Synthesis, Structure, and Properties of Compounds in the NaHSO_4−CsHSO_4 System. 2. The Absence of Superprotonic Transitions in Cs_2Na(HSO_4)_3 and CsNa_2(HSO_4_)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.