The solid is isostructural with (has the same structure as) fluorite (calcium fluoride), where each U is surrounded by eight O nearest neighbors in a cubic arrangement. In addition, the dioxides of cerium, thorium, and the transuranic elements from neptunium through californium have the same structures.  No other elemental dioxides have the fluorite structure. Upon melting, the measured average U-O coordination reduces from 8 in the crystalline solid (UO cubes), down to 6.7±0.5 (at 3270 K) in the melt. Models consistent with these measurements show the melt to consist mainly of UO and UO polyhedral units, where roughly  of the connections between polyhedra are corner sharing and  are edge sharing.