Transições de fase em LiKSO4

Abstract

In this work the LiKso4 crystal was studied by means of dielectrical measurements and Raman spectroscopy as a function of temperature in the range from lOK to 300K, at ambient pressure. Three discontinuities were observed in the dielectric constant and loss curves that were related to three first-order phase transitions. Raman spectra also displayed effects related to these modifications. The first transition occurs at 242K when increasing the temperature (201K when decreasing). There is no concordance in the bibliography on the structure of the crystal below 242K. Some workers suggest a trigonal symmetry belonging to space group P31 (c3 4 ) while others favor a hexagonal structure C V with space group P63(c:v). We present a model that explains all observations. A second transition occurs at 190K while increasing the temp􀁕rature (180K when decreasing). Again there are different interpretations abóut the structure and space group for this phase. A different model is proposed that is more consistent with the observations or Ramãn spectroscopy and dielectric measurements. The third transition is observed at 30K and is explained by a subtle re-arrangement of the sulphate ions that affects the bending normal modes, leading again to further lowering of symmetry in the unit cell.