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Our materials professor likes to do Kahoot quizzes and I am confused about why the following statement is true:

A defect in a crystal gives it higher energy.

I know that an imperfect crystal has a higher internal energy than a perfect one, in which case the statement would make sense. However, the lecture only introduced the concept of Gibbs energy so I am assuming the statement is referring to that one.

$$G = H - TS$$

Since a crystal with a defect has a higher entropy than a perfect one I am wondering what the net effect on $G$ is.

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I think you have a good understanding of the matter because what you say is correct. You might find the paper by Mosquera-Lois et al. [1] interesting where they go deeper in this balancing between enthalpy and entropy.

The enthalpy/internal energy increases with the introduction of defects. But indeed, you have an entropy effect as well. A balance arises between the energy and entropy effects.

Reference

  1. Mosquera-Lois, I.; Kavanagh, S. R.; Klarbring, J.; Tolborg, K.; Walsh, A. Imperfections Are Not 0 K: Free Energy of Point Defects in Crystals. Chem. Soc. Rev. 2023, 52 (17), 5812–5826. DOI: 10.1039/D3CS00432E.
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