The formula of a metal oxide with a metal deficiency defect in its crystal is $A_{0.8}O$. The crystal contains $A^{2+}$ and $A^{3+}$ ions. The fraction of metal existing as $A^{2+}$ ions in the crystal is -

The correct statement among the following,regarding defects in solids is:
$(1)$ Frenkel defect is favored by small difference in the sizes of cation and anion.
$(2)$ Frenkel defect is a metal excess defect.
$(3)$ Trapping of electron in the lattice leads to formation of $F$-centres.
$(4)$ Schottky defect has no effect on the physical property of solids.

Explain the following term with a suitable example:
$(iv)$ $F$-centres

It is observed that $Li_2O$ is soluble in $NiO$ in small amounts,where $Li^+$ ions occupy some of the cation sites. Which of the following conclusions can be drawn?

Schottky defect generally appears in

The correct statement regarding defects in solids is