Abstract:
The temperature dependence of halogen nuclear quadrupole resonance (NQR) in the series CH3HgX (X=Cl, Br, I) is measured with special emphasis on the structural phase transitions at T c =162 K, 310 K, and $\sim$400 K, respectively. In the temperature dependences of NQR frequencies similarities are observed and discussed in relation with the structure and thermal vibrations on both sides of the phase transition. On the basis of known data a mechanism for the three analogous phase transitions is proposed. The chlorine spin-lattice relaxation behaviour in CH3HgCl can be explained by a competition of fast thermal fluctuations of MMX molecules across the high temperature (h.t.) mirror plane and of infrequent transitions to the other equivalent fluctuation mode across the orthogonal h.t. mirror plane. Proton high temperature relaxation is probably dominated by the same slow motion, but at lower temperatures some other mechanism involving magnetic coupling prevails. Copyright Springer-Verlag Berlin/Heidelberg 2003
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