Over the past centuries, chemists have developed simple schematic models (and rules) to rationalize and possibly predict the chemical behavior of elements and compounds. However, the scaffold of chemical intuition just mentioned tends to fall down when one enters the realm of high pressure (tens or hundreds of GPa) [1]. To mention just a few among many examples, at pressures above 80 GPa unexpected compounds such as NaxCl (x=1.5,2,3) [2] and Mg3O2 [3] were predicted to be thermodynamically stable. In this contribution, we present the results of our investigation on the series of compounds NaxCl. We performed periodical calculations for all the compounds of the series at several different pressures and we have analyzed the resulting wavefunctions by exploiting quantum-chemistry-based tools such as the Quantum Theory of Atoms In Molecules, the Electron Localization Function and the deformation density analysis. Our thorough study allowed us to highlight and rationalize some peculiar features of the investigated compounds, e.g. the presence of Na atoms forming at the same time both metallic and ionic bonds, and the formation of non-nuclear charge density maxima. More importantly, by considering the results of chemical bonding analysis in NaxCl and other similar compounds, we will introduce a general model for the understanding and prediction of the outcome of high-pressure reactions between alkali halides and alkali metals.
References
[1] W. Grochala, R. Hoffmann et al. (2007) Angew. Chem. Int. Ed. 46, 3620
[2] W. Zhang, A. R. Oganov et al. (2013) Science 2013, 342, 1502
[3] Q. Zhu, A. R. Oganov, A. O. Lyakhov (2013) PCCP, 15, 7696