Résumé
(U2O)(PO4)2 is related to ultra-low expansion β-(Zr2O)(PO4)2 ceramics, but shows a continuous thermal contraction. High-temperature neutron diffraction has allowed to follow accurately the thermal variations of its cell edges and to give a structural explanation to the phenomenon: like in β-(Zr2O)(PO4)2, the dilatometric anomaly arises simultaneously from a contractive push–pull effect due to Coulombic repulsions and from a libration of the PO4 and UO7 polyhedra, but in the present case, the second mechanism predominates. The size of the tetravalent cation appears as a key parameter in monitoring the thermal expansion of ceramics of this family.