The investigation of the Zr_1-yTi_y(Cr_1-xNi_x) ₂-H₂ system 0.0≤y≤1.0 and 0.0≤x≤1.0 Phase composition analysis and thermodynamic properties

The Zr_1-yTi_y(Cr_1-xNi_x)₂ compounds were synthesized and characterized by means of X-ray powder diffraction. From the Rietveld analysis carried out on the alloys, we determined the phase abundancy and refined the crystal structure. The system crystallized with hexagonal C14-type structure for nickel content less than x=1.0 with some amount of additional phases (ZrNi, Zr₉Ni₁₁, Zr₇Ni₁₀, TiNi) and pure metals (Ni and Cr). For higher nickel concentrations, the system crystallized in the cubic C15-type structure. The amount of additional phases increased with increasing nickel and titanium content. The Pressure-Composition-Temperature measurements using a volumetric method were carried out at 313 K on some selected alloys. The single phase systems were found to absorb a large amount of hydrogen (H/M≤1.8 wt.%) but had very poor kinetics. The multiphase systems were found to behave as a single phase (presence of single plateau), have a relatively good hydrogen capacity (H/M≤1.4 wt.%) and exhibit faster kinetics. A linear correlation between the heat of formation of the Zr_1-yTi_y(Cr_1-xNi_x)₂ hydrides estimated from the band structure model and the unit cell volume of the alloys determined from the X-ray Rietveld analysis was obtained. It shows a decrease of hydride stability with increasing unit cell volume.