Transformações estruturais dos compostos Cs2HgBr4, LiKSO4 e N(CH3)4CIO4 devido a transições termotrópicas de fase

Abstract

The Cs2HgBr4 compound has been studied since 1979 and different structures have been proposed to describe its five known phases. X-ray diffraction and Raman Spectroscopy experiments have been used to investigate the room temperature phase (T > 245 K), the incommensurate phase (233 < T < 245 K) and the commensurate phase (167 < T < 233 K). Structural analysis shows that the crystal can be described, in the room temperature and in the incommensurate phases, both by ordered and disordered models but the latter one give more consistent results. Raman results corroborate the descriptions based on X-ray analysis; the presence of an extra peak in the Raman spectra, which according to group theory should be forbidden in an ordered structure, indicates the lack of local symmetry and is associated with an orientational disorder of HgBr4 tetrahedra. In order to characterize the modulation in the incommensurate phase, the position of some satellite reflections were measured as a function of temperature. In the transition from incommensurate to commensurate phase a multi-soliton behavior is observed. The Cs2HgBr4 crystal in the low temperature commensurate phase is composed of two kinds of ordered pseudomerohedral monoclinic twined domains. LiKSO4 crystals present several phase transitions from low temperature to the melting point. Some works published by different authors using different experimental techniques reveal contradictory results concerning physical and structural properties; even the phase transition sequence is not completely established. In particular, the symmetry description of the phase between 708 and 943 K (phase II) is controversial; incommensurate, commen- surate and twined models have been used to explain the observed diffraction pattern and the results of electrical conductivity, Raman and Infrared spectroscopy measurements. The first structure refinement of phase II based on single crystal X-ray diffraction is carried out at two distinct temperatures (723 and 803 K). All the reflections, including those considered as satellites by some authors, are used in the refinements. Fourier difference maps reveal a disorder of the oxygen atoms. Two disordered models and an anharmonic vibrations ordered model are refined. The dependence with temperature of the One Particle Potentials (OPP) is used to analyze the nature (static or dynamic) of the observed disorder. The model consi- dering two complete disordered SO4 groups in the refinements is found to be the best one in the structure description of phase II LiKSO4. A model for the movement of the SO4 group during the transitions phase III → phase II → phase I is presented. Due to the experimental and theoretical relevance of the phase transition mechanisms the search for compounds that present transitions that lead structural normal phases to disordered, modulated and twined phases, is performed in a number of research centers. In this sense synthesis and crystal growth of N(CH3)4YO4 (Y=Cl,Br, I) is developed in the Laboratório de Cristalografia da UFMG. DSC experiments have shown that N(CH3)4ClO4 presents one phase transition between 493 K and 110 K. X-ray diffraction experiments per- formed at room temperature lead to a tetragonal lattice with V≈ 400 Å3 (lattice A). A careful analysis show the presence of a small set of weak reflections which can be only indexed using a unit cell with V ≈ 1600 Å3 (lattice B). Their intensity increase with decreasing temperature and a discontinuous growth is observed during the phase transition. A disordered structural model described in the P ¯4 space group can be satisfactorily refined using lattice A. The set of reflections defining lattice B can be explained by considering an orientational correlation between nearest ClO4− molecules. The increase in their intensity is explained as a consequence of the increase in the correlation length due to crystal cooling. Results obtained with a simple theoretical model for the interactions between ClO4−1 and N(CH3)4+ in the crystal reinforce the conclusions obtained from the diffraction results.