Phase transformation in hydriding and dehydriding of LaCo5−xNix (x = 0-2) alloys

The hydriding and dehydriding behavior of LaCo5−xNix (x = 0, 1 and 2) was studied by the pressure differential scanning calorimetry (PDSC) under a hydrogen pressure of 1-5 MPa in the temperature range from 323-533 K. In the heating and cooling runs of the LaCo5-H2 system under a hydrogen pressure of 1-3 MPa, two endothermic and two exothermic peaks were observed, respectively. One peak at higher temperature was due to the transformation between the α phase (solid solution) and the βI phase (AB5H3.4-type hydride). The other at lower temperature was due to the transformation between the βI phase and the γI phase (AB5H4.3-type hydride). Furthermore, under hydrogen pressures of 4 and 5 MPa the second peak at higher temperature divided into two peaks. Combining the PDSC with the P-C isotherms, the division is attributed to the formation of a new hydride phase (LaCo5H2.2 hydride) between the α phase (solid solution) and the βI phase (LaCo5H3.4 hydride) in the desorption process above 470 K and in the absorption process above 465 K. On the other hand, in the PDSC of the LaCo3Ni2-H2 system an intermediate transformation mode between the Co-rich and the Ni-rich systems was observed. In the LaNi5-H2 system, the peaks correspond to the transformations between the α phase, the β phase (AB5H3-type hydride) and the γ phase (AB5H6-type hydride).