Among the following solids,Schottky defect is $NOT$ observed in-

Zinc oxide (white) is heated to high temperature for some time. Observe the following statements regarding the above process:
$I$. Zinc oxide colour changes to pale yellow
$II$. The type of defect formed is 'metal deficiency'
$III$. Some $Zn^{2+}$ and $e^{-}$ are present in interstitial places
The correct statements are:

Schottky defect generally appears in

$NaCl$ is heated in an atmosphere of sodium vapour. The resultant yellow colour is due to the formation of

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

Electron-deficient impurities are caused by the elements of which group?