In situ X-ray diffraction study of point defects in neptunium dioxide at elevated temperature

Lattice thermal expansion of NpO₂ has been measured up to 2000 K by in situ X-ray diffraction. Above 1643 K, the departure from linearity is interpreted on the basis of the calculation of the vacancy formation energy from the oxygen Debye temperature, as a combination of chemical reduction into NpO_2-x and the development of oxygen Frenkel pairs.