The conductivity of the binary system Cs2S2O7−CsHSO4 has been measured at 20 different molten compositions
in the full composition range and in the temperature range 430−750 K. From the obtained liquidus−solidus
phase transition temperatures, the phase diagram has been constructed. It is of the simple eutectic type with
the composition X(CsHSO4) = 0.86 and temperature of fusion of 470 K for the eutectic. The previously
unpublished temperature of fusion of CsHSO4 was found to be 491.7 K. The experimental phase diagram is
in good accordance with a calculated diagram based on measured thermodynamic properties of the pure
compounds and the heat of liquid−liquid mixing measured for X(CsHSO4) = 0.5 and 1.0. 133Cs and 17O
NMR spectra and Raman spectra of the liquid Cs2S2O7−CsHSO4 system indicate the presence of a temperature
sensitive equilibrium 2HSO4
- ⇌ S2O7
2- + H2O where the water molecules are strongly associated in the
melt. Fast exchange reactions take place between the species present, leading to the observed 17O NMR
single line with an averaged chemical shift. Super-ionic and thermodynamic stable phases and the temperatures
of phase transitions have been identified from the NMR measurements on CsHSO4, Cs2S2O7, and Cs2S2O7−CsHSO4 mixtures. For 11 selected compositions covering the entire composition range of the Cs2S2O7−CsHSO4 binary system, the conductivity of the molten state has been expressed by equations of the form
κ(X) = A(X) + B(X)(T − T
m) + C(X)(T − T
m)2 where T
m is the intermediate temperature of the measured
temperature range and X is the initial mol fraction of CsHSO4.
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