Low-temperature elastic anomalies in an anthracene single crystal

Longitudinal and transverse wave velocities, elastic parameter (elastic constants, Debye temperature and Gruneisen parameter), and dilational and shear internal frictions were observed between room temperature and 137 K for an anthracene single crystal, using an ultrasonic pulse method. We observed anomalies in transverse velocity, acoustic anisotropy factor, elastic parameters and internal frictions at 266 and 197 K. We also observed devitrification, shrinkage of 0.86% and partial collapse after measurement. These results suggest the possibility of a type I/type II phase transition from 181 K, caused by cooling and/or ultrasonic vibration. (C) 2000 Elsevier Science B.V.