Ionic crystal structures adopt a form which compromises between three factors: relative sizes of cations and anions, lowest possible potential energy, and charge balance.

 

 

Introduction

 

 

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.

 

 

 

array of anions and cations

 

Ionic compounds constitute a class of substances which find technological uses in almost every aspect of modern life: electronic components, catalysis, batteries, lasers,  solar energy, gas purification, to name only a few.

 

 

Binary Ionic Compounds

 

 

 

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

 

 

MX

 

 

These ionic compounds include all substances which have a 1:1 ratio of anions and cations. This can mean: KCl, MgO, or AlN

 

 

 

NaCl

 

 

 

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.

 

This structure applies to other common ionic structures like AgCl, and CsF. It also applies to ionic compounds when both the cation and anion both have the same charge and form from 1:1 stoichiometry: MgO, PbS, FeO, and TiO.

 

 

 

 

CsCl

 

 

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.

 

Other known examples of ionic compounds that adopt this configuration include NH4Cl, CsCN, and CsSH.

ZnS

 

 

Zinc sulfide can adopt one of two crystal structures: sphalerite or Wurtzite. Sphalerite is the more stable form with Wurtzite forming above 1020°CZnS finds technological uses like luminescent pigments and as water splitting catalyst.

 

 

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.

 

It shares common structure with CuCl.

 

 

Wurtzite

 

 

wirtzite
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.

 

Other ionic compounds which take a Wurtzite structure like BeO, SiC, AlN, and NH4F.

 

MX2

 

 

Many metal halides, oxides, and sulfides combine to form a 1:2 ration of cations to anions. These can take a variety of structures and often exhibit polymorphism: presenting more than one crystal phase.

 

CaF2 (Fluorite)

 

 

 

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

A single metal with a (+2) charge and a metal substituent with a (-1) charge, fluorite or CaF2, serves as an example where the cation assumes the role of a face centered cubic cell. Fluoride ions take the role of filling the tetrahedral holes, Figure 5. The red spheres show Ca+2 and the blue spheres represent fluoride ions.

 

Alternatively, the unit cell can be regarded as a face centered cubic cell of Ca+2 with a a simple cubic cell of fluoride ions imbedded inside.

 

 

Fluorite occupies the role of the most important starting material of fluorine containing compounds. It finds use for the production of HF. For ceramic applications CaF2 is used to produce a shiny glaze. It also plays in important role as a flu additive used during the smelting of metals.

 

 

HgF2, PbF2, ZrO, and HfO also take a fluorite structure.

 

 

MgCl2

 

 

Magnesium chloride crystal structure
Figure 6: Ionic crystal structure of magnesium chloride where chlorides adopt a face centered cubic structure and half the octahedral holes are occupied by Mg+2.

In magnesium chloride, like many other ionic crystals, the chloride ions adopt a cubic close packed structure. Magnesium cations, in order to establish the stoichiometry of 1:2, occupy half of the octahedral holes, Figure 6.

 

 

 

This also has an  interpretation where the corner three atoms from a cation cube overlap the corner of a cube formed from chloride anions.

 

 

This gives rise to a layered crystal structure, in which alternating layers of ions are arranged in an anti-parallel fashion, Figure 7.

 

MgCl2 layered structure
Figure 7: The MgCl2 ionic crystal structure shown as alternating layers

Magnesium chloride has its best known use as a diet supplement which alleviates the symptoms of magnesium deficiency. The deficiencies include muscle cramps and fatigue.  It is prescribed for high blood pressure and type 2 diabetes.

 

 

Other common ionic crystals which adopt the same structure are: CdCl2, MnCl2, NbS2, and TaS2.

TiO2

 

 

Titanium (IV) oxide exemplifies another common kind of ionic crystal structure, rutile and anatase. Like ZnS, TiO2 has more than one stable structure.

 

 

The majority of TiO2 finds use as an additive in pigments in:  paints, paper and food coloring. Thin films are employed as a ceramic glaze to improve surface luster. A common consumer product is as an additive in sun screen. 

 

 

Titanium dioxide shows great promise as photocatalyst for generating hydrogen from water, and as a photosensitizer in solar cells.

 

 

Rutile

 

 

 

rutile ionic crystal structure
Figure 8: Rutile crystal structure shows a rhombohedral unit cell: oxygen in blue and titanium in red.

 

Rutile forms a rhombohedral unit cell, which can also be seen as a distorted hexagonal structure. Oxygen occupies the the corners and the center of the unit cell. The Ti+4 ions fit into every other octahedral hole, Figure 8

 

Anatase

 

 

 

 

ionic crystal structure of anatase
Figure 9: Anatase has oxygen assume a tetragonal unit cell with Ti+4 ions filling half the octahedral holes

Ionic crystals of anatase are constructed from tetragonal unit cells. You can also imagine the structure as an elongated hexagon. Each kite shaped chevron connects in a perpendicular direction to its next kite shaped chevron, Figure 9

 

 

M2X

 

Low valent metals often combine with nonmetals in multiple proportions dictated by the necessity of achieving an overall neutral charge.

 

Li2O

 

 

Antifluorite ionic crystals
Figure 10: Li2O crystals assume antifluorite structure, where O-2 provides an fcc structure while Li+1 fills all the tetrahedral holes

Lithium oxide is used industrially as a starting material for lithium hydroxide and lithium carbonate.  In the ceramics industry, it used as an additive as a flux during the production of glazes.

 

 

It adopts an antifluorite structure. In this case, the O-2 ion takes a face-centered cubic structure where the Li+1 cations occupy all the tetrahedral holes.

 

 

 

 

M2X3

 

The majority of substances which conform to a 2:3 stoichiometry include metal oxides and metal sulfides. 

 

Al2O3

 

 

aluminum oxide crystal
Figure 11: Al2O3 has oxygen assume a hexagonal ionic crystal structure with Al+3 filling half the octahedral holes.

Alum, or aluminum oxide, has the anionic oxygen ions take a the lattice points of a hexagonal close packed structure. The Al+3 cations occupy two thirds of the octahedral holes. Each aluminum atom is surrounded with six oxygens while each oxygen is surrounded by four aluminums. 

 

 

Many other metal oxides share the same basic structure such as: α-Fe2O3, Cr2O3, V2O3, and Rh2O3.

 

There are so many applications for alumina that it requires a separate article to discuss them all. They include but are not limited to: glass, gems, paint, catalysis, and gas purification.

 

 

 

Conclusion

 

There are many other examples of binary ionic crystal structures. These represent the most common time most often encountered. At least a passing knowledge of them becomes necessary to understand how defects and doping are used to exploit the properties of ionic crystals.

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