A DFT study on the correlation between topology and Bader charges: Part IV, on the change of atomic charges in polymorphic transitions – A case study on CaCl2

• We present a DFT study of all modifications of CaCl2 in different specified volumes.
• The correlation between structural topology and Bader charges of the atoms is shown.
• We define a structural softness based on the variation of energy with volume.
• We define a hardness of charge as driving force for structural organization.

Referring to the experimental results of high pressure experiments of Léger et al. (1998) we have calculated the energies of all phases observed for CaCl2 within the DFT formalism using the VASP package, and we have retrieved enthalpies and transition pressures. All phases can be considerably compressed or dilated without much change in energy. This energetic “softness” could even be quantified. We classify the high temperature TiO2-type structure and the PbCl2-type one at highest pressures as the energetically “softest” ones and the SrI2-type one as the “hardest”. We furthermore discuss the energy density (E/V) of the different phases and redefine it as a fictive cohesive pressure within these structures.Pursuing our earlier approaches we have analysed the charges of the atoms in the different CaCl2 phases and their change on compression or dilation. On comparing the gradients of the charge curves we define a sort of “charge hardness” which will generally depend on the type of cation–anion pair but also on their topological connection in the respective structures. We speculate that exhausting the “charge softness or hardness” of individual ions in such arrangements may initiate the structural reorganization at the transition pressures.

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