NQR study of phase transitions in CH$mathsf{_3}$HgX (X = Cl,Br, I)

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 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.Received: 23 June 2003, Published online: 15 October 2003PACS: 64.70.Kb Solid-solid transitions - 61.66.Fn Inorganic compounds - 76.60.Gv Quadrupole resonanceSupported in part by the Ministry of Education, Science and Sport, Republic of Slovenia