Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 11504035), the Scientific and Technological Reseaech of Chongqing Municipal Education Commission, China (Grant Nos. KJ1703044, KJ1703062), and the Chongqing Science and Technology Project, China (Grant No. cstc2018jcyiAX0820).
Received Date:03 December 2018
Accepted Date:30 January 2019
Available Online:01 April 2019
Published Online:20 April 2019
Abstract:The lanthanide and actinide metals and alloys are of great interest in experimental and theoretical high-pressure research, because of the unique behavior of the f electrons under pressure and their delocalization and participation in bonding. Cerium (Ce) metal is the first lanthanide element with a 4f electron. It has a very complex phase diagram and displays intriguing physical and chemical properties. In addition, it is expected to be an excellent surrogate candidate for plutonium (Pu), one of the radioactive transuranic actinides with a 5f electron. The bulk properties and phase transformation characteristics of Ce-based alloys are similar to those of Pu and its compounds. Thus, the investigations of Ce-based alloys are necessary and can potentially advance the understanding of the behavior of Pu. In this work, the equation of state, phase transition, elastic and thermodynamic properties of Ce0.8La0.1Th0.1 alloy at high pressure are investigated by using first-principles calculations based on the density-functional theory. The structural properties of the Ce0.8La0.1Th0.1 alloy are in good agreement with the available experimental and theoretical data. The lattice constant a decreases with pressure increasing, while c shows an opposite variation. It is found that the lattice parameter c shows abnormal jump. And the critical volume is located at 20.1 ?3. The axial ratio jumps from a value of about $\sqrt 2 $ (corresponding to the fcc structure) to a higher value, which indicates that the fcc-bct transition occurs. And the corresponding transition pressure is located at ~31.6 GPa. When the pressure rises to 34.9 GPa, the bct structure displays a saturated c/a axial ratio close to about 1.67. The Young's modulus E, shear modulus G and the Debye temperature of the fcc phase tend to be " softened” around the phase transition pressure. The vibrational free energy is obtained by using the quasi-harmonic Debye model. And then the thermodynamic properties including the thermal equation of state, heat capacity and entropy under high pressure and high temperature are also predicted successfully. The results show that the heat capacity and entropy increase rapidly with temperature increasing, and decrease with pressure increasing. The high pressure can suppress part of the anharmonicity caused by temperature. Keywords:Ce-based alloy/ density functional theory/ phase transformation/ high pressure
表1零温零压下fcc相Ce-La-Th合金的平衡体积(V0)及体积模量(B0) Table1.Equilibrium volume (V0) and bulk modulus (B0) of Ce-La-Th of fcc phase at 0 GPa and 0 K.
对Ce0.8La0.1Th0.1合金的fcc—bct相变进行计算和分析, 将得到的fcc相和bct相的体积-压强关系, 与纯Th[19]、纯Ce[20]、Ce0.76Th0.24[21]及Ce0.875La0.125[12]的数据进行比对, 如图1. 本文的数据与已有的Ce基合金及纯Ce的体积-压强变化的规律相吻合. 图 1 体积随压强变化的规律(黑色实点为直接加压结构优化后的结果, 黑色实线为状态方程拟合结果), 并与已有的Ce[20], Th[19], Ce0.875La0.125[12]的计算值及Ce0.76Th0.24[21]实验值进行比较 Figure1. The EOS of fcc and bct Ce-La-Th together with the experimental data (the black solid point is the result of the structure optimization, the black solid line is the fitting result of the EOS), together with the experimental data for Ce0.76Th0.24[21] and the calculated results for Ce[20], Th[19], Ce0.875La0.125[12].
通过对总能的计算结果表明, fcc和bct两种相结构之间焓的差异非常小, 几乎接近计算误差值, 这意味着不能再通过比较焓变的差异来判断相变压强. 对bct相固定体积进行结构优化, 晶格常数的计算结果如图2所示. 随着体积的减小, 当体积减小至20.14 ?3时, 晶格常数a, c的线性规律发生变化, 这意味着晶体结构出现了变化, 相变开始发生. 计算得到的轴向比c/a与已有的Ce0.76Th0.24[21], Ce[6], Th[11], Ce0.875La0.125[12]数据进行了对比. 当压强小于31.6 GPa时, c/a的值约等于, 随着压强的增加, fcc相变得不稳定, 开始向bct相转变. 在34.9 GPa附近时, bct相趋于稳定 c/a的值约等于1.65, 与我们前期计算得到的CeTh, CeLa合金的fcc相, bct相的c/a的值在误差范围内吻合[11, 12]. 图 2 (a) 晶格参数随体积的变化关系; (b) 轴向比c/a随压强的变化关系, 并与已有的Ce0.76Th0.24[21]实验结果和Ce0.875La0.125[12]、纯Ce[6]、纯Th[11]计算结果进行比较 Figure2. (a) Lattice constants a and c of Ce0.8La0.1Th0.1 as functions of volume; (b) the calculated axial ratio (c/a) of bct phase as functions of pressure.
如图4所示, 随着压强的增加, 弹性模量呈线性增加的趋势, 而杨氏模量E和剪切模量G则在相变压强附近出现“变软”的趋势; 随着相变的完成, 新相趋于稳定, 杨氏模量E和剪切模量G随压强增加的关系再次趋于线性. 通过弹性模量,能够计算德拜温度. 如图5所示, 在0 GPa压强下, Ce0.8La0.1Th0.1合金的德拜温度为228.85 K, 高于纯Ce已有的德拜温度研究值[26, 27]. 根据已有的研究[12], 德拜温度随着La组分的增加而增加, Ce0.875La0.125在零温零压下德拜温度为140.9 K, 遂认为在Ce0.8La0.1Th0.1合金中, La和Th元素的掺杂导致了德拜温度的升高, 原子间作用力也会因此升高, 本文的计算结果是合理的. 在大约34.4 GPa时, fcc相德拜温度随压强增加呈减小的趋势, 而bct结构与fcc结构的德拜温度十分接近, 并随压强增加呈线性增加的趋势. 意味着结构相变开始发生, 在41.6 GPa附近, fcc结构转变为bct结构, 此时德拜温度为330 K. 图 4 剪切模量G、体模量B和杨氏模量E随压强的变化 Figure4. Shear modulus G, bulk modulus B and Young′s modulus E as functions of pressure.
图 5 德拜温度随压强的变化 Figure5. The Debye temperature as a function of pressure.
23.3.热力学性质 -->
3.3.热力学性质
利用准谐德拜模型获得了Ce0.8La0.1Th0.1的热力学性质和不同温度下的等温压缩曲线. 如图6所示, 当温度为300 K, 压强从0 GPa上升到40 GPa时, fcc相体积缩小了约34.6%. 而在零压下, 当温度从300 K上升到1000 K时, fcc的体积膨胀了约11.1%. 高压状态下温度对体积的影响逐渐减小, 高温的非谐效应在压力的作用下被抑制. 当压强大于40 GPa时bct相稳定存在, 当温度为300 K, 压强从40 GPa上升到80 GPa时, bct 相体积缩小了约13.9%, 而在零压下, 当温度从300 K上升到1000 K时, bct相体积膨胀了约8.2%. 图 6 不同温度下的等温线, 其中V0为零温零压下的体积, 小图为零压下体积随温度的变化 Figure6. Isotherms at different temperatures, where V0 is the volume at zero temperature and zero pressure; the volumes at zero pressure as functions of temperature (the insert) .
分别计算了fcc相和bct相定容热容CV随温度和压强变化的关系, 以及熵S随温度和压强的变化关系. 如图7所示, CV随着温度升高迅速增加, 在高温下接近25 J/(mol·K)的极限, 热容在不同温度下随压强的变化情况几乎是单调递减的. 当温度超过600 K后, 不同等温线之间的差距变小. 计算得到在常温常压下的熵值约为49.44 J/(mol·K), 随温度的升高, 熵值几乎迅速单增, 在不同的温度下, 熵值随压强的增加而减小. 图 7 定容热容CV随温度(a)和压强(b)的变化, 以及熵S随温度(c)和压强(d)的变化; 图中阴影区域包含fcc和bct两相的数据 Figure7. The constant volume heat capacity CV versus temperature (a) and pressure (b), and the entropy S versus temperature (c) and pressure (d).