Vibrational study of the superprotonic phase transition in the mixed crystal Rb2(HSeO4)(H2PO4)

Raman spectra (10–1200 cm−1) of polycrystalline samples of Rb2(HSeO4)(H2PO4) were studied at temperatures ranging from 300 to 423 K. An assignment of most of the observed bands is proposed. The first‐order phase transition previously detected at 382 K was characterized as: $$ \hbox{I} {\buildrel 382 \hbox{K} \over \longrightarrow} \hbox{II} $$ This superionic‐protonic transition is believed to be governed by librations of the HSe/PO42− ion and the AOH (A = Se, P) stretching mode. It corresponds to the weakening of Se(P)OH˙˙˙ HOSe(P)hydrogen bonds and to the melting of the proton sublattice into a quasi‐liquid state in which the protons and the HSe/PO42− ions contribute to the unusually high conductivity. The activation energy that was determined from the plot Δν1/2 versus temperature for the ν (AOH) band has the same order of magnitude as that determined from conductivity measurements. Copyright © 2006 John Wiley & Sons, Ltd.