ISSN:
1089-7690
Quelle:
AIP Digital Archive
Thema:
Physik
,
Chemie und Pharmazie
Notizen:
p-chlorobromobenzene (PCBB) and p-chloroiodobenzene (PCIB) crystallize in the centrosymmetric space group P21/c, Z=2. Since the molecules do not possess an inversion center, the crystals exhibit orientational disorder, in other words, the electrical dipoles of regularly distributed molecules irregularly point in opposite directions. In this study, the temperature dependence of the nuclear quadrupole resonance (NQR) line shape for PCBB and PCIB and the spin-lattice relaxation time (T1) for PCIB have been measured. Both compounds exhibit an inhomogeneously broadened spectrum, as is expected for this kind of disorder. However, the measured NQR profile strongly depends on temperature, particularly that of PCIB. To explain this behavior, each orientation of the molecular dipoles has been considered as a component of what we call a binary orientational alloy. Within this framework it is shown that the minimum free energy of the system always corresponds to a centrosymmetric array, but that short-range orientational correlations may arise, depending on the difference in binding energy between a parallel and an antiparallel pair of neighboring molecules. In this way, the overall temperature dependence of the experimental profile can be succesfully reproduced if a one-dimensional orientational correlation among molecules is considered. At high temperatures, 180° molecular reorientations completely destroy the short range order and the measured spectra can be reproduced in terms of a pure random disordered model. The presence of molecular reorientations has been inferred from the measured temperature dependence of the nuclear spin-lattice relaxation time. In addition, the effect of short-range orientational correlations on T1(T) has also been calculated and discussed. © 1999 American Institute of Physics.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1063/1.478641
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