Ferroelastic domain freezing and low-frequency elastic behaviour of (NH4)(4)LiH3(SO4)(4) (ALHS)

In this work elastic measurements on (NH4)(4)LiH3(SO4)(4) (ALHS) which were carried out in the low-frequency range between 1 and 50 Hz are presented. The temperature dependence of the inverse elastic compliance S-11(-1) has been determined between 90 K and 420 K. Distinct anomalies have been found in the temperature dependence of Si-11(-1), which are connected to the motion of domain walls in the ferroelastic phase below T-c = 232.5 K. Around T-f = 162 K a (partial) ferroelastic 'domain freezing' phenomenon has been observed. To the knowledge of the authors this is the first time that pure ferroelastic domain freezing has been reported. However, below T-f the domain walls seem to retain a certain vibrational degree of freedom which could be responsible for an additional anomaly of the loss modulus which was observed. The elastic behaviour of a crystal of ALHS is dependent on the 'history' of the given sample. During temperature cycling Si-11(-1) shows differences between the first run of heating and cooling and later runs. Finally, some basic insights concerning the domain wall motion were obtained; it was found that the domain wall mobility decreases by three orders of magnitude in the temperature region 170-230 K.