Amphoteric behavior of hydrogen (H+1and H-1) in complex hydrides from van der Waals interaction included ab-initio calculation

Abstract: In order to identify potential hydrogen storage materials, worldwide attention has been focused on hydrides with high gravimetric and volumetric capacity. Hydrogen is a unique element that possess positive, negative or neutral oxidation state in solids depending upon the chemical environment. If one can find hydrogen storage materials where hydrogen is present in both negative and positive oxidation state within the same structural framework then one can accommodate hydrogen with high volume density. So, it is fundamentally as well as technologically important to identify compounds in which hydrogen is in amphoteric nature and understand the necessary criteria for its origin. The experimental structural analysis of Cyclotriborazane and Diammonium dodeca hydro-closo-dodecaborate insinuate the presence of hydrogen with anionic and cationic behavior within the same structure. In order to understand the role of van der Waals (vdW) interactions on structural parameters, we have considered 12 different vdW corrected functionals and found that the optPBE-vdW functional predicts the equilibrium structural parameters reliably with less than 0.009 % accuracy. So, the optPBE-vdW functional is used to calculate charge density, electron localization function, total as well as the partial density of state, Bader and Born effective charge etc. We have observed that hydrogen is exhibiting amphoteric behavior with H closer to B in the negatively charged state and that neighboring to N is in a positively charged state.