Superprotonic Phase Transition in CsH(PO_3H)

High-temperature investigations of the compound CsH(PO₃H) (or CsH₂PO₃, cesium hydrogen phosphite) revealed that this material undergoes a transition, with an onset of 137 °C, to a phase of high proton conductivity. The transition is accompanied by a large heat of transformation, ΔH = 58 ± 2 J/g (12.4 ± 0.4 kJ/mol), and exhibits measurable hysteresis, occurring at 96 °C upon cooling. High-temperature X-ray powder diffraction showed that the high-temperature phase is cubic, with a₀ = 4.896(1)Å, and likely takes on a CsCl structure, with Cs atoms at the corners of a simple cubic unit cell, and PO₃H groups at the center. The conductivity in the high-temperature phase at 160 °C is 5.5 × 10⁻³ Ω⁻¹ cm⁻¹, and the activation energy for proton transport is 0.40 ± 0.01 eV. These values suggest that proton transport is facilitated by rapid PO₃H group reorientations in the cubic phase of CsH(PO₃H), as is known to occur in the high-temperature, tetragonal phase of CsHSO₄.