Structural stability studies of single-phase Ce2Ni7-type and Gd2Co7-type isomerides with La0.65Nd0.15Mg0.2Ni3.5 compositions

摘要

In order to find out the effect of lattice configuration on the structural stability of Rare earth magnesium nickel compounds, two La0.65Nd0.15 Mg0.20Ni3.50 isomerides with single Ce2Ni7 -type (2H) and single Gd2Co7 -type (3R) structure, respectively, have been prepared for the first time. It is found that, after hydrogenation/dehydrogenization cycles, the volume of [A(2)B(4)] subunit in 3R-type structure shrinks while that of [A(2)B(4) ] in 2H-type structure gets expanded. Meanwhile, the [AB(5) ] subunits in both structures expand, which have smaller initial volumes than [A(2)B(4)] subunits. The standard deviation of subunit volumes sigma(V-[A2B4] , V-[AB5]) in 3R-type structure is always smaller than that of 2H-type structure, and the sigma(V-[A2B4], V-[AB5]) in 3R-type structure is only 0.45 angstrom(3) , far less than that of 2H-type compound (2.11 angstrom(3) ) after 50 cycles. The lattice microstrain in 3R-type structure is quite lower than that in 2H-type structure and the 3R-type structure possesses a superior structural stability and higher anti-pulverization ability. After 50 cycles, the capacity retention of 3R-type compounds is 100% while that of 2H-type compounds is only 87.44%. Transmission electron microscope and X-ray diffraction analyses show that the crystal structure of 3R-type compound is almost unchanged after 50 cycles, while, severe lattice deformation is observed in 2H-type structure. (C) 2018 Elsevier B.V. All rights reserved.