Nanocrystalline samples were prepared using a modified citric acid synthesis method previously reported by Bergum et al. and Danmo et al., with composition and naming of all samples shown in Table . For the precursor solutions, metal acetates R(CH3CO2)3•xH2O (R = Y, Ho, Er, Dy, Tb, Mn) were each dissolved in a mixture of citric acid and deionized water, and stirred on a hot plate set to 150 °C for 3 hours. Titanium (IV) isopropoxide as the Ti 4+ precursor was added to a mixture of deionized water and citric acid. The precursor solutions were added in stoichiometric amounts together with ethylene glycol. The resulting solution was kept on a hot plate at 150 °C for 24-48 hours until a viscous gel was formed. The gel was then dried at 120 °C for 3 days before calcination in air at 600 °C for 6 hours, resulting in an amorphous powder. These powders were pre-reduced in 5 % H2 in N2 at 250 °C for 10 hours to promote formation of the desired hexagonal structure, and then crystallized at 900 °C in N2 atmosphere for 1 hour. More details about the synthesis are given in the Supporting Information. Mass changes were measured during heating to 800 °C and subsequent cooling to room temperature, using heating and cooling rates of 5, 10 or 20 °C min -1 . δ values were calculated relative to a reference point where all Mn is assumed to be found as Mn 3+ , chosen as the point of lowest mass at T > 700 °C, corresponding to δ = 0 for undoped and δ = 0.075 for Ti-doped samples. High temperature XRD of the samples HEO1-n and HEO2-n was done with a Bruker D8 Advance with Cu Kα radiation. Data were first collected in O2 at 25 °C intervals during heating to 725 °C, then upon subsequent cooling to 50 °C, and finally during re-heating to 725 °C. This was repeated in N2 atmosphere. Pawley refinements of lattice parameters and crystallite size was performed with the Bruker AXS TOPAS 5 software. Synchrotron HT-XRD and X-ray absorption near edge structure (XANES) measurements during in situ switching of atmosphere from N2 to O2 was performed using BM31 at the Swiss-Norwegian Beamlines (SNBL) at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. Samples were mounted in quartz capillaries and a 10 mL min -1 gas flow was used for all experiments. The wavelength was 0.38591 Å for diffraction while XANES fluorescence signal was measured at the Mn K-edge and the Tb L3-edge. Lattice parameters were determined by Pawley refinement, and Larch 42 was used for normalization of XANES data.