A influência estrutural nas propriedades físicas do sistema TeO2 – Li2O – MoO3 vítreo em função da composição

Abstract

In this work, vitreous samples of the (1-x-y)TeO2-xLi2O-yMoO3 system, where x and y are molar percentages (mol%), were prepared using the melting quenching method and their physical properties characterized according to the composition. The amorphicity was checked and confirmed by X-ray diffraction (XRD). Structural changes were investigated by Raman and Fourier Transform Infrared (FTIR) spectroscopy. For structural properties, it was evidenced that the modifier oxide Li2O influences in breaking Te – Oax bonds and the formation of new bonds (non-bridging oxygen transposition) occurs when adding the transition metal oxide MoO3. Another consequence of the adding both oxides are the change in coordination of units with Te atoms (TeO4 to TeO3+1 and TeO3, with or without NBO) and units with Mo atoms (MoO4 to MoO6). Besides, from the density ( ) results, determined by the Archimedes method, the parameters of molar volume (VM), oxygen packing density (O.P.D.) and, molar volume oxygen (VO) were obtained. The behavior of these data was attributed to the atomic radius of the constituent atoms according to the composition. For thermal properties, the behavior of increasing the glass transition temperatures (Tg), crystallization onset temperatures (Tx), crystallization temperatures (Tc) and, thermal stability (ΔT) was determined by Differential Scanning Calorimetry (DSC) data. This was attributed to the increase in the amount of MoO3 up to a limit of y ≤ 35 mol%. For optical properties, the bandgap (Eg) and Urbach tail (ΔE) energy values were determined by spectroscopy in Visible Ultraviolet (UV-Vis), which decreases with increasing x and y. Also, with the data of linear refractive index ( ), determined by the Brewster angle (at two wavelengths, 594 nm, and 633 nm), the values of electronic polarizability ( ) were calculated. In addition, the theoretical optical basicity values ( ) were calculated, according to Duffy’s theory, and compared with the optical basicity values as a function of the refractive index ( ), which decrease according to composition. The mechanical properties of hardness (H) and elasticity modulus (E) values were determined by the instrumented indentation technique. As the MoO3 increases, there is a reduction in glass H an increase in E, respectively. The Poisson’s coefficient ( ) data, longitudinal and transverse velocities ( and , respectively) and, elasticity modulus (Eecho) of elastic properties were obtained by the pulse-echo technique. Impedance Spectroscopy was used to characterize the electrical behavior of the samples. It was observed that the character of the samples with the highest concentration of the MoO3 is resistive (Rel). On the other hand, the capacitive character (C) was observed in the samples with a high concentration of Li2O. The increase in the conduction band, ie, the electrical conductivity ( ) and the activation energy (Ea), is a consequence of the increase in non-bridging oxygens transposed in glasses with a high concentration of MoO3 oxide.