Question

In Schottky defect (a) a lattice site is occupied by electron (b) the radius ratio, \(\mathrm{r}^{+} / \mathrm{r}^{-}\), is low (c) some of lattice sites are vacant (d) an ion occupies interstitial position between lattice points

Short answer

(c) some of lattice sites are vacant.

Step-by-step solution

Understanding Schottky Defect

A Schottky defect occurs when pairs of cations and anions are missing from their lattice sites in an ionic crystal. This type of defect conserves both the electrical neutrality and the stoichiometry of the crystal.

Recall Characteristics of Schottky Defect

In a Schottky defect, the primary characteristic is the presence of vacancies at lattice sites, leading to missing pairs of both positive and negative ions. This contrasts with Frenkel defects, where ions are displaced to interstitial positions.

Identifying the Correct Option

The correct characteristic of a Schottky defect is related to vacancies at lattice sites, which corresponds to option (c), 'some of the lattice sites are vacant.' Options involving electrons or interstitial ions do not describe Schottky defects.

Key concepts

Ionic Crystal Defects

In the world of chemistry and materials science, ionic crystal defects are a crucial concept for understanding how materials behave on an atomic level. These defects occur when the ideal arrangement of ions in a crystal lattice is disturbed. Ionic crystals are made up of cations and anions, which regularly arrange themselves in a repeating pattern to form a solid structure. However, in reality, these perfect arrangements are rarely found, as defects can naturally occur. There are several types of defects, and Schottky defects are one of the most common.

• Schottky defects involve missing ions from lattice sites.
• Such defects are considered stoichiometric, as they maintain the balance between cations and anions.
• By contrast, Frenkel defects involve ions dislocated to interstitial sites rather than being absent altogether.

These imperfections impact the physical properties of the materials, such as their electrical conductivity and structural stability. Understanding these defects helps in tailoring materials for specific industrial and technological applications.

Vacancies

A vacancy in a crystal lattice is essentially a missing atom or ion in that perfect array of atoms. In an ionic crystal, when a cation and its corresponding anion are absent from their normal lattice positions, it creates what is known as a Schottky defect. This type of defect results from the missing cations and anions that would normally occupy these lattice points.

• In Schottky defects, such vacancies occur in pairs to maintain charge balance.
• The presence of vacancies alters the density of the crystal because there are fewer ions present than the theoretical density predicts.
• These missing ions can affect the material's thermal and electrical properties by changing how ions and electrons move through the crystal.

Vacancies play a significant role in determining how materials behave, especially when it comes to conductivity and mechanical strength. They allow the atoms within a crystal to move more easily, affecting the crystal's overall stability and strength.

Crystal Lattice Sites

A crystal lattice site is the specific position within a crystal structure that is usually occupied by an ion or atom in a stable ionic crystal. The lattice is a three-dimensional structure that repeats periodically, with each node representing a potential position for an ion. The arrangement and occupancy of these lattice sites are crucial for determining the crystal's characteristics.

• Lattice sites provide a grid-like framework that supports the orderly structure of the crystal.
• Typically, in ionic crystals, lattice sites are alternately filled with cations and anions to maintain the electrical neutrality.
• Disturbances, such as vacancies, alter this ordered arrangement and can introduce changes in the material's properties.

Understanding crystal lattice sites helps scientists and engineers manipulate materials for specific uses, as changes in these sites directly affect the material properties like conductivity, hardness, and melting point.

Cation and Anion Pairs

In ionic crystals, cation and anion pairs are fundamental to the structure's stability and functionality. Cations are positively charged ions, while anions carry a negative charge. They arrange themselves in alternating patterns, creating an electrostatically attractive environment that holds the crystal together.

• Each cation is generally surrounded by anions, and vice versa, reflecting the balanced distribution of charge.
• In a Schottky defect, both cation and its paired anion are absent from the crystal lattice, which is crucial for maintaining the crystal’s electrical neutrality and stoichiometry.
• The stability of ionic crystals heavily relies on these paired structures, as their absence or disturbance leads to defects.

Grasping the importance of these ion pairs allows a deeper understanding of how ionic crystals maintain their structure and the potential impact when these pairs are disturbed. Such insights are invaluable for the development of materials used in electronics, sensors, and other technological applications.