Reversible topochemical exsolution of iron in BaFe(2+)2(PO4)2

BaFe(2+) 2 (PO4 )2 was recently prepared and identified as the first 2D-Ising ferromagnetic oxide with an original reentrant structural transition driven by high-spin Fe(2+) ions arranged in honeycomb layers. Both long-term air exposure and moderate temperature (T>375 °C) leads to topochemical oxidation into iron-depleted compounds with mixed Fe(2+) /Fe(3+) valence. This process is unique, as the exsolution is effective even from single crystal with preservation of the initial crystallinity, and the structure of the deficient BaFe2-x (PO4 )2 (x