Bulk and surface properties of ionic salt CsH5(PO4)2

We report a comparative study of transport and thermodynamic properties of single-crystal and polycrystalline samples of the ionic salt CsH₅(PO₄)₂ possessing a peculiar three-dimensional hydrogen-bond network. The observed potential of electrolyte decomposition ≈ 1.3 V indicates that the main charge carriers in this salt are protons. However, in spite of the high proton concentration, the conductivity appears to be rather low with a high apparent activation energy E_a ≈ 2 eV, implying that protons are strongly bound. The transport anisotropy though is not large, correlates with the crystal structure: the highest conductivity is found in the 001] direction (σ_130 °C ～ 5.6 × 10^? 6 S cm^? 1) while the minimal conductivity is in the 100] direction (σ_130 °C ～ 10 ^?6 S cm^? 1). The conductivity of polycrystalline samples appears to exceed the bulk one by 1–3 orders of magnitude with a concomitant decrease of the activation energy (E_a ≈ 1.05 eV), which indicates that a pseudo-liquid layer with a high proton mobility is formed at the surface of grains. Infrared and Raman spectroscopy used to study the dynamics of the hydrogen-bond system in single-crystal and polycrystalline samples have confirmed the formation of such a modified surface layer in the latter. However, no bulk phase transition into the superionic disordered phase is observed in CsH₅(PO₄)₂ up to the melting point T_melt ～ 151.6 °C, in contrast to its closest relative compound CsH₂PO₄.