Question

What are F-centres?

What are F-centres and how do they impart colour to a crystal?

What are F-centres in an ionic crystal?

Answer 1

1. The electrons trapped in anion vacancies are called F-centres (F stands for Farbe, a German word meaning colour) or colour centres because they impart colour to the crystals.
2. In fact, the trapped electron is delocalised and it may be able to capture light to quickly move from the ground state to the excited state. This imparts colour to the crystal.
3. The colourless crystals acquire a certain degree of colour as a result of the presence of F centres. For example, the violet colour of KCl is imparted by F-centres, while LiCl crystals are coloured pink.