Fluorite Structure

Fluorite structure, in general terms, is a common motif for compounds with the formula MX_2, wherein the X ions tend to occupy the eight tetrahedral interstitial sites. On the other hand, the M ions occupy the regular sites of a face-centred cubic (FCC) structure. The most common mineral, fluorite (CaF₂), has this structure.

Taking the example of calcium fluoride, it is a solid that crystallises isometric cubic habit, or in simple terms, it forms a cube-like structure. Now, this structure is centralised around calcium molecules. In other words, we can say that the crystal lattice structure that calcium fluoride form is the fluorite structure. Here, the Ca²⁺ ions are eight-coordinate, being centred in a cube of eight F^– ions. Each of the F^– ions are coordinated with four Ca²⁺ ions. Interestingly, fluorite (also called fluorspar) is the mineral form of calcium fluoride, CaF₂.

Some other examples of this include UO₂, ZrO₂, and CeO₂. Basically, the structural motif adopted by fluorite is so common that the motif, in general, is referred to as the fluorite structure. The substitution for the calcium cation is often done by other elements, such as strontium and even a few specific rare earth elements (REE) like yttrium and cerium. It is a type of ionic crystal structure.

Fluorite Structure Representation

A typical representation of the structure resembles cations making an FCC lattice and anions occupying the tetrahedral sites.

Fluorite Structure Important Points

• The cations are slightly bigger in comparison to other structures
• Co-ordination number: Cations – 8, Anions – 4
• Lattice: FCC
• Motif: M – 0 0 0, X – ¼ ¼ ¼, ¾ ¾ ¾
• The structure has a large void in the centre of the unit cell which is made by the cations.
• Due to these empty spaces, it makes such oxides that are good ionic conductors and is important in energy storage applications, such as in batteries.

Anti-fluorite Structure

On the other hand, magnesium compounds such as Mg₂X, where X is normally elements such as Si, Ge, Sn, or Pb, have an antifluorite structure. In this case, the locations of the anions and cations are reversed relative to fluorite (an anti-structure). The anions occupy the FCC regular sites, whereas the cations occupy the tetrahedral interstitial sites. The antifluorite structure is basically the opposite arrangement, with anions having coordination number 8 and cations having coordination number 4.

For instance, magnesium silicide, Mg₂Si, has a lattice parameter of 6.338 Å, where magnesium cations occupy the tetrahedral interstitial sites wherein each silicide anion is surrounded by eight magnesium cations. Each magnesium cation is surrounded by four silicide anions in a tetrahedral fashion.

Also Read: Hybridization

Fluorite Structure Video Lesson