Espectroscopia vibracional e de impedância em monocristais de SrAlF5

Abstract

SrAlF5 has been considered as one of the rare ferroelectric fluoride crystals, belonging to the I4 polar group. However, recent x-ray diffraction data suggest that the correct structure would be centrosymmetric (I41/a). Dielectric constant and ionic conductivity measurements have been performed in SrAlF5 single crystals, along the [100] and [001] directions, as functions of frequency (10 Hz to 10 MHz) and temperature (300 to 800 K). Ac conductivity showed power-law dependencies of the form ωn, with 0.6 < n[100] < 0.9 and 0.8 < n[001] < 1 in the real part of the conductivity at high frequencies. A phase transition was observed around 700 K with considerable thermal hysteresis in the measurements of real part of the dielectric constant and conductivity. Ionic conductivity vary about four orders of magnitude in the investigated temperature range and shows various regions with an Arrhenius type dependence with activation energies between 0.2 and 1.32 eV. Since ferroelectricity is forbidden in the I41/a structure, the existence of an inversion center was investigated by vibrational spectroscopy. Thus, SrAlF5 single crystals have been measured by polarized Raman scattering and infrared reflectance spectroscopy. The results, discussed on the basis of the factor group analysis of the proposed structures and those of others members of the ABF5 family, favor a centrosymmetric structure.