Correlation of the Madelung constant and I—M—I bonding angle with cohesive energy contributions in layered metal diiodides (MI₂) with CdI₂ (2H polytype) structure

This study uses theoretically methods to investigate, for metal diiodides MI₂ (M = Mg, Ca, Mn, Fe, Cd, Pb) with CdI₂ (2H polytype) structure, the mutual correlation between the structure-characterizing parameters (the flatness parameter of monolayers f, the Madelung constant A, and bonding angle I—M—I) and correlation of these parameters with contributions of the Coulomb and covalent energies to cohesive energy. The energy contributions to cohesive energy are determined with the use of empirical atomic potentials. It is demonstrated that the parameters f and A, and the bonding angle I—M—I are strictly correlated and increase in the same order: FeI₂ < PbI₂ < MnI₂ < CdI₂ < MgI₂ < CaI₂. It is found that with an increase of parameter A and bonding angle I—M—I the relative contribution of the Coulomb energy to cohesive energy increases, whereas the relative contribution of the covalent energy decreases. For a hypothetical MX₂ layered compound with the CdI₂ (2H polytype) structure, composed of regular MX₆ octahedra (angle X—M—X = 90°), the flatness parameter and the Madelung constant are found to be f_reg = 2.449 and A_reg = 2.183, respectively. Correlation of the covalent energy with the type of distortion of MI₆ octahedra (elongation or compression) with respect to regular configuration (angle I—M—I = 90°) is also analyzed.