Measurements are presented of the structural and magnetic properties of several series of pseudo-binary compounds formed between the (heavy) Rare Earth metals, or Yttrium, and the 3d transitional metals, in the Laves phase structure. This work has been undertaken in an attempt to obtain a better understanding of the nature of the moments and coupling mechanisms in these compounds, RX(_2).The magnetic results are discussed on the basis of a rigid band model for the 3d electrons in these compounds and this model is shown to provide a unified account of the properties of all the RX(_2) compounds. An elementary density of states histogram for the 3d band associated with the X component is presented. The C 36 phase is found to coexist with the C 15 terminal phase at intermediate compositions in some of the series. For the C 15 phases the lattice parameters show a positive deviation from Vegard's Law and these results are discussed in relation to the magnetic properties
