Despite many previous attempts, Sr4Fe6O12 (the expected fully reduced composition for Sr4Fe6O13-x) has not been synthesised. It is of interest because it is believed to contain equal amounts of Fe2+ and Fe3+ in edge-linked square pyramids. This material has now been synthesised and Mossbauer spectroscopy supports an electronic model that is related to that observed in Fe3O4: elctron delocalisation giving Fe2.5+ at 300 K, but localised electrons to form Fe2+ and Fe3+ at 20 K. The detailed structure with respect to the oxygen sublattice has not been established using XRD, so we propose to use NPD to determine the structure asa function of temperature to establish structural changes arising from electron localisation and possible charge order. The magnetic order will also be determined.