The complete set of optical parameters of micron-sized polycrystalline CH 3 NH 3 PbI 3−x Cl x films deposited by the vacuum co-evaporation of lead iodide and methylammonium chloride is determined by analysis of oscillating optical transmittance and reflectance spectra in the wavelength range 400-1000 nm. It is shown that for a medium and weak absorption region the envelope method is valid for the extraction of refractive index, extinction and absorption coefficient when is using only transmittance spectra. As well thickness of the film is determined from transmittance and reflectance spectra with interference-effect. The absorption coefficient for the strong absorption region and optical band gap (direct transition at E opt =1.62 eV) are calculated based on transmittance and reflectance spectra by using conventional approximated formulas.
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.
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 MX
2 layered compound with the CdI2 (2H polytype) structure, composed of regular MX
6 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.
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