The appearance of colour in solid alkali metal halides is generally due to:

The appearance of colour in solid alkali metal halides is due to specific defects in their crystal structure. Alkali metal halides, such as NaCl or KCl, are ionic compounds that are typically colourless when pure. However, under certain conditions, they develop colour, and we need to identify the cause from the given options.

Let us examine each option:

Option A: Schottky defect. This defect involves missing pairs of cations and anions from the lattice, maintaining electrical neutrality. It creates vacancies but does not involve any unpaired electrons or species that absorb visible light. Therefore, Schottky defects do not cause colour.

Option B: Frenkel defect. This occurs when an ion (usually a cation due to its smaller size) leaves its lattice site and occupies an interstitial position. It creates a vacancy and an interstitial ion, but no free electrons are involved. Since Frenkel defects do not introduce electrons that can absorb light, they do not produce colour.

Option C: Interstitial position. This refers to an atom or ion occupying a space between regular lattice sites. While interstitial positions can be part of defects like Frenkel defects, merely having an interstitial atom does not necessarily lead to colour unless it traps an electron. However, interstitial positions alone are not the primary cause of colour in alkali metal halides.

Option D: F-centres. The term "F-centre" comes from the German word "Farbzentrum," meaning "colour centre." An F-centre is formed when an anion vacancy is occupied by an unpaired electron. This electron is trapped in the vacancy and can absorb visible light, promoting it to an excited state. The absorption of specific wavelengths of light results in the complementary colour being observed. For example, NaCl appears yellow when heated in sodium vapour due to F-centres, as sodium atoms deposit electrons into chloride ion vacancies.

In alkali metal halides, F-centres are the primary reason for colour development. The process involves creating anion vacancies (e.g., by heating the crystal in alkali metal vapour), and these vacancies capture electrons, forming F-centres that absorb light.

Hence, the correct answer is Option D.