UNUSUAL LOW-FREQUENCY ELASTIC ANOMALIES AROUND THE UPPER INCOMMENSURATE PHASE OF [NH3C3H7]2MNCL4

Low-frequency elastic properties (1-15 Hz) of [NH3C3H7]2MnCI4 crystals have been investigated in the phase transition region to the upper in commensurate phase (gamma phase). We have found that the character of the elastic anomalies is essentially different near T-i and T-c althou gh the symmetry breaking at these phase transitions is equivalent. In addition the anomalies both in real and imaginary part of complex effe ctive elastic constants are strongly frequency dependent in the range of 1-15 Hz. This unusual elastic behavior in the region of the reentra nt beta-gamma-delta phase sequence is treated within the framework of a phenomenological theory.