An example of a non-stoichiometric oxide when heated is ZnO. Zinc oxide is white in colour at room temperature. On heating, it loses oxygen and turns yellow.
ZnO
heating
⟶
Zn2++
1
2
O2+2e−
Now, there is excess of zinc in the crystal and its formula become Zn1+xO. The excess Zn2+ ions move to interstitial sites and the electron to the neighbouring interstitial sites.