Synthesis, Structure, and Properties of Compounds in the NaHSO₄−CsHSO₄ System. 2. The Absence of Superprotonic Transitions in Cs₂Na(HSO₄)₃ and CsNa₂(HSO₄)₃

Abstract

Exploratory synthesis in the NaHSO4−CsHSO4 system, aimed at discovering novel proton-conducting solids, yielded the new compounds CsNa2(HSO4)3 and Cs2Na(HSO4)3. Thermal analysis demonstrated the absence of phase transitions for both compounds prior to melting. The conductivities of the two compounds vary monotonically with temperature, in a non-Arrhenius manner, from ambient to the respective melt temperatures of 398 and 413 K. At 363 K the conductivities are on the order of 10-8 Ω1-cm-1, which is comparable to that of other alkali acid sulfates. The absence of transitions to disordered, high conductivity phases, as is observed in compounds such as CsHSO4, is explained in terms of the stiffness of the Na−O bonds, which may serve to prevent rapid reorientation of SO4 tetrahedral groups.

Uncovering fast solid-acid proton conductors based on dynamics of polyanion groups and proton bonding strength

“…This corroborates earlier hypotheses that large cations generally provide an increased lattice flexibility. 19,75 However, as can be seen in Fig. 6a, the cation size alone is not a sufficient condition for quantitatively relating to flexibility; i.e.…”

“…This is in agreement with our earlier experimental study that observed no superprotonic transition in Cs 2 Na(HSO 4 ) 3 and CsNa 2 (HSO 4 ) 3 and suggested that strong Na–O bonds hinder structural flexibility. 75…”

“…6a Energy & Environmental Science Paper earlier hypotheses that large cations generally provide an increased lattice flexibility. 19,75 However, as can be seen in Fig. 6a, the cation size alone is not a sufficient condition for quantitatively relating to flexibility; i.e., there is a range of o cat values for a group of compounds with the same cation.…”

“…This is in agreement with our earlier experimental study that observed no superprotonic transition in Cs 2 Na(HSO 4 ) 3 and CsNa 2 (HSO 4 ) 3 and suggested that strong Na-O bonds hinder structural flexibility. 75 A second reason for finding many of the screened compounds (from Fig. 6) with low proton diffusion at 650 K can be the use of the NVT ensemble in our MD simulations.…”