Specific heat anomalies and phase transitions in the solid electrolyte Ag26I18W4O16

Changes in the heat capacity of the solid electrolyte Ag26I18W4O16 are measured in the temperature range 150–330 K by differential scanning calorimetry. Our measurements, done on a powder sample, showed that Ag26I18W4O16 has three phase transitions. At T1?199 and T2?248 K, this material exhibits a first order phase transition with heat of transformation H1=53±2 mcal/g and H2=56±4 mcal/g. The phase transition at T3?277 K is second order with characteristic lambda shape. The transition at T1 shows a broad onset and hysteresis (ΔT?4 K). Similarly, the transition at T2 is hysteratic but with ΔT?1 K. No hysteresis is associated with the continuous order–disorder transition at T3. It is concluded that Ag26I18W4O16 becomes a superionic conductor at 277 K, in agreement with F. Habbal, J.A. Zvirgzds, and J.F. Scott [J. Chem. Phys. 69, 4984 (1978)] and not at 247 K, as erroneously concluded by S. Geller, S. A. Wilber, G. F. Ruse, J. R. Akridge, and A. Turkovic, Phys. Rev. B 21, 2506 (1980).