Dielectric and Neutron Inelastic Scattering Measurements on Phenanthrene

Abstract

The static dielectric constants and the linear coefficients of thermal expansion of high purity single crystals of phenanthrene were measured as a function of temperature in three mutually perpendicular directions and indicate a small increase in the dielectric constant in the a direction at about 72°C. Cold neutron inelastic scattering from polycrystalline phenanthrene was investigated at room temperature, 60, 75, and 85°C, using the Brookhaven Slow Chopper facility. Changes in the density of phonon states above and below 72°C were examined by difference analysis and loss of some low-lying states above 72°C was detected. Some implications of these dielectric measurements and the neutron spectra as well as earlier electrical and heat capacity measurements are discussed. The conclusion is that phenanthrene cannot at present be described as one of the known types of solids. It is suggested that the observed phenomena are not inconsistent with a model in which hindered vibration of the 4, 5 hydrogens above 72°C introduce a new degree of freedom.