Pr2NiO4+d overstoichiometric oxides are one of the most prominent candidates in regard to oxygen membrane material for SOFCs at already moderate temperatures. Recent studies on La2CuO4+d and also Brownmillerite (Ca,Sr)FeO2.5 systems have shown a phonon-assisted diffusion mechanism to be at the origin of low temperature oxygen mobility in these type of frameworks. These findings are in agreement with what has been theoretically predicted in terms of diffusion pathways but at much more elevated temperatures for K2NiF4 type structures. This mechanism is essentially based on a dynamical disorder of the apical oxygen atoms being on a shallow potential on a ring of about 2 Å radius in the case of Pr2NiO4.25. Furthermore, Pr2NiO4+d is highly organized in terms of 3D-oxygen ordering implying large unit cells. Thus, in order to better understand the diffusion mechanism in K2NiF4-type oxides, the analysis of the complex incommensurate structure of Pr2NiO4.25 is a key issue. We therefore want to undertake structure analysis by single crystal neutron diffraction on D19.