BSTRACTIncreasing surface-to-volume ratios for nanoscale materials may cause metal(II)oxides phases to be thermodynamically unstable compared to their bulk counterparts. For instance

previous studies have found FeO to be unstable for nanoparticles with dimensions below 100 nm. In this study in-situ TEM was used to gradually reduce nanoparticles and nanochains of Fe

2

O

3

. Electron energy-loss spectroscopy and selected area diffraction at different temperatures not only confirm earlier predictions

but also reveal the unexpected stabilization of the FeO phase for nanochains with a minimal critical length. Hence

dimensionally constrained phases were stabilized on length-scales that were previously considered unattainable.