High temperature neutron diffraction investigation of U₃Si₂

The U-Si system is actively undergoing studies due to its promise as a component of an accident tolerant nuclear fuel. The crystal structure of the U₃Si₂ compound in the U-Si system was investigated as a function of temperature from room temperature to 1298 K for a sample of approximately 42.02 at.% Si composition and 1373 K for a sample with approximately 42.10 at.% Si using high temperature time-of-flight neutron diffraction on the High-Pressure Preferred Orientation (HIPPO) diffractometer at Los Alamos Neutron Science Center (LANSCE). The simultaneous Rietveld refinement of five histograms from the five HIPPO detector backs (40°, 60°, 90°, 120°, and 145°) provided datasets for the lattice parameters, anisotropic atomic displacement parameters, and atomic positions as a function of temperature. To explore the possibility of a homogeneity range, two sample compositions were analyzed, a 40.02 at.% Si (near stoichiometric) and a potentially hyperstoichiometric (40.10 at.% Si) sample. While minor differences in the anisotropic atomic displacement parameters between the two samples were observed, over the entire investigated temperature range no additional phases were found indicative of a single phase sample and the ability of the U₃Si₂ compound to accommodate nonstoichiometry, suggesting that the U₃Si₂ compound is not a line compound confirming previous determinations. However, the differences observed in the average coefficient of linear thermal expansion identified between the two compositions warrant future investigation. Synopsis High temperature time-of-flight neutron diffraction measurements were carried out on 2 different compositions of U₃Si₂ and detailed analyses of the U₃Si₂ crystal structure as a function of temperature to 1298 K (for a 42.02 at.% Si sample) and 1373 K (for a 42.10 at.% Si sample) were performed. Anisotropic atomic displacement parameters, bond lengths, thermal expansion, coefficient of thermal expansion, and related properties were assessed as a function of temperature as well as the location of excess Si atoms and compared with simulation predictions.