Ionic Crystals

Ionic crystal structures adopt a form which compromises between three factors: relative sizes of cations and anions, lowest possible potential energy, and charge balance.       Structures fall most often into either binary or ternary compounds. Binary compounds consist of an anion and a cation. Ternary compounds consist of three species.   A few examples illustrate common ionic compounds. A complete treatment requires a much longer text.   Regardless of the type of compound, the basic structure of ionic compounds derives from lattice structures already presented. How Atoms Pack. In most cases, anions occupy the lattice points, because anions are larger.  Cations occupy voids in the structure. Solid Voids.    

Binary Ionic Structures



  Binary ionic crystal structures contain a metal and a nonmetal. Structures occur based on the relative size of cation and anion.   



Figure 1: sodium chloride has chlorides take a face-centered cubic structure with sodium occupying the voids.

  Sodium chloride has the chloride anions adopt a face centered cubic structure, Figure 1. The chloride anions occupy the corners and the center of each face of a cube, (blue). Cationic sodium ions, (red) occupy each octahedral void  formed from the chloride ions.     The sodium chloride structure can also be seen as two interpenetrating face-centered cubes.              



Figure 2: Because cesium is much smaller than chloride, the chloride takes a body-centered cubic structure. . This is equivalent to two interpenetrating cubes where the opposite ion’s corner resides in the middle of the other ions simple cubic structure.

Cesium chloride shows another possible arrangement of ions, Figure 2. Given the small size of the cesium cation, it fits into an interstitial between four adjacent chloride ions.   The chloride ions forms a simple cubic structure with the cesium in the middle making the unit cell body-centered cubic.   Alternatively, the cesium chloride cells can be thought of as two mutually interwoven cubes with a corner of one cube at the center of the other cube.   CsCl is used in the purification of DNA, with its radioactive isotopes employed in cancer treatment. It finds uses in specialty electronic devices like electrically conductive glasses, activation of welding electrodes, and high temperature soldering fluxes.     


    Zinc sulfide can adopt one of two crystal structures: sphalerite or Wurtzite. Sphalerite is the more stable form with Wurtzite forming above 1020°C. ZnS finds technological uses like luminescent pigments and water splitting       Sphalerite (zinc blende)    
sphalerite crystal structure

Figure 3: Sphalerite structure made of face-centered S-2 ions with half the tetrahedral voids filled with Zn+2 ions.

In the most common form of ZnS, sulfide (-2) anions assume a face centered cubic structure. The Zn+2 occupy half the tetrahedral interstitial sites. It can also be seen as a face-centered cube with a tetrahedron imbedded inside, Figure 3.       Wurtzite      

Figure 4: alternating hexagonal faces with alternating counter ion tetrahedral sites build the structure of Wurtzite.

When ZnS adopts its other possible structure, it adopts an alternating hexagonal structure. The anions and cations alternate in hexagonal faces with alternating Zn+2 and S-2, Figure 4.   Between the hexagonal faces the opposite ion takes a tetrahedral geometry in three sectors of the hexagon. It can also be explained as sulfide ions adopting a hexagonal close packed structure, while zinc cations occupy half the tetrahedral holes. It can also be seen as corner shared tetrahedrons with alternating layers disposed in an opposite sense to each other.  


      CaF2 (Fluorite)  
fluorite crystal structure

Figure 5: CaF2 adopts an fcc structure for Ca+2 and the F-1 sits in tetrahedral voids

                        MgCl2 TiO2





Ternary Ionic Structures

  Perovskites   Rutile