Anomalous behavior of the Madelung constant and I–Fe–I bonding angle at high-pressure induced structural phase transition of FeI2

Abstract This study is devoted to theoretical analysis of the high-pressure experimental results available in literature that are associated with the structural phase transition of FeI2 in the pressure variation range up to ≈20 GPa. To establish the dependence of the interatomic interaction potentials of FeI2 on the pressure, theoretical relationships connecting the Madelung constant with the pressure-dependent lattice parameters, ionicity parameter, and I–Fe–I bonding angle are obtained. A strong correlation is revealed between the Madelung constant, ionicity parameter, and the bonding angle in their dependences on pressure. As a result of this correlation, with increase of pressure the Coulomb energy, which depends on the Madelung constant and ionicity parameter (i.e., ionic partial charges), tends to decrease, whereas the covalent energy dependent on the bonding angle exhibits a trend to increase. It is found that at the pressure of P = 16.9 GPa, the Madelung constant, ionicity parameter, and the bonding angle of the initial phase of FeI2 achieve their well-expressed minimum values. As a consequence, at above pressure the binding energy of FeI2 contributed by the Coulomb and covalent energies achieves its minimum value of 6.46 eV that results in structure destabilization and partial transition of the initial phase of the FeI2 to a new phase. It is concluded that, for FeI2 and compounds isostructural with this material, analysis of the pressure-dependent variation of the Madelung constant and bonding angle may be helpful in the prediction of possible structural phase transitions in high-pressure experimental investigations.