Phase transformation in hydrogenation and dehydrogenation of LaCo_5-xAl_x-H₂ (x = 0, 0.25) systems

The dehydrogenation behavior of the LaCo₅-H₂ system was studied by in situ X-ray diffraction (XRD) under a hydrogen pressure of 5 MPa in the temperature range of 433-505 K. A new hydride phase (LaCo₅H_2.2) which is formed between the α phase (solid solution) and the β phase (LaCo₅H_3.4 hydride) has a hexagonal structure and the crystal lattice expands mainly along the a-axis during the transformation from the α phase to the LaCo₅H_2.2 phase. To elucidate the effect of the substitution of aluminum for a part of cobalt in LaCo₅ on the phase transformation behavior in the LaCo₅-H₂ system, the hydrogenation and dehydrogenation behavior of the LaCo_4.75Al_0.25-H₂ system was studied by the pressure differential scanning calorimetry (PDSC) under a hydrogen pressure of 0.1-5 MPa in the temperature range of 323-593 K. In the heating and cooling runs of the LaCo_4.75Al_0.25-H₂ system under a hydrogen pressure of 0.1 MPa, one endothermic and one exothermic peaks were observed, respectively. These PDSC peaks were due to the transformation between the α phase and the β phase. Under a hydrogen pressure of 0.2 MPa both peaks divided into two peaks. Combining the PDSC with the P-C isotherms, the division of the PDSC peaks is attributed to the formation of the LaCo_4.75Al_0.25H_2.7 phase between the α phase and the β phase above 383 K in the LaCo_4.75Al_0.25-H₂ system, whereas the LaCo₅H_2.2 phase is formed above 465 K in the LaCo₅-H₂ system. By the substitution of aluminum for a part of cobalt in LaCo₅, the formation temperature of a new hydride phase lowers. The substitution of aluminum makes the β phase stable and the γ phase (LaCo₅H_4.5 hydride) unstable.