Structural, optical and thermal properties of glass and anti-glass phases in strontium tellurite system containing europium and boron
Rajinder Kaur1, Atul Khanna1*, Marina González-Barriuso2 , Fernando González2 Banghao Chen3
1
Department of Physics, Guru Nanak Dev University, Amritsar-143005, Punjab, India.
2
Department of Chemistry and Process & Recourse Engineering, University of Cantabria,
Santander-39005, Spain 3
Department of Chemistry & Biochemistry, Florida State University, Tallahassee, FL 32306-
4390, USA
*Email:
[email protected] Tel. +91-183-225-8802 (Ext. 3568) Fax +91-183-225-8820
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ABSTRACT The 10SrO-90TeO2 sample prepared by melt-cooling is found to contain triangular, rectangular and spherical shaped micro-inclusions of the average size ~946 µm in the glassy matrix. These inclusions grow (average size~1480 µm) and assume flower like morphology on adding 1 mol % Eu2O3. X-ray diffraction studies show that micro-inclusions exhibit sharp peaks but their Raman spectra have broad phonon bands that are indistinguishable from that of the glassy phase. Micro-Raman spectroscopy reveals the identical chemical composition and short-range structure of the anti-glass inclusions and glass-matrix. The addition of B2O3 into these samples inhibits the growth of anti-glass inclusions, and forms purely amorphous samples. The addition of 20 mol% B2O3 significantly enhances the luminescence properties of Eu3+ doped samples.
Additionally, B2O3 converts the four-fold co-ordinated Te—O
structural units into three-fold co-ordinated Te—O units.
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1. INTRODUCTION Glasses are non-periodic, hard and optically transparent materials that are produced by the rapid cooling of the melt without any crystallization [1, 2]. Glasses have short-range order on length scales < 5 Å, and can have substantial intermediate-range order at 5 to 10 Å length scale [3]. The study of the structure and properties of oxide glasses is of great significance due to some unique properties such as transparency at room temperature, physical isotropy, hardness, the absence of grain boundaries, good workability and excellent corrosion resistance [4]. Glasses have potential applications in various fields such as information technology for optical fibre communication [5], transparent radiation shielding windows [6], bone repair and replacements [7], nuclear waste disposal [8], lasers [9], solders [10], batteries [11], transformer cores [12], magnetic heads and sensors [13] magneto-optic discs [14] and other applications [15]. Among metallic, organic and semiconducting glasses, the oxide glasses offer most attractive features; they are impervious to gases and liquids, they are chemically inert, hard solids, which resist abrasion and scratching and can withstand very high temperatures [16] [17]. Tellurium oxide-based glasses have wide range of applications in gas sensors, memory switching devices, up-conversion laser, optical waveguides and in non-linear optical devices [18, 19]. These glasses have several attractive properties such as good glass stability against crystallization, chemical durability, low melting point, non-hygroscopic nature, low phonon energies, high refractive indices, rare-earth ion solubility, low process temperature, excellent transmission in the visible and near infrared light up to 4.5 µm and exceptional non-linear optical properties [19-22]. Earlier, it was believed that these properties were due to the electronic configuration of tellurium (IV) atom which has a lone pair of electrons (5s2) after its bonding with oxygen atoms but recent studies show that the nature of the Te–O–Te bridges make tellurite glass structure exhibit novel features [23].
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By resolving the TeO2 structure using different characterization techniques, it has been found that the TeOx short-range structure can be described in terms of
units, where
the superscript n gives the number of bridging oxygen, and the subscript m is the coordination number. More precisely, there are five tellurite polyhedral units namely [23-25].
unit represents a fully interconnected network of four coordinated tellurium with
four bridging oxygens,
polyhedra possess one non-bridging oxygens and
units represent the breaking of the TeO2 network, as they result in a terminating group or a lone contain only
group respectively. The crystalline TeO2 polymorphs (α, β, γ-TeO2) units, while the modified TeO2 crystals contain all the other units
[23, 24]. McLaughlin et. al. [24] showed that all the five tellurite polyhedra are necessary to obtain structural models consistent with the full data sets. Further, it was stated by these authors that the structural units present in glasses also exist in the crystals with similar composition. The structural variability of tellurite glasses i.e. presence of more than one structural units (TeO4 bi-pyramid, TeO3 trigonal pyramid, and TeO3+1 polyhedra), creates a range of dipole environments which make these glasses distinctive for the optical, spectroscopic and mechanical properties [26].
TeO2 is a conditional glass former and forms glass easily when combined with transition metal, alkali, alkaline-earth and rare-earth ions. When metal oxide is added to TeO2, there is strong polarizability of the tellurium lone pair electrons which breaks the axial Te—O bonds and leads to the formation of glass structure [27]. The transition metal ions are present in more than one valence state in the glass matrix due to which these glasses have interesting properties [28]. In the present work, SrO (alkaline earth ion) is added into TeO2 to prepare strontium tellurite samples. Modifiers usually enhances the glass forming ability by breaking the structural units, causing to change the local structure and promotes the subsequent stability against recrystallization in tellurites [29]. It is reported that the addition
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of SrO to the tellurite glasses remarkably increases their thermal stabilities [27]. The glass forming range of SrO-TeO2 system lies between 4.8 to 11 mol% of SrO [19]. The phase formed in SrO-TeO2 system depends very much on the method of preparation, maximum temperature of the melt and the cooling rate [30]. For instance, depending on these parameters, different types of phases can be obtained including SrTe2O5, α-SrTeO3, βSrTeO3, γ- SrTeO3, SrTeO4, SrTe5O11 etc.
Tellurite systems are reported to form ―anti-glass‖ phases with heavy metal oxides. An anti-glass is a material in which the cations are randomly distributed at their sites but, the anion sites are partly vacant [31]. More precisely, a prominent long-range order contrasts with highly disturbed short-range order i.e. there is order and disorder co-existence in anti-glass materials. Due to non-periodicity of the anion sites, anti-glass is also referred as the ―anion glass‖. An anti-glass system is reported to form CaF2 fluorite-type structure with cationic distribution over the Ca2+ positions but the F- positions are occupied by the vacancies and the oxygen ions. Therefore, modifiers must have suitable ionic radii to apt for the fluorite-type structure. Furthermore, all the ions are randomly distributed from their ideal positions causing large irregularities which results in apparently high temperature factors than those generated by thermal motion of atoms [31-35]. Very few anti-glass phases had been studied: Ln2Te6O11 [32], Bi2Te4O11 [36], SrTe5O11 [37], Ln2Te6O15 [33], LiHo0.045Y0.955F4 [38]. In the present system, SrTe5O11 anti-glass phase is synthesized and its inclusions grow within the glassy matrix on slow cooling of the melt. B2O3 is the best oxide glass former and borate glasses have applications in thermal shock-resistant, super ionic conductors and in non-linear optics [39, 40]. In the vitrified B2O3 system, each boron ion is bound to three oxygens, each of which is further bounded to another boron giving planar BO3/2 groups which exhibit local ordering i.e. intermediate range order. These groups form six-member boroxol rings which are interconnected to other BO3/2 network by bridging oxygen atom [3]. Additionally, BO3 triangular and BO4 tetrahedral units 5
are present in the borate glass network [41]. Higher the BO4 units, more is the glass forming ability of a borate material, unlike the tellurite glasses in which the triangularly coordinated TeO3 units represent the features of the glassy phase and TeO4 units corresponds to crystalline TeO2 [18]. The multicomponent tellurite glasses containing alkali and alkalineearth oxides and B2O3 show excellent thermal resistance against crystallization when compared to the binary glasses [18, 42]. Tellurite glasses are promising host materials for the rare-earth ions due to the aforementioned properties (rare-earth ion solubility, wide transmission window ~0.35–5 µm, the lowest vibrational energy ~780 cm-1, high refractive index etc.) which enhance the local fields at the rare-earth ion site, leading to the enhancement of the radiative transition rates. Rare earth-activated tellurite glasses have been proved to be very useful materials due to the various photonics applications namely: frequency up-converters, optical fibres for communication, self-frequency doubling lasers, white light emission glasses, light converters, sensors and optical amplifiers at 1.3 and 1.5 µm [22, 43-47]. Usually, it is difficult to fabricate highly concentrated rare-earth doped glasses. When rare-earth ion concentration increases in the glasses, these ions may be in different physical environments with greatly varying spectroscopic properties due to the inherent disordered nature of glasses and results in ion-ion energy transfer that affects the luminescence properties of the glasses. Thus, there is inhomogeneous broadening of the spectra, due to the multiplicity of rare-earth sites in glasses unlike in inorganic crystals [47-49]. Among all of the lanthanide ions, Eu3+ has been the preferred probe ion to study the inhomogeneity and symmetry of the host matrices because of the non-degenerate nature of the 7F0 (ground) state and 5D0 (excited) state, which makes its energy-level structure much simpler. Furthermore, it is the most reported dopant ion due to its properties in narrow line width emission in warm white LEDs for high chromatic index. The trivalent europium ion exists in f6 ground state. The spectra in glasses are obtained by the forced electric transitions within the f shell. Eu3+ ions show temperature dependence transitions. For instance, at low 6
temperature, the absorption spectrum is due to the transitions from 7F0 5D0,1,2,3 but at room temperature, transitions from the 7F1 level (~250 cm-1 above the 7F0 ground state) are also observed. Eu3+ is the most appropriate ion for the 5D07F2 emission and 7F05D2 absorption. The optical properties of Eu+3 ions are critically dependant on the local environment and chemical composition of the glass matrix, which makes them convenient to study the structure and nature of the bonds in disarrayed systems [44, 47, 50-53]. Eu3+ ions doped in various glasses and crystals have been used in glass fibres, amplifiers and up-convertors [45, 50], laser materials [54], phosphors [55] and visible convertors [56]. The present work aims at the elucidation of the thermal, optical and structural properties of SrO-TeO2, SrO-B2O3-TeO2 glasses containing Eu3+ ions. The thermal properties are measured by Differential Scanning Calorimetry (DSC). Photoluminescence spectroscopy (PL) is used to analyse the light emission properties. To measure the changes in Te—O speciation with the changes in metal oxide concentration, Raman spectroscopy is used. X-ray diffraction,
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B magic angle spinning nuclear magnetic resonance (11B MAS-NMR), optical
microscopy and density measurements have been performed on the samples.
2. EXPERIMENTAL 2.1. Glass preparation The tellurium oxide based glasses 10SrO-xB2O3-(90-x)TeO2 and 9SrO-1Eu2O3xB2O3-(90-x)TeO2, where x = 0, 10, 20 mol% were prepared by using TeO2 (Alfa Aesar 99%), SrCO3 (Sigma Aldrich India 99.9%), H3BO3 (Sigma Aldrich India 99%) and Eu2O3 (Sigma Aldrich India 99.9%) by normal quenching technique. The appropriate amount of these starting materials were weighed and ground in a mortar-pestle to obtain homogeneous mixtures. Each batch was then transferred to a platinum crucible (25 cm3) and melted at 750 °C in muffle furnace. Glass melt was kept at this temperature for ~ 30 minute. Afterwards, the melt was poured on heavy brass plate to obtain disk shaped samples. The samples were 7
immediately annealed at 250 °C for 1h to reduce thermal stresses in the samples. Clear, bubble free, and yellowish coloured samples were obtained. The colour of samples got lightened with increase in the B2O3 concentration. The samples were characterized by density, XRD, DSC, PL, optical microscopy,
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B MAS-NMR and Raman studies. The
composition, density and molar volume of samples are given in Table 1.
2.2 X-ray diffraction (XRD) XRD studies were carried out on powdered samples on Shimadzu XRD-7000 X-ray diffractometer with Cu Kα1 radiation (λ = 1.54056 Å) in the 2θ range of 10 °– 65 °. The X-ray tube was operated at 40 kV and 30 mA and the scattered X-ray intensity was measured with a scintillation detector.
2.3. Optical microscopy The optical micrographs of the 10SrTe and 10Sr1EuTe disk samples which contained anti-glass inclusions, were carried out on the Nikon Eclipse microscope at magnification of 40X and100X.
2.4. Density measurement Density of the disk samples was determined at room temperature by Archimedes method using dibutylphatalate as the immersion fluid. The error in density was in the range of ± 0.001 to 0.003 g cm-3. Density of each composition was calculated by using the following relation [57]: …… (1) where ρL is the density of the dibutylphatalate at the laboratory temperature, WA and WL are the weight of the glass sample in air and in the liquid respectively. The molar volume is calculated by using the following relation: ……… (2)
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where x, y, z are mole % of the components A, B and C and MA, MB, MC are their respective molecular weights.
2.5. Differential Scanning Calorimetry (DSC) DSC studies were performed on powdered samples on SETARAM SETYS 16 TGDSC system at heating rate of 10 °C min− 1 under air-flow conditions in the temperature range of 200-800 °C. About 20-40 mg powder sample was taken in a platinum pan to carry out the measurements. The maximum uncertainty in the measurement of the thermal properties including glass transition (mid-point), crystallization (peak-point) and melting temperatures (peak-point) is ± 1 °C.
2.6. Photoluminescence The photoluminescence spectra of Eu3+ doped strontium tellurite and strontium borotellurite disk samples were recorded at room temperature, on Perkin Elmer LS 55 fluorescence spectrometer. The excitation wavelength of 395 nm with an accuracy of 1 nm was used to record the dispersed luminescence spectra.
2.7. 11B MAS-NMR 11
B MAS-NMR studies were performed on powdered samples using Bruker AVIII
HD NMR spectrometer operating at magnetic field of 11.74 T with a 4 mm Bruker MAS probe. The sample spinning rate was 14 kHz and Larmor frequency of 11B nuclei is 160.5299 MHz. Short RF pulses (~15 °) with recycle delay of 20 s were used. After 4096 scans, spectra were collected and referenced to solid NaBH4 at -42.16 ppm
2.8. Raman spectroscopy Raman studies were performed on powdered samples using Renishaw In-Via Reflex micro-Raman spectrometer with 514 nm argon ion laser (50 mW) as excitation source and diffraction grating having 2400 lines mm-1. Measurements were carried out at room temperature using an edge filter for Stokes spectra and a Peltier cooled CCD detector in an 9
un-polarized mode using backscattering geometry within the wave number range of 30 to 4500 cm-1 at a spectral resolution of better than 1 cm-1.
3. RESULTS AND DISCUSSION 3.1. Structure The XRD patterns of as prepared samples are shown in Fig. 1. The pure 10SrTe and europium doped 9Sr1EuTe sample show sharp peaks at 27.2 °, 31.6 °, 45.2 °, 53.6 ° and 56.2 ° superimposed on broad hump, which is due to the blend of glass and anti-glass phases. The sharp peaks match well with cubic SrTe5O11 phase having face-centred lattice and ‗Fm3m‘ space group (powder diffraction file #36-1235) as reported by Burckhardt et. al. [37]. The addition of B2O3 into 10SrTe and 9Sr1EuTe samples destroys the anti-glass phase. Thus, in the strontium borotellurite and europium doped strontium borotellurite samples, XRD patterns show no sharp peaks; but only a broad hump in the angular range of 20 ° to 35° confirming its wholly amorphous nature.
Fig. 1. XRD patterns of strontium tellurite samples containing boron and europium.
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3.2. Optical Microscopy From the optical micrographs (Fig. 2), it is clear that the samples: 10SrTe and 10Sr1EuTe contain micro-inclusions of size of several microns in the predominant glass matrix. Due to the presence of inclusions, these samples show sharp crystalline peaks in the XRD patterns. In the 10SrTe sample, different rectangular and triangular shaped inclusions are observed. Some of the inclusions are overlapping. The inclusion size in 10SrTe varies between 550 µm to 1850 µm with average size of 946 ± 372 µm, where 372 µm is the standard deviation in size. With the addition of 1 mol% of Eu2O3 in 10SrTe sample, the shape of the inclusions gets modified to flower like and spherical shapes. The size of these inclusions is comparatively large in the range of 725-3075 µm with an average of 1480 ± 537 µm, and 537 µm is the standard deviation. The incorporation of B2O3 (10 and 20 mol %) into 10SrTe sample destroys the growth of these inclusions; making the samples (10Sr10BTe and 10Sr20BTe) purely amorphous.
Fig.2. Optical micrographs of 10SrTe (left) and 10Sr1EuTe (right) samples.
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3.3. Density Density of pure strontium tellurite sample decreases from 5.578 ± 0.002 g cm−3 to 4.774 ± 0.001 g cm−3 as the concentration of B2O3 is increased from 0 to 20 mol%, which is due to the lower molecular weight of B2O3 (69.6 u) in comparison to that of TeO2 (159.6 u) as the heavier TeO2 molecules are replaced by the lighter B2O3 molecules. Similar density variation is found in the europium doped strontium borotellurite glasses i.e. density decreases from 5.628 ± 0.002 g cm−3 to 4.937 ± 0.002 g cm−3 in SrO-Eu2O3-TeO2 system with addition of B2O3 (0, 10, 20 mol %). However, when density of SrO-B2O3-TeO2 and SrO-Eu2O3-B2O3TeO2 systems is compared, the density enhances with the addition of 1 mol% Eu2O3 into the SrO-TeO2 and SrO-B2O3-TeO2 systems at the expense of SrO, since lighter SrO (103.6 u) molecules are replaced by the significantly heavier Eu2O3 (351.9 u) molecules. Molar volume, Vm generally describes how compactly the constituent particles are placed in the glass structure. Molar volume increases by small amount from 27.604 ± 0.001 to 28.046 ± 0.015 cm3mol−1. The increase in Vm corresponds to the formation of more open structure and it is due to the transformation of trigonal bipyramidal TeO4 units into trigonal pyramidal TeO3 units containing both bridging and non-bridging oxygen atoms. Table 1. Composition, density, molar volume and co-ordination number of strontium tellurite samples containing boron and europium.
Sample Code
Composition (mol %)
Density [g cm-3]
VM [cm3mol-1]
NTe—O
NB—O
SrO
Eu2O3
TeO
B2O3
10SrTe
10
-
90 2
-
5.578 ± 0.001
27.604 ± 0.008
3.54
-
10Sr10BTe
10
-
80
10
5.237 ± 0.001
27.688 ± 0.003
3.50
3.62
10Sr20BTe
10
-
70
20
4.774 ± 0.001
28.484 ± 0.004
3.47
3.53
9Sr1EuTe
9
1
90
-
5.628 ± 0.002
27.801 ± 0.011
3.44
-
9Sr1Eu10BTe
9
1
80
10
5.285 ± 0.003
27.903 ± 0.016
3.43
3.61
9Sr1Eu20BTe
9
1
70
20
4.937 ± 0.002
28.046 ± 0.015
3.42
3.49
12
3.4. Thermal properties The DSC patterns for strontium tellurite, strontium borotellurite and europium doped glasses are shown in Fig 3. The glass transition temperature (Tg) is crucial to know the thermal stability of the glass system. In the DSC patterns, the glass transition temperature, Tg is taken at the mid-point of the incline of the endothermic curve, followed by an exothermic peak that corresponds to the crystallisation temperature (Tc). In some DSC plots, there is more than one Tc which may be due to the crystallization of different phases in the glasses. The glass transition temperature for pure strontium tellurite sample is 322°C which increases to 328°C on adding 1 mol% of Eu2O3. When 10 and 20 mol% of B2O3 is added into pure strontium tellurite glass, Tg increases up-to 366 °C. The similar trend is observed in europium doped strontium borotellurite glasses. The Tg value increases from 366 °C to 374 °C when Eu2O3 is doped in the strontium borotellurite glass containing 20 mol% B2O3. The increase in Tg value in the strontium borotellurite glasses, with increasing B2O3 content from 0 to 20 mol%, can be explained by using bond enthalpy values. The bond enthalpy is defined as the energy required for dissociating the chemical bonds in a molecular unit. The bond enthalpy is low for Te—O (391±8 kJ/mol) than B—O (806±5 kJ/mol) i.e. weaker Te—O—Te linkages are replaced by stronger B—O—B linkages. The average single bond enthalpy of all the glasses is calculated to quantitatively understand the effect of B2O3 on the glass network strength. The average single bond enthalpy (Eb) for 10SrTe can be calculated as [44]; ………………
(7)
For boron and europium doped glasses, Eb is calculated using the formula: (
)
……… (8)
where the bond enthalpies for different cation-oxygen bonds are: ESr-O = 454±14 kJ/mol, ETe-O = 391±8 kJ/mol, EB-O = 806±5 kJ/mol and EEu-O = 557±13kJ/mol [58].
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The values of the Eb for samples are given in Table 2. It is observed that the value of Eb increases with addition of B2O3 that corresponds to stronger glass structure. When Eu2O3 is doped into the strontium borotellurite glasses, the value of Tg increases which is due to the higher single bond enthalpies of these glasses. Thus, in the present system, Tg shows direct correlation with bond dissociation energies.
Fig. 3. DSC patterns of strontium tellurite samples containing boron and europium.
The thermal stability (ΔT) of the glasses is the measure of resistance against devitrification. The nucleation takes place in ΔT interval. It can be calculated using the relation [21]: …….. (3) The thermal stability of 10SrTe glass is 43 °C, which enhances by a large amount upto 119 °C and 144 °C with addition of 10 mol% and 20 mol% B2O3. When Eu is doped into these glass samples, the thermal stability decreases in both tellurite and borotellutite glasses.
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Table 2. Thermal properties and fluorescence intensity ratio of strontium tellurite samples containing boron and europium.
Sample Code
Tc [oC]
Tg [oC]
Tm [oC]
Eb [kJ mol-1]
Tc1
Tc2
Tc3
Tm1
Tm2
(
)
(
)
10Sr10BTe 10SrTe
341 322
460 365
440
--
581 627
664 680
119 43
439 397
--
10Sr20BTe
366
510
-
-
657
671
144
480
-
9Sr1EuTe
328
370
468
581
614
-
42
398
1.51
9Sr1Eu10BTe
354
468
-
-
556
657
114
440
1.45
9Sr1Eu20BTe
374
513
574
-
650
-
139
481
2.09
3.5. Photoluminescence The PL spectra of europium doped strontium tellurite and strontium borotellurite glasses are shown in Fig. 4. The radiative transitions of the local probes such as Eu3+ ions can provide understanding of the structural and optical properties of glasses, which depend on the chemical composition of the glass matrix and hence, determines the nature of the bonds. In the PL spectra, there are six different emission bands centred at 537 nm, 554 nm, 586 nm, 613 nm, 653 nm and 700 nm. The spectra consists of inhomogeneous broadened 5D0, 1→7FJ (where J=0, 1, 2, 3, 4) transitions. The six emission bands centred at 537 nm (green), 554 nm (deep green), 586 nm (yellow), 613 nm (orange-red), 653 nm (red) and 700 nm (deep red) are due to the optical transitions: 5D1 → 7F0, 5D1 → 7F1, 5D0 → 7F1, 5D0 → 7F2, 5D0 → 7F3 and 5D0 → 7F4 in Eu3+ respectively. The CIE (Commission Internationale de l‘Eclairage, or the International Commission on Illumination) chromaticity coordinates calculated from the emission curve for 9Sr1Eu90Te (0.53, 0.45), 9Sr1Eu10B80Te (0.46, 0.45) and 9Sr1Eu20B70Te (0.49, 0.44) falls in the red-orange region of the spectrum. 15
The 5D0→7F2 transition is an electric-dipole transition and it is hypersensitive to the local symmetry around the Eu3+ ions,
while 5D0→7F1 transition is a magnetic dipole
transition and its intensity is insensitive to the host matrix [59]. It can be seen from the Fig. 4, that the electric dipolar transitions are more intense than the magnetic dipolar transitions as is reported by Reisfeld et. al. [49] for glasses. The relative fluorescence intensity ratio, R is defined as the ratio of the intensities of the magnetic dipolar 5D0 → 7F1 (586 nm) transition to the electric dipolar 5D0 → 7F2 (613 nm) transition. R is defined as [60]: (
)
(
)
……… (9)
R delimits the degree of asymmetry in the neighbourhood of Eu3+ ions i.e. R is inversely related to the local ion symmetry. The value of R is less than or equal to one, when the ion is situated in water, cubic crystals or in a totally symmetric environment while its value can reach up to ten for solidified glass or in asymmetric environments. For the present system, the calculated R value is 1.51, 1.45 and 2.09 for 9Sr1EuTe, 9Sr1Eu10BTe and 9Sr1Eu20BTe samples respectively.
Fig. 4. PL spectra of strontium tellurite samples containing boron and europium.
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Fig. 5. CIE chromaticity diagram of strontium tellurite and borotellurite samples containing europium.
3.6. 11B MAS-NMR 11
B MAS-NMR spectra of strontium borotellurite and europium doped strontium
borotellurite glasses are shown in Fig. 6. In these spectra, there is one sharp peak at ~0.2 ppm, due to 4-coordinated boron units and another broader peak at 9.7 ppm representing BO3 units. The tetrahedral boron is in sp3 hybridized state and has no character but the BO3 units have sp2 hybridized state, showing character. The presence of axially symmetric character leads to strong electric field gradient followed by high quadruple coupling in the BO3 units. Retrospectively, trigonally connected barons give a broad resonance in the range: 3 to 17 ppm. In the europium doped samples, the peak at 9.7 ppm gets distorted. This is due to the fact that Eu3+ shows magnetic properties and there is interference between the applied magnetic field in the NMR spectrometer and the induced magnetic field. The spectra are deconvulated (Fig. 7) and the integrated area under these peaks is used to calculate the B-O co-ordination number, NB—O [40];
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…….. (4) where A4 and A3 represents the area under the peak due to BO4and BO3 units respectively.
Fig. 6. B11 MAS-NMR spectra of strontium-tellurite samples containing boron and europium.
Fig. 7. Deconvulated 11B MAS-NMR spectra of (a). 10Sr10BTe, (b). 10Sr20BTe, (c). 10Sr1Eu10BTe and (d). 10Sr1Eu20BTe samples.
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Increasing the concentration of B2O3 from 10 to 20 mol% leads to a significant decrease in the co-ordination of boron with oxygen (NB-O) from 3.62 ± 0.01 to 3.53 ± 0.01. The decrease in NB-O corresponds to the conversion of BO4 units into BO3. When 1 mol% Eu2O3 is added into 10Sr10BTe and 10Sr20BTe glasses, the peak due to BO4 units gets broader due to magnetic interactions of Eu3+. The NB-O values for 10Sr10BTe, 9Sr1Eu10BTe, 10Sr20BTe, 9Sr1Eu20BTe are 3.62, 3.61, 3.53 and 3.49 respectively. The addition of europium shows very small variation in NB-O. This small variation indicates that addition of Eu2O3 in these samples produce only a small conversion of tetrahedral to triangular boron units. Increase in B2O3 produces a much higher transformation of BO4 into BO3.
3.7. Raman spectroscopy Raman transitions take place between states of the same parity and it precisely gives the position of absorption bands to elucidate the short-range structure of tellurite glasses. The Raman spectra of glasses are displayed in Fig. 8. There is a strong low frequency peak at ~55 cm-1 which is the universal feature of the amorphous materials and commonly known as the boson peak. The latter band is originated by the collective motions of acoustic phonons within the medium-range order of glasses and its intensity depends upon the composition of the sample [61-63]. The intensity of boson band increases slightly with the addition of B2O3. In the strontium borotellurite glasses (0-20mol% B2O3), there are two Raman bands in the wavenumber ranges: 357—540 cm-1 and 550—810 cm-1. The band in the range of 357532 cm-1 is assigned to the bending vibrations of Te—O—Te linkages. In these spectra, the shoulder at ~722 cm-1 is assigned to the TeO3 units along with additional TeO3+1 units, which are created due to the formation of non-bridging oxygens. The band around 532—857 cm-1 is baseline corrected and deconvoluted with four peaks centred at ~617 cm-1, 662 cm-1, 720 cm-1 and 779 cm-1 as shown in Fig. 8. The peaks at 617 cm-1 and 662 cm-1 are due to asymmetric stretching vibrations of TeO4 units, whereas the peaks at 720 cm-1 and 779 cm-1 are due to the 19
stretching vibrations of TeO3 units. There is shifting of peak from 662 cm-1 to 672 cm-1 in the strontium tellurite glass when B2O3 is added into it. Shifting of peak towards higher wavenumber indicates that the concentration of TeO3 units increases at the expense of TeO4 units. In the Eu doped samples, the intensity of boson band increases slightly on adding Eu3+. Tikhomirov et.al. [63] suggests that the amplitude and energy of the boson band is determined by the presence of non-bridging oxygen atoms, since these anion atoms have strained bonds with increased polarizability. The rare-earth ions are incorporated at non-
(a)
(b)
Fig.8. Raman spectra of (a) strontium tellurite samples containing boron and europium in low wavenumber range, (b) europium doped strontium tellurite samples in high wavenumber range. Raman spectrum of crystalline Eu2O3 is also shown for comparison.
bridging anion sites to compensate total charge, followed with the rare-earth environment dominancy and thus provide symmetrical dopant sites, which contribute to the amplitude of boson band [44, 63]. The addition of Eu3+ to the strontium tellurite and strontium borotellurite
20
glasses has no significant effect on the Raman shift of the band: 350-857 cm-1 but, it supresses the intensity of this band to great extent. Moreover, Eu2O3 causes large changes in higher energy range causing additional bands: 860—1000 cm-1, 2107—2218 cm-1, 2255— 2798 cm-1, 2909—3628 cm-1 and 3978—4301 cm-1 (Fig. 8). All of the above bands corresponds to the Eu2O3 except 860—1000 cm-1 band, which might be due to the Te—O— Eu/ Eu—O—Te linkages. In order to calculate the Te—O coordination (NTe—O), the area under the four peaks of deconvluted Raman spectra was used (Fig. 9) and NTe—O is given by [64]: ……… (5) where A4 represents the area under the peaks due to TeO4 units and A3 corresponds to area under the peaks of TeO3 and TeO3+1 units respectively. NTe—O decreases steadily from 3.54 to 3.47 on adding 10-20 mol% B2O3 and the NTe-O value for 9Sr1EuTe, 9Sr1Eu10BTe, 9Sr1Eu20BTe samples is: 3.44, 3.43 and 3.42 respectively. The decrease in co-ordination number with addition of B2O3 corresponds to breaking of Te—O—Te bonds followed by the conversion of TeO4 into TeO3+1/TeO3 units along with the formation of non-bridging oxygens.
Fig. 9. Deconvulated Raman spectrum of 10SrTe sample.
21
To confirm the short-range structure of the inclusions inside the glass samples, microRaman studies were performed on 10SrTe and 9Sr1EuTe disk samples. For 10SrTe sample, the spectrum was taken at two different points, one point was on the inclusion and other was on glass matrix and for 9Sr1EuTe sample, three different points were taken, two points were on the inclusion and the third point was taken on the glass matrix. Since the spectra at the inclusions and matrix phases are very similar (Fig. 10), it indicates that the chemical composition and short range structure of the anti-glass inclusion and glass matrix is same.
(a)
(b)
Fig. 10. Micro-Raman spectra of glassy matrix and anti-glass inclusions in (a) 10SrTe and (b) 9Sr1EuTe samples.
4. CONCLUSIONS Strontium tellurite and borotellurite glasses with and without Eu2O3 and two B2O3 molar concentrations were prepared and studied for their short range structure, optical and thermal properties. The samples 10SrO-90TeO2 (10SrTe) and 10SrO1Eu2O389TeO2 (10Sr1EuTe) have sharp peaks in the XRD patterns, while their Raman spectra have broad
22
Raman bands, similar to those of glassy SrO-B2O3-TeO2 and SrO-Eu2O3-B2O3-TeO2 . Samples 10SrTe and 10Sr1EuTe samples are neither fully amorphous nor crystalline; rather they show the co-existence of glass and anti-glass micro-inclusions of various shapes and sizes. Micro-Raman spectroscopy confirms that the inclusions and glass matrix have the same chemical composition and short-range structure. The addition of B2O3 suppresses the formation of anti-glass inclusion. The thermal stability of the strontium tellurite glasses increases with the addition of B2O3, while europium containing samples are thermally less stable. PL studies found that the 9Sr1Eu20BTe sample contains the most un-symmetric environment around the Eu3+ ion and has maximum photoluminescence yield. The shortrange structural studies show that TeO4 TeO3 and BO4 BO3 transformation occur with increase in B2O3 concentration.
Acknowledgements
Atul Khanna thanks the UGC-DAE Consortium for Scientific Research, Indore and Mumbai Centers and Interuniversity Accelerator Centre, New Delhi, India for research grants that supported this work. REFERENCES [1]
Yamane, M. and Y. Asahara, Glasses for photonics. 2000: Cambridge University Press.
[2]
Donth, E., The glass transition: relaxation dynamics in liquids and disordered materials. Vol. 48. 2013: Springer Science & Business Media.
[3]
Youngman, R., et al., Short-and intermediate-range structural ordering in glassy boron oxide. Science-AAAS-Weekly Paper Edition, 1995. 269(5229): p. 1416-1419.
[4]
Halimah, M., et al., Optical properties of ternary tellurite glasses. Mater. Sci. Pol, 2010. 28(1): p. 173-180.
23
[5]
Nishimori, H., Statistical physics of spin glasses and information processing: an introduction. Vol. 111. 2001: Clarendon Press.
[6]
Singh, S., et al., Barium–borate–flyash glasses: as radiation shielding materials. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2008. 266(1): p. 140-146.
[7]
Gough, J., et al., Synthesis, degradation, and in vitro cell responses of sodium phosphate glasses for craniofacial bone repair. Journal of Biomedical Materials Research Part A, 2002. 59(3): p. 481-489.
[8]
Donald, I., B. Metcalfe, and R.J. Taylor, The immobilization of high level radioactive wastes using ceramics and glasses. Journal of materials science, 1997. 32(22): p. 5851-5887.
[9]
Jiang, S., M. Myers, and N. Peyghambarian, Er3+ doped phosphate glasses and lasers. Journal of Non-Crystalline Solids, 1998. 239(1): p. 143-148.
[10]
Nishikawa, H., et al., Solderability of bulk metallic glasses using lead-free solders. Materials transactions, 2009. 50(6): p. 1326-1329.
[11]
Feltz, A., Ionic conducting glasses for batteries and wave-guide devices. Journal of Non-Crystalline Solids, 1987. 90(1-3): p. 545-555.
[12]
Raskin, D. and L.A. Davis, Materials: Metallic glasses: Researchers are grappling with novel production techniques that would revolutionize transformer manufacturing. IEEE Spectrum, 1981. 18(11): p. 28-33.
[13]
Inoue, A. and N. Nishiyama, New bulk metallic glasses for applications as magneticsensing, chemical, and structural materials. Mrs Bulletin, 2007. 32(8): p. 651-658.
[14]
Mansuripur, M., Effects of high-numerical-aperture focusing on the state of polarization in optical and magneto-optic data storage systems. Applied Optics, 1991. 30(22): p. 3154-3162.
[15]
Rao, K.J, Structural chemistry of glasses. 2002: Elsevier.
[16]
Stanworth, J.E., Physical properties of glass. Vol. 6. 1950: Clarendon Press.
24
[17]
Bansal, N.P. and R.H. Doremus, Handbook of glass properties. 2013: Elsevier.
[18]
Kaur, A., et al., Structural, optical, dielectric and thermal properties of molybdenum tellurite and borotellurite glasses. Journal of Non-Crystalline Solids, 2016. 444: p. 110.
[19]
El-Mallawany, R.A., Tellurite glasses handbook: physical properties and data. 2011: CRC press.
[20]
Gupta, N., et al., Structure-property correlations in TiO2-Bi2O3-B2O3-TeO2 glasses. Journal of Non-Crystalline Solids, 2017.
[21]
Murugan, G.S. and Y. Ohishi, TeO2–BaO–SrO–Nb2O5 glasses: a new glass system for waveguide devices applications. Journal of Non-Crystalline Solids, 2004. 341(1): p. 86-92.
[22]
Rolli, R., et al., Optical spectroscopy of lanthanide ions in ZnO–TeO2 glasses. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2001. 57(10): p. 2009-2017.
[23]
Gulenko, A., et al., Atomistic simulations of TeO2-based glasses: interatomic potentials and molecular dynamics. Physical Chemistry Chemical Physics, 2014. 16(27): p. 14150-14160.
[24]
McLaughlin, J., et al., The structure of tellurite glass: a combined NMR, neutron diffraction, and X-ray diffraction study. Journal of Non-Crystalline Solids, 2000. 274(1): p. 1-8.
[25]
McLaughlin, J.C., S. Tagg, and J. Zwanziger, The structure of alkali tellurite glasses. The Journal of Physical Chemistry B, 2001. 105(1): p. 67-75.
[26]
Jha, A., et al., Rare-earth ion doped TeO2 and GeO2 glasses as laser materials. Progress in Materials Science, 2012. 57(8): p. 1426-1491.
[27]
Murugan, G.S. and Y. Ohishi, Structural and physical properties of a novel TeO2– BaO–SrO–Ta2O5 glass system for photonic device applications. Journal of NonCrystalline Solids, 2005. 351(5): p. 364-371.
25
[28]
Ghosh, A., Electrical properties of semiconducting amorphous copper-tellurite glasses. Journal of Physics: Condensed Matter, 1989. 1(42): p. 7819.
[29]
Kaur, A., et al., Preparation and characterization of lead and zinc tellurite glasses. Journal of Non-Crystalline Solids, 2010. 356(18): p. 864-872.
[30]
Mishra, R., et al., The standard molar enthalpies of formation of SrTeO3and SrTe2O5. The Journal of Chemical Thermodynamics, 1998. 30(7): p. 879-883.
[31]
Gupta, N. and A. Khanna, Glass and anti-glass phase co-existence and structural transitions in bismuth tellurite and bismuth niobium tellurite systems. Journal of NonCrystalline Solids, 2017.
[32]
Trömel, M., et al., Formation and luminescence of lower symmetrical tellurite antiglass phases. Journal of Solid State Chemistry, 1988. 76(2): p. 345-354.
[33]
Trömel, M., W. Hützler, and E. Münch, Anti-glass phases and other lanthanide tellurates with fluorite-related structures. Journal of the Less Common Metals, 1985. 110(1-2): p. 421-424.
[34]
Bertrand, A., et al., New Transparent Glass-Ceramics Based on the Crystallization of ―Anti-glass‖ Spherulites in the Bi2O3–Nb2O5–TeO2 System. Crystal Growth & Design, 2015. 15(10): p. 5086-5096.
[35]
Blasse, G., et al., Luminescence of tellurite anti-glass phases. Journal of Solid State Chemistry, 1986. 63(2): p. 148-153.
[36]
Masson, O., et al., On the structure of the disordered Bi2Te4O11 phase. Journal of Solid State Chemistry, 2004. 177(6): p. 2168-2176.
[37]
Burckhardt, H. and M. Tromel, Strontium Undecaoxotellurate, SrTe5O11, A CaF2 Defect Structure Its Relationship To The Structure Of Simple Glasses. Acta Crystallographica Section C-Crystal Structure Communications, 1983. 39(OCT): p. 1322-1323.
[38]
Rodriguez, J., et al., μSR study of the ―anti-glass‖ LiHo0. Condensed Matter, 2006. 374: p. 13-16.
26
045Y0. 955F4.
Physica B:
[39]
Becker, P., Borate materials in nonlinear optics. Advanced Materials, 1998. 10(13): p. 979-992.
[40]
Kaur, A., et al., B-O and Te-O speciation in bismuth tellurite and bismuth borotellurite glasses by FTIR,
11
B MAS-NMR and Raman spectroscopy. Journal of
Non-Crystalline Solids, 2017. 470: p. 19-26. [41]
Maheshvaran, K., P. Veeran, and K. Marimuthu, Structural and optical studies on Eu3+ doped boro-tellurite glasses. Solid State Sciences, 2013. 17: p. 54-62.
[42]
Murugan, G.S., T. Suzuki, and Y. Ohishi, Raman characteristics and nonlinear optical properties of tellurite and phosphotellurite glasses containing heavy metal oxides with ultrabroad Raman bands. Journal of applied physics, 2006. 100(2): p. 023107.
[43]
Rolli, R., et al., Erbium-doped tellurite glasses with high quantum efficiency and broadband stimulated emission cross section at 1.5 μm. Optical materials, 2003. 21(4): p. 743-748.
[44]
Kaur, A., A. Khanna, and L.I. Aleksandrov, Structural, thermal, optical and photoluminescent properties of barium tellurite glasses doped with rare-earth ions. Journal of Non-Crystalline Solids, 2017. 476: p. 67-74.
[45]
Mori, A., Y. Ohishi, and S. Sudo, Erbium-doped tellurite glass fibre laser and amplifier. Electronics Letters, 1997. 33(10): p. 863-864.
[46]
Wang, J., E. Vogel, and E. Snitzer, Tellurite glass: a new candidate for fiber devices. Optical materials, 1994. 3(3): p. 187-203.
[47]
Balda, R., et al., Time-resolved fluorescence-line narrowing and energy-transfer studies in a Eu 3+-doped fluorophosphate glass. Physical Review B, 1996. 54(17): p. 12076.
[48]
Jerrard, H., Laser spectroscopy of solids: Edited by WM Yen, PM Selzer Springer, topics in applied physics volume 49, 1981, pp x+ 310, $53.40. Optics & Laser Technology, 1982. 14(3): p. 162.
27
[49]
Reisfeld, R., E. Zigansky, and M. Gaft, Europium probe for estimation of site symmetry in glass films, glasses and crystals. Molecular Physics, 2004. 102(11-12): p. 1319-1330.
[50]
Capobianco, J., et al., Absorption and emission spectroscopy of Eu3+ in metaphosphate glasses. Physical Review B, 1990. 42(10): p. 5936.
[51]
Kumar, A., D. Rai, and S. Rai, Optical studies of Eu3+ ions doped in tellurite glass. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2002. 58(10): p. 2115-2125.
[52]
Khanna, A. and P. Dutta, Tunable color temperature solid state white light source using flux grown phosphor crystals of Eu3+, Dy3+ and Tb3+ activated calcium sodium molybdenum oxide. Optical materials, 2014. 37: p. 646-655.
[53]
Brecher, C. and L. Riseberg, Laser-induced fluorescence line narrowing in Eu glass: A spectroscopic analysis of coordination structure. Physical Review B, 1976. 13(1): p. 81.
[54]
Schimitschek,
E.J.,
Stimulated
emission
in
rare‐earth
chelate
(europium
benzoylacetonate) in a capillary tube. Applied Physics Letters, 1963. 3(7): p. 117-118. [55]
Klaassen, D., R. Van Ham, and T. Van Rijn, Energy transfer processes in yttrium oxide activated with europium. Journal of Luminescence, 1989. 43(5): p. 261-274.
[56]
Stevels, A., Recent developments in the application of phosphors. Journal of Luminescence, 1976. 12: p. 97-107.
[57]
Tagiara, N., et al., Synthesis, thermal and structural properties of pure TeO2 glass and zinc-tellurite glasses. Journal of Non-Crystalline Solids, 2017. 457: p. 116-125.
[58]
Dean, J., Lange's Handbook of Chemistry, McGraw-Hill Book Company. New York Google Scholar, 1985.
[59]
Winterstein-Beckmann, A., et al., Structure and properties of orthoborate glasses in the Eu2O3–(Sr, Eu) O–B2O3 quaternary. The Journal of Physical Chemistry B, 2015. 119(7): p. 3259-3272.
28
[60]
Babu, S.S., et al., Thermal, structural and optical properties of Eu3+-doped zinctellurite glasses. Journal of Physics D: Applied Physics, 2007. 40(18): p. 5767.
[61]
Anderson, A.C., et al., Amorphous solids: low-temperature properties. Vol. 24. 2012: Springer Science & Business Media.
[62]
Belançon, M.P. and G. Simon, Low frequency Raman study of the Boson peak in a Tellurite-tungstate glass over temperature. Journal of Non-Crystalline Solids, 2018. 481: p. 295-298.
[63]
Tikhomirov, V., et al., An interpretation of the Boson peak in rare-earth ion doped glasses. Journal of Non-Crystalline Solids, 1999. 256: p. 89-94.
[64]
Kaur, N. and A. Khanna, Structural characterization of borotellurite and aluminoborotellurite glasses. Journal of Non-Crystalline Solids, 2014. 404: p. 116-123.
29
