Medidas de constante dielétrica no CsLiSO4 submetido a deformações homogêneas

Abstract

Dielectric measurement was dane on CsLiso4 submited to variations of temierature and uniaxial stress near ihe structural phase transition that takes place at 202K. It is shown that the phase transition is observed through measurements of the dielectric constant in the c-axes direction and it is confirmed that in this direction there is no mechanism responsible for the ferroelastic phase transition. We also verify, for the first time, that the anomaly of the dielectric constant in a or b-axis directions has the sarne nature although it is much more pronounced in the b direction. The mechanism responsible for the phase transition appears to be in the abplane and this reflects the anomalous rotations of the so4 ians about the c-axes. The origin of this anomaly is explained taking into acount the second arder coupling between the soft TA mode and hard TO mode. The energy of the phonon is calculated and it is observed that it is in the same region that is attributed to the librational modes of the so4 ion in CsLiso4 cristals. We show that this phonon is the main responsiblé for the anomalous specific heat. The phase transition is not influenced when the force is parallel to the [001] direction, while a force parallel to the [100] direction changes the phase transition in the same way as a force parallel to the [010] transition. The energy variation of the TA phonon is calculated when we apply a stress of the arder of lKbar in the baxes directíon and it is shown that this change is sufficient to stabilize the monoclinic phase. The TO mode is uncoupled and it is seen that then we have a normal dependence whith temperature. A Landau Free Energy study was done and it is shown that it does not explain all experimenta:tfy observed results. This is attributed to the fact that this theory does not take into acount the changes in the sites of the ions in the primitive cell of the crystal. Implicit and explicit contributions of the anharmonicities were separated through the dielectric measurements as a function of the uniaxial stress and the temperature. The explicit term (6E2T)V dorninates at temperatures near the phase transition and this is associated with anharrnonicities coming from the librational mode of the so4 ions. Implicit terms -(AE1P) Tand -(AE3P)T dominate at all ternperatures, showing that the anharmonicities that exist in these directions are due only to thermal expansion.