Marashdeh Ali, Frankcombe Terry J
Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands.
J Chem Phys. 2008 Jun 21;128(23):234505. doi: 10.1063/1.2937917.
The dehydrogenation enthalpies of Ca(AlH(4))(2), CaAlH(5), and CaH(2)+6LiBH(4) have been calculated using density functional theory calculations at the generalized gradient approximation level. Harmonic phonon zero point energy (ZPE) corrections have been included using Parlinski's direct method. The dehydrogenation of Ca(AlH(4))(2) is exothermic, indicating a metastable hydride. Calculations for CaAlH(5) including ZPE effects indicate that it is not stable enough for a hydrogen storage system operating near ambient conditions. The destabilized combination of LiBH(4) with CaH(2) is a promising system after ZPE-corrected enthalpy calculations. The calculations confirm that including ZPE effects in the harmonic approximation for the dehydrogenation of Ca(AlH(4))(2), CaAlH(5), and CaH(2)+6LiBH(4) has a significant effect on the calculated reaction enthalpy. The contribution of ZPE to the dehydrogenation enthalpies of Ca(AlH(4))(2) and CaAlH(5) calculated by the direct method phonon analysis was compared to that calculated by the frozen-phonon method. The crystal structure of CaAlH(5) is presented in the more useful standard setting of P2(1)c symmetry and the phonon density of states of CaAlH(5), significantly different to other common complex metal hydrides, is rationalized.
在广义梯度近似水平下，利用密度泛函理论计算了Ca(AlH₄)₂、CaAlH₅和CaH₂ + 6LiBH₄的脱氢焓。采用帕林斯基直接法包含了简谐声子零点能（ZPE）校正。Ca(AlH₄)₂的脱氢是放热的，表明其为亚稳氢化物。对CaAlH₅包含ZPE效应的计算表明，对于在接近环境条件下运行的储氢系统而言，它不够稳定。经ZPE校正的焓计算后，LiBH₄与CaH₂的不稳定组合是一个有前景的体系。计算证实，对Ca(AlH₄)₂、CaAlH₅和CaH₂ + 6LiBH₄的脱氢在简谐近似中包含ZPE效应，对计算的反应焓有显著影响。将通过直接法声子分析计算得到的ZPE对Ca(AlH₄)₂和CaAlH₅脱氢焓的贡献与通过冻结声子法计算得到的结果进行了比较。以更有用的P2(1)c对称标准设置给出了CaAlH₅的晶体结构，并对CaAlH₅与其他常见复杂金属氢化物显著不同的声子态密度进行了合理解释。
