| Abstract
| - We have examined the relaxation that occurs in the supercooled plastic crystalline phases of pentachloronitrobenzene (PCNB), dichlorotetramethylbenzene (DCTMB), trichlorotrimethylbenzene (TCTMB) along withsome of their deuterated samples, and 1-cyanoadamantane (CNADM) in the presence of intentionally addeddopants. The experimental techniques used in the present study are dielectric spectroscopy and differentialscanning calorimetry (DSC). Only one relaxation process similar to that of the primary (or α-) relaxationcharacteristic of glass-forming materials is found, but there is no indication of any observable secondaryrelaxation within the resolution of our experimental setup. The α-process can reasonably be described by aHavriliak−Negami (HN) shape function throughout the frequency range. However, in the case of PCNB thedielectric strength (Δε) of the above said α-process does not change appreciably with temperature, thoughinterestingly, a small addition of a dopant such as pentachlorobenzene (PCB), trichlorobenzene (TCB), andchloroadamantane (CLADM) to the molten state of PCNB drastically lowers the dielectric strength by afactor of 4 to 8. Powder X-ray diffraction measurements at room temperature and DSC data do not indicateany appreciable change in the crystalline structure. It is noticed that the effect of PCB as a dopant on themagnitude of α-process of CNADM is moderate, whereas both PCB and TCB as dopants show a muchreduced effect on the relaxation in DCTMB and TCTMB. It is suggested that the drastic changes in thedielectric strength of the α-process is due to the rotational hindrance caused by the presence of a smallnumber of dopant molecules in the host crystalline lattice. In the above context, the possibility of a certaindegree of antiparallel ordering of dipoles is also discussed.
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