Analytic dependence of the Madelung constant on lattice parameters for 2D and 3D metal diiodides (MI2) with CdI2 (2H polytype) layered structure

Abstract

Abstract In this theoretical study, the Madelung constant (A M ) both for a 2D layer and parent 3D bulk crystal of metal diiodides MI2 (M = Mg, Ca, Mn, Fe, Cd, Pb) with CdI2(2 H polytype) structure is calculated on the basis of the lattice summation method proposed in author’s earlier work. This method enabled, both for a 2D layer and 3D bulk crystal of these compounds, to obtain an analytic dependence of the Madelung constant, A M (a, c, u), on the main crystallographic parameters a, c, and u. The dependence A M (a, c, u) reproduces with a high accuracy the value of the constant A M not only for metal diiodides MI2 with CdI2(2 H polytype) structure, but also for metal dihalides (MX2) and metal dihydroxides [M(OH)2] with the same structure. With the use of the high-pressure experimental results available in literature particularly for FeI2, it is demonstrated that the above analytic dependence A M (a, c, u) is also valid for direct and precise analysis of the pressure-dependent variation of the Madelung constant.