PHASE-BEHAVIOR AND THERMAL-CONDUCTIVITY OF UREA AT PRESSURES UP TO 1-GPA AND AT TEMPERATURES IN THE RANGE 50-370-K

The thermal conductivity of the solid phases I and III of urea was mea sured at temperatures in the range 50-370 K for pressures up to 1 GPa. Phase III, previously detected only at pressures above 0.5 GPa, was- observed here at low pressures (< 0.07 GPa) below about 230 K. Extrapo lation of the I-III phase line indicates that phase III might be obtai ned at 218 K at atmospheric pressure and, consequently, that urea migh t exhibit two solid phases at atmospheric pressure. The temperature de pendence of the thermal conductivity of both phase I and phase III cou ld be described by the Debye model for thermal conductivity assuming p honon scattering by three phonon umklapp processes only. Despite a vol ume decrease at the I --> Ill transition, the thermal conductivity dec reased by about 20%. Normally, thermal conductivity increases at a pha se transition at which volume decreases. This rather unusual behavior of urea might be due to an increase in the nearest-neighbor distance a t the I --> III transition.