Around 1930, the concept of point defects was established by Yakov Frenkel, Walter Schottky and Carl Wagner, including the development of point-defect thermodynamics by Schottky and Wagner; this helped explain ionic and electronic transport in ionic crystals, ion-conducting glasses, polymer electrolytes and nanocomposites.
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Ionic crystals may contain a mixture of covalent and ionic species, as for example salts of complex acids, such as sodium cyanide, NaCN . X-ray diffraction shows that in NaCN, for example, the bonds between sodium cations ( Na + ) and the cyanide anions ( CN " ) are " ionic ", with no sodium ion associated with any particular cyanide.
