Study on electronic properties of alpha-, beta- and gamma-AlH3 - The theoretical approach

AIH(3) polymorphs (alpha-, beta-, gamma-) are highly promising materials for hydrogen storage and hydride electronics applications. Given the recent developments in the synthesis and hydrogen desorption approaches, here presented detailed comparison study of three AIH3 polymorphs (alpha-, beta-, gamma-) is aimed to explain and potentially guide the improvements in applicability of these materials. We use electronic structure calculations based on the density functional theory (DFT) to address stability and bonding in alpha-, beta- and gamma-AlH3. For better understanding of stability of various polymorphs, formation enthalpy of alpha-AlH3 is also addressed. Electronic properties (electronic density distribution, density of states, band structure and Baders charge) are calculated using both generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) for exchange-correlation, as well as additional Tran-Blaha modified Becke-Johnson functional (TBmBJ) for exchange. Study shows interesting correlation of electronic structure and bond strength, not observed in previously reported studies of alanes, and presents results obtained using TBmBJ method applied on beta- and gamma-alanes. Band gaps, calculated using TBmBJ, are increased up to 96% as compared to the GGA-PBE values. Due to the lack of experimental data, strong conclusion on the applicability of TBmBJ for alanes cannot be made, although good agreement to G(0)W(0) value and overestimation of GW value is seen in case of alpha-AlH3. Band structure calculations lead to conclusions on electron mobility and other types of application beside hydrogen storage, while based on Baders theory we compare bonding in all investigated polymorphs. (C) 2017 Elsevier B.V. All rights reserved