High-temperature phase transitions in K3H(SO4)2

Two phase transitions in K3H(SO4)(2) were discovered in the temperature range 25-300 degreesC. The transition temperatures for the mid and high-temperature phases are T-c = 190 degreesC (DeltaH = 17.0 kJ/mol) and T-c = 227 degreesC (DeltaH = 7.4 kJ/mol), respectively, for freshly heated samples. A slow decomposition process begins above 270 degreesC. The conductivity of K3H(SO4)(2) in these phases (sigma = 1.68 X 10(-3) at 198 degreesC and 2.19 X 10(-2) Omega (-1) cm(-1) at 251 degreesC) is comparable to that of other M3H(XO4)(2) compounds (M = Cs, NH4, Rb and X = S. Se) in their super-protonic 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), 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), 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. (C) 2001 Elsevier Science B.V. All rights reserved.