NEUTRON-DIFFRACTION STUDY OF FE SUBSTITUTIONS IN ND(2)FE(17-DELTA)X(DELTA) (X=AL,SI,GA,MO,W)

In Nd(2)Fe(17-delta)X(delta) (X = Al, Si, Ga, Mo, W and delta = 0 to 1 .5, depending on the concentration), the X elements substitute for Fe atoms without changing the crystal structure. Neutron diffraction meas urements show that Al, Si, and Ga elements tend to substitute for Fe w ith a strong preference for the 18h site while Mo, and W tend to subst itute for Fe with a strong preference for the 6c site. This substituti on can be explained by considering the enthalpy of mixing using Miedem a's model. We find that the enthalpy of mixing for Nd-(Al,Si,Ga) compo unds is more negative than that for Fe-(Al,Si,Ga) compounds and this d rives Al, Si and Ga to occupy the 18h site which has the largest numbe r of Nd nearest neighbours. The enthalpy of mixing for Nd-(Mo,W) has a large and positive value and the enthalpy of mixing for Fe-(Mo,W) is zero. This drives Mo and W to the site with the smallest number of Nd nearest neighbours. We show that the site volume plays a minimal role in the substitution of the X elements, only affecting the substitution in the 9d site.