A neutron scattering investigation of some aspects of the structural phase transition in (CD3)4NMnCl3(TMMC)

A series of measurements on the nature of the order-disorder molecular phase transition at T c approximately 126.5K in tetramethylammonium manganese trichloride (TMMC) is reported. These measurements involve the use of diffraction, diffuse elastic and quasistatic, and inelastic, coherent neutron scattering techniques. The transition is found to be driven by a strong coupling between the librational motion of the tetramethylammonium molecule and the TA (100) phonon mode. This coupling gives rise to quasielastic critical scattering at the low-temperature superlattice peak positions and to an over-damping of the phonon modes. Estimates of parameters involved in the coupled pseudospin-phonon theory of Yamada et al. (1974) are made from the nature of the spectrum near T c . Below T c the acoustic translational and librational modes become clearly resolved. Some observations on the nature of the low-temperature structure are given.