Superprotonic conductivity in β-Cs_3(HSO_4)_2(H_x(P,S)O_4)

Investigations of the system CsHSO_4-CsH_2PO_4 led to discovery of the new compound, β-Cs_3(HSO_4)_2(H_x(P,S)O_4), with a superprotonic phase transition at 125 degrees C. The room temperature structure of β-Cs_3(HSO_4)_2(H_x(P,S)O_4) is related to that of α-Cs_3(HSO_4)_2(H2PO_4) and that of CsHSO_4-II. It is unique, however, in that P and S occupy the same crystallographic position in the structure, and there is accordingly a variable H content and a partially occupied hydrogen bond. This leads to room temperature conductivity that is significantly higher than either of the two related compounds. The conductivity in the high temperature phase is comparable to that of CsHSO_4-I. Furthermore, the transition exhibits a relatively large heat of 30 J g^(-1) and some hysteresis. These an explained by probable disordering between S and P sites upon transition to the superprotonic state.