Low-temperature elastic anomalies in an anthracene single crystal

Longitudinal and transverse wave velocities, elastic parameter (elastic con stants, Debye temperature and Gruneisen parameter), and dilational and shea r interval frictions were observed between room temperature and 137 K for a n anthracene single crystal, using an ultrasonic pulse method. We observed anomalies in transverse velocity, acoustic anisotropy factor, elastic param eters and internal frictions at 266 and 197 K. We also observed devitrifica tion, shrinkage of 0.86% and partial collapse after measurement. These resu lts 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. All rights.