Elastic properties of ferroelastic LiKSO4 in the temperature range from 20 K to 150 K

Abstract

Brillouin spectroscopy was used to investigate elastic properties of ferroelastic lithium- potassium sulfate (LiKSO₄) single crystal in the temperature range from 20 K to 150 K. Particular attention has been paid to the temperature region where structural phase transformations have been reported to occur. -- Samples used in the experiment were prepared from untwinned parts of large single crystals and were not treated thermally prior to the experiments. Due to cracking of samples at approximately 50 K, only the temperature dependence of longitudinal modes is reported. -- The longitudinal mode propagating in the [100] direction was temperature independent over the whole temperature range. The modes propagating in [101] and [011] directions show small anomalies below 50 K. Nothing can be concluded about the nature of the changes or the possible phase transition temperatures from these data. However, the mode propagating along the [001] direction shows a local minimum at 52 K which is associated with a structural phase transition. Cracking of most of the samples used in this work at this tempearture suggests the phase transition to be of the first order. Moreover, a small anomaly of the sound wave velocity propagating in this direction occurs at 38 K. Also a pronaunced increase of Brillouin shift occurred below 28 K. These anomalies disappeared after performing the measurements for the second time on the same sample. This behavior can be attributed to the coexistence of different phases over the whole temperature range, or the influence of phase transitions on the spatial distribution of defects and related local internal stresses. -- The temperature of the ferroelastic phase transition reported earlier in the literature was confirmed to be 185 K. The changes of elastic properties at this temperature are in good agreement with previously reported results. -- Suggestions for further work are included.