Estudo de vidros nióbio-fosfatos dopados com ferro por espectroscopia Mossbauer e corrente de depolarização termicamente estimulada (TSDC)

Abstract

Niobophosphate glasses P2o5-PbO-Nb2O5-IS_ü-xFe2o3 were studied using Mõssbauer Spectroscopy and TSDC (Th-errnally Stimulated Depolarization Currentes) techniques, aiming its structural determination (specially the coordination number of Noobium. During the preparation of these glasses the concentrations of Niobium and Plumbum were varied. The concentrations of P2o5 e K2O were kept constant, while the relation PbO/Nb2O5 was varied from 4.33 to 0.12 getting a total of nine different compositions and chosen for the present study the rate PbO/Nb2O5 equal to 0.82. It was added to this composition 0.5%, 3.0% and 5.0% in mass of Fe2o3, therefore rcsulting in three samples with different dopping percentages. The effect of dopping iron was studied under conditions of oxidation and redúction with low pressure, in temperatures of 200ºC, 400ºC and 600ºC. The Mõssbauer spectra presented high eficiency in the conversion F +2 ➔ F +3 as annealing temperature increases. The Quadrupole Splitting suggests that the sample in its original satate before annealing presents Fe+2 in octahedral coordination (A = 2.18mm/s), and Fe +3 in octahedral (A = 0.44 mm/s) and tetrahedral (A = 0.92mm/s) coordinations. After the reduction process in which Fe +3 band decreases until it disappears and the Fe +2 increases to 100%, we have therefore Fe +2 in octaheclral and tetrahedral coordenations. ln the oxidation process, in which we have the inverse situation on the fim al result presents Fe+3 an tetrahedral octahedral coordinations. The TSDC results showed that the generation process of vacancies is associated to dopping iron, having the Fe+2 ion presented a large inhomogeneous linewidth that carried to an average increase of three orders of magnitude in relaxation time comparing to Fe +3. It was also observed the variation effect of dopping as well as polarization field effect associated to arficial charge due to the Maxwell-Wagner effect.