Brillouin scattering investigation of structural phase transitions in TMMC and TMMC:Cu

Structural phase transitions of tetramethyl ammonium manganese trichloride (TMMC) are investigated by using the Brillouin scattering method. The temperature dependences of the elastic coefficients C 11 and C 33 extracted from the experimental data are analysed by using the Landau theory of phase transitions. The step anomaly of C 33 observed at the order-disorder transition temperature T c can be attributed to a coupling between the order parameter and totally symmetric strains. An anomalous softening C 33 above T c indicates an interaction also between tetramethyl ammonium (TMA) molecules and the MnCl 3 chains. A new lattice anomaly, which the authors attribute to a change in the rotational degree of freedom of the TMA molecules, is observed at about 395K. Doping of TMMC with 2% of Cu 2+ creates a sequence of phases with transition temperatures at 92, 124 and 130K. An incommensurate phase stabilised by impurities may be involved in this sequence. Strong elastic scattering of light indicating a formation of needle-like domains along the c axis is observed in the doped crystals between 90 and 130K.