Structure and magnetism in RNi1-xCuxAl(R = Er, Dy) compounds

Results of crystal and magnetic structure study of DyNi1-xCuxAl quasiternary compounds are presented and discussed in context with results of previous investigations of analogous compounds with Th and Er. This allows to draw conclusions with more general validity for this type of materials with heavy rare earths showing common anomalies of lattice parameters (a and c) and evolution of magnetism with respect to the composition of the Ni,Cu sublattice. The opposite steps in the a(x) and c(x) dependences compensate and have no effect on the lattice volume. On the other hand, these anomalies yield discontinuity of the c/a concentration dependence and manifest existence of a "forbidden" c/a ratio around c/a (approx 0.56-0.57) in RTX compounds with the hexagonal ZrNiAl structure. The loss of magnetic long-range order, which is generally observed around x= 0.8, is not coupled with the structure anomaly but the number of 3d electrons in the Ni, Cu sublattice seems to be the crucial parameter.