High pressure single crystal diffraction of UO2 and UN*

Uranium dioxide is an important material which is the primary fuel used in comercial nuclear reactors. and uranium nitride has similar properties that make it a candidate as a possible new fuel material. Both have antiferromagnetic magnetic transitions at low temperatures that corespond to unusual physical behavior, indicating strong magneto-elastic coupling. Below 30.8 K, UO₂ shows a small volume collapse due to a latice distortion, as well as rarely seen piezomagnetic behavior. Below 50 K, UN also orders antiferromagnetically and switchs to negative thermal expantion. UN crystalizes in the cubic rocksalt structure, but ongoing magnetostriction measurements show signs of distotions below T_N. Using High quality and unifrm single crystals of UO₂ and UN, we have systematically examined the crystal structure and change in compressability to high pressures above and below the magnetic transiton temperatures.