Transição de fase e efeitos anarmônicos no BaC1 2.2H20

Abstract

Raman spectra of BaCl2.2H2 o were studied as a function of temperature in the range l0K-300K, for all possibile polarizations related to the irredutible representations of the symmetry group to which tbe crystal belongs. From these experiments we observed qualitative changes in the Raman spectra, mainly in the frequency range of the normal medes associated to librations and stretching of the crystal water molecules. These changes occurring between 150K and 200K can only be understood if one assumes that the crystal is undertaking a phase transition that modjfy the status of half of the water molecules in the unit cell. This is a transition that shows all the characteristics of a second-order transition of the order-disorder type. From these experiments we also obtained quantitative results for the dependence of the frequencies and line-widths of the Raman- active optical phonons with temperature. Such measurements showed the existence of anharmonic effects due to interaction processes involving three or four phonons that modify some of the normal modes related to the water molecules. We also studied the dependence of the dielectric constant of the crystal with the temperature in the interval 77.K. to 300K. These measurements showed an anomalous behaviour of the dielectric constant curve in the range l50K-200K which confirms the existence of a second-order phase transition in the BaC12.2H2 o crystal, as seen by our Raman results.