Question

At zero kelvin, most of the ionic crystal possess
a) Frenkel defect
b) Schottky defect
c) Metal excess defect
d) No defect

Answer 1

Ionic crystals have different kinds of vacancy and interstitial defects. They have a highly ordered crystal arrangement. We should know the properties of ionic crystals well and must have knowledge about the defects.

Complete solution:
Ionic crystals are the solid which have a high melting point. The arrangement of ions in the crystals is highly ordered. Thus they have a hard but brittle structure.
Ionic crystals are made of ions. Ions of opposite charge are highly attracted towards each other due to the coulombic force of attraction. The melt of ionic crystals conduct electricity due to the presence of ions. Ionic compounds have a very higher enthalpy of fusion and vaporization than other molecular compounds. Some examples of ionic compounds are $NaCl,KCl,etc$.
They are considered as good conductors of electricity when in water.
They have defects like the Frenkel defect where atoms present at the lattice site leave their places and get adjusted in the interstitial positions. In this defect there is no loss of electrical neutrality and no decrease in density. It also has a Schottky defect where equal numbers of cation and anion are missing from their lattice site. This defect causes a decrease in density.
But at zero kelvin the ionic crystals came in highly ordered arrangement and thus the different kinds of defect have no place in them.
Hence ionic crystals do not have any defect at zero kelvin.
Hence the correct option is (D).

Additional Information: Point defects in solids can be categorized into three types. Frenkel and Schottky defect comes under the Stoichiometric defect. They are also called intrinsic defects or thermodynamic defects.

Note: Stoichiometric defects are those which do not disturb the stoichiometry of solids. They are of two types- is) Vacancy defect and ii) Interstitial defect. Interstitial defects are those when constituent particles occupy an interstitial site. Due to this defect the density of solid increases.