Thermodynamic and structural properties of LaNi 5− yAl y compounds and their related hydrides

Abstract

Among the intermetallic compounds able to absorb reversibly large amounts of hydrogen in convenient conditions, LaNi 5 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− y Al 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 LaNi 5 and LaNi 4Al 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.