It should be noted that the FeO-ZnO system is significantly different from the previously described systems. This is mainly due to the fact that FeO (wüstite) is a non-stoichiometric compound and at ambient pressure is only stable above 873 K. At lower temperatures it must decompose to a mixture of Fe + Fe 3 O 4 (magnetite) , which is, however, kinetically hindered. According to Xray diffraction data, the initial "FeO" (Aldrich) sample is a mixture of 96 wt% rs-Fe 0.925 O (wüstite) with lattice parameter a = 4.308(1) Å and 4 wt% α-Fe. After high pressure -high temperature treatment, the iron impurity dissolves to form cubic Fe 0.985 O with lattice parameter a = 4.312(1) Å, which is in good agreement with literature data .