Is There an Excess Wing in the Dielectric Loss of Plastic Crystals?

The plastic crystals ortho- and meta-carborane and 1-cyanoadamantane are investigated by dielectric spectroscopy for frequencies 10^-3≤ν≤10⁹Hz, paying special attention to the spectral shape of the loss peaks. For the carboranes, we provide clear experimental evidence that the high-frequency wing follows a single power law over many decades in frequency. The excess wing, which shows up in a variety of glass-forming liquids as a second power law at high frequencies, is completely absent in the carboranes. In 1-cyanoadamantane, a weak excess wing shows up which can be ascribed to a Johari-Goldstein β process. We conclude that the excess wing is a property of supercooled liquids only.