In a solid with Schottky defect atoms are missing from their position. Thus, the effective number of formula units per unit cell will decrease. so, density will also decrease.

For FCC lattice with $0.1\%$ Schottky defect, the effective atoms per unit cell,

${\mathrm{Z}}_{\mathrm{effective}}=4\left(1–\frac{0.1}{100}\right)=4–0.004 =3.996$

Now, the formula for density $\mathrm{d}=\frac{\mathrm{ZM}}{{\mathrm{N}}_{\mathrm{A}}{\mathrm{a}}^3}$

$\mathrm{Z}$$\to$ Effective atoms per unit cell

$\mathrm{M}$$\to$ Atomic mass in $\mathrm{g}$ ${\mathrm{mole}}^{–1}$

${\mathrm{N}}_{\mathrm{A}}\to 6\times {10}^{23}, \mathrm{a}\to 4\times {10}^{–8} \mathrm{cm}$$\mathrm{d}=\frac{3.996\times 56}{6\times {10}^{23}\times (4\times {10}^{–8}{)}^3}=5.8 \mathrm{g} {\mathrm{cm}}^{–3}$