Research PapersStructural, magneto-optical and dielectric properties of phosphate tellurite glasses
Graphical abstract
Introduction
Transition metals like Te4+ have attracted particular attention due to their electron structure with the characteristic of parity-allowed s-p transitions [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. This fact is supported by the emission properties that vary between visible to near-infrared regions, very often used for medical applications. These properties may be changed and controlled by the concentration of Te4+ and the melting temperature of their subsequent oxides in the glass compounds. For example, in the Te-borate glasses, the emission is centered at 460 nm [17], while in the TeO2-Li2O glasses, the emission is shifted toward 645 nm [18]. In phosphate tellurite glasses, the emission moves between 850 and 1000 nm [19], but the higher melting temperatures of these materials should be taken into account. Over 1000 °C, the phosphate tellurite glasses contain tellurium colloids due to the oxidation process accompanied by elemental clustering, which leads to aggregation processes toward metallic colloids [20,21]. Increasing the melt temperature up to 1200 °C changes the color of the phosphate tellurite glasses from red to deep purple due to the gradual changing of Te4+ in metallic tellurium, starting with Te2 clustering toward precipitation of Te metallic colloids [22].
Metallic colloids are marked by a prominent peak in the visible part of the absorption spectrum, centered at 532 nm (2.33 eV) and an A1-term with an asymmetric shape in magnetic circular dichroism, generated by localized surface plasmon resonance [23]. These colloids change the structure of the phosphate tellurite glasses by adding their dielectric properties to the glass matrix.
The tellurium metallic colloids were investigated in other glasses such as Y2O3-Al2O3 powder by using X-ray photoelectron spectroscopy (XPS) photoluminescence and lifetime, where the emission is changing with the melting temperature (between 1100 °C to 1500 °C) from the yellow-orange to the red. Te-doped zinc phosphate and phosphate tellurite glasses display broadband near-infrared luminescence from 900 to 1500 nm, making their promising applications in broadband optical amplifiers and tunable lasers [22].
Dielectric spectroscopy is described by the interaction between alternating current and electric charges or electric dipole moments from samples. From the measured impedance, phase shift, and current, some specific parameters of these samples can be evaluated, such as capacitance, inductance, complex permittivity, and electrical conductivity.
Dielectric spectroscopy of phosphate glasses has been studied in BaO–ZnO–B2O3–P2O5 glasses due to their properties that make them suitable for the barrier ribs in plasma display panels [24]. Here, the dielectric constant of the glass increases with the adding compounds like ZnO or decreases by adding Al2O3. The same is true for the (90-x)TeO2-10Nb2O5-xZnO, especially for x>10 mol% [25]. The explanation was connected with the gradual increase of Zn+ ions, which replaces the Te4+ ions. Meanwhile, the TeO bonds change the structures from TeO4 in TeO3, which increases the non-bonding oxygens. In borotellurite glasses, the AC conductivity and dielectric constant decreased with increasing TeO2 content from 0 mol.% to 15 mol.% [26]. These behaviors can be combined when some crystallization processes occur in the glass matrix. This is the case of vanado-tellurite glasses containing BaTiO3, where thermal annealing leads to gradual crystallization of BaTiO3 and the constant dielectric increase to 318 K then decreases, following a Curie-Weiss dependence at 1 kHz [27].
Tellurium dioxide is known as conditional glass former and, without the addition of other glass formers like P2O5, does not transform into the glassy state [28]. The main problems of these glasses, based on a conditional glass former (TeO2), a glass former (P2O5), and a spacer (ZnO), is connected with their different melting points: 340 °C for P2O5, 732 °C for TeO2, and around 2000 °C for ZnO. Phosphate pentoxide is highly hygroscopic, but along with TeO2, it induces superior mechanical and chemical stability for the resulting glasses [29].
The main purpose of this article is to examine the changes in the structural, magneto-optical, and dielectric properties of the 20%TeO2+40%ZnO+40%P2O5 glass samples that occurs during the melting procedure at different temperatures and not compositional as usual. This melting temperature becomes a key parameter for the samples melted from 1000 °C to 1200 °C. Over 988 °C, tellurium reaches the boiling point from the phase diagram of TeO, where oxygen solubility is minimal [30]. The coloration of these glasses occurs due to the elemental clustering of Te2/Te2−, where Te4+ is reduced at high temperatures, leading to aggregation processes toward metallic colloids [20]. Going from 1000 °C to 1200 °C, phosphate-tellurite glasses change their color from pale red to deep purple. This fact promotes the reduction in the redox equilibrium of glass melt in which a part of Te4+ is converted into metallic tellurium. The Te2 clustering leads to the precipitation of Te metallic colloids, which in turn, modify the magneto-optical, structural, and electric properties of these glasses [22].
Section snippets
Experimental
The conventional melt quenching technique was applied to prepare glasses of 20%TeO2+40%ZnO+40%P2O5 (10 g). High purity chemicals of TeO2 (99.9% purity, Merck Company), ZnO (99.99% purity, Alfa Aesar Company), and P2O5 (99% purity, pro analysis Merck Company) were used as received. The phosphate pentoxide was maintained at 100 °C for 1 h before weighing to eliminate the water, because this compound is highly hygroscopic. All chemicals were weighed and set into alumina (Al2O3) crucible, starting
Magneto-optical measurements
All magneto-optical measurements were performed on the sample melted at 1200 °C because the MCD signal is relatively low for the other two samples. However, the absorption spectra at room temperature show a broad band centered at 532 nm for the samples obtained at 1000 °C with a significant increase for the sample obtained at 1200 °C (Fig. 1).
The temperature dependence of the absorption spectrum performed at room temperature (295 K) suggests an asymmetric band shape slightly deformed toward
Discussions
Magneto-optical measurements were performed to investigate the low field magnetic properties of the phosphate tellurite glasses, especially magnetic circular dichroism (MCD), which was compared with the absorption spectra. The increase in the area below the absorption curves suggests a higher density of the absorbing centers produced during the melting procedure. While the prominent peak centered at 532 nm have the same intensity at 110 and 295 K and the same area, the second one is shifted and
Conclusions
The combination of one glass former, such as phosphate pentoxide, having a low melting point (340 °C) and highly hygroscopic with a classical conditional glass former like TeO2 but with a higher melting point (723 °C), improves the chemical stability of the melt quenched glasses.
Increasing the melting point above 1000 °C induces an oxidation process of Te4+ ions toward Teo atomic tellurium, followed by a clustering process that changes the structural, magneto-optical, and dielectric properties
Authors’ statement
PS carried structural analysis, MCD measurements and writing the paper; NA performed the experimental part concerning the sample preparation; PG performed dielectric measurements.
CRediT authorship contribution statement
Silviu Polosan: Conceptualization, Formal analysis, Methodology, Writing - original draft, Writing - review & editing. Paul Ganea: Investigation, Methodology. Andrei Nitescu: Investigation, Methodology.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by a grant of the Romanian Ministry of Research and Innovation, CCCDI-UEFISCI, project number PN III-PI-1.2-PCCDI-2017-0871 within PNCDI III.
References (41)
- et al.
Composite thermoluminescent detectors based on the Ce3+ doped LuAG/YAG and YAG/LuAG epitaxial structures
Radiat. Meas
(2019) - et al.
Radiation induced defects in Pb2+-doped LiF crystals
Phys. Proc.
(2009) Luminescence of Sb3+ in closed shell transition metal oxides
J. Lumin
(2019)- et al.
The luminescence of Sb3+ in LaOCl
J. Alloy. Comp.
(1996) - et al.
Crystal structure and luminescence studies of microcrystalline GGG:Bi3+ and GGG:Bi3+,Eu3+ as a UV-to-VIS converting phosphor for white LEDs
J. Lumin.
(2019) - et al.
Near-infrared quantum cutting via energy transfer in Bi3+, Yb3+ co-doped Lu2GeO5 down-converting phosphor
J. Alloy. Comp.
(2019) - et al.
Blue-emitting Bi-doped double perovskite Gd2ZnTiO6 phosphor with near-ultraviolet excitation for warm white light-emitting diodes
J. Alloy. Comp.
(2019) - et al.
Structure and luminescence properties of Bi3+ activated Ca12Al14O32Cl2 phosphors
J. Alloy. Comp.
(2013) - et al.
Photoluminescence, photoacoustic, and scintillation properties of Te4+-doped Cs2HfCl6 crystals
Mater. Res. Bull.
(2018) Structure and low field magnetic properties in phosphate-tellurite glasses
J. Non-Cryst. Solids
(2019)