On Lattice Excitations in the Modulated Structures of [N(CH3)4]2CuCl4

Brillouin spectroscopy has been used to investigate the acoustic excitations in [N(CH 3 ) 4 ] 2 CuCl 4 . This crystal is known to have successive transitions from the prototype phase ( D 2 h 16 ) to the incommensurate phase at T 1 = 297 K, to the ferroelastic commensurate phase ( C 2 h 5 ) at T 2 = 291 K and to another monoclinic phase at T 3 = 263 K. Upon transition to the incommensurate phase no anomaly could be detected in the velocities of the longitudinal phonons propagating in the [100], [010] or [001] directions. Within the incommensurate phase the velocity of the mode propagating along [001] showed a decrease of about 1.2%. The Brillouin line width of this mode was found to depend on temperature both in the incommensurate and in the commensurate phases, attaining the maximum value near T 2 . The results are compared with the existing ultrasonic data and discussed by assuming the phason-phonon interaction in the incommensurate phase.