Medidas de constate dielétrica no KH2PO4 submetido a condições de temperatura e pressão uniaxial

Abstract

Measurernents of the dielectric constant done on KH2Po4 (KDP) as a function of the uni-axial pressure and ternperature.The phase transition P.E. ➔ F.E. was studied as a function of ternperature at various fixed pressures with forces applied in [ 001 ] , [ 100 ] and [ 110 ] directions of the tetragonal structure of KDP. With the force on [ 001] and [ 100 J directions we could determine the variation of the phase transition ternperature Tc with the pressure. The value of dTc/dP is in good concordance with the one obtained frorn hydrostatic pressure rneasurernents. The ferroelectric direction is responsible by the lowering of Te when the force is applied on the [ 001] directions of the paraelectric phase of KDP. We also deterrnined the variation of the Curie-Weiss constant with the pressure when the force is applied on the direction of the ferroelectric axis. Frorn the rneasurernents of dielectric constant agãinst ternperature for various fixed pressures with F parallel to the [ 110] direction of the tetragonal structure when T > T and parallel to the [100] direction of the e orthorhornbic structure when T < Te' we observed structural phase transitions predicted by the deforrnation potential thepry. Our measurements confirmed the results frorn Rarnan scattering that we had previously obtained. The new phase (N.F.E.) is ferroelectric and is observed when the crystal is under pressure at temperatures near the transition temperature Te = 122K. The phase transition P.E. ➔ N.F.E. is due to a shearing force when F is parallel to [ 100] of D2d and the crystal goes from the tetragonal structure, with a n2d group symmetry to an orthorhombic structure with a symmetry group cs=192v. This transition was expected since c66 goes to zero when the temperature gets near to Te = 122K. At the F.E. ➔ N.F.E. phase transition, when ➔F is parallel to [ 100] of c2v' the crystal rnaintains the orthorhornbic structure but changes the syrnrnetry group from Cs192v Cs=192v. This transition is understood as due to changes in the symmetry sites of·the ions. From these measurements the point where the three crystalline phases coexist was deterrnined as (0,15Kbar, 121K). From the measurernents of the dielectric constant when F is parallel to [ 110] of n2d, changing both the temperature and the pressure, we observed that the N.F.E. ➔ P.E. transition is reversible while the F.E.➔N.F.E. transition is irreversible; This is also a confirrnation of the Raman results already obtained. We also did measurements of the dielectric constant as a function of pressure at 125K and 118K. Finally we discuss the rneasurernents of dielectric constant as a function of both uni-axial pressure and ternperature taking into account the implicit and explicit contributions (as related to the thermal expansion) that reflect the changes in the polarizability of the crystalline lattice and contribute to the anornaly of the dielectric constant in the ferroelectric phase transition that KDP undergoes at 122K.