SHORT-RANGE ORIENTATIONAL CORRELATION IN THE DISORDERED CRYSTAL OF 1-CHLORO-4-IODOBENZENE

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 orientati...