A theoretical explanation of RbBH4's fractionally occupied ground-state phase and the reorientational motion of the [BH4](-) group

作者：Chen, Lei; Song, Qi^*; Zhou, Bo; Zhang, Xiaodong; Jiang, Zhenyi

来源：Journal of Physics D: Applied Physics , 2017, 50(45): 455501.

DOI：10.1088/1361-6463/aa86f7

• first-principles calculations
• fractional occupied ground-state phase
• hydrogen storage
• alkaline metal borohydrides

In this work, a pseudo-structure model is adopted to describe the fractionally occupied phase of RbBH4 at low temperatures. The fractionally occupied cubic ground state with Fm3m symmetry can be explained as a superposition and average of three well-defined pseudo-structures with P43m, F43m and P4(2)/nmc symmetries. First, both mechanical and dynamical stability of these pseudo-structures are supported by the calculated elastic constants and phonon spectra, respectively. By comparing the electronic structure, elastic properties and thermodynamic properties of these well-defined pseudo-structures, it can be found that all pseudo-structures exhibit a certain degree of similarity. Second, the system is stabilized by charge transfer. Bader charge analysis of the pseudo-structures shows that charge transfer in RbBH4 contributes a constant shift in formation energy. Coulomb interaction energy and total electronic energy variations with the unit cell volume have been obtained. The results show that the larger the volume of the unit cell, the lower the difference of Coulomb interaction energy between the pseudo-structures P43m, F43m and P4(2)/nmc. Third, the barrier energies calculated for paths of transitions between these pseudo-structures vary significantly with the volume. As the cell volume decreases from 512 to 216 angstrom(3), the B-H covalent bonds are compressed from 1.242 to 1.208 angstrom. Meanwhile, the rotational barrier of [BH4](-) anion increases rapidly, from 0.04 to 0.97 eV. Last, considering the temperature effect, the free energy and entropy contributions are calculated based on the rigid rotor harmonic oscillator approximation. The Gibbs free energy shows that at low temperature the P42/nmc structure is the preferred structure, and with increasing temperature, the P43m structure becomes the preferred structure. We found that the fractionally occupied phase of RbBH4 can be explained by the size of the Rb+ cation and a subtle balance between the Coulomb interaction energy, electronic total energy and thermal vibration energy.

• 出版日期2017-11-15
• 单位西北大学