The appearance of colour in solid alkali metal halides is generally due to F-centres.

Explanation:

Alkali metal halides are compounds formed by combining alkali metals (such as sodium, potassium, etc.) with halogens (such as chlorine, bromine, etc.). These compounds have a crystalline structure and exhibit different colors, ranging from white to yellow, orange, and even deep red.

The appearance of color in solid alkali metal halides is primarily due to the presence of F-centres. F-centres are color centers formed when an electron from the valence band of the alkali metal is excited and occupies an interstitial position within the crystal lattice.

Why F-centres cause color:

When an electron is excited and occupies an interstitial position, it creates an unpaired electron in the crystal lattice. This unpaired electron absorbs energy from the visible light spectrum and imparts color to the compound. The absorbed energy excites the electron to a higher energy level, and the color observed is complementary to the absorbed wavelength.

For example, in sodium chloride (NaCl), F-centres are formed when a sodium ion loses an electron and occupies an interstitial position. The presence of F-centres gives rise to the yellow color of sodium chloride. Similarly, other alkali metal halides exhibit different colors based on the energy absorbed by the F-centres.

Other factors:

While F-centres are the primary reason for the color in alkali metal halides, other factors such as lattice defects, impurities, and crystal structure also play a role in determining the observed color. Schottky defect and Frenkel defect are lattice defects that can affect the color of a compound, but they are not the main reason for the appearance of color in alkali metal halides.

Overall, the presence of F-centres in solid alkali metal halides is responsible for the observed colors, and the specific color depends on the energy absorbed by the F-centres.