a	unit cell parameter (Å) or (nm);
D*	oxygen tracer diffusion coefficient (cm2 s−1);
f 16–18	C16O18O atomic fraction;
n(H+)/n(O2−)	molar ratio of the proton content to the oxygen;
T	temperature (°C) or (K);
V	unit cell volume (Å3);
Greek letters
α	18O atomic fraction;
2θ	diffraction angle (°);
ρ	true density (g cm−3);
σH+	protonic conductivity (Ω−1 cm−1).