We have used several experimental methods to study how a large extrinsic oxygen vacancy density in pure tetragonal ZrO2 powders depends on details of how those powders are made. Samples were made from oxychloride and nitrate precursor solutions, We used perturbed angular correlation spectroscopy to determine ill situ phase structure and the density of oxygen vacancies at 1200 degreesC, XRD and SEM to determine the grain size and morphology of samples annealed at temperatures ranging from 200 degrees -1200 degreesC, and neutron activation analysis (NAA) to investigate purity of samples, NAA results showed that samples contain cation impurities at levels <<100 ppm, The XRD and SER I measurements showed that grains were nanometer-size, had a broad distribution, and grew from similar to 10 nm at 200 degreesC to similar to1 mum at 1200 degreesC. The most striking process dependence is on presence of chlorine during processing. The grain size and phase above 600 degreesC, and both the morphology and the density of oxygen vacancies at 1200 degreesC were strongly affected by presence of chlorine-containing vapor during annealing, Samples processed in a chlorine-free atmosphere had large well-sintered grains and large (>500 ppm) oxygen vacancy concentrations at 1200 degreesC, whereas samples processed in flowing. H2O/HCl vapor had smaller grains, porous morphology, and small (<100 ppm) vacancy density. All samples were loose powders consisting of single grain particles at <1000 degreesC and multiple-grain particles at 1200 degreesC.