A crystal has Schottky defects in 1.0% of its formula units (each such unit has lost one cation and one anion) and, in addition, Frenkel defects in 2.0% of the cation sites. Assuming the cell volume is unchanged, the density of this crystal compared with the ideal one

Ais lower by 1.0%
Bis lower by 3.0%
Cis lower by 2.0%
Dis unchanged

Answer: A

Frenkel defects only displace cations to interstitial positions, so the number of ions in the crystal does not change and the mass is unaffected. Only Schottky defects remove mass: 1.0% of the formula units are lost, so the density falls by 1.0%.

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