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.