Estudos de efeitos lineares e não lineares e transição de fase no KDP induzidas por pressão uniaxial, campo elétrico e temperatura

Abstract

Raman spectra of KH2PO4KDP under uniaxial pressure are studied at various temperatures around the transition point from the paraelectric to the ferroelectric phases. For stress along the [110] direction in the parelectric phase and along the [100] direction in the ferroelectric phase, large qualitative modifications are observed in the Raman spectra. These modifications are interpreted as phase transitions due to changes in the symmetry sites of the ions, induced by the uniaxial stress. The reversibility of the processes leading to these transitions is studied. At low temperatures, KDP undergoes a phase transition induced by an applied static electric field. This phase transition was studied by light scattering. It was found an increase in the symmetry of the crystal with the applied field, going from a monoclinic to an orthorhombic structure. A comparison is made with a transition already reported induced by uniaxial stress. The reversibility of the transition is discussed. When the crystal is in the orthorombic (electri-field induced) phase, it remains in this phase when the field is removed at constant temperature. If now the field is reversed the Raman spectra obtained is identical to that of the high temperature paraelectric phase (D2d) . This phase persists even if the field is removed or the temperature is lowered to as much as 20K. Measurements of the dielectric constant were performed on KDP as a function of the uniaxial pressure and temperature. The ferroelectric phase transition was studied as a function of temperature at various fixed pressures with forces applied along the [001], [100] and [110] directions of the tetragonal structure of KDP. With the force along the [001] and [100] directions we could determine the variation of the phase transition temperature Tc with the pressure. From the measurements of dielectric constant as a function of temperature for various fixed pressures with the force along the [110] direction of the tetragonal structure, we observed the structural phase transitions predicted by the deformation potential theory. Our measurements confirmed the results from Raman scattering that we had previously obtained. The measurements of dielectric constant as a function of static electric field are discussed. Ferroelectric phase transitions of KDP and Rochelle Salt are studied by piezoelectric resonance. The results show that the phase transitions in both materials obey different dynamics. Large nonlinearities were observed in KDP as a function of temperature, frequency and amplitude of the electric field. Nonlinearities of harmonics and sub harmonics generation are explained by meaans of a phenomenological model involving an acoustic soft mode and piezoelectric resonances.