Hexagonal close-packed lattice interstitial holes

When the radius ratio is less than 0.59 the alloy is normal and the mclal—interstitial atom arrangement is face-centred cubic, close-packed hexagonal or body-centred cubic. The complex interstitial alloys have a radius ratio greater than 0.59 and are less stable. Carbon and nitrogen always occupy octahedral holes in interstitial alloys hydrogen always occupies the smaller tetrahedral interstices. In face-centred cubic and close-packed hexagonal lattices there are as many octahedral holes as metal atoms and twice as many tetrahedral holes. 

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