Interstitial compound

The term interstitial compound, or interstitial alloy, is used to describe a compound that is formed when an atom of sufficiently small radius sits in an interstitial “hole” in a metal lattice. Examples of small atoms are hydrogen, boron, carbon and nitrogen. These compounds are important industrially, for example some transition metal carbides and nitrides.

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History

The idea of interstitial compounds was mooted in the late 1920's and they are often called Haag phases after G.Hägg ^[1]. Transition metals generally crystallise in either the hexagonal close packed or body 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:

• 2 tetrahedral holes per metal atom , i.e. the hole is between four metal atoms
• 1 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 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.

An 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.

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