Physics:Interstitial compound

In this two-dimensional representation, small atoms sit in some of the interstitial sites of the lattice of the red atoms

An interstitial compound, or interstitial alloy, is a compound that is formed when an atom with a small enough radius sits in an interstitial “hole” in a metal lattice. Examples of small atoms are hydrogen, boron, carbon and nitrogen. The compounds are industrially important. They are neither typically ionic nor covalent.

History

The idea of interstitial compounds was started in the late 1930s and they are often called Hagg phases after Hägg. Transition metals generally crystallise in either the hexagonal close packed or face centered cubic structures, both of which can be considered to be made up of layers of hexagonally close packed atoms. In both of these very similar lattices there are two sorts of interstice, or hole:

• Two tetrahedral holes per metal atom, i.e. the hole is between four metal atoms
• One octahedral hole per metal atom, i.e. the hole is between six metal atoms

It was suggested by early workers that:

• the metal lattice was relatively unaffected by the interstitial atom
• the electrical conductivity was comparable to that of the pure metal
• there was a range of composition
• the type of interstice occupied was determined by the size of the atom

These were not viewed as compounds, but rather as solutions, of say carbon, in the metal lattice, with a limiting upper “concentration” of the smaller atom that was determined by the number of interstices available.

Current

A more detailed knowledge of the structures of metals, and binary and ternary phases of metals and non metals shows that:

• generally at low concentrations of the small atom, the phase can be described as a solution, and this approximates to the historical description of an interstitial compound above.
• at higher concentrations of the small atom, phases with different lattice structures may be present, and these may have a range of stoichiometries.

One example is the solubility of carbon in iron. The form of pure iron stable between 910 °C and 1390 °C, γ-iron, forms a solid solution with carbon termed austenite which is also known as steel.