Step 1: Understand the problem setup.
In the FeO crystal (ideal formula), some Fe2+ ions are missing from their lattice sites. To maintain electrical neutrality (since each missing Fe2+ leaves an excess negative charge from surrounding O2- ions), other iron atoms compensate by adopting the Fe3+ oxidation state.
Step 2: Set up the charge balance for Fe0.95O.
The actual formula is Fe0.95O, meaning there are only 0.95 iron atoms per oxygen atom instead of the expected 1.00. Let y be the number of Fe3+ ions among the 0.95 total Fe ions. The rest are Fe2+ ions, so Fe2+ count = (0.95 - y).
Step 3: Apply electrical neutrality condition.
For each formula unit of Fe0.95O, the O2- ion carries a charge of -2. The total positive charge from iron must equal +2: \[ 2(0.95 - y) + 3y = 2 \] \[ 1.90 - 2y + 3y = 2 \] \[ 1.90 + y = 2 \] \[ y = 0.10 \]
Step 4: Interpret the result.
So 0.10 of the iron ions are Fe3+ and 0.85 are Fe2+, totalling 0.95 iron ions per oxygen. This means 5 out of every 100 Fe2+ sites are vacant, and to compensate, 10 Fe2+ ions have been oxidised to Fe3+ (each Fe3+ compensates for 0.5 missing Fe2+ in terms of charge).
Step 5: Identify the type of defect.
This is a metal deficiency defect (a type of non-stoichiometric defect). It occurs because iron has a variable oxidation state (can be Fe2+ or Fe3+). Cation (Fe2+) sites are vacant in the lattice, and some Fe3+ ions substitute for Fe2+ to maintain charge balance. The crystal remains electrically neutral but the metal-to-anion ratio is less than 1 (metal deficient). This also makes FeO a p-type semiconductor.
Step 6: State the final answer.
The formula is Fe0.95O, and it exhibits a metal deficiency defect (non-stoichiometric defect with cation vacancies compensated by higher-valence cations).
\[ \boxed{\text{Fe}_{0.95}\text{O, metal deficiency defect}} \]