High-temperature phase transitions in K3H(SO4)2

Two phase transitions in K3H(SO4)2 were discovered in the temperature range 25–300°C. The transition temperatures for the mid and high-temperature phases are Tc=190°C (ΔH=17.0 kJ/mol) and Tc=227°C (ΔH=7.4 kJ/mol), respectively, for freshly heated samples. A slow decomposition process begins above 270°C. The conductivity of K3H(SO4)2 in these phases (σ=1.68×10−3 at 198°C and 2.19×10−2 Ω−1 cm−1 at 251°C) is comparable to that of other M3H(XO4)2 compounds (M=Cs, NH4, Rb and X=S, Se) in their superprotonic phases. The activation energy for proton transport in the highest temperature phase of K3H(SO4)2 is 0.45 eV, a value slightly higher than in the related compounds. Despite the similarity between the electrical properties of K3H(SO4)2 and other M3H(XO4)2 compounds, the structural properties are quite distinct. Specifically, high-temperature X-ray powder diffraction measurements show that neither of the high-temperature phases of K3H(SO4)2 is trigonal, indeed, the symmetry of the structure decreases at the first transition, in contrast to the superprotonic phases in other M3H(XO4)2 compounds.