Why does white ZnO(s) becomes yellow upon heating ?

Explanation

Zinc oxide (ZnO) crystals upon heating decompose as follow :
`ZnO(S)hArr Zn^(2+)(S)+1//2O_(2)(g)+2e^(-)`
The excess of `Zn^(2+)` ions formed are trapped in the interstitial spaces and the electrons are alos trapped in the neighbourhood. These electrons absosrb radiations corresponding to certain specific colour from the light and emit yellow light.