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On the structural and thermodynamic properties of the RENi4Ga-hydrogen (RE=La, Ce, Nd and Sm) systems

The RENi4Ga (RE=La, Ce, Nd and Sm) intermetallic compounds were prepared and studied by means of x-ray powder diffraction. All compounds are single phase and exhibit the same hexagonal symmetry (CaCu5 type, space group P6/mmm) as do their prototype RENi5 binaries. The interaction with hydrogen was also studied. It was found that all ternary intermetallics react readily and reversibly with hydrogen to form hydrides with high hydrogen contents of up to four hydrogen atoms per alloy formula unit. The pressure composition desorption isotherms were measured. The entropy, the enthalpy and the Gibbs free energy of formation have been extracted from the pressure composition desorption isotherms. The hydrogen capacity and the equilibrium pressure of the RENi4Ga-hydrogen systems were compared with the corresponding values for their aluminium analogues and with the values for the RENi5-hydrogen systems and briefly discussed. The hydride properties of gallium and aluminium containing compounds show great similarities whereby both series of ternary compounds form more stable hydrides and exhibit smaller hydrogen capacities than do the corresponding binaries.

hydrogen sorption; intermetallic compounds; crystal structure; thermodynamics

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