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When talking about conductivity in combination with lattice defects you can often see that AgX is taken as an example for a Frenkel defect where the $\ce{Ag^+}$ moves to intersticial voids. I remember that my professor once called this a property of silver, that silver likes itself so much that it can be close to other silver cations. And I think we see trends like this in for example $\ce{Hg2}$-dimers. Is this some sort of relativistic effect or can I explain it with certain orbitals overlapping that I can get a quite close contact for two $\ce{Ag^+}$ ions?

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(partial answer)

Interesting question. Silver also forms an unusual "subfluoride", Ag2F. My understanding is the Ag is NOT in dimers. But the structure has sorts of "sheets" of silver in that all of the octagonal holes are filled but only in alternating layers. This allows silver atoms to be very close to each other, similar to the metal. Presumably there is a lot of silver-silver bonding going on. But more like sheets of metal than dimers.

The explanation of Frenkel defects you usually hear is that the cation is small enough versus the structure to allow dislodging by radiation (i.e. not a thermodynamics distribution of defects but a provoked one). See the Wikipedia page for instance. However defect models are idealizations.

It would further be interesting to know how often the Frenkel defect is associated with some higher conductivity. A purely billiard ball dislodged model would be less prone to conductivity than one where silver-silver bonding is the driver.

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