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Why do defects in crystal lattices of various molecules and compound occur ?
Is it due to the continuous contact or it arises due to impurities and human error during the formation?

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  • $\begingroup$ There are many reasons for the many different kinds of defects. Vacancies are part of the thermodynamic equilibrium of 'perfect' crystals because of configurational entropy. The Simmons-Baluffi experiments are the classic example. $\endgroup$ – Jon Custer Feb 17 '15 at 21:39
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First of all, there are no such thing as a "perfect" crystal.

Defects occur from both human error and the experimental setup.

Defects from human error are primarily due to handling issues, which can never be perfected, especially not if the crystals are solvated.

Experimental defects can occur from either the diffraction experiment or the crystal growth experiment. Impurities can affect crystal growth by displacing the molecule of interest in the lattice and unless you are attempting neutron diffraction, the crystal will be significantly damaged during the data collection.

In my field of protein x-ray crystallography, the typical crystals can have diffraction ranging from nothing at all to ~0.8 Angstrom range (best cases) and as such you can have beautiful crystals that have no diffraction and ugly crystals that diffract very well.

Crystal defects can be perfectly acceptable and are a so common that we usually don't think about them too much.

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A perfect crystal can exist only at 0K. Above this temperature, there is always some energy associated with the crystal which causes some particles to leave the crystal as in Schottky or Frenkel defects, or it allows some impurities to enter.

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  • $\begingroup$ Even at 0K (which is impossible to reach) perfect crystals cannot exist, unless you can crystalize a single isotope of each element present... the isotopes will otherwise be randomly distributed... And you better not be thinking of crystalizing anything with radioactive decay products! $\endgroup$ – Jeppe Nielsen Jun 15 '17 at 15:09

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