Title: Thermal Transport in Molecular Disordered Crystals with Long-range Order.
When: Thursday, January 31, (2019), 12:00.
Place: Department of Condensed Matter Physics, Faculty of Sciences, Module 3, Seminar Room (5th Floor).
Speaker: Daria Szewczyk, Institute of Low Temperature and Structure Research, PAS, 2 Okólna Str., 50-422, Wrocław, Poland.
The nature of the amorphous state is recognized as one of the greatest challenges of condensed matter physics in the XXI century. In the quest for simple systems exhibiting glassy behavior, which may at least partially be used as models, some disordered molecular crystals know as plastic crystals, seem to meet the requirements.
The focus of the talk is on studies of singularities in thermal transport. For that reason, as an introduction the different experimental methods of determining the thermal conductivity coefficient κ(T) and the heat capacity Cp(T) will be presented. The main part will be devoted to elucidating the particular processes and mechanisms governing the heat transport of materials for which their molecules can retain a translational order, being orientationally disordered between them, upon cooling. The case studied are the derivatives of nitrobenzene crystal (C6H5NO2), namely the orientationally disordered pentachloronitrobenzene and two phases of parachloronitrobenzene- orientationally ordered and a disordered one. Although most of the results of κ(T) may be assign to the typical dependences expected in crystalline and amorphous states, some anomalous behavior is observed. A detailed analysis of the experimental data concerning the peculiarities, especially in the high temperature range, will be presented. Additionally, the conclusions will be supported with heat capacity results.