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

Abstract

In this theoretical study, the Madelung constant ( A _M ) both for a 2D layer and parent 3D bulk crystal of metal diiodides MI _2 (M = Mg, Ca, Mn, Fe, Cd, Pb) with CdI _2 (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 MI _2 with CdI _2 (2 H polytype) structure, but also for metal dihalides (MX _2 ) and metal dihydroxides [M(OH) _2 ] with the same structure. With the use of the high-pressure experimental results available in literature particularly for FeI _2 , 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.

Keywords

• layered compounds
• metal diiodides
• the madelung constant
• 2D layer
• high-pressure phase transition