FTIR, Raman, and UV-Vis spectroscopic and DFT investigations of the structure of iron-lead-tellurate glasses |
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Authors: | Rada Simona Dehelean Adriana Culea Eugen |
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Institution: | (1) Department of Physics, Technical University of Cluj-Napoca, 400641 Cluj-Napoca, Romania;(2) National Institute for Research and Development of Isotopic and Molecular Technologies, Cluj-Napoca, Romania |
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Abstract: | In this work, the effects of iron ion intercalations on lead–tellurate glasses were investigated via FTIR, Raman and UV-Vis
spectroscopies. This homogeneous glass system has compositions xFe2O3·(100−x)4TeO2·PbO2], where x = 0–60 mol%. The presented observations in these mechanisms show that the lead ions have a pronounced affinity towards TeO3] structural units, resulting in the deformation of the Te–O–Te linkages, and leading to the intercalation of PbO
n
] (n = 3, 4) and FeO
n
] (n = 4, 6) entities in the TeO4] chain network. The formation of negatively charged FeO4]1− structural units implies the attraction of Pb2+ ions in order to compensate for this electrical charge. Upon increasing the Fe2O3 content to 60 mol%, the network can accommodate an excess of oxygen through the formation of FeO6] structural units and the conversion of TeO4] into TeO3] structural units. For even higher Fe2O3 contents, Raman spectra indicate a greater degree of depolymerization of the vitreous network than FTIR spectra do. The bands
due to the Pb–O bond vibrations are very strongly polarized and the TeO4] structural units convert into TeO3] units via an intermediate coordination stage termed “TeO3+1]” structural units. Our UV-Vis spectroscopic data show two mechanisms: (i) the conversion of the Fe3+ to Fe2+ at the same time as the oxidation of Pb2+ to Pb+4 ions for samples with low Fe2O3 contents; (ii) when the Fe2O3 content is high (x ≥ 50 mol%), the Fe2+ ions capture positive holes and are transferred to Fe3+ ions through a photochemical reaction, while the Pb2+ ions are formed by the reduction of Pb4+ ions. DFT calculations show that the addition of Fe2O3 to lead–tellurate glasses seems to break the axial Te–O bonds, and the TeO4] structural units are gradually transformed into TeO3+1]- and TeO3]-type polyhedra. Analyzing these data further indicates a gradual conversion of the lead ions from covalent to ionic environment.
There is then a charge transfer between the tri- and tetracoordinated tellurium atoms due to the capacity of the lead–tellurate
network to form the appropriate coordination environments containing structural units of opposite charge, such as iron ions,
FeO4]1−. |
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Keywords: | |
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