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) less than or equal to nu less than or equal to 10(9) Hz, 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 beta process. We conclude that the excess wing is a property of supercooled liquids only.