Red the passage given below and answer the following question
Point defect play an important part in determining the physical properties of most crystalline substance, most notably those controlling the transport of matter and the properties that stem from it. Even crystl of high purity under conditions of no irradiation contains point defects in thermal equilibrium. Some lattice sites are vacant, and some atoms are displaced from their normal lattice sites into interstitial positions or onto "wrong" lattice sites. For stoichiometric compounds of high purity, the concentrations of these point defects are very low, even at temperatures up to the melting point. A meaningful model, then, is to consider the crystal as a solvent containing a very dilute solution of simple, individual vacancies and interstitials, Long-range interactions among the defects and with impurity atoms, and short-range interactions that produce pairs or other clusters can be introduced in a first order approximation.
Source: https:`////`www.sciencedirect.com`//`topics`//`physics-and-astronomy`//`point-defects
This type of defect arises due to absence of equal number of cations and anions from lattice sites in the crystalline solid of the type `A^+B^(-)` and it lowers the density of the crystal.

Options

A. Vacancy defect
B. Schottky defect
C. Interstitial defect
D. Frenkel defect

Answer

To solve the question regarding the type of point defect in crystalline solids, we will follow these steps:

### Step 1: Understand the Concept of Point Defects
Point defects are imperfections in the crystal lattice that occur at a single point. They can include vacancies (missing atoms) and interstitials (extra atoms in the lattice). In this case, we are focusing on a specific type of defect that involves the absence of cations and anions.

**Hint:** Remember that point defects can significantly affect the properties of a crystalline solid.

### Step 2: Identify the Type of Defect
The question mentions the absence of equal numbers of cations and anions from the lattice sites in a crystalline solid of the type \(A^+B^-\). This indicates that both types of ions are missing from their expected positions in the lattice.

**Hint:** Think about how the absence of ions would affect the structure and density of the crystal.

### Step 3: Relate the Defect to Density
The passage states that this type of defect lowers the density of the crystal. When cations and anions are missing, the overall mass of the crystal decreases while the volume remains the same, leading to a lower density.

**Hint:** Consider how the mass-to-volume ratio changes when components of the crystal lattice are missing.

### Step 4: Name the Defect
The specific defect described, where equal numbers of cations and anions are missing, is known as a "Schottky defect." This defect typically occurs in ionic solids and is characterized by the absence of ions in pairs (one cation and one anion).

**Hint:** Recall the names of common types of point defects in ionic crystals.

### Conclusion
Based on the analysis, the type of defect arising from the absence of equal numbers of cations and anions in a crystalline solid of the type \(A^+B^-\) is called a **Schottky defect**.

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