Charge transfer on the metallic atom-pair bond, and the crystal structures adopted by intermetallic compounds

Rajasekharan, T.; Seshubai, V.

doi:10.1107/S0108767311044151

Figure 3
R_A/R_B is plotted versus $[\Delta{N}^{1/3}]$ for MgCu₂ and AsNa₃-type compounds. Compounds exist in both systems over a wide range of R_A/R_B values. No preference is shown for any particular metallic radii ratios. A is the minority element. R_A and R_B are the radii of the atoms in the elemental state. $[\Delta{N}^{1/3}]$ = N_A^1/3 - N_B^1/3 is negative for MgCu₂-type compounds and so is $[\Delta\varphi]$ = $[{\varphi}_{A}]$ - $[{\varphi}_{B}]$ . $[\Delta\varphi]$ [proportional to]

$[\Delta{N}^{1/3}]$ 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 R_A/R_B 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 R_A/R_B $[\gg]$ 1 and with $[|\Delta{N}^{1/3}|]$ [asymptotically equal to]

1; the minimum change is when R_A/R_B [asymptotically equal to]

1.0 at $[|\Delta{N}^{1/3}|]$ [asymptotically equal to]

0. The variation of R_A/R_B with $[\Delta{N}^{1/3}]$ is almost linear. A similar argument will show that radii ratios will shift from R_A/R_B values which are in the range less than 1.0 for AsNa₃-type compounds, to values close to 1.0 by charge transfer.