ARTICLE pubs.acs.org/JPCC
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 Lucille Lemort,†,‡ Michel Latroche,*,† Bernard Knosp,‡ and Patrick Bernard‡ †
Institut de Chimie et des Materiaux Paris-Est (ICMPE), Chimie Metallurgique des Terres Rares (CMTR) Centre National de la Recherche Scientifique (CNRS) UMR 7182, 2-8 rue Henri Dunant, 94320 Thiais Cedex, France ‡ SAFT, Direction de la Recherche, 111-113 Boulevard Alfred Daney, 33074 Bordeaux, France ABSTRACT: 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 R3m). 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 R3m). 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.
’ INTRODUCTION Since the discovery of the hydride-forming intermetallic alloys,1 many applications have been considered for these compounds. Until now, the most successful of these applications is still hydrogen storage in NiMH alkaline batteries. However, to keep NiMH batteries competitive to other energy storage systems, enhanced performances need to be obtained by developing new generations of compounds. In the last 10 years, much attention has been paid to the La Mg Ni system. Indeed, recent studies have shown that La1 yMgyNix (3 < x < 4, 0 < y
