Theory of superionic conduction in hydrogen-bonded systems

Among superionic conduction phenomena in various ionic materials, the conduction phenomenon associated with the motion of protons in hydrogen-bonded systems has aroused considerable interests with regard to a problem of whether the proton motion should be treated quantum mechanically or classically. In this paper we first describe a quantum mechanical mechanism of proton-induced ionic conduction in the superionic phase in zero-dimensional hydrogen-bonded M3H(XO4)2[M = K, Rb, Cs, X = S,Se] materials, by giving a brief review on the theory developed by Ito and Kamimura. Then we discuss the characteristic difference between quantum mechanical and clasiical mechanisms in the case of proton-induced superionic condcuction, in paticular, by comparing characteristic time scales in quantum mechanical and classical diffusions in hydrogen-bonded systems. Finally a prediction is made on an anomalous behavior of terahertz spectra for Rb3H(SeO4)2.