Zero-temperature bulk modulus of alpha-plutonium

Abstract

Using resonant-ultrasound spectroscopy, we measured alpha-plutonium’s bulk modulus $B$ between 298 and $18\mathrm{K}$. Fitting the measurements to an Einstein-oscillator-based function gave the zero-temperature bulk modulus $B_o=70.9\mathrm{GPa}$. We compare our measurement with numerous previous measurements and with numerous theoretical estimates ranging from $41\text{to}227\mathrm{GPa}$. From $0\text{to}300\mathrm{K}$, $B\left(T\right)$ is regular and smooth, evincing no phase transition (electronic, magnetic, structural). The bulk modulus decreases to $54.4\mathrm{GPa}$, about 30%, a very large change compared with typical materials. We attribute this large decrease to electron localization during warming. High-temperature $dB∕dT$ yields a Gruneisen parameter $\gamma =5.1$, too high we believe because of temperature-induced electron localization. From the low-temperature elastic constants, averaged in the usual Debye $⟨v^{-3}⟩$ manner, we obtain a Debye temperature ${\Theta }_D=205\mathrm{K}$.