Synonymous terms in solid-state chemistry: “solid electrolyte,” “fast ion conductor” and “superionic conductor”

Chapter 13 with the title “Ionic Conductivity and Solid Electrolytes” in the Solid State Chemistry textbook by Anthony West [1] is a good introduction to the named subject. I found the three terms solid electrolyte, fast ion conductor and superionic conductor mentioned there. Such materials are special salts, composed of ions, with one set of ions able to move around easily.  West describes the materials as having a special crystal structure with open tunnels or layers through which mobile ions may move.

Typical electrical conductivity values of solid electrolytes are between 10^-3 and 10¹ S cm^-1, about the range found for common molten salts.  The conductivity of solid electrolytes is higher than that of  “normal” ionic crystals with values from below 10^-18 up to 10^-4 S cm^-1[1].  In contrast to metals (typical conductivity range: 10¹ to 10⁵ S cm^-1), the conductivity of solid electrolytes and ionic crystals increases with increasing temperature. Solid electrolytes are a special class of ionic crystals and—to highlight this relation— the term superionic crystal is used in addition to the above given phrases [2].

Notice that these terms and distinctions refer to phases at low and ambient temperatures—often not well defined. Some materials become appreciable solid electrolytes at elevated temperatures, such as the high temperature oxide ion conductor zirconia (ZrO₂, above ~600 °C). However, all salts—unless they evaporate—transform into electrically conductive ionic liquids above their melting point.

Keywords: physics, materials science, solid-state ionics, ionic conductivity, electrical conductivity, temperature dependence

References
[1] Anthony R. West: Solid State Chemistry and its Applications. John Wiley & Sons, Chichester, 1984; page 453 and others.
[2] Angus Gray-Weale's Research - Superionic conductors: www.gusgw.info/superionics/.