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Figure 3
RA/RB is plotted versus [\Delta{N}^{1/3}] 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. [\Delta{N}^{1/3}] = NA1/3 − NB1/3 is negative for MgCu2-type compounds and so is [\Delta\varphi] = [{\varphi}_{A}] − [{\varphi}_{B}]. [\Delta\varphi] ∝ [\Delta{N}^{1/3}] for the compounds as can be seen from Fig. 2[link]. The direction of charge transfer has to be from the more electronegative atom to the less electronegative atom according to Pauling (1950BB27), 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 [\gg] 1 and with [|\Delta{N}^{1/3}|] ≃ 1; the minimum change is when RA/RB ≃ 1.0 at [|\Delta{N}^{1/3}|] ≃ 0. The variation of RA/RB with [\Delta{N}^{1/3}] 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.

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ISSN: 2053-2733
Volume 68| Part 1| November 2012| Pages 156-165
doi:10.1107/S0108767311044151
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