Frenkel defect..

This statement is true.

Frenkel defect is the point defect in which an ion is missing from its normal position and occupies an interstitial site between lattice points.

This defect is observed in the case of ions which have low radius ratio (r₊/ r_-). This means that the anions are larger in size than the cations.

The cations are small size so that the ions can go into the interstitial site. Large size ions cannot get into the interstittial positions so they do not show Frenkel defect.