MOLECULAR ORIENTATIONAL CORRELATION AND ORIENTATIONAL GLASS IN PLASTIC CRYSTAL

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In a plastic phase, 1-cyanoadamantane C10H15CN has a positional order and reorients among <001> and its equivalent five orientations (<001> six-fold reorientation) rotating around dipolar axis (uniaxial rotation). On cooling from the plastic phase slowly, the compound transforms to a metastable phase, and then to an ordered phase. The <001> six-fold reorientation and uniaxial rotation freezes in the ordered phase. A glassy phase can easily be obtained by quenching the molecules in the plastic phase below glass transition temperature T_g=170 K. In the glassy phase, the <001> six-fold reorientation freezes, the molecules take randomly one of the six orientations rotating around the dipolar axis. In this study, the plastic, glassy and metastable phases have been studied by NMR and MD simulation to clarify the origin of glass forming.^<13>C NMR shows clearly that the molecules undergo the <001> six-fold reorientation in the plastic phase, and undergo the <111> eight-fold reorientations tog … More ether with the six-fold reorientation in the metastable phase,. In addition, the metastable phase is found to relax toward the ordered phase. The structural relaxation time is determined to be 3200 s. In the gassy phase, the spin-lattice relaxation time tells that the correlation time of the uniaxial rotation distributes. By assuming Davidson-Cole function, the parameter for motional distribution β is determined to be 0.2. This means that the motion is largely poly-dispersive.The MD simulation shows that the correlation time of the six-fold reorientation is poly-dispersive even in the plastic phase. The correlation function calculated for the six-fold reorientation is analyzed at several temperatures by using a stretched exponential function. Obtained results show that β increases with decreasing temperature. From the analysis of dispersion curves, the motional distribution is fund to arise from the dispersion of the correlation time. Moreover, the dispersion becomes remarkable with decreasing temperature. The temperature behavior of the dispersion almost agrees with that of β. Less