Modeling of complex hydrides: Lattice anomaly in NaAlH4

Following exptl. observation that the c-axis expands anomalously faster than the a-axis in NaAlH4, we undertook an ab initio incursion into the nature of this anomaly. We have found that contrary to the exptl. deductions the anomaly is even more pronounced in a than in c. There is also an interdependence of in-plane and inter-planar chem. bonding leading to compensating effects in terms of changes in the lattice parameters. An energy loss caused by expansion of the axial lattice parameters is exactly compensated by the contraction of the basal lattice parameter. Other possibilities we are investigating in this interesting arena includes phase transition. We have ascertained that our results is not an artifact of the potential by using both the Ultra Soft (US) pseudo-potentials and the much hyped PAW potentials as implemented in VASP, with the same conclusions. Moreover, during our simulations we allowed the structure to break symmetry. The immediate implication of our results apart from adding rich information on the structural properties of NaAlH4 is that it gives ideas on how to load(unload) hydrogen from the material. In addn. these results also spectacularly shows that our structure is stable along the (001) plane without the need to do surface energy calcns.!.