Abstract
Brillouin and Rayleigh scattering techniques have been employed to study the elastic behavior of single crystals of urea over the temperature range 15–300 K. An extensive investigation of the room temperature elastic constants is presented yielding a complete evaluation of their anistropy. The temperature dependence of these elastic modes along the [001] and near the [110] directions is followed to 15 K; no evidence of a structural phase transition has been found. Rayleigh scattering experiments reveal a correlation time in the ms range and an intensity behavior that are temperature dependent but show no anomalies as a function of temperature for a number of directions in the crystal. It is found that scattering intensity and correlation time are caused by the modulation and driving of defects by an external periodic applied stress pulse. Neither the correlation time nor the intensity behavior supports the conclusion that urea undergoes a structural second order phase transition in the temperature range 1...
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