Superprotonic phase transition in CsH(PO3H)

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 degreesC, to a phase of high proton conductivity. The transition is accompanied by a large heat of transformation, DeltaH = 58+/-2 J/g (12.4+/-0.4 kJ/mol), and exhibits measurable hysteresis, occurring at 96 T upon cooling. High-temperature X-ray powder diffraction showed that the high-temperature phase is cubic, with a(o) = 4.896(1) Angstrom, 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 degreesC is 5.5 x 10(-3) Omega(-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.