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

Abstract

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]− and [AlH6]3– octahedral anions. Structures were produced with Al–H 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]− anions. The Na–Al–H 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 1–2 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–H 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. J. Phys. Chem. Lett. 2010, 1, (15), 2412−2416).