RA/RB is plotted versus for MgCu2 and AsNa3-type compounds. Compounds exist in both systems over a wide range of RA/RB values. No preference is shown for any particular metallic radii ratios. A is the minority element. RA and RB are the radii of the atoms in the elemental state. = NA1/3 − NB1/3 is negative for MgCu2-type compounds and so is = − . ∝ for the compounds as can be seen from Fig. 2. The direction of charge transfer has to be from the more electronegative atom to the less electronegative atom according to Pauling (1950), i.e. from B to A, and the A atoms become smaller owing to the enhanced attraction by the positive charges at the core. The radius ratios of the atoms shift from RA/RB which are in the range above 1.0 to values suitable for ideal packing in the MgCu2-type structure. The maximum change in radii is for compounds with RA/RB 1 and with ≃ 1; the minimum change is when RA/RB ≃ 1.0 at ≃ 0. The variation of RA/RB with is almost linear. A similar argument will show that radii ratios will shift from RA/RB values which are in the range less than 1.0 for AsNa3-type compounds, to values close to 1.0 by charge transfer.