Ionic crystalline solids are formed by ionic bonds of positive and negative ions. The primary contribution to the binding energy of ionic crystals is electrostatic and is named the Madelung energy. The total crystal lattice energy is the energy needed to separate a crystal into individual ions at infinite distance from each other and is the sum of all interactions between individual ions. The total crystal lattice energy U can be written as the following.
Here, N is Avogadro’s number; zi and zj are the integral charges on the ions (in units of e); n is the Born exponent; and e is the charge on the electron in electrostatic units (e = 4.803 × 10–10 esu). r is the shortest distance between cation–anion pairs in centimeters, and M is called the Madelung constant. Then U is in ergs (1 erg = 10–7 J).
Table 1 contains the value for the Madelung constant for several different crystal forms and ion types.
Substance | Synonym | Formula | Ion type | Crystal forma | M |
Aluminum oxide (α) | Corundum | Al2O3 | 2M3+, 3X– – | Rhombohedral | 4.1719 |
Cadmium chloride | Cadmium dichloride | CdCl2 | M++, 2X– | Hexagonal | 2.244b |
Cadmium iodide | Cadmium diiodide | CdI2 | M++, 2X– | Hexagonal | 2.355b |
Calcium chloride | Hydrophilite | CaCl2 | M++, 2X– | Cubic | 2.365 |
Calcium fluoride | Fluorite | CaF2 | M++, 2X– | Cubic | 2.51939 |
Cesium chloride | CsCl | M+, X– | BCC | 1.76267 | |
Copper(I) oxide | Cuprite | Cu2O | 2M+, X– – | Cubic | 2.22124 |
Magnesium fluoride | Sellaite | MgF2 | M++, 2X– | Tetragonal | 2.381b |
Silicon dioxide (β-quartz) | Smoky quartz | SiO2 | M4+, 2X– – | Hexagonal | 2.2197b |
Sodium chloride | Halite | NaCl | M+, X– | FCC | 1.74756 |
Titanium(IV) oxide (anatase) | Anatase | TiO2 | M4+, 2X– – | Tetragonal | 2.400b |
Titanium(IV) oxide (rutile) | Rutile | TiO2 | M4+, 2X– – | Tetragonal | 2.408b |
Zinc oxide | Zincite | ZnO | M++, X– – | Hexagonal | 1.4985b |
Zinc sulfide (sphalerite) | Sphalerite | ZnS | M++, X– – | FCC | 1.63806 |
Zinc sulfide (wurtzite) | Wurtzite | ZnS | M++, X– – | Hexagonal | 1.64132b |
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The total crystal lattice energy is a balance between the electrostatic (Madelung) potential and a short-term repulsive potential, which is proportional to 1/n, where n is the Born exponent. Table 2 gives values of the Born exponent for a number of different ion types. For a crystal with a mixed-ion type, an average of the values of n in this table is to be used (6 for LiF, for example).
Ion type | n |
He, Li+ | 5 |
Ne, Na+, F– | 7 |
Ar, K+, Cu+, Cl– | 9 |
Kr, Rb+, Ag+, Br– | 10 |
Xe, Cs+, Au+, I– | 12 |