A brown sample of zinc oxide was found to have the hexagonal wurtzite structure with a = b = 3.2495 Å, c = 5.2069 Å (α = β = 90°;γ = 120°). Chemical analysis gave 80.765% Zn by mass. Density measurements gave 5810 kg/m³. Determine the formula of the material and state whether it contains oxygen vacancies or interstitial metal atoms.