Activation of <i>d</i> orbitals under pressure and stability of alkali metal iodides

High pressure has emerged as a powerful tool for synthesizing novel compounds and initiating specific chemical reactions in materials science. In this work, we employed first-principles calculations and the CALYPSO structure search technique to systematically investigate the crystal structure stability of MexIy (Me = Li, Na, K, Rb, and Cs) compounds at high pressures. Our predictions unveiled several new phases with both conventional and unconventional stoichiometries. Notably, we observed an opposing trend in the stability of alkali metal iodides at high pressures, and we explored this phenomenon from multiple perspectives. Our analysis revealed that the activation of inner-shell d orbitals in heavy alkali metals is a key factor driving the decomposition of these compounds. In particular, in alkali metal iodides, a significant number of electrons transfer from the 5p orbital of iodine to the d orbitals of the heavy alkali metals, leading to weakening of the ionic bonds and triggering of the decomposition of these compounds. This research not only provides valuable insights into the decomposition mechanisms and high-pressure properties of Me-I compounds but also significantly contributes to our knowledge of chemical reactions in geological systems under extreme conditions.