Difference Between Stoichiometric and Nonstoichiometric Defects

The key difference between stoichiometric and nonstoichiometric defects is that stoichiometric defects do not disturb the stoichiometry of the compound whereas nonstoichiometric defects disturb the stoichiometry of the compound.

There are two main types of defects present in crystal structures; namely, stoichiometric defects and nonstoichiometric defects. In a stoichiometric compound, its chemical formula indicates the ratio between the cations and anions in the compound.

CONTENTS

1. Overview and Key Difference
2. What are Stoichiometric Defects
3. What are Nonstoichiometric Defects
4. Side by Side Comparison – Stoichiometric vs Nonstoichiometric Defects in Tabular Form
5. Summary

What are Stoichiometric Defects?

Stoichiometric defects are those that do not disturb the stoichiometry of a compound. That means the stoichiometric defects do not change the ratio between cations and anions present in the crystal structure. There are several different types of stoichiometric defects;

1. Interstitial Defects

In crystal structures, usually, there are vacant interstitial sites. Small atoms can occupy these sites in a configuration that is energetically favorable (Usually, the presence of interstitial sites increases the total energy of the crystal). Therefore, the presence of ions in the interstitial sites causes interstitial defects.

2. Schottky Defects

Schottky defects form when the cations and anions removed from the crystal structures are in equal numbers. However, the electrical neutrality of the crystal remains unchanged because the numbers of charges removed from the crystal are equal. This type of defects occurs in crystals having cations and anions of similar sizes.

3. Frenkel Defects

A Frenkel defect arises when an ion of the crystal lattice removes and occupies an interstitial site of the crystal structure. However, the electrical charge of the crystal remains unchanged because no ions are removed or added from outside.

What are Nonstoichiometric Defects?

Nonstoichiometric defects are defects in crystal structures that disturb the stoichiometry of the crystal. In other words, nonstoichiometric defects change the stoichiometry of the crystal system. When nonstoichiometric defects are present in a crystal structure, the ratio of the constituent ions of the compound becomes nonstoichiometric. There are two main types of nonstoichiometric defects;

1. Metal Excess Defect

There are two types of metal excess defects. First one is the metal excess defect due to anionic vacancies. In this, the defect arises due to missing anion from a lattice. However, the electrons of the lattice remain unchanged. The second type is the metal excess defects due to the presence of extra cations in the interstitial sites. Here, the defect is observable when positive ions occupy the interstitial sites of the lattice.

2. Metal Deficiency Defect

These defects are also of two types; defects due to cation vacancies and extra anions occupying the interstitial sites of the lattice. When a positive charge is missing from a lattice, the nearby cations balance the extra negative charge. This type of defects is called cation vacancy defects. Meantime, when an extra anion occupies the interstitial sites of the lattice, the nearby cations balance the extra negative charge. This type of defect is the second type of metal deficiency defects.

What is the Difference Between Stoichiometric and Nonstoichiometric Defects?

Summary – Stoichiometric vs Nonstoichiometric Defects

Defects are unusual points in crystal structures. There are two basic forms of defects named as stoichiometric defects and nonstoichiometric defects. The difference between stoichiometric and nonstoichiometric defects is that the stoichiometric defects do not disturb the stoichiometry of the compound whereas nonstoichiometric defects disturb the stoichiometry of the compound.

Reference:

1. “Non-Stoichiometric Defects.” AMAZING SOLIDS, 29 July 2013. Available here   
2. VibeThemes. “Stoichiometric Defects.” EMedicalPrep. Available here  

Image Courtesy:

1.’Schottky v frenkel defect’ By Aminba1376 – Own work, (CC BY-SA 4.0) via Commons Wikimedia