Crystal Structure Variations in Alpha Uranium at Low Temperatures

Abstract

The unusual temperature dependence of the elastic moduli and other physical properties of alpha uranium near 43\ifmmode^\circ\else\textdegree\fi{}K which have been previously reported are found to be correlated with changes in atomic position and with cell dimensions. The atom position parameter, $y$, decreases to a minimum at 43\ifmmode^\circ\else\textdegree\fi{}K and then rapidly rises on further cooling, as shown both by x-ray and neutron data. The ${a}_{0}$ and ${b}_{0}$ cell dimensions decrease to a minimum at 43\ifmmode^\circ\else\textdegree\fi{}K and then rapidly increase, giving a large negative linear expansion coefficient in the range 43-18\ifmmode^\circ\else\textdegree\fi{}K; the ${c}_{0}$ dimension contracts on cooling to 43\ifmmode^\circ\else\textdegree\fi{}K and decreases even more rapidly below this temperature thus giving a higher positive linear expansion coefficient in the 43-18\ifmmode^\circ\else\textdegree\fi{}K range. Neutron diffraction experiments disclose the existence of additional reflections at the lower temperatures which are presumably due to a magnetic moment arrangement.