HEAT CAPACITY MEASUREMENTS OF RbAg₄I₅ AND Rb₂AgI₃

Vatlin D.A., Chernyuk S.D., Reznitskikh O.G., Bushkova O.V.

Institute of Solid State Chemistry UB RAS

620990, Ekaterinburg, Pervomaiskaya st., 91

Solid-state superionic conductors with general formula MAg₄I₅ (Me = K, Rb, NH₄) are known as solid electrolytes with very high (~10^-1 Ohm^-1×cm^-1) silver-ion conductivity at room temperature and below. Discovered in 1960s, RbAg₄I₅ is still the most interesting among them as it has the highest silver ion conductivity of 0.33 Ohm^-1·cm^-1 at 25 °C. Moreover, it was experimentally confirmed unambiguously that high values of conductivity remain in the wide temperature range due to the low activation energy of conduction (0.10 eV), being 0.11 Ohm^-1×cm^-1 at -50 °C [1]. However, accumulated over decades experimental information contradicts the available literature data. It was stated [2] that RbAg₄I₅ is thermodynamically unstable below 27 °C and tends to decompose to low-conductive phases according to the reaction

RbAg₄I₅ ⇄ Rb₂AgI₃ + AgI

In order to solve this contradiction and theoretically confirm experimentally observed rather high stability of RbAg₄I₅ one need to obtain full thermodynamical characterization of the system RbI – AgI. In addition to RbAg₄I₅, it contains another compound Rb₂AgI₃. The goal of this work was to determine temperature dependencies of heat capacity from -196 to 230 °C (RbAg₄I₅) and 290 °C (Rb₂AgI₃), using differential scanning calorimetry (DSC) experimental techniques. The temperature dependences of the molar heat capacity at constant pressure C_p(T) for the compounds RbAg₄I₅ and Rb₂AgI₃ were measured by the comparison method [3] using high-purity silver as a heat capacity standard. The obtained results relate to reference thermodynamic data and can be used in thermodynamic calculations.

1. Reznitskikh O.G., Yaroslavtseva T.V., Glukhov A.A., Popov N.A., Urusova N.V., Bukun N.G., Dobrovolsky Yu.A., Bushkova O.V. Synthesis and Comparative Investigation of the Electrochemical Characteristics of CsAg₄Br_2.5I_2.5 and RbAg₄I₅ Solid Electrolytes // Russ. J. Electrochem. 2022. Vol. 58, Nr 10. P. 927–937. https://doi.org/10.1134/S102319352210010X

2. Topol L.E., Owens B.B. Thermodynamic Studies in the High-Conducting Solid Systems RbI-AgI, KI-AgI, and NH₄I-AgI // J. Phys. Chem. 1968. Vol. 72, Nr 6. P. 2106-2111. https://doi.org/10.1021/j100852a038

3 GOST R 56754-2015 (ISO 11357-4:2005) "Plastics. Differential scanning calorimetry (DSC). Part 4. Determination of specific heat capacity".

This work was carried out according to the state assignments for ISSC UB RAS (theme No. 124020600047-4).