Raman spectra of single crystals of silver iodide tetratungstate (Ag26I18W4O16) have been obtained as a function of temperature from 77 to 350 K. These data indicate the presence of structural phase transitions near 199 and 285 K. Dielectric and conductivity measurements perpendicular to the unique twofold axis confirm the presence of these two transition temperatures. Plots of log (σT) versus 1/T reveal three distinct regions of linearity with activation energy 2.4 eV below 199 K; 0.07 eV between 199 and 285 K; and 0.42 eV above 285 K.
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Habbal et al. (1978) studied this question.
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