Heat Capacity and Thermal Expansion of Uranium-Gadolinium Mixed Oxides

Uranium-gadolinium mixed oxides of four different compositions, (U1-y Gdy) O2±x (y = 0.1, 0.2, 0.5, and 0.8), were synthesized. Single-phase fluorite structure was observed for the compositions (U1-y Gdy) O2±x (y = 0.1, 0.2, and 0.5). The room temperature lattice constants measured for (U0.9 Gd0.1) O2.02, (U0.8 Gd0.2) O2.00, and (U0.5 Gd0.5) O1.98 are 0.5463, 0.5454, and 0.5433 nm, respectively. Heat capacity measurements in the temperature range 298 – 800 K were carried out using a differential scanning calorimetry. Considerable anomalous increases in the heat capacity values were observed for compositions (U1-y Gdy) O2±x with y = 0.1, 0.2, and 0.5. The heat capacity values of (U0.9 Gd0.1) O2.02, (U0.8 Gd0.2) O2.00, and (U0.5 Gd0.5) O1.98 at 298 K are 63.5, 61.1, and 65.7, respectively. Thermal expansion characteristics of (U0.9 Gd0.1) O2.02, (U0.8 Gd0.2) O2.00, and (U0.5 Gd0.5) O1.98 were studied using a high temperature X-ray diffraction in the temperature range 298 – 1973 K.