Superprotonic Phase Transition in CsH(PO3H)

High-temperature investigations of the compound CsH(PO 3 H) (or CsH 2 PO 3, 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, Δ H = 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 a o = 4.896(1) Å, and likely takes on a CsCl structure, with Cs atoms at the corners of a simple cubic unit cell, and PO 3 H 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 PO 3 H group reorientations in the cubic phase of CsH(PO 3 H), as is known to occur in the high-temperature, tetragonal phase of CsHSO 4 .