Elaboration and Characterization of New Pseudo-Binary Hydride-Forming Phases Pr1.5Mg0.5Ni7 and Pr3.75Mg1.25Ni19: A Comparison to the Binary Pr2Ni7 and Pr5Ni19 Ones

New pseudobinary phases have been synthesized in the ternary system Pr–Mg–Ni. The Pr1.5Mg0.5Ni7 compound is obtained as a pure phase, but it exists under two polymorphic structures: hexagonal Ce2Ni7-type (2H; space group P63/mmc) and rhombohedral Gd2Co7-type (3R; space group R3̅m). The Pr3.75Mg1.25Ni19 compound is found in equilibrium with the PrNi5-type secondary phase, and it adopts also two polymorphic structures: hexagonal Sm5Co19-type (2H; space group P63/mmc) and rhombohedral Ce5Co19-type (3R; space group R3̅m). The crystallographic structures as well as the thermodynamic properties of these hydrogen-absorbing compounds have been determined and are compared to those of the binary compounds Pr2Ni7 and Pr5Ni19. The two compounds are able to form hydrides at room temperature, in the pressure range of 0–10 MPa. The replacement of 25 at. % of the rare earth by magnesium for both stoichiometries leads to an increase of the weight capacity and a decrease of the equilibrium pressures with the formation of well-defined plateaus.