Critical slowing down of low-frequency dielectric relaxation in incommensurate [N(CH3)4]2ZnCl4

The complex dielectric constant of tetramethylammonium tetrachlorozincate (TMA-ZnCl4) in the incommensurate phase has been measured in the frequency range from 5 kHz to 13 MHz. Two relaxation processes were observed, one fast and one slow, which is common in A2BX4 crystals. In contrast to earlier investigations of TMA-ZnCl4, however, we find that both the slow relaxation process, which dominates (0) near the lock-in transition, and the fast process exhibit critical slowing down. This may indicate the coexistence of incommensurate and commensurate domains, or be related to a recent suggestion of Holakovsky, Dvorak, Fousek, and Kroupa concerning the pinning of solitons.