Phase transformation in hydrogenation and dehydrogenation of LaCo5−xAlx–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 (LaCo5H2.2) which is formed between the α phase (solid solution) and the β phase (LaCo5H3.4 hydride) has a hexagonal structure and the crystal lattice expands mainly along the a-axis during the transformation from the α phase to the LaCo5H2.2 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 LaCo4.75Al0.25–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 LaCo4.75Al0.25–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 LaCo4.75Al0.25H2.7 phase between the α phase and the β phase above 383 K in the LaCo4.75Al0.25–H2 system, whereas the LaCo5H2.2 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 (LaCo5H4.5 hydride) unstable.