which point defect may (1) lower the density of an crystal (2) does not alter the density of the relevant solid

schottky defect may lower the density of the crystal as in this defect there r cations nd anions missing from the crystal lattice although the numbr of cations nd anions missing are equal !!! but frenkel defect  does not decrease the density of the crystal , even if the cations r missing frm their positions in the crystal lattice because the missing particles occupy the interstitial positions !!