Theoretical Study on Structural Stability and Superionic Phase Transition of UH5 under High Pressure
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摘要: 采用第一性原理方法，系统研究了UH5在30.0 GPa内的热力学、力学、动力学稳定性以及电子性质。研究发现，实验合成的正交、六角、立方相均为磁性材料，自旋极化率分别达到82%、100%、100%，热力学稳定性依次降低。弹性与声子模拟表明，3个相均具备力学与动力学稳定性，化学键分析表明，这主要源于晶格中普遍存在共价性较强的U-H相互作用。此外，预测了实验上截留至1.0 GPa的正交相在
1200 K转变为H离子在U亚晶格间隙中快速扩散的超离子态，扩散系数达到1.2×10–4 cm2/s。Abstract: The thermodynamic, mechanical, and dynamical stability, along with the electronic properties of UH5 within 30 GPa, are systematically investigated using first-principles calculations. The experimentally synthesized orthorhombic, hexagonal, and cubic phases are all found to be magnetic materials, with spin polarizations of 82%, 100%, and 100%, respectively, and their thermodynamic stability decreases sequentially. Elastic constant and phonon calculations demonstrate that all three phases are mechanically and dynamically stable. Chemical bonding analysis indicates that this stability primarily originates from the prevalent covalent U-H interaction within the lattice. Furthermore, it is predicted that the orthorhombic phase, which has been experimentally quenched to 1 GPa, transforms into a superionic state at1200 K, where hydrogen ions undergo rapid diffusion within the uranium sublattice interstices, achieving a diffusion coefficient of 1.2×10−4 cm2/s.-
Key words:
- high pressure /
- hydrides /
- lattice dynamics /
- superionic state /
- first-principles
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图 1 F$ \overline{4} $3m、P63mc和Cmcm相：(a) 能量随体积的变化，(b) 体积随压强的变化，(c) 内能随压强的变化，(d) 焓值随压强的变化
Figure 1. F$ \overline{4} $3m, P63mc and Cmcm phases: (a) energy changes with volume; (b) volume changes with pressure; (c) internal energy changes with pressure; (d) enthalpy changes with pressure
图 2 Cmcm、F$ \overline{4} $3m和P63mc的XRD图谱及晶体结构：Kruglov等[14]在(a) 1 GPa、(c) 8.5 GPa、(e) 29 GPa下测得的同步XRD图谱，Cmcm、F$ \overline{4} $3m和P63mc分别在(b) 1 GPa、(d) 8.5 GPa和(f) 29 GPa下的理论XRD图谱，Cmcm (g)、$F\overline{4}3m $ (h)和P63mc (i) 的晶体结构
Figure 2. XRD patterns and crystal structures of Cmcm, F$ \overline{4} $3m and P63mc: synchrotron XRD patterns measured by Kruglov, et al.[14] at (a) 1 GPa, (c) 8.5 GPa, (e) 29 GPa; theoretical XRD patterns of Cmcm, F$ \overline{4} $3m and P63mc at (b) 1 GPa, (d) 8.5 GPa, (f) 29 GPa, respectively; crystal structures of Cmcm (g), $F\overline{4}3m $ (h), and P63mc (i)
图 3 F$ \overline{4} $3m、P63mc和Cmcm相在0和30 GPa下的声子色散曲线以及声子态密度
Figure 3. Phonon-dispersion curves and the PHDOS of Cmcm, P63mc and $ F\overline{4} $3m phases at 0 and 30 GPa
图 4 (a)～(c) Cmcm、F$ \overline{4} $3m和P63mc相分别在1、8.5 和29 GPa下的能带和电子态密度，(d) 三相U-H键的晶体轨道哈密顿布居
Figure 4. (a)–(c) Band structures and partial densities of states of the Cmcm, F$ \overline{4}3m $ and P63mc phases at 1, 8.5 and 29 GPa, respectively; (d) crystal orbital Hamilton populations (COHP) of three-phase U-H bond
图 5 Cmcm相： (a) 体积随温度的变化、(b) 晶胞参数随温度的变化、(c) 离子的部分径向分布函数、(d) 均方根位移随时间的变化，以及(e) 300 K和(f)
1200 K下的分子动力学轨迹Figure 5. Cmcm phase: (a) volume changes with temperature; (b) unit cell parameters changes with temperature; (c) partial radial distribution function (RDF) of ions; (d) mean squared displacement changes with time; molecular dynamics trajectory changes with time at (e) 300 K and (f)
1200 K表 1 Cmcm、F$ \overline{4} $3m和P63mc相在1、8.5和29 GPa下的晶胞参数及体积
Table 1. Lattice parameters and volumes of Cmcm, F$ \overline{4} $3m and P63mc phases at 1, 8.5 and 29 GPa
Phase p/GPa a/Å b/Å c/Å Volume/Å3 Exp. Calc. Exp. Calc. Exp. Calc. Exp. Calc. Cmcm 1 3.438 3.379 7.150 7.126 6.200 6.363 38.102 38.303 F$ \overline{4} $3m 8.5 5.319 5.252 5.319 5.252 5.319 5.252 37.625 36.210 P63mc 29 3.668 3.644 3.668 3.644 5.766 5.727 33.600 32.930 
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表 2 F$ \overline{4} $3m、P63mc、Cmcm相在0 和30 GPa下的弹性常数和平均弹性模量
Table 2. Elastic constants and average elastic moduli of F$ \overline{4} $3m, P63mc, and Cmcm phases at 0 and 30 GPa
GPa Phase p C11 C12 C13 C22 C23 C33 C44 C55 C66 B G F$ \overline{4} $3m 0 251 94 114 147 98 30 333 175 136 228 109 P63mc 0 289 85 67 312 97 148 103 30 422 150 115 416 115 224 131 Cmcm 0 242 33 114 297 64 275 94 87 59 136 85 30 335 93 162 477 100 409 119 110 96 214 120
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