Figure: Left - Average structure of pentachloronitrobenzene (PCNB). Each of the six substituent sites in every molecule contains 1/6 NO2 and 5/6 Cl, as derived from X-ray and neutron diffraction [L.H. Thomas et al., Acta Cryst., 2007]. Right - Debye-reduced specific heat data Cp /T3 for the low-temperature crystalline phase of PCNB, exhibiting “glassy anomalies”: two-level systems and the boson peak.
The low temperature thermal and transport properties of an unusual kind of a crystal which exhibits minimal molecular positional and tilting disorder have been measured in this work, as well as inelastic neutron scattering spectra. The material, namely the low-dimensional, highly anisotropic pentachloronitrobenzene (PCNB) exhibits a layered structure of rhombohedral parallel planes in which the molecules execute large-amplitude in-plane as well as concurrent out-of-plane librational motions. Our study reveals that the universal low-temperature glassy anomalies can be found in a system with minimal disorder as that due to the freezing of mostly in-plane reorientational jumps of molecules between crystallographic equivalent positions with partial site occupation. Our findings will pave the way to a deeper understanding of the origin of above-mentioned universal glassy properties. [Full article]