Non-exponential orientational relaxation in dipolar solids: the role of dipolar interactions and dielectric friction

Abstract

Extensive Brownian dynamics simulations have been performed on a dipolar cubic lattice to study the dynamics of orientational relaxation in the presence of dipolar interactions. In particular, detailed calculations are carried out to obtain the single-particle orientational correlations C_l(t), C_l(t) = [4π/(2l + 1)]Σ^l_m=_-1<Y*_lm[Ω(0)]Y_lm[Ω(t)]> where Y_lm(Ω) are the usual spherical harmonics and the dielectric friction on a rotating dipole. It is found that the orientational correlation functions become progressively non-exponential as the polarity of the system is increased. At the highest polarity studied (with a dielectric constant of 19.0), the C₁(t) factor is strongly non-exponential, indicating a considerable effect of dipolar interactions on orientational relaxation. The correlation functions obtained from simulations are compared with the molecular theory of Madden and Kivelson and the continuum-model based theories of Nee and Zwanzig and Hubbard and Wolynes. At low polarity the molecular-based theory of Madden and Kivelson fares better than the continuum theories. At intermediate and higher polarity, however, agreement between the simulation and the theories becomes unsatisfactory. The important prediction of Hubbard and Wolynes on the rank dependence of dielectric friction and its effect on the higher correlation functions also has been examined through simulations and compared with the theories mentioned above. The dielectric friction was found to decrease rapidly with increasing l, in qualitative agreement with the predictions of Hubbard and Wolynes. The observed effect however is much stronger than the predictions of the existing theories. The present simulations provide unambiguous proof of the important role of dipolar interactions on orientational relaxation in dipolar systems.