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

Abstract

Structural phase transitions of tetramethyl ammonium manganese trichloride (TMMC) are investigated by using the Brillouin scattering method. The temperature dependences of the elastic coefficients C11 and C33 extracted from the experimental data are analysed by using the Landau theory of phase transitions. The step anomaly of C33 observed at the order-disorder transition temperature Tc can be attributed to a coupling between the order parameter and totally symmetric strains. An anomalous softening C33 above Tc indicates an interaction also between tetramethyl ammonium (TMA) molecules and the MnCl3 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 Cu2+ 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.