Non-stoichiometry of iron oxide

Four iron oxide (FeO) samples were heated at the same temperature under the atmosphere of pure oxygen. After a sufficiently long time, the samples are quenched and kept at the ordinary temperature to optimize their properties, which they had acquired at high temperature. 

Iron is titrated in each sample. Cell parameters are measured from X-ray diffraction pattern and density is determined. 

Can you say whether iron deficit is due to an excess of oxygen or a lack of iron in the crystal lattice  

Taking into account the preceding result, propose in the Wagner approximation, a model of point defects for this iron oxide, involving iron ions with valency of +3. 

Calculate the deviation from stoichiometry and the site fractions of each structural element related to the point defect. 

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