The clamping effect in the complex hydride Mg2NiH4 thin films

The thermodynamic properties and hysteresis behavior of Mg2NiH4 thin films were studied using hydrogenography in the 303-513 K temperature range. For the enthalpy of hydride formation and decomposition, as estimated from a Van't Hoff analysis, we find Delta H-abs = -51.2 kJ mol(-1) H-2 and Delta H-des = -76.3 kJ mol(-1) H-2. These values are affected by the clamping of the film to the substrate. However, the average enthalpy of the (de-)hydrogenation reaction, Delta H = (Delta H-abs + Delta H-des)/2 = -63.5 kJ mol(-1) H-2, is in remarkable agreement with the average bulk data, -64 kJ mol(-1) H-2. The (de-)hydrogenation pressure hysteresis in Mg2NiH4 thin films is more than 3 orders of magnitude at 303 K, which is substantially larger than in MgH2 films. This behaviour is believed to originate from the fact that the phase transition in Mg2NiH4 films occurs between two preferentially oriented phases. We assume that the energy involved in the necessary re-orientation is related to the monoclinic to hexagonal structure transformation. By adding this term, the stress-strain analysis is able to explain the enormous difference between absorption and desorption equilibrium pressures in Mg2NiH4.