surface

The magnetic stability of E′ centers and the peroxy radical on the surface of α-quartz is investigated with first-principles calculations to understand their role in magnetic flux noise in superconducting qubits (SQs) and superconducting quantum interference devices (SQUIDs) fabricated on amorphous silica substrates. Paramagnetic E′ centers are common in both stoichiometric and oxygen deficient silica and quartz, and we calculate that they are more common on the surface than the bulk. However, we find the surface defects are magnetically stable in their paramagnetic ground state and thus will not contribute to 1/f noise through fluctuation at millikelvin temperatures.