Investigation on the Decomposition Enthalpy of Novel Mixed Mg(1–x)Znx(BH4)2Borohydrides by Means of Periodic DFT Calculations

The combination of Mg(BH4)(2) and Zn(BH4)(2) compounds has been theoretically investigated as a possible mixed borohydride prone to give an enthalpy of decomposition around 30 kJ/mol(H2), that is, suitable for a dehydrogenation process close to room temperature and pressure. The total energy of pure compounds and solid solutions has been computed by means of periodic DFT calculations. To generate the Mg(1x)Znx(BH4)(2) solid solutions, the alpha-phase of Mg(BH4)(2) (space group P6(1)22) has been considered in which Mg2+ ions have been progressively replaced with Zn2+, without lowering the symmetry of the crystalline structure. A charge density topological analysis is reported to better understand the chemical bonding in the pure and mixed metal borohydrides. The decomposition enthalpy of the mixed borohydrides according to two different reaction paths that lead to MgH2, Zn, H-2, and alpha-B or B2H6, respectively, as products has been estimated. As regards the former, a value of about 30 kJ/mol(H2) has been predicted for a Mg(1x)Znx(BH4)(2) solid solution with x = 0.20.3