Elsevier

Journal of Non-Crystalline Solids

Volume 499, 1 November 2018, Pages 49-57
Journal of Non-Crystalline Solids

Influence of Eu2O3 on phase crystallization and nanocrystals formation in tellurite glasses

https://doi.org/10.1016/j.jnoncrysol.2018.07.018Get rights and content

Highlights

  • Heat treatment at 420 °C leads to crystallization of ZnTeO3 nanocrystals and several phases.

  • Raman spectra revealed the presence of interstitial molecular oxygen in the glass.

  • Eu2O3 concentration increment decreases the formation of crystalline phases.

  • Photoluminescence shows that Eu3+ ions are not reduced after heat treatment.

Abstract

Glasses of the 17ZnO·32CdO·51TeO2 system doped with Eu2O3 (0–0.75 mol per 100 mol of oxides) were obtained. Non-isothermal differential scanning calorimetry (DSC) tests showed that the glass transition temperature varied in the range of 354° to 371 °C with the addition of Eu2O3. The samples were exposed to a thermal treatment in a laboratory oven at 420 °C. X-ray diffraction analysis of the treated samples showed crystalline phases of a variety of metallic oxides, the intensities of which decreased as the Eu doping amount in the samples increased. In all samples, the strongest peak corresponded to ZnTeO3 with a crystallite size of about 36 nm. Fourier Transform Infrared Spectroscopy showed that CdO and ZnO change their role from modifier to a glass former. Raman spectra showed characteristic bands corresponding to γ–TeO2. The observations suggest that Eu2O3 induced structural changes which in turn led to the formation of molecular oxygen in the interstitial network. Deconvolution analysis of Raman spectra and High-Resolution Transmission Electron Microscopy images confirmed the formation of Te2O5, ZnTeO3 and Te nanocrystals. Photoluminescence analysis showed that Eu ions are incorporated as Eu3+ and no reduction occurred after heat treatment.

Introduction

Tellurite glasses are of interest due to their capability to host rare earth ions, which show high intensity and narrow peak in photoluminescence emission spectra [[1], [2], [3], [4], [5]]. The low phonon energy of TeO2 based glasses decreases the probability of occurrence of non-radiative energy transfer; this property makes these materials good candidates for laser applications [[6], [7], [8], [9], [10], [11]]. Specifically, the properties of tellurites, as their wide transmission in the range of 0.4–5.0 μm, high refractive index, low melting temperature, good corrosion resistance, mechanical and chemical stability, make these materials of interest for optoelectronic applications [12,13].

Moreover, TeO2 belongs to the intermediate class of glass-forming oxides. When this species is blended with a secondary component such as Na2O, WO3, BaO, PbO, CdO, and ZnO, the capability of the resulting mixture to form a glass melt improves significantly. Several studies on the use of TeO2 as a glass-forming oxide have been reported in the literature. Sidek et al. [14] found that the ZnO–TeO2 binary system presented good glass stability. In their experiments, the authors noted that the physical and optical properties of the glass were affected by the increment in ZnO concentration, causing the formation of non-bridging oxygens (NBOs) breaking the Te–O–Te network. In general, modifier oxides lead to the formation of Te–O–M (M = cation, like Na+, K+, Ca2+, Fe3+, Mg2+) terminal bonds or NBOs leading to the depolymerization of the Te–O–Te structure, changing the glass properties. In contrast with the conventional alkali or alkaline earth oxide modifiers, CdO is thermally stable, appreciably covalent, and shortens the time necessary for solidification of glasses during quenching [15]. In this laboratory, the glass-forming zone for the ternary system ZnO–CdO–TeO2 was determined experimentally [16]. Further, the properties of the vitreous matrix were measured upon the incorporation of rare earth compounds (REC). During the experiments, the proportion of REC in the ternary system was maintained constant while the proportions of CdO and TeO2 were varied. The formation of CdTeO3 and CdTe2O5 crystalline phases were found when the glass was doped with Tb4O7 and EuCl3 [17,18]. However, when the system was doped with YbBr3 and Nd2O3 no presence of crystalline phases were detected [17,19]. Also, it has been reported that for ZnO–TeO2 glasses, ZnTeO3 and Zn2Te3O8 are the crystalline phases that appear around 428° and 458 °C respectively, according to DTA analysis [20]. Thus, there are several crystalline phases that can be expected to appear in the ZnO–CdO–TeO2 system.

Glasses with embedded nanocrystals are of interest because of the optical properties of such materials like photoluminescence, refractive index, and non-linear optics [[21], [22], [23], [24]]. However, the information regarding some crystalline phases like ZnTeO3, Zn2Te3O8 or CdTeO3 embedded in TeO2 glasses is scarce [[25], [26], [27], [28], [29]]. The goal of this investigation was to test the effects of europium concentration and thermal treatment on the development of crystalline phases in a glass of 17ZnO·32CdO·51TeO2. For that purpose, an experimental program was followed as described below.

Section snippets

Experimental procedure

A glass matrix consisting of 17ZnO·32CdO·51TeO2 was doped with varying concentrations of europium nitrate. Reagent grade of zinc oxide (ZnO, Fluka Analytical), cadmium oxide (CdO, 99.5%), tellurium dioxide (TeO2 ≥ 99%), and europium nitrate hexahydrate (Eu(NO3)3·6H2O, 99.99%) from Sigma Aldrich were used to prepare the glass batches. The content of the metallic oxides in the glass matrix was maintained constant as indicated in the chemical formula: 17ZnO·32CdO·51TeO2 as mol%, whereas Eu2O3 was

Composition

The final composition of the as-cast glasses was determined by SEM/EDS analysis, and the results are presented in Table 2. Upon addition of europium nitrate, the concentration of Cd and Te slightly increased while O and Zn content decreased as compared with to the undoped glass. Among the glasses with Eu2O3, there is no significant variation in Cd, Zn and Te concentration, and Eu content increased as expected. For all glasses, a low quantity of aluminum was detected, which possibly originated

Conclusions

In this work, DSC analysis for the obtained 17ZnO·32CdO·51TeO2:xEu2O3 glasses indicate that they are thermally stable. The glasses showed several structural changes at 420 °C which are related on the europium concentration. The Raman spectra analysis showed that the addition of europium at different concentrations promoted the appearance of interstitial molecular oxygen and NO2 in the glass. The latter is associated to the europium nitrate decomposition. The intensity of the band associated

Acknowledgements

The authors thank to CONACYT for their support through the Cátedra-CONACYT (1959) project. We gratefully acknowledge the use of TEM facilities at the TEM Laboratory of the University of Sonora and the Geology Department of the University of Sonora for their assistance in XRD characterization.

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