Pr2NiO4+d exhibits extremely high oxygen diffusivity. The crystal structure of Pr2NiO4+d consists of PrNiO3 perovskite and PrO rock salt units. The exceptional high oxygen diffusivity was ascribed to a combination of interstitial diffusion in PrO rock salt layers and vacancy diffusion in PrNiO3 perovskite structure. A-site deficiency further improves the oxygen diffusivity in the Pr2-xNiO4+d (x > 0). This increase in oxygen diffusivity was ascribed to the creation of vacancies at A-site positions which lower the diffusion barrier for oxygen atoms. A complete understanding of the diffusion mechanism as well as accurate knowledge of the composition with highest oxygen diffusivity does not exist until now. We will, therefore, create nuclear density maps based on neutron diffraction data to determine the oxygen positions in the crystal lattice. The concentration of vacancies on A-site position will be changed systematically by variation of the A-site deficiency.