New insights into the thermal expansion of Neptunium dioxide up to 2000 K

摘要

We report for the first time the thermal expansion of NpO_2 up to 2000 K and compare it to the one of UO_2 and PuO_2. Lattice parameters were measured as a function of temperature by in situ X-ray diffraction with neutral, reducing and oxidizing atmospheres. Data for NpO_2 under neutral and reducing conditions perfectly superpose. Up to 1550 K, thermal expansion varies linearly with increasing temperature. Above, thermal expansion presents a curvature towards higher lattice parameters. No bibliographic data were available for this temperature range for comparison. As for the thermal expansion of UO_2, we propose this curvature is certainly inferred by the formation of lattice defects in the oxygen sub-lattice. Expression of thermal expansion and coefficients of thermal expansion at given temperatures for NpO_2, are presented.