Low-Temperature Phase Transition in Alpha Uranium

Abstract

A phase transition in alpha uranium at 42\ifmmode^\circ\else\textdegree\fi{}K\ifmmode\pm\else\textpm\fi{}1\ifmmode^\circ\else\textdegree\fi{}K is indicated by measurements of the single-crystal elastic moduli using ultrasonic pulse techniques. The indicated transition coincides closely to anomalies noted in the thermal expansion, Hall coefficient, electrical resistivity, and thermoelectric power. An examination of the literature specific heat data shows that the transition is associated with an inflection point in the entropy vs temperature curve. On the basis of the elasticity and available lattice parameter data it is postulated that the temperature dependence of the $y$ positional parameter becomes relatively large in this range and the phase instability is a result of a change in the nature of certain interatomic bonds.