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Arrange the following interstitial voids in decreasing order of their size:

1. cubic > octahedral > tetrahedral > trigonal
2. trigonal > tetrahedral > octahedral > cubic
3. trigonal > octahedral > tetrahedral > cubic
4. cubic > tetrahedral > octahedral > trigonal

Correct answer: cubic > octahedral > tetrahedral > trigonal

Solution

The radius ratio r/R (where r is the radius of the atom fitting in the void and R is the radius of the host atom) increases with the size of the void: cubic (r/R = 0.732) > octahedral (r/R = 0.414) > tetrahedral (r/R = 0.225) > trigonal (r/R = 0.155). Hence cubic > octahedral > tetrahedral > trigonal.

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