Ionicity scale based on the centers of maximally localized Wannier functions

The maximally localized Wannier functions (MLWFs) of 32 A(N)B(8-N) compounds, ranging from elemental group-IV solids to I-VII compounds, have been constructed according to the method of Marzari and Vanderbilt [Phys. Rev. B 56, 12847 (1997)]. The considered systems crystallize in the diamond, zinc-blende, or rocksalt structures. A different bond ionicity scale has been introduced based on the deviation of the centers of the MLWFs from the corresponding bond centers, which involves only physical constants. The present bond ionicity of the considered compounds can be considered, to a very good approximation, as the best fit to the previous empirical and self-consistent ionicity scales. The critical value of the bond ionicity that separates the fourfold and sixfold coordinate structures is found to be of about 0.7, which is smaller than previous theoretical values of about 0.8. The volume variation of the bond ionicity is found to be much smaller than previously thought, except for SiC.