ULTRASONIC STUDY OF NORMAL-INCOMMENSURATE-COMMENSURATE PHASE-TRANSITIONS IN [N(CH3)4]2MNCL4

The complete set of elastic constants of the single crystal [N(CH3)4]2 MnCl4 has been measured at 15 MHz in the temperature range 259-310 K, including the normal-incommensurate transition at T(I) = 292.3 K, the incommensurate-improper ferroelastic lock-in transition at T(L) = 291. 7 K, and the improper ferroelastic-improper ferroelastic transition at T1 = 266.7 K. The Landau theory has been used to explain the experime ntal data. In addition, the attenuation of the longitudinal wave assoc iated with the C-11 mode was measured as a function of temperature and frequency (12.7-71 MHz) around T(I) and analyzed in terms of a dynami c-scaling model. Above T(I) the results agree with both three-dimensio nal-Ising and mean-field theory. In the commensurate phase the critica l exponents have values consistent with mean-field theory. The relaxat ion time of the amplitudon below T(I) has been determined to be tau(A) = tau(A0)/(T(I) - T) with tau(A0) almost-equal-to 4 X 10(-10) s K.