ESTUDO ESTRUTURAL E VIBRACIONAL DO CRISTAL TRIS (GLICINATO) DE CROMO (III) MONOHIDRATADO EM FUNÇÃO DA TEMPERATURA.

Abstract

Amino acids complexed with transition metals have increasingly attracted the interest of researchers because they have a wide range of transparency in the region of the visible spectrum, high thermal stability, high mechanical strength, ease of growth and low production cost. In this work we present the synthesis and characterization of the Tris (glycinate) chromium (III) monohydrate crystal (Cr(C2H4NO2)3H2O). This crystal was grown by technique of slow evaporation of the solvent, for an average period of 4 weeks and characterized by X-ray Diffraction (XRD), UV-Vis Spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR), Raman Spectroscopy, Thermogravimetric Analysis (TG), Differential Thermal Analysis (DTA) and Differential Scanning Calorimetry (DSC) techniques. According to measurements UV-Vis spectroscopy the metal-amino acid bond is more favorable when the amino acid is in its anionic form, that is, in basic pH. The XRD analysis together with the Rietveld method confirmed that the sample crystallized in the space group monoclinic structure P21/C, Z = 4, lattice parameters a = 6.2601 (5) Å, b = 14.6455 (1) Å, c = 12.273 (1), β = 100.411 (8)° and V = 1106.74 (6) Å3. From the results of Raman Spectroscopy and FTIR, it was possible to assign the probable normal modes of crystal vibration, including the modes that prove the occurrence of complexation. From the analysis of the TG curve, we observed the presence of four endothermic events of loss of mass of the sample; the first event is between 348.29 and 367.11 K, the second between 578.50 and 587.31, the third event between 594.84 and 606.12 and the last between 660.89 and 671.77 K. The first event is related to the loss of hydration water, while the others relate the decomposition of glycine. The DTA curve corroborates with the TG curve, respectively showing peaks at 361.39, 584.68, 599.01 e 666.26 K, which also indicate gradient and significant mass loss of the material, that is the decomposition of the crystal. The DSC result confirm the loss of water in the material near 370 K. From the DRX at high temperature was observed the disappearance of some peaks and the emergence of others. The standards measured up to 363 K exhibit the monoclinic phase, but from 363 K occur changes in the measured diffraction pattern, indicating the beginning of the phase transformation, which is completed at 393 K. Another fact observed was the hygroscopicity, after a period of 12 hours the material had absorbed the lost water, showing that the resulting transformation is reversible. With the results of Raman Spectroscopy at high temperature was possible to observe all changes in the material due to phase transformation, including the disappearance of the band referring to water molecule. The measurements of XRD and Raman at low temperature do not show changes that characterize a phase transformation, thus showing that the material remains with the same structure when subjected to low temperatures.