Elaboration and characterization of unreported (Pr,Nd)5Ni19 hydrides

In this study two new compounds have been synthesized: Pr5Ni19 and Nd5Ni19. The crystallographic structures as well as the thermodynamic properties of the hydrogen absorbing compounds Pr5Ni19 and Nd5Ni19 have been determined. Both compounds exist under two polymorphic types that can be described as the stacking along the c axis of two different subunits [(Pr,Nd)2Ni4] and [(Pr,Nd)Ni5]: the hexagonal (2H) Pr5Co19-type structure (space group P63/mmc) and the rhombohedral (3R) Ce5Co19-type structure (space group R-3m). The two compounds are able to form hydrides at room temperature, in the pressure range of 0–10 MPa. They show desorption pressure plateaux around 0.8 MPa for Pr5Ni19 and 1 MPa for Nd5Ni19 and exhibit capacities under 10 MPa of 1.33 wt% for Pr5Ni19 and 1.17 wt% for Nd5Ni19 at the first cycle.

Research highlights▶ Two new compounds Pr5Ni19 and Nd5Ni19 have been synthesized and their crystallographic structures have been determined. ▶ Two polymorphic types are reported to coexist for the same composition, one rhombohedral and one hexagonal. ▶ The hydrogen sorption properties of these two novel compounds have been measured and they exhibit capacities of 1.33%(wt) for Pr5Ni19 and 1.17%(wt) for Nd5Ni19 under 10 MPa.