Abstract
Longitudinal and transverse wave velocities, elastic parameter (elastic constants, Debye temperature and Gruneisen parameter), and dilational and shear internal frictions were simultaneously observed between room temperature and 137 K for an anthracene (C_<14>H_<10>) single crystal with lattice imperfections such as vacancy and dislocation, using an ultrasonic pulse method with 1.7 MHz. The transverse wave velocity and Debye temperature dropped suddenly at 266 K, and then increased somewhat down, to 197 K, and gradually increased with decreasing temperature. The Gruneisen parameter showed two large positive peaks at 194 K and 175 K, and one large negative peak at 261 K, indicating lattice expansion and shrinkage, respectively. The dilational friction showed one small jump at 266 K and one large peak at 181K. The 266 K jump and the 181 K peak are probably due to rotation between interplanar stacks and intermolecular readjustment. The 181 K valley is due to the accumulation of strain. We also observed devitrification, shrinkage of 0.86 % and partial collapse after measurement. These results suggest a possibility of a type I/type II phase transition at 181K, caused by cooling and/or ultrasonic vibration. Thus, we can semi-quantitatively explain the low-temperature elastic anomalies of ASC, using linear chain and double potential models based on van der Waals and Lennard-Jones interactions, respectively. If no previous precise measurements of phonon velocities, VI and Vt, for organic crystals has been done, we are delighted to be pioneers in this field.
