In order to understand the plateau slope and hysteresis in the Zr-based Laves phase intermetallic compound-$H_2$ system, the thermodynamic properties, the hydrogen site occupancies, and the structural behaviors of the quaternary $Zr_{1-x}Ti_xCr_{1-y}Fe_{1+y}$ (0≤x≤0.5, 0≤y≤0.5), $Zr_{1-x}Ti_xMnFe$ (0≤x≤0.3), and $Zr_{1-x}Ti_xV_{0.5}Fe_{1.5}$(0≤x≤0.3) alloys are investigated as a function of composition and temperature. And then, the thermodynamic characteristics of the $Zr_{0.8}Ti_{0.2}(Mn_xCr_{1-x})Fe$-$H_2$ system are investigated in order to find the interrelation between the slope behavior and the hysteresis factor, which would be caused by the difference in starting material (e.g. $ZrCr_2$-base or $ZrMn_2$-base alloy). Also, the hydrogen site occupancy in the $ErFe_2$-H system is carried out by the hydrogen thermal desorption study and the structural changes during hydrogen absorption because $ErFe_2$ exhibits 'multi-plateau' behavior that is a series of plateaux in P-C-isotherms indicating that at two hydride phases are formed, which should be different from 'sloping' plateau behavior in most Zr-based Laves phases.
The slope behavior of $Zr_{1-x}Ti_xCr_{1-y}Fe_{1+y}$ is investigated by the changes of P-C-T curves. The increasing iron substitution for chromium does not change the plateau slope although hydride stability of the alloy is some lowered. The Ti substitution for Zr, however, is the determining factor for the plateau slope in the two phase region.
When the Ti concentration range is lower than 0.1, the slopes of linearly increasing plateaus with H/M become lower as the Ti concentration is increased. The well-defined plateau appear at x=0.1. However, a further increase in Ti content above x=0.1 results in the increase of plateau slope. The enthalpies are strongly affected by the iron substitution but the enthropies are independent of iron concentration, while the replacement of zirconium by titanium changes both the enthalpies and entropies. The hydrogen site occupation in the lattices is altered by Ti substitution and not by Fe substitution. Thermal desorption experiment of $Zr_{1-x}Ti_xCrFe$ hydrides shows that one peak in hydrogen evolution rate vs. temperature plot is farmed at x=0.1, and it developers into three peak structure as the titanium concentration is further increased. From the comparison with thermal desorption spectra of $ZrCr_2H_x$ hydrides, it is suggested that, in $Zr_{1-x}Ti_xCrFe$ hydride, one peak at lower temperature is associated with the hydrogen from ZrTiB2-type site and two peak at high temperature side is closely related with Zr2B2-type sites. The lattice strain observed from X-ray diffraction data of $Zr_{1-x}Ti_xCr_{1-y}Fe_{1+y}$ alloys reveals that Ti substitution minimize the strain in the lattice at x=0.1 concentration where well-defined plateau is formed. This implies that the strain energy in the interstitial sites contributes the sloping mechanism.
In $Zr_{1-x}Ti_xMnFe$ alloy system, the Ti effect on the slope behavior is similar to $ZrCr_2$-base system, while the degree of sloping in P-C-T curve is minimum at x=0.2 and the hysteresis factor is larger than that of $ZrCr_2$-base system. In $Zr_{1-x}Ti_xV_{0.5}Fe_{1.5}$ system, there is no dependence of plateau slope on Ti concentration.
From the results of the above work, the sloping mechanism in the Zr-based Laves phase is originated from the site energy difference among the interstitial sites where the energy level of each site is determined from the combination of chemical energy and strain energy.
And P-C-T results of $Zr_{0.8}Ti_{0.2}(Mn_xCr_{1-x})Fe$-H2 system reveals that the slope behavior and the hysteresis have the interrelation. As the Mn concentration is increased, the P-C-T behavior exhibits low sloping and large hysteresis. And the reverse trend is observed as the Cr concentration is increased. This fact is resulted from the more sensitive change of hydride formation pressure than decomposition pressure on the alloy composition. The hysteresis factor affected by alloy composition shows that the hysteresisis differ in nature and magnitude from alloy system to alloy system. In $Zr_{1-x}Ti_xCrFe$ system, the change in hysteresis factor depending on Fe concentration can be explained by strain energy model. However the hysteresis behavior affected by Ti substitution exhibits reverse trend, from which it is suggested that the lattice strain of alloy itself affect the hysteresis factor. And the hysteresis factor is also varied depending on composing element of Laves phase, e.g.V, Cr, Mn. This fact is explained from the difference in elastic lattice strain of each alloy system. It is proposed that the mechanical property of the alloy itself determine the nature and magnitude of hysteresis.
From the hydrogen site occupancy in the $ErFe_2H_x$, it is shown that two thermal desorption peaks are correlated with the A2B2 site and one is associated with the AB3 site. The rhombohedral distortion appears above x ＞ 3, but it disappears, causing a return to the cubic as the hydrogen concentration is further increased. It is suggested that the progressive occupancy of hydrogen atoms in the A2B2 sites is affected by two factors, one arising from a strong Er-H bond and the other from the combined effect of a reduced binding energy and an increased strain energy.
