# Why does ZnO turn yellow when it is hot?

Why does ZnO's color become yellow when it is hot? I have read that it is because of physical properties, but I need a more elaborate explanation.

• I read on wikipedia that it is a thermochrome compound (see: en.wikipedia.org/wiki/Thermochromism ) – user2117 Jan 9 '14 at 17:05
• If anyone is interested, this video shows the effect very well, and also touches on the explanation. – Nicolau Saker Neto Apr 2 '15 at 15:53

The thermochromism of $\ce{ZnO}$ results from a minor loss of oxygen upon heating to temperatures around 800 °C, i.e. a non-stoichiometric $\ce{Zn$_{1+x}$O}$ with $x = 7 \times 10^{-5}$ is formed.

Under air, this effect is reversible. Heating (and cooling) of the material while hooked up to a vacuum pump might result in a more persistent colour change.

• So heating causes $\ce{2O^{2-}}$ to be oxidized to $\ce{O2}$? That seems unlikely to me. Does something else happen to the $\ce{O^{2-}}$? – SendersReagent Mar 11 '16 at 8:09
• This may explain the chemical change but it doesn't explain the reason behind the colour change. What causes the colour? – matt_black May 23 '16 at 18:43

When you heat $\ce{ZnO}$ then Oxygen leaves as $\ce{O2}$ leaving behind $\ce{Zn^{2+}}$ and 2 electrons. $\ce{Zn^{2+}}$ and the 2 electrons move to the interstitial sites of the crystal . This provides excess electrons in the crystal lattice of $\ce{ZnO}$. When light falls on these crystals then these electrons absorb a part of the light in the visible region and hence impart a yellow colour to the $\ce{ZnO}$.

Most probably because the band gap closes, thus allowing electrons from the valence band to be elevated into the conduction band (by absorbing blue light, which makes the reflected color yellow).

Upon heating zno anionic vacancies are created and when an electron occupies that vacancy it imparts yellow colour(metal excess defect/f-centre).

• It would be nice to flesh this answer out a bit, particularly on the color centers and their population vs temperature in thermodynamic equilibrium. – Jon Custer Mar 10 '16 at 14:37

In $\ce{ZnO}$, $\ce{Zn}$ is present in the 2+ oxidation state, and the d subshell has 10 electron, and the s subshell has 0. When $\ce{ZnO}$ is heated there is a transition of electron from d to s subshell, causing it to become yellow, and on cooling it becomes white.

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