Which defect leads to an increase in the density of a solid?

Analysis shows that nickel oxide has the formula $Ni_{0.98}O_{1.00}$. What fractions of nickel exist as $Ni^{2+}$ and $Ni^{3+}$ ions?

In an ionic solid,equal number of cations and anions are missing from their regular positions in the crystal lattice,creating vacancies. This is called-

What type of crystal defect is indicated in the diagram below?
$Na^{+} \, Cl^{-} \, Na^{+} \, Cl^{-} \, Na^{+} \, Cl^{-}$
$Cl^{-} \, \Box \, Cl^{-} \, Na^{+} \, \Box \, Na^{+}$
$Na^{+} \, Cl^{-} \, \Box \, Cl^{-} \, Na^{+} \, Cl^{-}$
$Cl^{-} \, Na^{+} \, Cl^{-} \, Na^{+} \, \Box \, Na^{+}$

When $SrCl_2$ is added to molten $NaCl$ and crystallized,the defect formed is ........

Which of the following ionic crystals exhibits Frenkel defect?