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We are taught in school that ionic substances don't conduct electricity, and when they do, it is either because they are in a molten state or because they are in solution. I understand these concepts.

Are there any exceptions? By exceptions, I am looking for ionic compounds that may not conduct electricity well, but they conduct electricity better in comparison to other ionic compounds. Additionally, what makes such a compound a better conductor of electricity?

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  • $\begingroup$ Mentioning that you are looking for ionic substances that conduct electricty in 'their solid state' would make the question more clear. The standalone statement that ionic substances don't conduct electricity is false as (like you mentioned) their molten state (which still is the same substance) conducts electricity. $\endgroup$ – Aumkaar Pranav Mar 23 '20 at 11:04
  • $\begingroup$ Does not fit perfectly into your question but you can have molecules and macromolecules whose ions - often present in pending groups - have mobility, or at least leads to electrons / hole with mobility. Example of the first case is PEDOT, examples of the second kind are the Os of OLEDs. $\endgroup$ – Alchimista Mar 23 '20 at 11:51
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  • $\begingroup$ Yes there are - take a look at the second paragraph in the answer to chemistry.stackexchange.com/questions/67481/… $\endgroup$ – Ian Bush Mar 23 '20 at 21:04
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Yes, there are.

Ionic conductivity greatly varies between compounds. For some of them is much higher then for others. They are called fast ion conductors, or superionic conductors.

Well known example is silver iodide. While it may be called a "salt", it's probably more covalent then ionic, that is, before it's heated to 146 °C, when its structure changes and Ag ions are "set loose" to conduct electricity. In result conductance rises a few thousand times.

If rubidium iodide is added to $\ce{AgI}$ we can get rubidium silver iodide which is even better, and is sometimes classified as one of "advanced" superionic conductors.

Many substances can be superionic in right circumstances, even water ice can, if temperature and pressure are high enough.

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Yes.

Another example is VO2. Over 67 °C its crystal structure changes to rutile with a band gap of ~0.1 eV. Dopants of other substances can reduce the temperature at which it becomes a conductor.

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