First-principles Prediction Of Low-energy Structures For AlH₃

During the past decades,new energy storage materials and corresponding technology are being developed to handle the environments problems which were caused by large amounts of fossil fuel use,such as petroleum and natural gas. Hydrogen,the most abundant element in the earth,has great potential as an energy source.Unlike the petroleum,it can be easily generated from renewable energy sources.It is also nonpolluting,and the byproduct water is harmless.However,it is difficult to store.One solution is to store hydrogen in solid materials.The key requirements for hydrogen storage material candidates in automotive applications are a high gravimetric and volumetric hydrogen density,a release of hydrogen at moderate temperatures and pressures,and a low-cost method to recharge the material.The US Department of Energy(DOE)hydrogen storage system goals for the year 2010 are a 6.0wt.%gravimetric capacity and a volumetric capacity of 45g/L.A new type of hydride is AlH₃,which has an appreciable gravimetric and a volumetric hydrogen capacities and may be able to meet the DOE targets.Aluminum hydride,AlH₃,is potentially an attractive storage material due to the large amount of hydrogen that can be contained in a relatively small,light-weight package.It contains 10wt.%H by weight and has a theoretical H density of 148 g/L, which is more than double the density of liquid H₂.Theoretically,AlH₃ is thermodynamic unstable and will decompose to H₂ and Al at room temperature.At least seven different AlH₃ phases are known to exist.At present,only four phases were found in the polymorph.And each phase has a unique arrangement of H and Al atoms.In this paper we predict ten different low-energy structures for aluminum hydride with space groups of Pnma,I4/mcm,P4/mbm,P6/mmm,P4/nmm, Pm-3m,P2₁/c,P2₁/m,P4/n,and Pbcm with density functional calculations. Mechanical stabilities of these low-energy structures are studied by phonon calculations.Pnma,P4/n,and Pbcm turn out to be stable since there is no soft phonon mode existing in each structure.Thermodynamic stability calculations indicate that Pnma,P4/n,and Pbcm are overall close to the existingαandγphases.From x-ray powder-diffraction patterns,the simulated main-peak positions for AlH₃(P4/n) are in good agreement with experimentalδ-phase.A full Rietveld analysis reveals that the fitting R-3c,Pnma,and Pbcm to experimental x-ray powder-diffraction pattern ofα-AlH₃ gives almost the same satisfactory result.