Room-temperature proton transport and its effect on thermopower in a solid ionic semiconductor, TTFCOONH₄

Ammonium proton in a solid ionic semiconductor, TTFCOONH₄, is shown to be mobile under anhydrous conditions at room temperature by the hydrogen concentration cell method. Isotope substituted TTFCOOND₄ exhibits a 2.2 H/D isotope effect in ion carrier mobility with TTFCOONH ₄. First-principles calculations reveal that an efficient proton-transfer pathway via low-barrier N⋯H⁺⋯N hydrogen bonds reduces the activation energy to 0.12 eV, which is quite small and comparable to that reported in a bulk water system. The ac conductivity of TTFCOONH₄ and TTFCOOND₄ is similar at room temperature, reflecting similar hole carrier concentrations. In sharp contrast, the thermopower exhibits a large isotope effect: TTFCOONH₄ shows 260 μV K^-1, which is twice as large as that predicted by the hole carrier concentration and the value of TTFCOOND₄, with 138 μV K^-1. The 1.9 H/D isotope effect in thermopower closely relates to the 2.2 H/D isotope effect in ion carrier mobility. Proton carriers in the temperature gradient enhance thermopower without cancelling out the effect of holes in the solid state owing to possession of the same positive charge.