EXAFS investigation of the destabilization of the Mg–Ni–Ti (H) system

Ti doping destabilizes the Mg2NiH4 system, reducing the hydrogenation enthalpy from – 64 kJ/(mol H2) to around – 40 kJ/(mol H2). However, the equilibrium pressure is hardly affected, as also the entropy of reaction changes. To understand this thermodynamic behavior it is essential to understand the structure of the phases present in the Ti-doped Mg2Ni system in metallic and hydrogenated state. We used Extended X-ray Absorption Fine Structure (EXAFS) to investigate the local coordination of Ni and Ti atoms in Mg–Ni–Ti thin films both in the as-prepared as well as in the hydrogenated state. In the as-prepared state two phases, Mg2Ni and TiNi or TiNi3, are formed, which transform in a single Ti-doped Mg2NiH4 phase in the hydrogenated state. These results are consistent with previous DFT calculations for this system.

► EXAFS confirms the unexpected doping of Ti in Mg2NiH4. ► This was predicted by DFT to be a possible reason for the destabilization found in thin film samples. ► While the DFT hints at a meta-stable state, the destabilization remains present on repeated cycling. ► On the other hand a simultaneous reduction in the entropy change on cycling prevents a strong increase in vapor pressure.