Reaction of hydrogen with the Laves phase (C14) TiCr1.78−xMnx compounds

The lattice constants a and c of the as cast hexagonal (C14) Laves phase TiCr1.78−xMnx (x = 0, 0.4, 0.6 and 0.8) have been measured by X-ray diffraction at room temperature as a function of the Mn content. It has been found that a decreases with increasing x while c remains almost unchanged. Being this alloy interesting as material for solid state hydrogen storage, pressure–composition isotherms have been traced on desorption mode for the H2 gas pressure and temperature varying from 10 to 0.01 MPa and from 209 to 307 K, respectively. From the temperature dependence of the plateau pressure the molar quantities ΔH¯H2 and ΔS¯H2 associated with the hydride decomposition have been determined as a function of the Mn content. With increasing x  , both ΔH¯H2 and ΔS¯H2 progressively decrease, and the desorption pressure initially decreases (for x ≤ 0.4) then slightly increases. At the same time, the pseudo-plateaus become flatter.