Fig. 1: Thermal expansion and magnetostriction in the paramagnetic state of UO₂.

a The relative change of the lattice constant in the <111> direction of the UO₂ single crystal as a function of temperature, measured by dilatometry of the bulk crystal (dashed line) and X-ray diffraction of the [888] peak (red squares). The X-ray data were obtained by single crystal diffraction in high-angle back-reflection geometry using 15.85 keV X-rays. An abrupt collapse in unit cell volume can be seen at T_N = 30.8 K, marked by an arrow. Insets show the structure of the UO₂ cubic unit cell above T_N (paramagnetic phase) and below T_N (ordered phase). b The magnetic response of the [888] peak in the paramagnetic phase (T = 40 K) to an applied pulsed magnetic field in the <111> direction. The initial state at 0 T (black line) shifts to higher 2θ at maximum applied field of 21.2 T (red line), then back to the same initial value after the pulse (blue line). The shift in 2θ of the peak corresponds to a small ~50 p.p.m. strain contraction in the <111> direction, consistent with bulk single crystal magnetostriction measurements².