Question

expression of schottky defects in a TiO2 crystal

Answer 1

A Schottky defect is a type of point defect in a crystal lattice named after Walter H. Schottky. In non-ionic crystals it means a lattice vacancy defect.

In ionic crystals, the defect forms when oppositely charged ions leave their lattice sites, creating vacancies. These vacancies are formed in stoichiometric units, to maintain an overall neutral charge in the ionic solid. The vacancies are then free to move about as their own entities. Normally these defects will lead to a decrease in the density of the crystal.

Lattice structures (or crystals) undergoing point defects experience one of two types:

1. atoms or ions leaving their spot (thus creating vacancies).
2. atoms or ions slipping into the little gaps in between other atoms or ions; those little gaps are known as interstitials--since atoms or ions in the crystals are occupying interstitials, they inherently become (create) interstitials.

The Schottky Defect is often visually demonstrated using the following layout of anions and cations:

a Schottky defect will always result a crystal structure in equilibrium--where no crystal is going to be too positive or too negative; thus in the case of MX2: one Schottky defect equals one cation and two anion vacancy.

For the TiO2, the chemical equations in Kröger-Vink Notation for the formation of Schottky defects:
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