Since oxidation is correlated with lattice parameters through chemical expansion, the time it takes for the lattice parameters to stabilize reflects the oxidation kinetics. Refined lattice parameters as a function of time after in situ switching of atmosphere from N2 to O2 at different temperatures are shown in Figure . Both samples show a net contraction in the a and c axis during oxidation, and the time to reach equilibrium decreases as expected with increasing temperatures. The contraction along c is lower in magnitude for HEO1-n than for HEO2-n, which is related to the former being slightly oxidized (indicated by appearance of the R3c reflections in Figure ) prior to exposure to O2. The net contraction in c also decreases with temperature for HEO1-n, as TGA measurements (Figure ) shows that the sample has a smaller OSC at T > 350 °C. When comparing the lattice parameters measured before oxidation in Figure with the lattice parameters found for the as-prepared HEO1-n in Table (a = 6.181(8) Å, c = 11.412(0) Å), we find a net expansion, not contraction, of the abplane during oxidation. During initial oxidation the ab-plane expands, while contraction is observed as the oxidation progresses. This is also observed for the Ti-doped sample, which displays a subtle expansion of the a parameter before contraction during oxidation.