Superprotonic conductivity in MH(PO₃H) (M = Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, NH₄⁺)

Electrical conductivity studies have been performed of solid acid phosphites MH(PO₃H) with M = Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, NH₄⁺. Superprotonic conductivity following a phase transition in the temperature range from 120 to 190 °C is observed for the monoclinic forms, including Na⁺, K⁺, Rb⁺, Cs⁺ and NH₄⁺ derivatives. At temperatures beyond the superprotonic transition temperature, thermal decomposition occurs in the case of the Na⁺, Rb⁺, and NH₄⁺ salts. No superprotonic phase transition is observed for orthorhombic LiH(PO₃H). The magnitude of the superprotonic conductivity of KH(PO₃H) and CsH(PO₃H), which is known to be stable under hydrogen atmospheres from literature, amounts to 4.2 · 10^{- 3} Ω^{- 1} cm^{- 1} (at 140 °C) and 3 · 10^{- 3} Ω^{- 1} cm^{- 1} (at 160 °C), respectively, which values are comparable to that of well known CsHSO₄.