Theoretical study of phase forming of NaZn13-type rare-earth intermetallics

By using the interatomic pair potential obtained with the lattice inversion method, the stability of RT13-xMx (R = La, Ce, Pr and Nd; T = Co and Fe; M = Si, Al, Cr, V and Ti) of the NaZn13 type and its derivative structure are studied. The structural transition of LaT13-xSix (T = Co and Fe) between the cubic one with the space group Fm3c and the tetragonal one with 14/mcm is imitated from the viewpoint of energy. As for the function of the third elements, Al and Si are beneficial to the phase stability of RT13-xMx, whereas Cr, Ti and V are unfavourable to the stability. In the calculation, the range of x, with which RT13-xMx could crystallize in the cubic or tetragonal structures, agrees with the experiments very well. The calculated crystallographic parameters coincide with the experimental observation. In the cubic structure, Si and Al prefer the 96i site, and in the tetragonal structure Si first occupy the 16l(2) site, then the 16k site. In addition, all the site positions of the compounds with either the cubic or tetragonal structure are really congruent with the experimental one.