Study on phase formation mechanism and electrochemical properties of La sub(0.75-x)Nd sub(x)Mg sub(0.25)Ni sub(3. 3) (x = 0, 0.15) alloys prepared by powder sintering

An investigation of the phase formation mechanism and electrochemical properties of the La sub(0.7-x)Nd sub(x-) Mg sub(0.25)Ni sub(3.3) (x = 0, 0.15) alloys has been conducted in this paper. As explored by Rietveld analysis of the XRD data, the La sub(0.75)Mg sub(0.25)Ni sub(3.3) alloy is composed of Ce sub(2)Ni sub(7)-type and Gd sub(2)Co sub(7)-type phases accompanied by minor CaCu sub(5)-type phase. The partial substitution Nd for La benefits the formation of Gd sub(2)Co sub(7)-type phase, indicating that the atoms with smaller radius favor the formation of the Gd sub(2)Co sub(7)-type phase. However, CaCu sub(5)-type phase disappears with Nd substitution. Because part of the replaced La atoms transform into [A sub(2)B sub(4)] subunits with the Ni atoms, and the formed [A sub(2)B sub(4)] subunits crystallize with [AB sub(5)] subunits to produce A sub(2)B sub(7)-type phase, resulting in the depletion of the minor CaCu sub(5)-type phase. The electrochemical P-C isotherms present that the La sub(0.60)Nd sub(0.15)Mg sub(0.25)Ni sub(3.3) alloy only containing A sub(2)B sub(7)-type phase has one sin-gle plateau while the La sub(0.75)Mg sub(0.25)Ni sub(3.3) alloy which contains minor LaNi sub(5) phase has two plateaus. It is beneficial for the elevation of equilibrium plateau pressure ascribing to the contraction in cell volumes by Nd substitution. Electrochemical measurements show that the substitution of La by Nd improves the discharge capacity and the high rate dischargeability owing to the increase of A sub(2)B sub(7)-type phase abundance.