Structure and proton conductivity of mechanochemically treated 50CsHSO4·50CsH2PO4

Mixtures of CsHSO4 and CsH2PO4 were mechanochemically treated using a planetary type of ball mill. The changes in structure and proton conductivity of the solid acid compounds with the treatment have been investigated. Cs3(HSO4)2(H2PO4) and Cs5(HSO4)3(H2PO4)2 were formed during milling. The mechanochemically treated composite consisting of Cs3(HSO4)2(H2PO4) and Cs5(HSO4)3(H2PO4)2 showed higher conductivity than the untreated mixture. In addition, a high temperature phase of Cs2(HSO4)(H2PO4) was generated from the composite at around 100 °C on heating. Conductivity of the mechanochemically treated composite significantly increased at temperatures around 90 °C on heating. The value becomes 2 × 10− 3 S cm− 1 at around 180 °C. On the other hand, no steep decrease is observed on cooling. The activation energies of the mechanically milled sample with high conductivities were estimated to be about 0.3 eV for both heating and cooling processes. The relatively high proton conductivity and a low activation energy for the proton conduction should be ascribed to the presence of the high temperature phase of Cs2(HSO4)(H2PO4).