engleski

Raman study of the Ag8W40l6 in the superionic system AgI + Ag8W40l6

Polarized Raman spectra have been obtained on oriented single crystals of Ag8W40l6 from 293 K to the melting point (893 K) and also of the melt, both in air in an oxygen atmosphere. The temperature dependences of the low-frequency Ag+-ion vibrations show no anomalies, in contrast to those in solid electrolytes such as a-AgI. Polarization versus temperature measure- ments have been made on the crystals under the same conditions as the Raman measurements. The results indicate that there is decomposition of the crystals in air which produces conducting phase irreversibly. The oxygen atmosphere impedes this decomposition, and the results in this case confirm an earlier prediction that there is no crystallographic transformation of the crystals up to the melting point and that the (W40l6)8- ions continue to be noncentrosymmetric in the melt. Results of measurements of electrical conductivity of single crystals of Ag26I18W40l6 in the temperature range 473-175 K show existence of two first order transitions at 246 and 197 K ; the phases are labeled α , β , γ in order of decreasing temperature. At 298 K the average conductivity, 0.0825 (Ω cm)-l, is 1.4 times the best value obtained from conductivity measurements on polycrystalline material. From the log (σ T) vs. T-l data for the a phase the activation energies of motion are of magnitude 10-1 eV as for other solid electrolytes. The magnitudes of the activation energies of motion of the β and γ phases appear to be constant, and suggest that there is greater order in the lower phases (β and γ ) than in the α phase.

Raman; superionic conductor; impedance spectroscopy

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