Distinction of disorder, classical and quantum vibrational contributions to atomic mean-square amplitudes in dielectric pentachloronitrobenzene

Abstract

The solid-state molecular disorder of pentachloronitrobenzene (PCNB) and its role in causing anomalous dielectric properties are investigated. Normal coordinate analysis (NCA) of atomic mean-square displacement parameters (ADPs) is employed to distinguish disorder contributions from classical and quantum-mechanical vibrational contributions. The analysis relies on multitemperature (5-295 K) single-crystal neutron-diffraction data. Vibrational frequencies extracted from the temperature dependence of the ADPs are in good agreement with THz spectroscopic data. Aspects of the static disorder revealed by this work, primarily tilting and displacement of the molecules, are compared with corresponding results from previous, much more in-depth and time-consuming Monte Carlo simulations; their salient findings are reproduced by this work, demonstrating that the faster NCA approach provides reliable constraints for the interpretation of diffuse scattering. The dielectric properties of PCNB can thus be rationalized by an interpretation of the temperature-dependent ADPs in terms of thermal motion and molecular disorder. The use of atomic displacement parameters in the NCA approach is nonetheless hostage to reliable neutron data. The success of this study demonstrates that state-of-the-art single-crystal Laue neutron diffraction affords sufficiently fast the accurate data for this type of study. In general terms, the validation of this work opens up the field for numerous studies of solid-state molecular disorder in organic materials.

Glassy Anomalies in the Low-Temperature Thermal Properties of a Minimally Disordered Crystalline Solid

“…The reduced VDOS, g()/ 2 , shown in the inset, is consistent with the presence of a weak feature or "boson peak" centered at about 2 meV superimposed to the Debye VDOS, g()   2 . It is worth mentioning that the crystal nature of the material enables an unambiguous identification of features appearing in g() to discrete normal modes and in fact, the observed extrema match the librational frequencies derived from the normal mode analysis of the atomic displacements [40], yielding 4.26 meV, 5.36 meV and 5.98 meV, as well as a peak at 8.55 meV assigned to molecular torsional oscillations.…”

“…Here we study a minimally disordered crystalline system in which only a few degrees of freedom make it to depart from a fully ordered state. The chosen system is a quasiplanar hexasubstituted benzene derivative, pentachloronitrobenzene (PCNB, C 6 Cl 5 NO 2 ), that exhibits a layered structure of rhombohedral (space group 3 R ) parallel planes in which the molecules can rotate around a six-fold-like axis [21,[39][40][41][42]. Nevertheless, the molecular dipole has two nonzero (in-plane and a small out-of-plane) components with a highly anisotropic dynamical coupling.…”

“…The pioneering study of Weiss highlighted the molecular pseudo-symmetry of the halogenobenzenes as the main cause for the subtle disorder [43] and some further studies accounted for that orientational disorder [44,45]. According to recent works [39,40], the constituent PCNB molecules exhibit remarkably large (temperatureindependent) atomic displacements giving rise to a highly distorted crystal structure. Such motions are allowed because of anomalously large values for some of the intermolecular contact distances within a crystal structure which exhibits molecular positional and tilting disorder.…”

Accurate data processing for neutron Laue diffractometers

“…T1 shows the refinement results from Cole et al (2011), where LAUENORM was used for normalization and merging, while LAUE4 was used for refinements T2-T5. T1-T3 use the integrated intensities from the published refinement, which were calculated using an earlier version of ARGONNE_ BOXES, while T4 and T5 use integrated intensities calculated by a newer version of ARGONNE_BOXES.…”

“…The final comparison is VIVALDI data for a 5 K data set for pentachloronitrobenzene from Cole et al (2011), shown in Table 3 and Fig. 6.…”