Thermal Expansion of Alpha-Uranium below 10°K

The thermal-expansion coefficient of $\alpha -\mathrm{U}$ (orthorhombic) remains negative in the $a$ and $b$ directions and positive in the $c$ direction below 43°K, the reported temperature for a suggested phase transition. Below 5°K, the temperature dependence of the three expansion coefficients can be described by $\{{\alpha }_1,=(\frac{1}{a})\frac{\partial a}{\partial T}=-21\mathrm{}\times {10}^{-8\mathrm{}}T-27\mathrm{}\times {10}^{-10\mathrm{}}{T}^3,\}\{{\alpha }_2,=(\frac{1}{b})\frac{\partial b}{\partial T}=-3.7\mathrm{}\times {10}^{-8\mathrm{}}T-3.1\mathrm{}\times {10}^{-10\mathrm{}}{T}^3,\}\{{\alpha }_3,=(\frac{1}{c})\frac{\partial c}{\partial T}=+5.0\mathrm{}\times {10}^{-8\mathrm{}}T+3.2\mathrm{}\times {10}^{-10\mathrm{}}{T}^3.\}$ It is shown that these results are thermodynamically consistent with the pressure dependence of the specific heat observed by Ho et al. Hysteretic behavior is observed in that the linear dimension of the crystal at 0°K depends on the speed of cooling in the range 25-15°K.