Harrison Neil
Los Alamos National Laboratory, Los Alamos, NM 87545
Proc Natl Acad Sci U S A. 2020 Mar 3;117(9):4480-4485. doi: 10.1073/pnas.1918281117. Epub 2020 Feb 18.
Plutonium metal exhibits an anomalously large softening of its bulk modulus at elevated temperatures that is made all the more extraordinary by the finding that it occurs irrespective of whether the thermal expansion coefficient is positive, negative, or zero-representing an extreme departure from conventional Grüneisen scaling. We show here that the cause of this softening is the compressibility of plutonium's thermally excited electronic configurations, which has thus far not been considered in thermodynamic models. We show that when compressible electronic configurations are thermally activated, they invariably give rise to a softening of the bulk modulus regardless of the sign of their contribution to the thermal expansion. The electronically driven softening of the bulk modulus is shown to be in good agreement with elastic moduli measurements performed on the gallium-stabilized δ phase of plutonium over a range of temperatures and compositions and is shown to grow rapidly at small concentrations of gallium and at high temperatures, where it becomes extremely sensitive to hydrostatic pressure.
钚金属在高温下其体模量呈现出异常大的软化现象，而更不同寻常的是，无论热膨胀系数是正、负还是零，这种软化都会出现——这与传统的格林爱森标度有极大的偏离。我们在此表明，这种软化的原因是钚热激发电子构型的可压缩性，而这在热力学模型中迄今尚未被考虑。我们表明，当可压缩电子构型被热激活时，无论它们对热膨胀的贡献符号如何，都会不可避免地导致体模量的软化。结果表明，由电子驱动的体模量软化与在一系列温度和成分下对镓稳定的钚δ相进行的弹性模量测量结果吻合良好，并且在镓浓度较低和温度较高时迅速增大，此时它对静水压力变得极其敏感。
