Customized structural and mechanical characteristics of Eu3+ imbued boro-telluro-dolomite glasses: effect of Dy3+ co-doping

Inorganic oxide glasses with high mechanical strength and durability became demanding. Thus, a new type of Eu3+/Dy3+ co-doped boro-telluro-dolomite glass system with customized structures and mechanical properties were prepared by the melt-quenching method and characterized. The densities, FTIR and Raman spectra of the glasses revealed a significant modification in the network structure with the increase of Dy3+ contents. The values of microhardness and elastic moduli of the glasses were increased with the increase of co-doping contents. In addition, a correlation between the glass hardness and applied load was established. The proposed glasses were shown to withstand about 300 g applied force without any distortion, confirming their usefulness to design mechanically stable screen shield and high strength surfaces.


Introduction
Nowadays, distinct class of oxide glasses obtained by blending various natural minerals with usual synthetic glass formers has drawn much attention [1][2][3].The evergrowing interest on these glasses is mainly due their enhanced chemical and thermal stability, high rareearth ions (REIs) solubility, and moderate mechanical strength [4,5].It was reported that any glass host incorporated with natural minerals by virtue of their excellent hardness, high Young's modulus, high fracture toughness together with high resistance against scratches and sharp contact damage find countless applications in the protective covers of electronic displays such as smartphones, laptops, tablets, and wearable devices [6].Ideally, the cover glasses are the outermost layer of the electronic displays and remain the most vulnerable part of these wearable devices under external force.Extreme hardness and higher Young's modulus can reduce the glass thickness while retaining sufficient durability [7].Despite the promises of this kind of glass system, nevertheless the selection of abundant minerals that can suitably be amalgamated into the existing synthetic based glass formers to form a new class of glass matrix for the aforementioned usage remains challenging.
Repeated studies showed that the structures, mechanical and optical properties of these glasses can appreciably be improved by europium (Eu 3+ ) and dysprosium (Dy 3+ ) ions co-doping.Besides, efforts have been made to incorporate the dolomite mineral (calcium rich stone) and other by-products such as wollastonite, cement dust, and ashes from sugarcane, coconut husk, palm oil, soda lime silica, and white rice husk into different oxide glass hosts for improving their structural, mechanical and optical performance wherein Abdellaoui et al. [8] studied the influence of natural raw barite and dolomite minerals on the glass forming ability of borate system.The results showed an increase in the glass density, molar volume, and the ratio of BO 4 to BO groups with the increase of BaO contents.Furthermore, the glass transition temperature and optical band gap were decreased, and the UV cut-off was moved towards higher wavelength with the increase of BaO contents.The gamma ray shielding features of lithium borate glasses doped with dolomite, hematite and goethite minerals were also studied [9].The results revealed much higher increase in the glass density, transparency, exposure buildup factor and radiation shielding properties with the inclusion of dolomite than hematite and goethite.The elastic and optical properties of CeO 2 -doped borotellurite glasses containing two dolomite minerals (pebble and marble) was reported, indicating their usefulness for the development of active filter in optical devices [10].Zakaly et al. [11] prepared a series of dolomite-borate glasses using the melt-quenching method and determined their physical, structural, and optical characteristics.A comprehensive overview of recent literatures showed that majority of the studies focused on the thermo-luminescence features of these glasses [12][13][14][15].The incorporation of dolomite in the lead-zinc borate glasses was observed to improve their gamma radiation shielding capacity.Despite an excellent shielding potency shown by the glass composed of optimum dolomite content, yet their physical and mechanical properties still need further improvement wherein REIs co-doping may be viable strategy.
The amalgamation of boron oxide having high phonon energy and tellurium oxide with low thermal stability as foundational glass former can impart desired practical properties to the final product like low phonon energy, high thermal stability and negligible hydroscopic nature [16][17][18].Moreover, the physical, structural and mechanical properties of La 3+ infused zinc borate [19], Ag/Eu 3+ -magnesium zinc sulfophosphate [20] and Nd 2 O 3 :SrO-Bi 2 O 3 -B 2 O 3 [21] glasses were meaningfully enriched due to the integration of REIs.However, studies on the structural and mechanical properties of Eu 3+ /Dy 3+ incorporation in boro-tellurate glass system containing dolomite mineral to the best of authors knowledge has not been reported.
Based on the aforementioned facts, we made a series of Eu 3+ /Dy 3+ co-doped boro-telluro-dolomite glasses and determined their structural and mechanical characteristics.The main purpose was to develop a high strength glass system with improved structural and mechanical properties suitable for mobile devices screen protector applications.As-quenched samples were characterized using different analytical instrument to determine some essential parameters like density, molar volume, N 4 ratio, microhardness and elastic moduli.Furthermore, the effects of various Eu 3+ /Dy 3+ codoping concentrations on the network structures and mechanical traits of the boro-telluro-dolomite glasses were examined.The obtained experimental results were analyzed, discussed, and compared with similar findings reported in the state-of-the-art literature.

Preparation of glass samples
In this study, analytical grade chemical reagents (99.99% purity, Sigma Aldrich) of tellurium dioxide (TeO 2 ), boron oxide (B 2 O 3 ) europium oxide (Eu 2 O 3 ), dysprosium oxide (Dy 2 O 3 ) oxide and dolomite (CaMg (CO 3 ) 2 ) were acquired.Dolomite minerals (pebble and marble) were collected from various sites of Johor (Malaysia) and crushed into fine powder using the (100 ×60) Mini-Jaw Crusher.Eu 3+ /Dy 3+ co-doped boro-telluro-dolomite (BTD) glasses of composition (where b = 0.3, 0.3, 0.5, 0.7 and 0.9 mol%, Dp = Dolomite pebble and Dm = Dolomite marble) were made using the conventional melt-quenching route and coded as BTDEu bDy (Table 1).First, about 15 g of the glass constituents (called batch composition) were weighed and homogeneously mixed in an alumina crucible, and melted at 1200°C for 1 h 30 min in a high temperature electrical furnace.Next, the molten fluid was dispensed onto a pre-heated stainless-steel mold and subjected to annealing in another furnace at 400°C for 3 h to release the residual strain that may cause glass embrittlement.

XRD and SEM-EDX analyses
The amorphous nature of the as-quenched samples was confirmed using X-ray diffraction analyses (Siemens X-Ray Diffractometer D5000 that used Cu-K α line of wavelength ≈ 1.54 Å at 40 kV and 30 mA).The samples' XRD patterns were recorded in the angular (2θ) range of 10 • to 80 • at scan rate of 2 • per minute.The microstructures, surface morphology and presence of chemical elements in the dolomite minerals and samples were examined using the energy dispersive X-ray (EDX, SwiftED3000) spectrometer attached to a table top scanning electron microscope (SEM, TM3000 Hitachi).All the prepared samples were characterized at room temperature.

Density measurement
The density (ρ) of the studied samples was measured using Archimedes principle with distilled water (density, ρ w = 1 gcm −3 ) as immersion liquid and calculated using [22]: where W a and W b are the sample's weight in air and water, respectively.The molar volume (V m ) in terms of molecular weight (M w ) of the glass samples were calculated using [23]:

FTIR and Raman spectral analyses
The FTIR spectra of the prepared samples were recorded in the range of 400-4000 cm −1 at a resolution of 4 cm −1 .Small amounts of powdered samples were mixed thoroughly with KBr at a ratio of 1:100 mg.The resultant mix was then hard-pressed by 5 ton per square inch pressure for one minute to get a transparent pellet of approximately 1 mm thick.The obtained pellet was used to record the FTIR spectra.The Raman spectra of the glasses in the range of 200-2000 cm −1 at a resolution of 1 cm −1 were recorded (HR800 UV Horiba Jobin Yvon Raman Spectrometer).FTIR and deconvoluted Raman spectra were used to estimate the N 4 ratio [24]: where A 4 and A 3 are the area of the IR and Raman bands in the range of 800-1200 cm −1 and 1200-1700cm −1 due to the vibration of BO 4 and BO 3 units, respectively.

Vicker's microhardness and ultrasonic velocity test
Micro-indentation measurement was performed on each sample with the applied loads of 50, 100, 200, 300 and 500 g (at a retention time of 15 s) using a digital micro-hardness tester (Shimadzu HVM-2).The values of glass microhardness were estimated using [5]: where H V is the Vickers hardness (in Hg/mm 2 ), F is the applied force (in Newton) and l is the average length (in metres) of the indentation's diagonals.
The longitudinal (V L ) and shear (V S ) ultrasonic velocities in the glasses at resonant frequency of 4 MHz were measured by a Ultrasonic flaw detector (RIGOL DS2202 digital storage oscilloscope).The values of V L and V S was calculated using [25]: where d is the glass thickness and t is the time interval (time taken between the start and retrieval of the pulse appear on the screen during round-trip).
The obtained values of ρ, V L and V S were used to estimate the Lame's constants (λ and μ) [26]: The values longitudinal (L), shear (G), bulk (K) and Young (E) modulus, Poison ratio (σ ) and fractal bond connectivity (D) were evaluated via [26]:

Physical and structural characteristics of glasses
Figure 1 shows the XRD patterns of the Eu 3+ /Dy 3+ co-doped as-quenched samples which did not show any distinct crystalline peaks, rather revealed a broad hump at lower Bragg angles, affirming their amorphous nature.This observation is in agreement with the one reported by Saddeek et.al [27].table of Figure 2(a)).Furthermore, the existence of Al in the studied glass confirmed the successful incorporation of dolomite pebble minerals into the BT glass network.
Figure 3 illustrates the glass density and molar volume as a function of Dy 2 O 3 contents.To determine the degree of the geometric configuration, network structures compactness, interstitial space facets and coordination number of the glasses, the density was measured [29].The REIs concentration dependent variation in the glass density provided a basic insight related to the structural transformation of BO 3 units into four-fold BO 4 units.The numbers of BO 4 structural units were more than BO 3 units, responsible for increasing the network binding and density (more compact structure) of the glasses.The observed increase in the glass density with the increase of Dy 2 O 3 contents was mainly due to the replacement of B 2 O 3 with lower molecular weight (69.62 g mol −1 ) by Dy 2 O 3 with much higher molecular weight (372.99 gmol −1 ) in the glass matrix.Conversely, the molar volume of the glasses was decreased with an increase of doping levels.Clearly, the inclusion of dopants with higher molecular weight than the host was enabled to increase the density of oxygen packing, transforming the loose network structures of the glass into more compact one [29] useful for the improvement of mechanical performance and durability.The obtained enhancement in the glass density and network structure packing with the increase of Dy 2 O 3 contents can be ascribed to the conversion of Sp 2 hybridized planar (triangular) BO 3 units into more stable Sp 3 hybridized BO 4 tetrahedral units.Compared to the  planar BO 3 units, BO 4 tetrahedral units are strongly connected together, suggesting a shrinking of the amorphous network structure and thus improvement in the glass density.In addition, the co-dopants (Eu 2 O 3 and Dy 2 O 3 ) can occupy the lattice interstitials, leading to a complete modification of the glass network structures [30].
Figure 4 shows the FTIR spectra of the glasses in the range of 380-4000 cm −1 .The IR absorption spectral analyses allowed us to distinguish the presence of various chemical functional groups in the proposed glass network.The measured absorbance of the glasses showed the bonding vibrations corresponding to the network former (B 2 O 3 ), conditional network former (TeO 2 ) and natural minerals (dolomite) network modifier (MgCa(CO 3 ) 2 ) used in the compositions, wherein the inclusion of co-dopants was found to influence the glass network structures appreciably.However, the IR spectral band positions were not affected significantly with the variation of Dy 2 O 3 concentrations.Various literature reports suggested that for any glass host made with B and Te show the IR bands of TeO 4 , TeO 3 , BO 4 as well as BO 3 units around 600-650, 650-700, 800-1200 and 1200-1400 cm −1, respectively [31].Herein, the bands  [33].Earlier study reported that pure borate shows a sharp IR peak at 806 cm −1 due to the vibration of boroxol ring [34].However, we did not observe the IR band corresponding to the boroxol ring vibration, indicating the dominance of BO 3 and BO 4 structural units in the proposed glass network.The band at 1630-1667 cm −1 was due to the stretching vibration of B-O bonds from isolated pyroborate groups.The IR band at 3444-3469 cm −1 was to the stretching vibration of H-O linkage from hydroxyl groups [35].
Figure 5 displays the Raman spectra of some selected samples in the range of 2000-200 cm −1 .The Raman analysis complemented the IR results because some of the forbidden transitions in the IR absorbance (zero net dipole moment) became allowed in the Raman spectra due to nonzero net polarizability [36].Table 2 shows the Raman peak positions and their assignments.The overall Raman profiles for different glasses remain similar wherein the shift in the Raman peak positions with the increase of Dy 2 O 3 content was insignificant.The observed peak at 349 cm −1 was due to the vibrations of the Te-O bonds from TeO 4 tetrahedra characteristic of α-TeO 2 [37].The weak peak (shoulder) at 682-696 cm −1 was due to the vibrations of Te-O linkages among TeO 4 units and bridging oxygen (BOx) [38].In addition, the intense Raman peak at 770 cm −1 was due to the symmetrical and asymmetrical vibration modes of (Te eq -O) in TeO 3 + 1 or TeO 3 units [39].The weak Raman peak at 922 cm −1 was due to the stretching vibration of BO 4 unit from diborate (B 2 O 5 −4 ) groups [40].The extremely weak peak at 1220 cm −1 for all the studied glasses corresponded to the stretching vibration of BO 4 groups.The Raman peak at1357 cm −1 was due to the asymmetrical stretching vibration of BO 3 groups in different borate units [41].In brief, both FTIR and Raman spectra of the glasses showed the existence of functional chemical groups of boron and tellurium in the network structures and did not reveal any spectrum related to Dy 2 O 3. Some significant FTIR and Raman spectral peaks were deconvoluted into many Gaussian functions (Figures 6 and 7) to assess the relative area (A) of each component proportional to the structural group abundance [46].The fraction of the 4-coordinated boron atoms (N 4 ) was evaluated using the expression described in Section 2.5.Together with the N 4 ratio, the vibrational spectral deconvolution provided a deeper insight into glass network structure whether densely packed or loosely bound matrix.The relative areas of the bands due to the IR vibration of BO 4 units in the range of 800-1200 cm −1 (A 4 ), BO 3 units in the range of 1200-1600 cm −1 (A 3 ), TeO 4 units (in the range of 600-650 cm −1 ) and TeO 3 units (in the range of 650-800 cm −1 ) were calculated.Table 3 shows the achieved relative areas and N 4 ratio of the glasses.Comparable observations were made for the fluoride borate glasses [47] and nickel-doped bismuth borate glasses [48].It was shown that by tuning the doping contents, the relative fraction of BO 4 and TeO 4 units in the studied glasses can be increased, enabling better control on the glass network structure, density and mechanical performance.

Mechanical characteristics of glasses
The glass-based screen protectors of various portable electronic devices must be thin, high strength, durable, and light-weight [49].These glasses should be resilient to external applied stress wherein their resistance to external pressure (shock shielding capacity) can be gauged through the mechanical hardness and elastic moduli measurement.Microhardness test provides basic knowledge on the glass resistance to permanent distortion stimulated via harder materials when subjected to stress.In the present study, the capacity of the proposed glass system in resisting plastic or permanent distortion under intense mechanical loading was examined using the Vickers hardness test [50].The hardness (H V ) of the glasses were measured at various applied loads (50, 100, 200, 300 and 500 g) with retention time of 15 s.The glass hardness was increased with the decrease of Dy 2 O 3 contents as shown in Figure 9.This observed increase in microhardness can be ascribed to the enhancement of glass network rigidity due to the reduction of NBO.The results (Figure 9) of hardness against applied load can be interpreted by diving into two regions AB (with lower loads) and BC (with higher loads).The region AB is predominantly an active zone wherein the stress-strain behaviour is nonlinear.Conversely, the region BC is essentially a plateau or saturation zone in which the stress-strain response is somewhat independent of the loads variation.The observed non-linear response at lower load was mainly due to the increase of glass resistance toward additional indentations wherein some newly formed dislocations may be nucleated at  the indentation region, thus enhancing the microhardness.The plastic zone (active region) was extended up to 3 N of load matching the plateau limit of the glass hardness.Furthermore, an additional increase in the applied load could lead to the phonon modes softening of the glass, making the microhardness constant.This result indicated that the proposed glass composition can resist up to 300 g (3 N) of applied load without being permanently deformed, making them potential candidates for high performance glass protector in mobile devices.
Deeper insight into the elastic modulus of various glass hosts became significant for the design and fabrication of high strength and durable screen shield [51].Thus, various elastic parameters of the studied glasses were recorded and compared with the existing-state-ofthe-art glass hosts reported in the literature (Table 4).The values of V L and V S of the glasses were increased with the increase of Dy 2 O 3 concentrations (Figure 10), confirming the increase of glass network compactness and rigidity.In is worth noting that with the increase of glass density, the values of V L and V S for most materials generally show a significant increase [52], thus the proposed glasses followed the expected trend.This observation was supported by the improvement of the glass elastic moduli (L, G, K, and E) with the increase of Dy 2 O 3 contents (Figure 11).Yet again, the achieved improvement in the elastic moduli of the prepared glass system indicated their good mechanical stability which agreed with the other reported findings on borotellurite glass [53].It was argued that the observed increase in both ultrasonic velocities and elastic moduli of the glasses with the increase of doping levels was mainly due to the continuous conversion of TeO 3 , and BO 3 groups into TeO 4 and BO 4 groups and thereupon the generation of more BOx at the cost of NBOx reduction.In short, the BOx atoms generated from the conversion of BO 3 into BO 4 structural units in turn strengthen the network bonding and density of the cross-links among different constituent atoms in the glass matrix, significantly improving the glass network rigidity and mechanical qualities.These data supported the results of density, FTIR and Raman spectra.
The variation in the Poisson's ratio (σ ) with the increase of Dy 2 O 3 contents clearly reflected the glass network structures modification (called dimensionality changes) in terms of cross-linking density.Eevon et al. [58] reported that the Poisson's ratio changes can be a good indicator for the glass network structure alterations.A glass system with σ values in the range of 0.1 to  0.2 is characterized as dense cross-linked network.Conversely, a glass system with σ values in the range of 0.3 to 0.5 is characterized as low density cross-linked network structure.According to Bergman and Kantor [59], fractal bond connectivity (D) can elucidate the effectual dimensionality of a specific material (for example inhomogeneous random mix of fluid states and solid backbones) near the percolation threshold.Interestingly, the value of D for the network structures in 1D, 2D and 3D are approximately 1.0, 2.0 and 2.0, respectively.In the current glasses, the values of σ was ≥ 0.3, indicating their low cross-linking network density.In addition, the values of D of the studied glasses found to be 2.0, confirming their 2D network structures.
Figure 12 depicts the correlation between fractal bond connectivity and Poisson's ratio of the glasses.In pure B 2 O 3 glass, most of the triangular BO 3 units are organized as boroxol rings wherein 3 O atoms are within the ring and 3 O atoms occur at the exterior of the ring.When the BO 3 units undergo constant transformation into BO 4 units due to the addition of Dy 2 O 3 as substitute of B 2 O 3 , these boroxol rings suffer random interconnections.Consequently, the values of Poisson ratio decrease and dimensionality of glass network increase (Table 4).This observation is consistent with the proposed semi-empirical relation μ = A−zD [59].The obtained σ − D correlation was fitted to σ = 0.474 − 0.095D, with correlation constant of 0.09, wherein the constants A and z validated the earlier claim made from the study on the WO 3 -B 2 O 3 -MgO-TeO 2 glass system with σ = 0.466 − 0.09D [60], oxyfluorozinc-tellurite with σ = 0.45 − 0.083D [61] and oxyfluoride tellurite AlF3-ZnO-TeO 2 with σ = 0.54 − 0.085 [62].

Conclusion
The possibility of customizing the structure and mechanical properties of some newly developed boro-tellurodolomite glasses by co-doping with Eu 2 O 3 /Dy 2 O 3 was determined for the first time.These glasses of composition (34 − b) B 2 O 3 -TeO 2 -20Dp-15Dm-1Eu 2 O 3 -bDy 2 O 3 (where b =0.1, 0.3, 0.5, 0.7 and 0.9 mol%) were made using melt-quenching method and thoroughly characterized.The incorporation of Dy 2 O 3 into the glass matrix was found to appreciably enhance the glass stiffness without affecting the borate entities, leading to an increase in the density and network compactness, thus improving the mechanical qualities useful for high performance screen shield.The FTIR and Raman spectral deconvolution confirmed a significant improvement in the structural and mechanical characteristics of the glassed due to Eu 2 O 3 /Dy 2 O 3 concentration tuning, wherein the N 4 ratio was shown to increase.This structural enhancement of the glasses was ascribed to the isomerization process of BO 3 + NBOs ↔ BO 4 and TeO 3 + O → TeO 4 .The high elastic modulus and strong correlation between fractal bond connectivity and Poisson's ratio of the glasses affirmed their high resistance against permanent deformation under external applied pressure, making them protective for mobile devices screen protector.

Figure 2 Figure 1 .
Figure1shows the XRD patterns of the Eu 3+ /Dy 3+ co-doped as-quenched samples which did not show any distinct crystalline peaks, rather revealed a broad hump at lower Bragg angles, affirming their amorphous nature.This observation is in agreement with the one reported by Saddeek et.al[27].Figure2displays the EDX spectraand maps of BTDEu0.5Dyglass, indicating the uniform distribution of elements like B, Te, O, C, Ca, Al, Mg, Eu and Dy existed as glass constituents[28].The white spots in the SEM image (inset of Figure2(a)) were the artifacts of measurement.The weight percent of the chemical elements were matched to the nominal glass composition (inset

Figure 3 .
Figure 3. Variation of density and molar volume of the glasses against Dy 2 O 3 contents.

Figure 4 .
Figure 4. FTIR spectra of the prepared glasses.

Figure 5 .
Figure 5. Raman spectra of the selected glasses.

Figure 8
illustrates the Dy 2 O 3 contents dependent N 4 and TeO 4 ratio in the glasses.The N 4 ratio was increased gradually with the increase of Dy 2 O 3 contents in the glass, indicating an improvement in the glass network compactness through BO 3 + O → BO 4

Figure 7 .
Figure 7. Deconvoluted Raman spectra of a selected glass.

Figure 8 .
Figure 8. Dy ions contents dependent (a) TeO 4 ratio and (b) N 4 ratio of the studied glasses obtained from FTIR and Raman analysis.

Figure 12 .
Figure 12.Correlation between the fractal bond connectivity and Poisson's ratio of the produced BTDEubDy glass system.

Table 1 .
Glass code, compositions, density (ρ) and molar volume (V m ) of the studied glasses.

Table 2 .
Comparison of the obtained Raman peak positions and assignments with other studies.

Table 3 .
Relative area of the deconvoluted FTIR and Raman bands alongside N 4 .

Table 4 .
Ultrasonic velocities, elastic modulus, Poisson ratio and fractal bond connectivity of the present glass system compared to other reported works.