Which of the following statements for crystals having Frenkel defect is not correct? (a) Frenkel defects are observed where the difference in sizes of cations and anions is large (b) The density of crystals having Frenkel defect is lesser than that of a pure perfect crystal (c) In an ionic crystal having Frenkel defect may also contain Schottky defect (d) Usually alkali halides do not have Frenkel defect

Defects in crystal lattices are deviations from the perfect, regular arrangement of atoms or ions that make up the structure of a crystal. These imperfections can arise from various processes, such as irregularities during the crystallization process or the introduction of impurities.

There are several types of lattice defects, including point defects, line defects, and planar defects. Point defects are localized disruptions represented by vacancies (where an ion or atom is missing), interstitials (additional atoms or ions inserted into spaces that are not regular lattice positions), Frenkel defects (a combination of a vacancy and an interstitial when an ion moves away from its position to a nearby gap), and substitutional defects (where one type of atom or ion replaces another).

Each type of defect can affect the properties of the crystal, such as its electrical conductivity, strength, and color. Frenkel defects, in particular, do not significantly change the density of the crystal as atoms or ions are not lost from the structure; they merely move to different locations within the crystal lattice.

Ionic crystals are a type of crystalline solid where the constituent particles are ions held together by strong electrostatic forces known as ionic bonds. These structures are characterized by a repeating array of positive and negative ions that typically form a highly organized crystal lattice.

Due to the nature of ionic bonding, ionic crystals usually have high melting points, are brittle, and when dissolved in water or melted, they conduct electricity. In their solid state, however, they are poor conductors because the ions are fixed within the lattice and cannot move freely. Common examples of ionic crystals include table salt (sodium chloride) and many minerals.

In the context of defects, the rigid and tight lattice structure of ionic crystals means that any imperfections can significantly affect their properties. For instance, ionic crystals are more susceptible to lattice defects due to inherent characteristics like ionic size and charge, which can create predispositions for certain types of defects such as Frenkel or Schottky.

A Schottky defect is a type of point defect that occurs in ionic crystals when an equal number of cations and anions are missing from the lattice, creating pairs of vacancies without introducing any new ions into the structure. The elcetrationic neutrality of the crystal is maintained due to the loss of an equal number of positive and negative ions.

This type of defect can result in a decrease in the density of the crystal material since there is an actual loss of mass with the ions missing from the lattice structure. Schottky defects are common in ionic compounds with ions of similar size, such as certain alkali halides.

Contrary to what one might expect, ionic crystals with Schottky defects can demonstrate increased ionic conductivity because the vacancies allow for easier migration of ions through the crystal lattice. This property is important in the application of certain materials within the field of solid-state ionics, such as in solid electrolytes for batteries.