First-Principles Study of Structural Prototypes for NaAlH4: Elevated Pressure Polymorph in Symmetry Fmm2 Leads to a Single-Step Decomposition Pathway

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• 作者：E. H. Majzoub ; E. Hazrati ; G. A. de Wijs
• 刊名：The Journal of Physical Chemistry C
• 出版年：2013
• 出版时间：May 2, 2013
• 年：2013
• 卷：117
• 期：17
• 页码：8864-8870
• 全文大小：349K
• 年卷期：v.117,no.17(May 2, 2013)
• ISSN：1932-7455

文摘

In situ high-temperature X-ray diffraction and high-temperature and pressure nuclear magnetic resonance studies of the complex metal hydride NaAlH₄ have strongly suggested an intermediate phase or phases. In order to search for these phases, structural prototypes of NaAlH₄ were generated using the method of prototype electrostatic ground states (PEGS). Structural motifs included tetrahedral [AlH₄]^{鈭?/sup> and [AlH₆]3鈥?/sup> octahedral anions. Structures were produced with Al鈥揌 coordination numbers from 4 to 6 and were examined for thermodynamic stability. We present a high-temperature phase in symmetry Fmm2 that is lower in total free energy than all currently known structures, including a structure presented in space group Cmcm, due to low rotational barriers for [AlH₄]鈭?/sup> anions. The Na鈥揂l鈥揌 phase diagram, calculated in the harmonic approximation, indicates that at higher pressures, the I4₁/a NaAlH₄ phase transforms to Fmm2 symmetry, which then decomposes directly into NaH and Al, eliminating the Na₃AlH₆ intermediate. The transition to Fmm2 is predicted to occur near 320 K at calculated pressures of about 1鈥? bar. We also present calculations of NMR 27Al and 23Na chemical shifts for the structural prototypes. These results indicate a monotonic decrease of the 27Al chemical shift with increasing Al鈥揌 coordination number with an unambiguous identification of an Al/H ratio of 1:4 for the unknown high-temperature phase S105 identified by Conradi and co-workers (Conradi, M. S. et al. e>J. Phys. Chem. Lett.e> 2010, <em>1em>, (15), 2412鈭?416).}