Center of Excellence in the Forum for Theoretical Science, Department of Physics, Faculty of Science, Chulalongkorn University, Bangkok, Thailand. ThEP, Commission on Higher Education, 328 Si-Ayutthaya Road, 10400 Bangkok, Thailand.
J Phys Condens Matter. 2014 Jan 15;26(2):025405. doi: 10.1088/0953-8984/26/2/025405. Epub 2013 Dec 10.
We used density functional theory to calculate the phase stability of the hcp (hexagonal close packed) and the fcc (face centered cubic) structures of ScH3. The hcp form is stable up to 22 GPa according to the generalized gradient approximation calculation. Then the fcc form becomes energetically more stable. In order to provide insight into the phase transition mechanism, we modeled the hcp to fcc transition by sliding the hcp basal planes, i.e. (001)h planes, in such a way that the ABABAB sequence of the hcp form is altered into the ABCABC sequence of the fcc form. This sliding was suggested by the experiment. The configurations of these sliding steps are our proposed intermediate configurations, whose symmetry group is the Cm group. By using the Cm crystallography, we can match the d-spacings from the lattice planes of the hcp and fcc forms and the intermediate planes measured from the experiment. We also calculated the enthalpy per step, from which the energy barrier between the two phases at various pressures was derived. The barrier at 35 GPa is 0.370 eV per formula or 0.093 eV/atom. The movements of the hydrogen atoms during the hcp to intermediate phase transition are consistent with the result from the Raman spectra.
我们使用密度泛函理论计算了 ScH3 的 hcp（六方密堆积）和 fcc（面心立方）结构的相稳定性。根据广义梯度近似计算，hcp 形式在 22 GPa 以下是稳定的。然后，fcc 形式变得更稳定。为了深入了解相变机制，我们通过滑动 hcp 基面（即 (001)h 面）来模拟 hcp 到 fcc 的转变，从而将 hcp 形式的 ABABAB 序列改变为 fcc 形式的 ABCABC 序列。这种滑动是由实验提出的。这些滑动步骤的构型是我们提出的中间构型，其对称群为 Cm 群。通过使用 Cm 晶体学，我们可以匹配 hcp 和 fcc 形式的晶格平面以及从实验测量的中间平面的 d 间距。我们还计算了每个步骤的焓，从中得出了不同压力下两相之间的能垒。在 35 GPa 时，每个公式的能垒为 0.370 eV，或每个原子 0.093 eV。在 hcp 到中间相转变过程中，氢原子的运动与拉曼光谱的结果一致。
