Protonic conductivity measurements are reported for H3PW12O40x21H2O single crystals in the temperature range 77 to 303 K. At room temperature, the conductivity is 0.18 Sm-1 and falls to a minimum of 0.26x10-3 Sm-1 at 188 K. An anomalous behavior in conductivity observed in the temperature range 263 to 283 K is reported and it is essentially due to the disordered structure of water molecules. The activation energy determined from the least squares analysis in the temperature range 278 to 303 K and 188 to 273 K are 0.38 and 0.15 eV respectively. The observed conductivity parameter results support the vehicle mechanism as the proton conduction mechanism in this single crystal. Using the Nernst-Einstein relation, the proton diffusion coefficient is calculated and found to be 1.29x10-11 m2s-1 at room temperature. Steady state photoconductivity is measured at room temperature for various intensities and the material is found to be photosensitive. The variation of photocurrent with different illumination levels is found to be linear in these single crystals. The transient photoconductivity measurement shows that the photo-induced responses are moderate in the beginning and slow during decay process with respect to time.
Keywords: protonic conductivity, photoconductivity, FT-IR, thermal analysis, polyoxometalate, dodecatungstophosphoric acid
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