Thermodynamic and structural properties of LaNi5−yAly compounds and their related hydrides

Among the intermetallic compounds able to absorb reversibly large amounts of hydrogen in convenient conditions, LaNi₅ presents large possibility of substitution involving a change of the plateau pressure of the related hydrides in a wide range of pressure. The results presented concern the metallurgical, structural and thermodynamic properties of the compounds LaNi_5?yAl_y and their related hydrides:The limit of existence of pseudobinary intermetallic compounds with hexagonal structure P6/mmm is determined (Y_A1 = 1.25).The lattice parameters and cell volume of the intermetallic compounds are determined in terms of Y_A1. The enthalpy of formation of LaNi₅ and LaNi₄Al are measured by a calorimetric method.The lattice parameters and cell volume of the hydride are determined by X-ray diffraction.The study of the absorption and desorption hydrogen isotherms show a decrease of the plateau pressures with the increasing rate of aluminium and decrease of the capacity. The enthalpy and entropy of formation of hydrides are deduced from the study of the change of their equilibrium pressures as a function of temperature. Correlations are established between structural and thermodynamical properties of intermetallic compound of this series and their related hydrides.