Short-Range Structure, Thermal and Elastic Properties of Binary and

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Short-Range Structure, Thermal and Elastic Properties of Binary and Ternary Tellurite Glasses Nagia S. Tagiara, Elham Moayedi, Apostolos Kyritsis, Lothar Wondraczek, and Efstratios I. Kamitsos J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.9b04617 • Publication Date (Web): 21 Aug 2019 Downloaded from pubs.acs.org on August 23, 2019

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The Journal of Physical Chemistry

Short-Range Structure, Thermal and Elastic Properties of Binary and Ternary Tellurite Glasses

N.S. Tagiara1, E. Moayedi2, A. Kyritsis3, L. Wondraczek2, and E.I. Kamitsos1,*

1Theoretical

and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou Ave., 116 35 Athens, Greece.

2Otto

Schott Institute of Materials Research, Friedrich-Schiller-Universität Jena, Fraunhoferstrasse 6, 07743 Jena, Germany.

3National

Technical University of Athens, Zografou Campus, 15780, Athens, Greece.

*Corresponding

author. E-mail address: [email protected] (E.I. Kamitsos). 1 ACS Paragon Plus Environment

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Abstract: Glasses Al2/3O-TeO2, ZnO-TeO2 and R2/3O-ZnO-TeO2 (R=Al, B) were prepared by melting in Pt crucibles and studied for correlations between structure and thermal as well as mechanical properties, whereby the glass composition is varied to tailor the short-range speciation of tellurite, aluminate and borate groups. The glass structure was studied by Raman and infrared spectroscopy and the measured properties include glass transition temperature (Tg), density (ρ) and ultrasonic longitudinal (VL) and transverse (VT) velocities. In addition, atomic packing density (Cg), elastic moduli and Poisson’s ratio (σ) were evaluated from the measured properties. It was found that Al2/3O leads to cross-linked alumino-tellurite networks by strong Te-O-Al bonds, which cause a profound enhancement in Tg. The influence of ZnO and B2/3O on Tg is relatively smaller, due to the weaker cross-linking effects of ZnO4 tetrahedra and of Te…O-B bonds. Short-range bonding characteristics, interatomic bonding energy differences and atomic packing density were found to have a strong effect on VT and mostly on the VL sound velocity. The combined effects of structure and bonding are nicely expressed in the composition dependence of Poisson’s ratio; it exhibits decreasing trends with Al2/3O content in the binary and ternary glasses studied here, but increasing trends with ZnO and B2/3O additions in glasses ZnO-TeO2 and B2/3O-ZnO-TeO2, respectively. The results for Poisson’s ratio and atomic packing density for the studied glasses were found to fit nicely in the global σ versus Cg correlation established previously for a range of glasses not including tellurites so far. Finally, the sound velocities and Poisson’s ratio of pure TeO2 glass were determined for the first time and found to differ markedly from those in the literature for TeO2 glass melted in alumina crucible; this is because the latter glass is highly doped by Al2O3 leached from the alumina crucible.

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1.

Ιntroduction Pure tellurium dioxide, TeO2, is known as a conditional glass-forming oxide.1,2 Thus,

the formation of tellurite glasses by conventional melt-quenching is usually achieved only through adding glass-former or glass-modifier oxides to TeO2.3-8 Such multicomponent tellurite glasses attract interest because of the easiness of glass formation, due to low melting temperatures, and their unusual physical and chemical properties which open new opportunities for applications in optical devices.9-16 In this context, a variety of TeO2-based glass systems have been developed over the years and the investigation of their structure and properties constitutes a subject of continuing interest.17-27 Despite its conditional glass-forming characteristics, TeO2 glass was prepared without additives by employing rapid melt-quenching techniques albeit in small quantities. The various forms include blobs formed by splat quenching,28 thin plates by dip-quenching the bottom of the crucible into a freezing mixture at -10 oC,29 and thin flakes obtained by twin-roller quenching.30 Recently, Tagiara et al. 31 showed that pure TeO2 glass can be synthesized in bulk form using an intermittent quenching technique, which involves rapid and repeated immersion of the bottom of the Pt crucible in water at room temperature. The density of such pure TeO2 glass (ρ=5.62 g/cm3) was found considerably higher than that of TeO2 glass melted in alumina crucible (ρ=4.86 g/cm3). However, the glass transition temperature (Tg) was found to exhibit the opposite behavior; the onset-Tg was measured at 303 and 380 oC for TeO2 glass melted in Pt and alumina crucibles, respectively. In the same study,31 binary zinc-tellurite glasses xZnO-(1-x)TeO2 were investigated in a broad composition range, 0≤x≤0.50; such glasses are suitable host matrices for new materials for applications like second harmonic generation after electro-thermal poling,12,13 ultra-low loss optical fibers 14 and glass fiber amplification.15 In line with the results for TeO2 glass, zinctellurite glasses synthesized by melting in Pt crucible were found to exhibit very different 3 ACS Paragon Plus Environment


density and glass transition temperature compared to glasses melted in alumina crucibles; the latter method is used broadly in the literature for the synthesis of tellurite glasses. To reveal the role of Al2O3 in tellurite glasses in terms of structure and properties, we have extended our previous work by developing binary alumino-tellurite glasses in a broad composition range (up to 20 mol% Al2O3) after melting in Pt crucibles. The glass notation yAl2/3O-(1-y)TeO2 (0≤y≤0.43) is used here to allow for the comparison of properties and structure with those of zinc-tellurite glasses having the same oxygen content (x=y), considering that the amount of oxygen added to TeO2 is the primary cause of glass modification. Besides the binary glasses, we have melted in Pt crucibles and quenched glasses in the ternary system z1Al2/3O-(0.30-z1)ZnO-0.70TeO2 (0≤z1≤0.30) where ZnO is replaced gradually by Al2/3O. As B2O3 is an excellent glass former, ternary glasses z2B2/3O-(0.30-z2)ZnO-0.70TeO2 (0≤z2≤0.30) were also developed and studied in comparison to the alumino-zinc-tellurite glasses. In the present work, we report density, glass transition temperature and elastic properties of the synthesized binary and ternary TeO2-based glasses. Velocities of sound, and from these, elastic moduli, are essential properties for characterizing glasses since they are related to the details of atomic bonding and structure. The composition dependence of the measured properties is considered here in terms of the evolution of glass structure as investigated by Raman and infrared spectroscopy. It was found that Al2O3 and B2O3 play different roles when substituting for ZnO in terms of transition temperature, structure, and Poisson’s ratio. The present results on tellurite glasses enrich the global correlation established by T. Rouxel32 between Poisson’s ratio and atomic packing density for a range of glassy materials, which now include tellurite glasses.

2.

Experimental

2.1

Sample preparation


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The glasses investigated here were prepared from high-purity starting materials; polycrystalline TeO2 (Alfa Aesar, 99.99%), ZnO (Alfa Aesar, 99.9%), Al2O3 (Alfa Aesar, 99.99%), and B2O3 (Sigma-Aldrich, 99.98%). Thirteen glass compositions (Table 1) were synthesized in the system yAl2/3O-(1-y)TeO2 where y=0.43 was determined as the upper glassformation limit (0≤y≤0.43), and six compositions in each of the glass systems z1Al2/3O-(0.30z1)ZnO-0.70TeO2 and z2B2/3O-(0.30-z2)ZnO-0.70TeO2 (0≤z1,z2≤0.30, Table 1). Appropriate amounts of the starting materials were thoroughly mixed to form batches of about 8 g, and then transferred into Pt crucibles where they were melted in an electric furnace for about 30 min in the temperature range 800-900 oC, depending on composition. The melts were stirred several times during heating, and then glasses in the binary and ternary systems were obtained by quenching the melt between preheated copper molds and steel plates, to avoid cracking of the glass specimen. This technique gives transparent glass discs of 15 mm in diameter and 2-3 mm in thickness. Pure TeO2 glass was prepared by melting in Pt crucible and employing the intermittent quenching technique described in ref. 31. All glasses were annealed for 24 h at about 40 oC below Tg to remove residual stresses. After polishing, glasses were kept under dry atmosphere. Volatilization losses during melting were checked systematically by weighing each prepared glass. Comparison with the initial batches showed that the glass compositions correspond to the nominal ones within ±0.5-1 mol%. Therefore, we use in the following the nominal glass compositions given in Table 1.

2.2

Density measurements The density of glasses was measured by the Archimedes principle, using the density

determination kit for solids of a Mettler high-accuracy balance of sensitivity 10-5 g, and employing distilled water as the immersion liquid.31 For each glass composition, density was



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measured on at least three different samples resulting in an error of about ±0.04 g/cm3. Density values are reported in Table1 for the studied glasses.

2.3

Differential scanning calorimetry The

glass

transition

temperature

was

measured

by

differential

scanning

calorimetry (DSC) under nitrogen flow on a TA Q20 instrument, calibrated with indium (for temperature and enthalpy) and sapphire (for heat capacity) and operating from room temperature to 700 oC. Powders of glasses were placed in aluminum pans and heated at a rate 10o C/min. The value of the transition temperature was determined as the onset or the midpoint of the heat capacity step during glass transition, and Tg values are reported in Table1.

2.4

Vibrational spectroscopy Raman spectra were recorded at the backscattering geometry on a Renishaw inVia

Raman Microscope equipped with a 2400 lines/mm diffraction grating, a high-sensitivity Peltier-cooled charge coupled device (CCD), a motorized xyz microscope stage and an x50 magnification lens. All spectra were measured at room temperature with 2 cm-1 resolution. The 514.5 nm line of an Ar ion laser was used for excitation, and care was taken to avoid sample damage by employing low levels of laser power (about 0.10 mW/μm2 at the sample). The measured Raman intensity, Ι(ω), was reduced as described in ref. 31. The Raman spectra presented in this work are in the form of reduced isotropic spectra, 𝐼𝑟𝑒𝑑,𝑖𝑠𝑜(), calculated from the expression: 4

(1)

𝐼𝑟𝑒𝑑,𝑖𝑠𝑜(𝜈) = 𝐼𝑉𝑉,𝑟𝑒𝑑(𝜈) ― 3 𝐼𝑉𝐻,𝑟𝑒𝑑(𝜈)

In the above expression, 𝐼𝑉𝑉,𝑟𝑒𝑑(𝜈) and 𝐼𝑉𝐻,𝑟𝑒𝑑(𝜈) are the reduced polarized Raman spectra measured under VV and VH polarizations, where the first letter indicates the polarization of


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the exciting laser beam and the second letter the polarization of the scattered light. The reduced isotropic spectrum, 𝐼𝑟𝑒𝑑,𝑖𝑠𝑜, gives the symmetric vibrational modes of the scattering structural units as it involves only the diagonal elements of the Raman tensor; the reduced anisotropic spectrum, 𝐼𝑟𝑒𝑑,𝑎𝑛𝑖𝑠𝑜(𝜈) = 𝐼𝑉𝐻,𝑟𝑒𝑑(𝜈), gives only the asymmetric vibrational modes because of the contributing off-diagonal elements of the Raman tensor.33 The infrared (IR) spectra were measured on a vacuum Fourier transformation spectrometer (Bruker, Vertex 80v), in quasi-specular reflectance mode (11o off-normal). For each spectrum 400-2000 scan were collected at room temperature with 4 cm-1 resolution and averaged for evaluation. Reflectance spectra were measured separately in the far- and mid-IR ranges and then merged to form a continuous spectrum in the range 30-7000 cm-1. Analysis of the reflectance spectra by Kramers-Krönig transformation34 yielded the absorption coefficient spectra, α(ν), presented in this work.

2.5

Ultrasonic measurements The elastic properties of glasses were investigated by ultrasonic echography using an

Echometer 1077 (Karl Deutsch GmbH & Co KG). The travel times of the longitudinal, tL, and transverse, tT, sound wave echoes were measured with an accuracy of ±1ns using a piezoelectric transducer with a central frequency ranging from 8 to 12 MHz. All measurements were performed on co-planar, optically polished glass discs of 15 mm in diameter and 1-2 mm in thickness, d, after polishing. The thickness of the samples was measured with a micrometer screw with an accuracy of ± 2µm. The longitudinal and transverse wave velocities were derived from the sample thickness and the sound travel times, VL=2d/tL and VT=2d/tT. The elastic properties were calculated using the following expressions,35 where L, G, K, and E are the longitudinal, shear, bulk and Young’s modulus, respectively, and σ is the Poisson’s ratio:



𝐿 = 𝜌 𝑉2𝐿

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(2)

𝐺 = 𝜌 𝑉2𝑇

(3)

(

)

4

𝐾 = 𝜌 𝑉2𝐿 ― 3𝑉2𝑇 = 𝐿 ― 𝐸=𝜌

[(

3𝑉2𝐿 ― 4𝑉2𝑇 𝑉𝐿 𝑉𝑇

𝑉2𝐿 ― 2𝑉2𝑇

)

2

]

( )𝐺 4

(4)

3

3L ― 4G

=𝐺

(5)

L―G

―1

𝐿 ― 2𝐺

(6)

σ = 2(𝑉2 ― 𝑉2) = 2(𝐿 ― 𝐺) 𝐿 𝑇 In the above expressions, ρ is the glass density.

3.

Results and discussion

3.1

Glass density The density values of glasses yAl2/3O-(1-y)TeO2 of this work are listed in Table1 and

plotted in Figure 1a as a function of the Al2/3O content. Comparison of the present results with values reported earlier36 for glasses in the range 6.7-16 mol% Al2O3 (18-36 mol% Al2/3O) shows reasonable agreement, with both sets of data exhibiting a decreasing trend of density with increasing Al2/3O content. The density of ZnO-TeO2 glasses decreases also with addition of ZnO to TeO2 (Figure 1a), although the rate of density decrease in this system is smaller compared to the Al2/3O-TeO2 system. These trends of density are well-consistent with differences in molar weight (MW) of the constituent oxides (MW: TeO2=159.60 g/mol, ZnO=81.38 g/mol, Al2/3O = 33.99 g/mol). Ternary glasses of composition z1Al2/3O-(0.30-z1)ZnO-0.70TeO2 and z2B2/3O-(0.30z2)ZnO-0.70TeO2 were also found to exhibit a decrease of density as ZnO is substituted by Al2/3O or B2/3O as shown in Table1 and Figure 1b. These trends are due to the lower molecular weight of B2/3O (23.21 g/mol) and Al2/3O compared to that of the substituted ZnO. It is noted that the density of boro-tellurite glasses is comparable to that of the alumino-tellurites for B2/3O 8 ACS Paragon Plus Environment

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contents below 15 mol%. However, this trend is reversed above this composition. This result may be related to structural differences which are manifested in atomic packing density variations. These aspects will be explored in the following sections.

Figure 1. Composition dependence of density for (a) binary glasses yAl2/3O-(1-y)TeO2 (0≤y≤0.43) and xZnO-(1-x)TeO2 (0≤x≤0.45),31 and (b) ternary glasses z1Al2/3O-(0.3-z1)ZnO0.7TeO2 and z2B2/3O-(0.3-z2)ZnO-0.7TeO2 (0≤z1,z2≤0.3). Lines are drawn to guide the eyes.

3.2

Glass transition temperature Typical DSC thermograms are illustrated in Figure S1 (Supporting Information) for

alumino-tellurite glass compositions covering the entire glass-forming range. The glass transition endotherm moves progressively to higher temperatures, with Tg changing from 303 oC

to 382 oC (onset values) as the Al2/3O content increases from y=0 to y=0.43. The Tg values 9 ACS Paragon Plus Environment


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were determined as shown in the inset of Figure S1 for y=0.43 and the onset values are listed in Table1. The present results are in very good agreement with those of our previous study for glasses of low Al2/3O contents, y≤0.03,31 and the results of Kaur et al.37 for glasses of higher Al2/3O contents. We note though that the latter study reports two glass transition events for the glass with 20 mol% Al2O3 (y=0.43) with Tg’s at 392 and 440 oC (midpoint values); this was attributed to phase separation for this glass.37 Consideration of the DSC thermogram of the y=0.43 glass prepared in this work (inset of Figure S1) shows one glass transition event with Tg=382 oC (onset value) or Tg=386 oC (midpoint value). This difference between the two studies may result from quenching rate variations and possible composition differences between the two studies. The evolution of glass transition temperature with composition, Figure 2a, demonstrates a linear correlation between the Tg and Al2/3O content in the entire glassformation range; thus extending our previous findings for y≤0.03.31 The linear fit of the data for the Al2/3O-TeO2 glasses gives:

Tg (oC) = 302.8 (±1.0) + 193.8 (±5.0) y

(7)

with R2= 0.997. The effectiveness of Al2/3O addition in increasing the Tg as expressed by eq.7 can be understood in terms of the strength of the Te-O and Al-O bonds which contribute to the formation of the alumino-tellurite network. The bond strength energies EM-O of the relevant bonds are ETe-O=390.8±8.4 kJ/mol and EAl-O=512.1±4.2 kJ/mol,38 and suggest that addition of Al2/3O should strengthen continuously the glass network as the Al-O bonds are stronger than the replaced Te-O bonds. In Figure 2a are also shown the Tg values of the zinc-tellurite glasses for comparison with the aluminum-tellurite system. It is found that Tg of the former glasses increases with a


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lower rate than in the aluminum-tellurites, and this should be due to the lower bond strength of the Zn-O bond compared to Al-O, EZn-O=284.1 kJ/mol.38 We note also that the Tg of zinctellurite glasses with ZnO content below x=0.1 is practically constant (303 oC, Table 1). This may reflect the mixed role of ZnO, i.e. as network-modifier causing a decrease in Tg like the alkali metal oxides,39 and as network-forming oxide leading to cross-linking of the tellurite network by ZnO4 tetrahedra40 and, thus, to an increase in Tg. The net effect of these two mechanisms is a composition independent Tg below about 10 mol% ZnO. The network-forming role of ZnO prevails above 10 mol% and leads to a steady increase in Tg.

Figure 2. Composition dependence of the onset-glass transition temperature (Tg) for glasses (a) yAl2/3O-(1-y)TeO2 and xZnO-(1-x)TeO2, and (b) z1Al2/3O-(0.3-z1)ZnO-0.7TeO2 and z2B2/3O-(0.3-z2)ZnO-0.7TeO2. The line for glasses yAl2/3O-(1-y)TeO2 is a linear fit to the data; the other lines are drawn to guide the eyes. Error bars in (a) are of the size of symbols.



The effect of replacing ZnO by Al2/3O or B2/3O on the glass transition is shown in Figure 2b, and the corresponding Tg values are given in Table1 for glasses z1Al2/3O-(0.30-z1)ZnO0.70TeO2 and z2B2/3O-(0.30-z2)ZnO-0.70TeO2. The increase of Tg as Al2/3O is introduced in the ternary glass is spectacular, as for the binary glasses considered above, and reflects the difference in bond strength values between the Al-O and Zn-O bonds. However, this is not the case for glasses B2/3O-ZnO-TeO2 where a mild increase of Tg is observed in Figure 2b despite the fact that the addition of B2/3O introduces B-O bonds which are the strongest bonds in the system, EB-O=806.3 ±5.0 kJ/mol.38 The question now is whether glass structure can provide an explanation for the big difference in Tg trends between the two ternary glass systems. To this objective, the vibrational spectra will be considered in the following.

3.3.

Short-range structure by Raman and infrared spectroscopy

3.3.1

Al2/3O-TeO2 and Al2/3O-ZnO-TeO2 glasses The reduced isotropic Raman spectra of glasses yAl2/3O-(1-y)TeO2 are shown in Figure

3 for Al2/3O contents spanning the range 0≤y≤0.43. Starting with the spectrum of TeO2 glass we note the presence of a strong peak at 660 cm-1, a broad envelope in the range 700-850 cm-1 and lower-frequency features at about 440 and 490 cm-1. As discussed earlier,31 the 660 cm-1 band is characteristic of stretching vibration localized on TeO4 trigonal bipyramids (tbp’s), the 700-850 cm-1 envelope involves stretching localized mainly on Te-O-Te bridges connecting tbp’s with some contribution from TeO4 tbp’s, while the bending vibrations of O-Te-O and TeO-Te are active at about 440 and 490 cm-1, respectively.41-43 Addition of Al2/3O to TeO2 leads to the decrease in intensity of the 660 cm-1 band and its gradual upshift to 684 cm-1 (y=0.43), suggesting the progressive destruction of TeO4 tbp’s as Al2O3 interacts with the tellurite network. At the same time the high-frequency envelope gains intensity, peaking first at ca. 740 cm-1 for y=0.10, 762 cm-1 for y=0.30 and at 780 cm-1 for y=0.43, while a new feature grows in


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as a shoulder at ca. 850 cm-1. Also, the low frequency profile becomes more symmetric upon increasing Al2/3O content and peaks at 460 cm-1 for y=0.43.

Figure 3. Reduced isotropic Raman spectra of glasses yAl2/3O-(1-y)TeO2 for y=0 to 0.43.

Based on the assignments for ZnO-TeO2 glasses,31 we associate the new 740 cm-1 feature in Figure 3 with the formation of TeO3+1 polyhedra. The terminology TeO3+1 indicates a TeO3 trigonal pyramid (tp) where tellurium forms three bonds with one bridging and two terminal oxygen atoms and a fourth-weak bond with an oxygen at a distance 2.2-2.5 Å.5 At higher Al2/3O contents, the weak bond in TeO3+1 polyhedra breaks up and leads to formation of isolated TeO3 tp’s where all oxygen atoms are terminal (TeO32-), as suggested by the appearance of the 762-780 cm-1 band.31 Therefore, the evolution of the Raman spectra with 13 ACS Paragon Plus Environment


increasing amount of Al2/3O is consistent with a progressive modification of the tellurite network, which involves first the conversion of TeO4 tbp’s with bridging oxygens to TeO3+1 polyhedra with two terminal oxygens and then to TeO32- tp’s with three terminal oxygen atoms, in general agreement with earlier studies.31,37 Comparison of the Raman spectra of Al2/3O-TeO2 glasses with those of ZnO-TeO2 glasses having the same oxygen content31 shows that the tellurite network in the former glasses has lower degree of modification. For example, at 25 mol% Al2/3O the 660 cm-1 band (TeO4) is the strongest Raman band, while at 25 mol% ZnO the 740 cm-1 band (TeO3+1) is the strongest Raman feature. This comparison suggests that part of the oxygen amount introduced by Al2/3O does not modify TeO2; instead, it is retained by aluminum to form strong Al-O bonds, which built up AlOn polyhedra. This hypothesis is supported by the shoulder developing at about 850 cm-1, and the fact that TeO32- tp’s give practically no Raman scattering at this frequency. 31 On these grounds, the 850 cm-1 feature can be attributed to Al-O stretching, ν(Al-O), in AlO4 tetrahedral units. Indeed, Tarte44 reported strong infrared activity in the range 700-900 cm-1 for ν(Al-O) in bonded (“condensed”) AlO4 tetrahedra in inorganic aluminates, and McMillan et al.45 measured Raman activity at about 800 cm-1 for AlO4 tetrahedra in alkali-silicate glasses. Bonding of AlO4 tetrahedra with each other through Al-O-Al bridges should be rather excluded for the tellurite glasses of this study, since the bending vibration of Al-O-Al bridges would be in the range 520-560 cm-1, 45 where there is limited or no activity in the Raman spectra of Figure 3. In turn, Al-(O-Te)4 linkages should develop and cross-link the tellurite units through Al-OTe bonds introduced into the glass. The frequency of the bending vibration of Al-O-Te bridges is expected to be close to that of Te-O-Te bridges, considering the similarity in bond strengths of the Al-O and Te-O bonds. Therefore, the Raman band developing at 460 cm-1 upon increasing the Al2/3O content can be associated with the formation of mixed Al-O-Te bridges which substitute for Te-O-Te linkages. Besides the Al-O-Te bridges, AlO6 octahedra would


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also contribute to the Raman band at 460 cm-1 since the Al-O stretching vibration in octahedral aluminate units is active in the 400-530 cm-1 range.45,46 The development of the 460 cm-1 band in Al2/3O-TeO2 glasses is in contrast with the progressive decrease of Raman activity in this spectral region in ZnO-TeO2 glasses as the Te-O-Te bridges break up at increasing ZnO contents.31 These structural differences between the two glass systems are reflected on differences in Tg as discussed above (see Figure 2a). The coordination of Al in Al2/3O-TeO2 glasses was studied earlier by 27Al MAS-NMR spectroscopy by Kaur et al.37 and Sakida et al.47 They showed that AlO6 octahedra are the main species at low Al2/3O contents, which are replaced by AlO4 tetrahedra at higher Al2/3O contents and minor amounts of AlO5 species at each glass composition. Youngman and Aitken48 applied also

27Al

MAS-NMR spectroscopy to glasses in the ternary system Al2O3-Ta2O5-TeO2 and

found 4-, 5- and 6-coordinated Al in all glasses. Al is primarily in octahedral sites in TeO2-rich glasses and tetrahedral sites are preferred in glasses with smaller amounts of TeO2, while the fraction of 5-coordinated Al remains constant in all AlTa-tellurite glasses. In addition, the aluminum speciation was correlated with the glass transition temperature which was found to increase monotonically with increasing percentage of the network-forming AlO4 tetrahedra, i.e. upon decreasing TeO2 content, while keeping the Ta:Al ratio fixed at Ta:Al=1.48 We note at this point that the composition dependence of Tg found in Fig. 2 for the binary and ternary tellurite glasses containing Al2/3O is in line with the results of ref. 48. Our Raman findings are in general agreement with the above NMR results considering the bands developing at about 850 cm-1 and 460 cm-1 in Fig. 3, which were associated with AlO4 and AlO6 units, respectively. The formation of AlO5 species in these tellurite glasses is difficult to assess from the Raman spectra alone, because Al-O stretching in AlO5 units is active in the 600-650 cm-1 range46 and thus it would be overmasked by the 660 cm-1 band of TeO4 trigonal bipyramids.



Figure 4. Reduced Raman spectra of glasses z1Al2/3O-(0.3-z1)ZnO-0.7TeO2 for z1=0 to 0.3.

The Raman spectra of glasses z1Al2/3O-(0.3-z1)ZnO-0.7TeO2 in Figure 4 show a smooth evolution between the spectra of the two end-member glasses z1=0 (0.3ZnO-0.7TeO2) and z1=0.3 (0.3Al2/3O-0.7TeO2). In particular, the 745 cm-1 band (TeO3+1) loses intensity in favor of the 670 cm-1 band (TeO4) and shifts to about 762 cm-1 (TeO32-). These trends show again a lower average modification of the tellurite network as ZnO is replaced by Al2/3O, and this should result from the creation of aluminate AlOn units, including AlO4 tetrahedra (shoulder at ca. 850 cm-1) and AlO6 octahedra (band at 460 cm-1, with contribution from Al-O-Te bridges). These differences between ZnO and Al2/3O in terms of glass structure and strengths of Zn-O and Al-O bonds result in the clear Tg trend observed in Figure 2b for Al2/3O-ZnO-TeO2 glasses. 3.3.2

B2/3O-ZnO-TeO2 glasses 16 ACS Paragon Plus Environment

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The evolution of the Raman spectra of glasses z2B2/3O-(0.3-z2)ZnO-0.7TeO2 is shown in Figure 5. The main effects of replacing ZnO by B2/3O are exhibited by the 745 and 660 cm1

bands, the first decreasing in intensity in favor of the second band. Also, the relative intensity

of the 440 and 490 cm-1 bands changes towards that for TeO2 glass as the composition approaches the end-member glass 0.3B2/3O-0.7TeO2 (z2=0.3). In fact, the Raman spectrum of the latter glass appears to be the superposition of the spectra of TeO2 glass (inset of Figure 5) and of a new envelope at about 880 cm-1, indicating the re-establishment of a TeO2 glass-like tellurite structure involving TeO4 tbp’s as the main network building units. It is noted also that there is no obvious evidence for Raman activity of borate units in these tellurite glasses even at the highest B2/3O content (z2=0.3). For example, a possible arrangement of [BO3/2]0 triangles in boroxol rings as in pure B2O3 glass would be signaled by a sharp Raman band at 805 cm-1 due to the ring breathing mode49 and three-member borate rings with one or two [BO4/2]- tetrahedral units would give relatively sharp Raman bands at about 780 and 770 cm-1, respectively, due to the analogous breathing modes50 (O1/2 indicates a bridging oxygen atom). Such an apparent absence of borate-related bands from the Raman spectra of Figure 5 could originate from the large difference in polarizability, , between the Te4+ and B3+ ions, 𝛼𝑇𝑒4 + =1.595 Å3 and 𝛼𝐵3 + = 0.002 Å3,51 which makes the Te-O bond much more polarizable than the B-O bond and, thus, leads to large differences in the Raman cross section between tellurite and borate vibrational modes. In addition, borate ring structures may not exist in such glasses either due to the insufficient boron content or to mixing between B and Te centers.



Figure 5. Reduced isotropic Raman spectra of glasses in the system z2B2/3O-(0.3-z2)ZnO0.7TeO2 for z2=0 to 0.3. The inset shows the corresponding Raman spectrum of pure TeO2 glass prepared in Pt crucible.31

To explore the origin of the ca. 880 cm-1 envelope which grows in at high B2/3O contents, we note that spectral deconvolution shows that this envelop can be described by one band at 875 cm-1 (Figure 6a), which is in addition to the lower-frequency components observed for pure TeO2 glass (Figure 6b) and other tellurite glasses.25,52 Vibrations of tellurite species above 850 cm-1 are usually associated with the presence of trigonal pyramid units having one terminal double bond and two single bonds with bridging oxygen atoms, i.e. units of the form O=TeO2/2. The frequency of Te=O stretching, ν(Te=O), was predicted by ab initio calculations 18 ACS Paragon Plus Environment

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at 868 cm-1 for TeO2 glass42 and at 915 cm-1 for (TeO2)n polymers.53 On these grounds we assign the 875 cm-1 band for z2=0.3 to ν(Te=O) in O=TeO2/2 tp’s, which are chemically isomeric to the TeO4 tbp’s (TeO4/2 ⇌ O=TeO2/2). It is noted again that no evidence is available from Raman spectra for the boron coordination in these glasses.

Figure 6. Deconvoluted reduced isotropic Raman (a, b) and absorption coefficient (c, d) spectra of glasses 0.3B2/3O-0.7TeO2 (a, c) and TeO2 (b, d). Thick black lines are the experimental spectra, blue lines the simulated spectra and thin black lines the Gaussian peak components of the spectra.



An alternative method to probe the boron coordination is IR spectroscopy, where changes in the dipole moment of the vibrating structural units determine their IR activity. The infrared spectra of glasses B2/3O-ZnO-TeO2 are shown in Figure 7.

Figure 7. Absorption coefficient spectra of glasses z2B2/3O-(0.3-z2)ZnO-0.7TeO2 for z2=0 to 0.3. The inset shows the absorption coefficient spectrum of pure TeO2 glass.

While the isotropic Raman spectra are dominated by the symmetric stretching or bending vibrations of tellurite species, the corresponding asymmetric vibrational modes are strongly active in the infrared (Figure 7). Therefore, the band at 675 cm-1 for z2=0 can be related to TeO3+1 units in analogy to the Raman band at 745 cm-1. Increasing the B2/3O content leads to the downshift of the 675 cm-1 band to 650 cm-1 as in TeO2 glass (inset of Figure 7), while two new absorption profiles develop at 830-1160 cm-1 and at 1160-1500 cm-1. The 830-1160 20 ACS Paragon Plus Environment

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cm-1 envelop can be assigned to the asymmetric stretching vibrations of tetrahedral borate units, (BO4/2)-, and the one at 1160-1500 cm-1 to the corresponding vibration in trigonal borate species.54 For the compositions studied here, the trigonal borate units can be either neutral (BO3/2)0 and/or charged (BO2/2O)- units (O is a non-bridging oxygen atom). To study further the evolution of borate speciation we have deconvoluted the infrared spectra as reported earlier for borate glasses.54,55 Figure 6c shows the deconvoluted spectrum for z2=0.3 in comparison to that for pure TeO2 glass (Figure 6d). Two Gaussian component bands at 923 and 1113 cm-1 describe the IR envelop for (BO4/2)- tetrahedra, and three bands at 1252, 1346 and 1423 cm-1 are resolved in the region of borate triangles. As it was argued earlier, 55

the 1346 cm-1 band can be attributed to neutral (BO3/2)0 triangles and the 1252 and 1423 cm-1

components to charged (BO2/2O)- triangles, noting that the latter species are isomeric to (BO4/2)tetrahedra, because of the metaborate equilibrium (BO4/2)- ⇌ (BO2/2O)-. Figure 8 shows the composition dependence of the sum of relative integrated intensities of the 923 and 1113 cm-1 bands (A11+A12) for (BO4/2)-, the sum of the 1252 and 1423 cm-1 bands (A13+A15) for (BO2/2O)- and that of the 1346 cm-1 band (A14) for (BO3/2)0 species, noting that normalization was done with respect to the total integrated intensity in the 880-1500 cm-1 region. As observed in Figure 8, the relative population of (BO4/2)- tetrahedra decreases and that of (BO2/2O)triangles increases with B2/3O content, while (BO3/2)0 triangles show a mild increase. Considering that (BO4/2)- tetrahedra have a positive effect on Tg due to their cross-linking action on the glass network, as opposed to (BO2/2O)- triangles which cause the depolymerization of the network,56 the evolution of the borate speciation in Figure 8 and the progressive change of the tellurite structure towards that of TeO2-like glass (Figures 5 and 7) provide a structural basis for the composition dependence of Tg in Figure 2b. The increase of Tg at z2=0.05 relative to the z2=0 glass should be related to the presence of (BO4/2)- tetrahedra; however, the



subsequently increasing population of (BO2/2O)- units (Figure 8) combined with the reestablishment of a TeO2-like structure lead to an overall reduction in the rate of increasing Tg.

Figure 8. Evolution of the relative infrared intensity of bands at 923 and 1113 cm-1 (A11+A12) for (BO4/2)- tetrahedra, 1252 and 1423 cm-1 (A13+A15) for (BO2/2O)- triangles and 1346 cm-1 (A14) for (BO3/2)0 triangles, in the system z2B2/3O-(0.3-z2)ZnO-0.7TeO2 for z2=0 to 0.3 (for the deconvoluted infrared spectrum see Figure 6c). Atot is the total integrated intensity in the 8801500 cm-1 region. Lines are linear fits to the data.

The initial formation of the charged (BO4/2)- and (BO2/2O)- borate units for z2ZnO>B2O3), (ii) the bonding strength of the glass network (B2O3>Al2O3>ZnO), and (iii) the atomic packing density (B2O3>Al2O3>ZnO). The first factor affects directly the glass transition temperature, while the second and third ones affect mainly the elastic properties. As a result, BZT glasses show the steepest increase of Young’s modulus with glass transition temperature among the studied tellurite glasses.

Supporting Information Additional figures (Figures S1-S4) are included in the Supporting Information file.

Acknowledgements This work was supported by the project “Advanced Materials and Devices” (MIS 5002409) which is implemented under the “Action for the Strategic Development on the Research and Technological

Sector”,

funded

by

the

Operational


Program

“Competitiveness,

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Entrepreneurship and Innovation” (NSRF 2014-2020) and co-financed by Greece and the European Union (European Regional Development Fund).

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Table 1 Density (ρ), atomic packing density (Cg), glass transition temperature (Tg, onset), longitudinal velocity (VL), shear velocity (VT), Poisson’s ratio (σ), longitudinal modulus (L),Young’s modulus (E), bulk modulus (K), and shear modulus (G) for glasses xZnO-(1-x)TeO2 (0x0.5), yAl2/3O(1-y)TeO2 (0y0.43), z1Al2/3O-(0.3-z1)ZnO-0.7TeO2 (0z10.30), and z2B2/3O-(0.3-z2)ZnO-0.7TeO2 (0z20.30) prepared in platinum crucibles.

Glass x 0 0.03 0.04 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 y 0 0.03 0.05 0.06 0.07

ρ±0.04 (g/cm3)

Cg±0.001

Tg±1.0 (oC)

VL±50.0 (m/sec)

5.62 5.61 5.60 5.59 5.58 5.56 5.54 5.53 5.51 5.48 5.44 5.41

0.512 0.511 0.510 0.509 0.507 0.505 0.500 0.498 0.496 0.492 0.485 0.486

303 302 303 303 306 309 313 318 324 331 338 352

3228±15 3229 3241 3240 3257 3292 3341 3357 -

5.62 5.56 5.47 -

0.512 0.510 0.505 -

303 309 311 312 316

3228±15 -

VT±25.0 σ±0.015 (m/sec) xZnO-(1-x)TeO2 1863±10 0.250 1867 0.248 1873 0.249 1865 0.252 1862 0.257 1878 0.259 1878 0.269 1877 0.273 yAl2/3O-(1-y)TeO2 1863±10 47

ACS Paragon Plus Environment

L±2.0 (GPa)

E±0.4 (GPa)

K±2.0 (GPa)

G±0.5 (GPa)

58.6 58.3 58.6 58.4 58.8 59.9 61.6 61.8 -

48.8 48.7 48.9 48.4 48.3 49.1 49.3 49.1 -

32.6 32.3 35.5 32.6 33.2 33.9 35.7 36.0 -

19.5 19.5 19.6 19.3 19.2 19.5 19.4 19.3 -

58.6 -

48.8 -

32.6 -

19.5 -

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

0.08 0.10 0.15 0.20 0.25 0.30 0.35 0.43 z1 0 0.05 0.075 0.15 0.225 0.30 z2 0 0.05 0.075 0.15 0.225 0.30

5.36 5.33 5.26 5.15 5.09 5.02 4.91 4.87

0.498 0.497 0.498 0.495 0.498 0.501 0.500 0.515

318 322 333 343 352 365 382

3288 3333 3372 3418 3476 -

5.51 5.47 5.41 5.32 5.20 5.02

0.496 0.501 0.500 0.509 0.511 0.507

324 334 336 347 359 364

3341 3335 3368 3438 3512 3476

5.51 5.43 5.40 5.35 5.24 5.21

0.496 0.499 0.502 0.517 0.523 0.538

324 328 326 325 326 329

3341 3323 3358 3461 3616 3650

1922 0.240 1954 0.238 1986 0.234 2024 0.230 2053 0.232 z1Al2/3O-(0.3-z1)ZnO-0.7TeO2 1878 0.269 1903 0.259 1936 0.253 1967 0.257 2015 0.255 2053 0.232 z2B2/3O-(0.3-z2)ZnO-0.7TeO2 1878 0.269 1904 0.255 1906 0.262 1907 0.282 1987 0.284 2023 0.278


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57.1 58.4 58.6 59.5 60.6 -

48.9 49.7 50.2 51.3 52.1 -

31.3 31.6 31.5 31.7 32.4 -

19.7 20.1 20.3 20.9 21.2 -

61.6 60.8 61.4 62.9 64.1 60.6

49.3 49.9 50.8 51.7 52.9 52.1

35.7 34.4 34.3 35.5 36.0 32.4

19.4 19.8 20.3 20.6 21.1 21.2

61.6 59.9 60.9 64.1 68.5 69.4

49.3 49.5 49.5 49.9 53.1 54.5

35.7 33.7 34.7 38.1 40.9 41.0

19.4 19.7 19.6 19.5 20.7 21.3

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Figure captions

Figure 1. Composition dependence of density for (a) binary glasses yAl2/3O-(1-y)TeO2 (0≤y≤0.43) and xZnO-(1-x)TeO2 (0≤x≤0.45), 31 and (b) ternary glasses z1Al2/3O-(0.3-z1)ZnO0.7TeO2 and z2B2/3O-(0.3-z2)ZnO-0.7TeO2 (0≤z1,z2≤0.3). Lines are drawn to guide the eyes.

Figure 2. Composition dependence of the onset-glass transition temperature for glasses (a) yAl2/3O-(1-y)TeO2 and xZnO-(1-x)TeO2, and (b) z1Al2/3O-(0.3-z1)ZnO-0.7TeO2 and z2B2/3O(0.3-z2)ZnO-0.7TeO2. The line for glasses yAl2/3O-(1-y)TeO2 is a linear fit to the data; the other lines are drawn to guide the eyes. Error bars in (a) are of the size of symbols.

Figure 3. Reduced isotropic Raman spectra of glasses yAl2/3O-(1-y)TeO2 for y=0 to 0.43.

Figure 4. Reduced isotropic Raman spectra of glasses z1Al2/3O-(0.3-z1)ZnO-0.7TeO2 for z1=0 to 0.3.

Figure 5. Reduced isotropic Raman spectra of glasses in the system z2B2/3O-(0.3-z2)ZnO0.7TeO2 for z2=0 to 0.3. The inset shows the corresponding Raman spectrum of pure TeO2 glass prepared in Pt crucible.31

Figure 6. Deconvoluted reduced isotropic Raman (a, b) and absorption coefficient (c, d) spectra of glasses 0.3B2/3O-0.7TeO2 (a, c) and TeO2 (b, d). Thick black lines are the experimental spectra, blue lines the simulated spectra and thin black lines the Gaussian peak components of the spectra.



Figure 7. Absorption coefficient spectra of glasses z2B2/3O-(0.3-z2)ZnO-0.7TeO2 for z2=0 to 0.3. The inset shows the absorption coefficient spectrum of pure TeO2 glass.

Figure 8. Evolution of the relative infrared intensity of bands at 923 and 1113 cm-1 (A11+A12) for (BO4/2)- tetrahedra, 1252 and 1423 cm-1 (A13+A15) for (BO2/2O)- triangles and 1346 cm-1 (A14) for (BO3/2)0 triangles, in the system z2B2/3O-(0.3-z2)ZnO-0.7TeO2 for z2=0 to 0.3 (for the deconvoluted infrared spectrum see Figure 6c). Atot is the total integrated intensity in the 8801500 cm-1 region. Lines are linear fits to the data.

Figure 9. Composition dependence of the longitudinal VL and transverse VT sound velocities for glasses xZnO-(1-x)TeO2 and yAl2/3O-(1-y)TeO2 melted in Pt crucibles. Solid lines are polynomial fits to the experimental data and extrapolate to pure TeO2 glass (x,y=0). The values of the VL and VT velocities (green points) for TeO2 glass melted in alumina crucible are also included for comparison (data from ref. 64).

Figure 10. Composition dependence of the Poisson’s ratio, σ, in (a) binary glasses xZnO-(1x)TeO2 and yAl2/3O-(1-y)TeO2, and (b) ternary glasses z1Al2/3O-(0.3-z1)ZnO-0.7TeO2 and z2B2/3O-(0.3-z2)ZnO-0.7TeO2. Lines are guides to the eyes. The σ value for TeO2 glass melted in alumina crucible is also included in (a) (from ref. 64).

Figure 11. Composition dependence of the longitudinal VL and transverse VT sound velocities for ternary glasses z1Al2/3O-(0.3-z1)ZnO-0.7TeO2 and z2B2/3O-(0.3-z2)ZnO-0.7TeO2. Lines are drawn to guide the eyes.


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Figure 12. Evolution of atomic packing density Cg in (a) binary tellurite glasses xZnO-(1x)TeO2 and yAl2/3O-(1-y)TeO2, and (b) ternary tellurite glasses z1Al2/3O-(0.3-z1)ZnO-0.7TeO2 and z2B2/3O-(0.3-z2)ZnO-0.7TeO2. Lines are guides to the eyes. Error bars are of the size of symbols.

Figure 13. Young’s modulus and glass transition temperature for tellurite glasses studied in this work: xZnO-(1-x)TeO2 (ZT), yAl2/3O-(1-y)TeO2 (AT), z1Al2/3O-(0.3-z1)ZnO-0.7TeO2 (AZT) and z2B2/3O-(0.3-z2)ZnO-0.7TeO2 (BZT).

Figure 14. Poisson’s ratio (σ) and atomic packing density (Cg) for glassy materials. The inset shows σ versus Cg values determined for tellurite glasses studied in this work: xZnO-(1-x)TeO2 (ZT), yAl2/3O-(1-y)TeO2 (AT), z1Al2/3O-(0.3-z1)ZnO-0.7TeO2 (AZT) and z2B2/3O-(0.3z2)ZnO-0.7TeO2 (BZT). The present tellurite data fit well in the global σ versus Cg correlation established by Rouxel 32 for a range of glasses, which includes now tellurite glass compositions.



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Short-Range Structure, Thermal and Elastic Properties of Binary and

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