1 Introduction

In recent years, glasses made of silicates, germinates, borates, vanadates, mixed lattices of aluminum silicates, phosphates, boron silicate, and others have been the subject of numerous research studies (Bouabdalli et al. 2021; Bouabdalli et al. 2023a, b; Bouabdalli et al. 2022a, b, c, d, e, f, g, h; Elbatal et al. 2014; El Jouad et al. 2020; Mariscal-Becerra et al. 2017; Rada et al. 2011; Ramzi et al. 2016). These glasses have the ability to incorporate a wide variety of chemical compounds as dopants into their structure, and at the same time, these dopants may incorporate significant amounts of rare earth into their own structures, imparting their (optical) characteristics. Phosphate glasses are among those that have been the subject of the most research because of their intriguing characteristics, such as low melting temperature, high thermal expansion coefficient, high transparency, low viscosity, low refractive index, low dispersion, and low phonon energy (Linganna and Jayasankar 2012; Mahdi Bouabdalli et al. 2022; Mariscal-Becerra et al. 2017). This makes them important for many applications such as optical data transmission, sensing, detection, laser technologies, and solar cells (Bouabdalli et al. 2022a, b, c, d, e, f, g, h; Bouabdalli et al. 2022a, b, c, d, e, f, g, h; Li et al. 2016; Linganna and Jayasankar 2012). Furthermore, phosphate glasses are not as effective as borosilicate glasses in vitrifying some nuclear wastes (Bouabdalli et al. 2022d, e, f, g, h, a, b, c; Karaçoban and Özdemir 2012). The vitrification of some nuclear waste is being considered using phosphate glasses. Moreover, as is well known, phosphate glasses are characterized by low chemical durability and high hygroscopic, therefore the addition of heavy metal oxides such as SrO to the phosphate-based glasses network matrix can enhance their physical, chemical durability, and mechanical strength, and increase glass formation capacity (Bouabdalli et al. 2022a, b, c, d, e, f, g, h; Bouabdalli et al. 2022a, b, c, d, e, f, g, h; Deepa et al. 2018; Sidek et al. 1998; Sreedhar et al. 2013). The structure of phosphate glasses is used to determine their characteristics. According to Brow's review, phosphate glasses have a network-like structure made up of phosphor-oxygen tetrahedrons joined by one to three bridging oxygen atoms (BOs) (Brow 2000). The number of bridging oxygen atoms in the PO4 tetrahedron is expressed as 'n' in the Qn notation, indicating network connectivity, which is influenced by the composition of glass (Liang et al. 2011). Due to their low processing temperatures (less than 1000 °C) and lower glass transition temperatures, phosphate glasses have attracted the attention of both the technological and scientific domains (Makhkhas et al. 2013; Shih 2004).

According to research on conductivity in on glasses containing rare earth elements, the conductivity of glass decreases as the amount of rare earth ions increases because these ions move slowly and have large masses (Devidas et al. 2008; Gedam and Ramteke 2013).

Due to their high electrical conductivity, elements ranging from lanthanum to actinium are collectively referred to as rare earth elements (17 elements). Scandium and yttrium, in addition to these 15 lanthanides, also show 3 + oxidation states. We have selected yttrium oxide (Y2O3) as the dopant for the premade phosphate glasses from the many rare earth elements. Y2O3, one of the rare earth elements, is essential for changing several physical and chemical characteristics of glasses and ceramics. Studies reported that mixing Y2O3 with alkaline earth oxides such as SrO in glass composition improves its chemical, thermal, physical, and mechanical properties (Kaur et al. 2011; Singh, Kalia, and Singh 2015). These glasses have applications in the disciplines of photonics, optics, and biomedicine and have been shown to be extremely effective luminescence materials (Bouabdalli et al. 2022a, b, c, d, e, f, g, h; Sadek et al. 2022). Heavy metal oxide-based glass matrices, such as Y2O3, have attracted a lot of attention. This is a result of their characteristics, as well as structural and physical qualities such as low transformation temperatures, strong infrared transmission, high density, high refractive index, and high thermal expansion. In addition, they can be used to protect nuclear waste from radiation. Singh and al reported that the addition of Y2O3 to glasses generally increases their thermal stability (Singh et al. 2007).

In addition, glasses that include yttrium oxide are of special interest since Y2O3 has been found to significantly improve applications such as selective internal radiation therapy (SIRT) to treat liver tumors (Riaz et al. 2009, 2010; Salem and Hunter 2006), as well as the endurance and physical qualities of glass (Bouabdalli et al. 2022d, e, f, g, h, a, b, c; Fu and Christie 2017). It is well known that the composition of yttrium aluminosilicate glass used for in situ radiation therapy, with a formulation of 17.1 mol.% Y2O3, 18.9 mol.% Al2O3, and 64.0 mol.% SiO2, is very strong and non-resorbable (Erbe and Day 1993; Hyatt and Day 1987). Beta particles (from the Y90 isotope), which have a relatively short half-life of 64.2 h, are released when this glass (in the form of solid microspheres) is activated by neutron bombardment (Burrill et al. 2011; Kawashita et al. 2011; Triller et al. 1995).

Incorporating Y3+ into the smaller Zr4+ crystal lattice helped form stable cubic zirconia polymorphs, which are thus advantageous for dental implants; According to Borgonovo et al., yttrium-stabilized zirconia can be used as a dental implant material (Borgonovo et al. 2012). The use of neodymium-doped yttrium aluminum garnets (Nd: YAG) to cure cancer (Schena et al. 2017). In addition, the function of yttrium oxide nanoparticles for nerve cells has been investigated (Schubert et al. 2006). The Y2O3 nanoparticles functioned as antioxidants that can protect cells from cell death induced by oxidative stress. Although its insoluble compounds, such as yttrium oxides, are non-toxic, its water-soluble compounds, such as yttrium chlorides and yttrium nitrates, are regarded as somewhat hazardous (Cochran et al. 1950). According to a study by Dubois et al. on the toxicity of yttrium chloride, nitrate, and oxide in rats, insoluble yttrium oxide (500 mg/kg) exhibited a relatively nontoxic behavior compared to soluble yttrium chloride (450 mg/kg) and yttrium nitrate (350 mg/kg) (DuBois 1956).

The literature has reported on the structural consequences of the addition of Y2O3 to silicate glasses. As Y2O3 concentrations exceeded 5 mol%, Singh et al. analyzed the structural alterations of lithium borosilicate glasses with the addition of 0 to 15 mol% of Y2O3 and demonstrated that Y2O3 operated as a network modifier (Singh et al. 2015). Simon et al. studied iron-containing yttrium aluminosilicate glasses using XPS analysis and mentioned that Y2O3 functioned as a network modifier oxide in the silicate glass network (Simon et al. 2005). In their study of borosilicate glasses doped with Y2O3, Fayad et al. observed that, especially at concentrations greater than 1% by weight, Y2O3 preferred to behave as a network modulator rather than a former network (Fayad et al. 2018). Moreover, the structure of glasses based on yttrium-doped phosphate was simulated using classical molecular dynamics, and Fu et al. found that the depolymerization of the phosphate network caused yttrium oxide to operate as a network modifier (Fu and Christie 2017).

The present work is devoted first to characterizing the structural composition of phosphate glass containing strontium oxide by doping it with yttrium oxide at different concentrations; second, to discover the physical properties of the strontium phosphate glasses doped with various concentrations of yttrium; third, to study the effect of the addition of yttrium content on the optical properties of the prepared glasses.

2 Materials and methods

2.1 Glass preparation

Four different glass formulations based on strontium phosphate in system 65 (P2O5)–(35 − x) (SrO)–x (Y2O3) (where x varied from 0 to 1.5 mol.%) were prepared using the classic melt quenching technique using an ammonium dihydrogen phosphate (NH4H2PO4) for P2O5, Strontium carbonate (SrCO3) for SrO, and yttrium chloride (III) (YCl3∙6H2O) for Y2O3. The precursors were weighed, combined in an agate mortar, and then transferred to an alumina crucible, which was then placed in a furnace at 120 °C for 24 h to evaporate the water and ammoniac. Finally, the temperature of the furnace was increased to 1100 °C to reach the melt for about 2 h as highlighted in Table 1. The resultant molten glass was then poured onto a steel plate for quenching and left to cool to room temperature. Figure 1 presents the schematic of the synthesis process method used to prepare the desired glasses as reported in (Bouabdalli et al. 2023a, b).

Table 1 Glass codes, drying and melting temperature used throughout the study
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Fig. 1
figure 1

Schematic illustration of the method used to synthesize the phosphate glasses

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2.2 Characterization technique

2.2.1 Powder X-ray diffraction

XRD analysis was used to explore the amorphous character of each glass formulation. Data were collected using the X-ray diffractometer (Shimadzu model: XRD 6000 using CuKa radiation, λ = 0.154 nm), in the region of 5° to 70°.

2.2.2 FTIR analysis

Infrared spectroscopy of the glass particles was also performed using a Brüker Tensor 27 spectrometer (Brüker Optics, Germany) which was operated in absorbance mode. Spectra were recorded in the range of 400 to 1500 cm−1 utilizing a Standard Pike ATR cell (Pike Technologies, Inc., UK). OPUS software version 5.5 was used for the analysis.

2.2.3 Physical parameters of the studied glasses

The density of pure and yttrium-doped strontium phosphate-based glasses was calculated using the mass/volume formula.

$${\rho } = \frac{{\rm M}_{\rm T}}{{\rm V}_{\rm T}}$$
(1)

where MT is the total mass of each glass sample, VT is the total volume of each glass sample.

The molar volume of the glass of phosphate according to the molecular weight (M) and density yields the following expression (Ahmadi et al. 2017):

$$V_m = \frac{M}{\rho }$$
(2)

where Vm is the molar volume and M is the total molecular weight of the multicomponent glasses system given by:

$$M = x_{SrO} Z_{SrO} + x_{P_2 O_5 } Z_{P_2 O_5 } + x_{Y_2 O_3 } Z_{Y_2 O_3 }$$
(3)

where \({x}_{SrO}\), \({x}_{{P}_{2}{O}_{5}}\), and \({x}_{{Y}_{2}{O}_{3}}\) are the mole fractions of the constituent oxides; \({Z}_{SrO}\), \({Z}_{{P}_{2}{O}_{5}}\), and \({Z}_{{Y}_{2}{O}_{3}}\) are the molecular weights of the constituent oxides.

The following quantities such as polaron radius (rp), ion concentration (Ni), field strength (F), and internuclear distance (ri) were evaluated by means of the relations.

$$N_i = \frac{N\rho X}{{M_{av} }}$$
(4)

where N is the Avogadro number, X is the mole fraction of the dopant, and \({M}_{av}\) is the average molecular volume (Ahmadi et al. 2017).

$$r_p \left( A \right) = \frac{1}{2}\left( {\frac{\pi }{6N_i }} \right)^\frac{1}{3}$$
(5)

Here, Ni is the concentration of the ions.

$$F = \frac{Z}{r^2 }$$
(6)

Here, r is the radius of the polaron and Z is the atomic number of yttrium.

$$r_i \left( A \right) = \left( {\frac{1}{N_i }} \right)^{{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 3}}\right.\kern-0pt}\!\lower0.7ex\hbox{$3$}}}$$
(7)

Here, Ni is the concentration of the ions.

2.2.3.1 UV–vis spectral measurements

The diffuse reflectance spectra of the glass powders obtained by grounding small glass blocks in a ceramic mortar were collected in the range of 100–1100 nm using a near-infrared UV–Vis spectrophotometer (UV-1800, Shimadzu, El Jadida, Morocco). The spectra were obtained with a resolution of 0.3 nm at room temperature.

The Kubelka–Munk function F(R) has a direct relationship with absorbance. As a result, the following relation was used to convert the F(R) values to the linear absorption coefficient (α):

$$\alpha = \frac{F\left( R \right)}{t}$$
(8)

where ‘’t’’ is the thickness of the sample.

In the limiting case of an infinitely thick sample, thickness and sample holder have no influence on the value of reflectance (r). In this case, the Kubelka–Munk equation at any wavelength becomes (Yakuphanoglu 2010; Zheng et al. 2011):

$$F\left( {R\infty } \right) = \frac{{\left( {1 - R\infty } \right)^2 }}{2R}$$
(9)

where \({\text{R}}\infty = {\text{R}}_{{\text{sample}}} /{\text{R}}_{{\text{standard}}}\). The following relational expression proposed by Tauc et al. (Brow 2000; Rada et al. 2012) was used.

$$\alpha hv = A \left( {hv - E_g } \right)^\frac{1}{2}$$
(10)

Here, Eg is the band gap, h is Planck's constant, v is the vibrational frequency, α is the absorption coefficient, and A is the proportional constant.

The acquired diffuse reflectance spectra were converted to the Kubelka–Munk function. Therefore, the vertical axis was converted to the quantity F (R∞), which was proportional to the absorption coefficient. The α in the Tauc equation is substituted with F (R∞). Thus, in the actual experiment, the relational expression becomes (Zheng et al. 2011):

$$\left[ {F\left( {R\infty } \right)hv} \right]^2 = B\left( {hv - E_g } \right)$$
(11)

Therefore, the band gap Eg of a powder sample can be easily extracted by getting F(R∞) from Eq. (9) and plotting the [F(R∞)hv]2 against hv.

3 Results and discussions

3.1 XRD analysis

XRD of pure and yttrium-doped strontium phosphate glasses was recorded in the range of 5° to 70°. The results showed that the XRD pattern of the prepared phosphate glasses exhibited extensive diffusion with decreased scattering angles. It showed a long-range structural irregularity, a characteristic of an amorphous nature, as seen in Fig. 2. The XRD characterization graphs, which did not have identifiable or sharp peaks, confirmed that the prepared samples were amorphous. This behavior is consistent with earlier research (Arafat et al. 2020; El-Rehim et al. 2022).

Fig. 2
figure 2

XRD patterns for glass of the 65 P2O5–(35 − x) SrO–xY2O3 phosphate system (where x = 0, 0.5, 1, and 1.5 mol.%)

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The broad halo-peak XRD positions and their full width at half maximum (FWHM) for each of the examined glasses, determined by the origin program, are shown in Table 2. When the dopant impact on the strontium phosphate glass system was compared, it was discovered that, as the concentration of yttrium increases, the position of the XRD broad halo-peak shifts toward lower angles of diffraction, as shown in Table 2 and Fig. 2. Additionally, it has been found that as the amount of yttrium in phosphate glasses rises, so does the FWHM of the XRD broad halo-peak. This behavior may be explicated by the change in interatomic distances due to the yttrium dopant. It is also important to note that the amorphous nature of the glass structure was not affected by the addition of a dopant to the glass system. The results obtained are in good agreement with previous work published in the literature (Omar et al. 2016).

Table 2 The position and full width at half maximum (FWHM) of the XRD broad-halo peak of the strontium phosphate glasses as a function of the yttrium concentrations
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3.2 FTIR analysis

Fourier transform infrared (FTIR) spectroscopy was performed to explore any structural changes within the glasses produced. Figure 3 shows the FTIR spectra of 65 P2O5–(35 − x) SrO–xY2O3 glasses in the frequency region between 400 and 1500 cm−1. The spectra of the pure and yttrium-doped strontium phosphate glasses have similar spectral features. The FTIR spectra can be deconvoluted using Gaussian lines to the component bands for each one to study the origin of this characteristic FTIR symmetry. Other peaks have been obtained by deconvolution in the 400–1500 cm−1 wavenumber region. Table 3 lists the band centers, the relative area of the deconvoluted peaks, and their assignments to the appropriate bands.

Fig. 3
figure 3

FTIR spectra for strontium phosphate-based glasses in the 65 P2O5–(35 − x) SrO–x Y2O3 system (with x = 0, 0.5, 1 and 1.5 mol.%)

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Table 3 The assignments and relative areas of different vibrational bands from FTIR spectra of 65 P2O5–(35 − x) SrO–xY2O3 glasses
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The specific deconvoluted spectrum for pure strontium phosphate glass is given in Fig. 4. The band around 534 cm−1 is due to the harmonics of the bending vibrations of the O–P–O linkages of basic structural units of phosphate glasses (Ahmed et al. 2004; Doweidar et al. 2005). The wavenumber localized at 564 cm−1 is attributed to bending and torsional vibrations that can be assigned to overlapping vibrations involving the strontium oxygen polyhedral and (P2O7)4− groups that are characteristic of a structure dominated by Q1 tetrahedrons (Lai et al. 2012). In addition, at 738 cm−1, there is another band caused by the symmetric stretching vibrations of the P–O–P bridge in the Q1 units (Sene et al. 2004). Moreover, the strong IR absorbance centered at 907 cm−1 is attributed to the asymmetric stretching vibration of the P–O–P links of PO4 units (Sene et al. 2004). The broad band centered at 1046 cm−1 is assigned to the symmetric stretching mode of the O–P–O non-bridging bond in the Q1 groups (Silva et al. 2010), indicating that the Q1 units are a completely major component of the structure of the glass of non-doped strontium phosphate. The absorption band located at 1209 cm−1 is attributed to asymmetric stretching motions of the O–P–O non-bridging oxygen in the Q2 groups (Lu et al. 2015).

Fig. 4
figure 4

Deconvoluted FTIR spectra of 65 P2O5–(35 − x) SrO–x Y2O3 glasses for x = 0 mol.%

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As examples represent for the nominal molar composition 0 ≤ x ≤ 1.5% range, the deconvolution in Gaussian bands of the spectrum for the x = 0.5% and x = 1% glasses, are given in Figs. 5 and 6, respectively. The incorporation of Y2O3 into the structural network of the parent-modified strontium phosphate glass is unmistakably shown by changes in the FTIR spectra of 65 P2O5–(35 − x) SrO–xY2O3 glasses (Fig. 4), which change as the Y2O3 content increases. The vibrations of structural units containing Y–O bonds can be observed in the lower frequency section of the FTIR spectra, while structural alterations in the phosphate network are particularly evident in the high-frequency portion of the spectrum. The additions of yttrium content to the base glass result in a change of the absorbance band localized at 733 cm−1 to a higher wavenumber (738 cm−1). The shift of the band was attributed depolymerization of the phosphate network, subsequently reducing the chains and increasing Q1 pyrophosphate groups. Furthermore, the band localized at 900 cm−1 also showed similar behavior, changing from 900 to 907 cm−1 to characterize the transition from chain to pyrophosphate, increasing the disorder in the glass network (RULMONT et al. 1991). The shift of νas(P–O–P) bands toward a higher wave number when the content of the yttrium content increases, a similar trend is observed for the IR spectra for glasses of P2O5-CaO-Na2O-Y2O3 when the amount of Na2O decreases (Arafat et al. 2020). In addition, the position of a prominent band at 1262 cm−1 (produced by the asymmetric stretching of PO2−) is relocated to 1266 cm−1 by adding the amount of yttrium concentrations. Additionally, we notice that as the Y2O3 content in the strontium phosphate glasses increases, the location of the IR band's peaks shifts to the lower wavenumber side. It is in line with earlier research (Sene et al. 2008; Shajan et al. 2017).

Fig. 5
figure 5

Deconvoluted FTIR spectra of 65 P2O5–(35 − x) SrO–x Y2O3 glasses for x = 0.5 mol%

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Fig. 6
figure 6

Deconvoluted FTIR spectra of 65 P2O5–(35 − x) SrO–x Y2O3 glasses for x = 1 mol.%

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3.3 Study on physical properties

The physical characteristics of rare-earth-ions-doped strontium phosphate-based glasses depend on the physical glass matrix. The nature and quantity of glass that forms and the network modifier used to affect the physical structure. As a result, the physical characteristics of the glasses examined were calculated using the formula in (Dejneka et al. 1995) and are shown in Table 4. The density measurement of the strontium phosphate-based glasses decreased from 2.108 to 2.099 g/cm3 when the content of the yttrium concentrations increased from 0.0 to 1.5 mol.% as demonstrated in Fig. 7. The reduction in the density of the strontium phosphate glasses can be attributed to the replacement of a larger SrO density (4.7 g/cm3) with a smaller relative density of YCl3 (2.67 g/cm3). Moreover, a decrease in densities was probably caused by the formation of non-bridging oxygen (NBO) atoms around the doping ions (Aryal et al. 2018; Mohan et al. 2007). With increasing yttrium concentrations, the molar volume of the glasses based on strontium phosphate increased from 60.974 to 62.641 cm3/mol. This increase in the molar volume of the strontium-phosphate-based glasses may be explained by a decrease in the compactness of the glasses. In addition, the increase in volume can be indicated by changes in the structural units of the glass, which can be confirmed by FTIR.

Table 4 Physical properties of yttrium-doped phosphate glasses
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Fig. 7
figure 7

Change in molar volume and density of the strontium phosphate-based glasses in relation to the yttrium concentration

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The radius of the polaron shrank as the concentration of trivalent yttrium ions increased, as seen in Table 4. The decrease in the polaron radius as the content of yttrium trivalent ions increased in the strontium phosphate glasses was a signal of decreased deformed lattice sites of the glasses studied. Similarly, as doping concentrations increased, the inter-ionic distances between Y3+ ions reduced, indicating a rise in the field strength around the ions (Table 4). As doping concentrations increase, attractive forces between ions and the nearby structural elements become stronger, causing field strengths to increase and the inter-ionic distance to decrease (Ramesh et al. 1997). The increased field strength increased as the amount of Y3+ ion availability per unit volume increased as the concentration of rare earth ions increased (Fig. 8).

Fig. 8
figure 8

The variation of the inter-ionic distance and field strength parameters according to the Y3+ ion concentrations

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The average phosphorus-phosphorus distance, which may be determined using the formula below, can also be used to study the impact of the yttrium concentration on the phosphate glass system.

$$\left\langle {{\text{d}}_{{\text{P}} - {\text{P}}} } \right\rangle = \left( {\frac{{{\text{V}}_{\text{m}} }}{{2{\text{N}}_{\text{A}} \left( {1 - {\text{X}}_{\text{P}} } \right)}}} \right)^{1/3}$$
(12)

where, NA is the number Avogadro and XP represents the molar fraction of P2O5. \(\langle {\mathrm{d}}_{\mathrm{P}-\mathrm{P}}\rangle\) is found to vary from 406.72 × 10−02 to 410.02 × 10−02 Å with increasing the content of yttrium.

The oxygen packing density (OPD) and the packing density are also important parameters that explain the system of the glasses. Packing density, VP (Veeranna Gowda 2013) is the measure of the rigidity of any oxide system that could be determined by the following equation:

$${\text{V}}_{{\rm P}} = \frac{1}{{{\text{V}}_{{\rm m}} }}\sum {\text{V}}_{{\rm i}} {\text{X}}_{{\rm i}}$$
(13)

Here, Vi represents the packing factor and Xi is the mole fraction of the composition. The packing factor (Vi) of an oxide MXOY having M and O ions of ionic radii rM and rO can be given by the following relationship:

$${\text{V}}_{\text{i}} = \frac{{4{\pi N}_{\text{A}} }}{3}{ }\left( {{\text{Xr}}_{\text{M}}^{3} + {\text{Yr}}_{\text{O}}^3 } \right)$$
(14)

The Vi of P2O5 = 34.8 cm3/mol and Y2O3 = 24.8 cm3/mol are available in the literature (Inaba et al. 1999). For SrO, it is 10.46 cm3/mol, calculated in this work via Eq. (10). It is clearly confirmed that the packing density decreases from 0.4293 to 0.4216 with increasing yttrium concentration, increasing the molar volume of the glasses prepared.

The OPD describes the association of oxygen atoms in the glass network, which can be calculated using the formula (Singh et al. 2015). It is discovered that as the yttrium level in the studied phosphate glass increases, the OPD varies between 59.04 and 57.95 mol/l.

The number of bonds per unit volume, nb, of the glasses can be calculated by the following relationship (Hager and El-Mallawany 2010; Lide 2004):

$${\text{n}}_{\text{b}} = \sum {{\text{n}}_{\text{s}} } {\frac{{{\text{N}}\rho }}{{\text{M}}}}$$
(15)

where ns is the number of bonds per unit of glass formula, ns = (coordination number of each cation)*(number of cations in the glass formula unit), N represents the number Avogadro, M is the molecular weight of the glass and \(\rho\) its density. Table 4 shows the computed values of nb.

Figure 9 demonstrates the variation observed in the packing density \(\left({\mathrm{V}}_{\mathrm{p}}\right)\), \(\langle {\mathrm{d}}_{\mathrm{P}-\mathrm{P}}\rangle\) and the oxygen packing density (OPD) with the change in the composition of yttrium. The formation of additional non-bridging oxygen and small bond modifications are responsible for this change. In addition, the number of bonds per unit of glass volume (nb) is determined using CNi, the coordination number of the cations immediately taken from the literature (Lide 2004). The nb value is observed to decrease as the amount of yttrium in the glass increases, confirming the variance in the molar volume and density values.

Fig. 9
figure 9

Variation of packing density, < dP−P > × 10−2 Å and OPD (mol/l) with yttrium concentration

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3.4 Optical properties

3.4.1 Reflectance spectra

Diffuse reflectance UV–Vis spectra of pure and yttrium-doped strontium phosphate glasses were recorded to characterize their structural features, as shown in Fig. 10. The glass matrices (PSrY0.0) presents the lowest reflectivity in the Ultraviolet region compared to the yttrium-doped glasses and it has the highest reflectivity in the visible region. The Kubelka–Munk transformed reflectance spectra of these prepared glasses are presented in Fig. 11. By extrapolating the linear portion of these spectra to meet the hν axis at [F (R∞)hν]2 → 0, the values of the band gap (Eg) have been determined. The optical band gaps as-quenched samples fall in the insulator region. The undoped strontium phosphate glasses (PSrY0.0) exhibit the highest value of the energy band gap around 6.528 eV. When the yttrium concentration was increased from 0.0 to 1.5 mol.% in strontium phosphate glasses, the optical band gap decreased from 6.528 eV for PSrY0.0 to 5.754 eV for PSrY1.5.

Fig. 10
figure 10

Reflectance spectra of strontium phosphate glasses according to yttrium concentrations

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Fig. 11
figure 11

Energy band gap of the synthesized glasses according to the content of yttrium

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The decrease in the optical band gap of the strontium phosphate glasses with the addition of the yttrium content can be explained by the increase in the non-bridging oxygen, which was reported in previous studies (Novatski et al. 2008; Singh et al. 2015). Moreover, the decrease of the Eg values in these yttrium-doped glasses may be due to the increasing cationic field strength (CFS) of the yttrium element (Li et al. 2019). The Urbach energy (Eu) of the glass samples was determined using the following relationship:

$$\alpha \left( {\text{v}} \right) = \beta \exp ({\text{hv}}/{\text{E}}_{\text{U}} )$$
(16)

The Urbach energy can be obtained from the graph of ln F (R) versus the photon energy, taking the reciprocal of the slope of the linear portion (Waring and Hsu 1983). The energy band gap and Urbach energy values are given in Table 5. We observed that the Urbach energy of the synthesized samples increased when the yttrium concentration also increased from 0.0 to 1.5 mol.%. Yttrium is an intermediate oxide that may be employed as both a modifier and a former. In this case, however, it appears to function as a network modulator in the glass phosphate network. In conclusion, the optical band gap and Urbach energy also depend on other factors than non-bridging oxygens (NBOs). The addition of Y2O3 leads to a shift in the broad halo peak toward lower diffraction angles, which modifies the glass network. The XRD spectra clearly indicated that the change in the broad halo peak is moving toward lower diffraction angles, where Y2O3 is present in glasses. This could be due to the reduction in the length bond and a higher coordination number with oxygens, which exhibits a transformation of the structural unit of the glassy phosphate network from metaphosphate to pyrophosphate and leads to a higher Urbach energy value compared to the PSrY1.5 glass.

Table 5 Optical band gap, Urbach energy, refractive index, molar refractivity, molar polarizability, reflection loss, metallization criterion, electronegativity, electron polarizability, optical basicity and oxide ion polarizability of the pure and yttrium-doped strontium phosphate glasses
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3.4.2 Calculation of the refractive index (n) of the prepared glasses

The refractive index (n) for all glass samples in the current study was determined using the following equation (Dimitrov and Sakka 1996; El-Mallawany et al. 2008):

$${\text{n}} = { }\left[ {6\sqrt {{\frac{5}{{{\text{E}}_{\text{g}} }}}} - 2} \right]^\frac{1}{2}$$
(17)

The refractive index (n) values obtained for the prepared glasses are given in Table 5 and presented in Fig. 12. It is clearly observed that the refractive index increased with increasing yttrium content, increasing from 0.0 to 1.5 mol.% in the strontium phosphate glasses.

Fig. 12
figure 12

The energy band gap (Eg) and the refractive index (n) of the strontium phosphate glasses according to the yttrium content

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As demonstrated in Fig. 12, we observed that the value of the refractive indices was oppositely corresponding to the optical band gap of the synthesized samples. The refractive indices obtained are greater than unity and also higher than the index of refraction of silicate glasses, which is approximately 1.458 (Naftaly and Miles 2007). Moreover, the glass samples have a high value (n), which indicates that they can be used as promising materials for photoelectronic and optical filter systems (Rammah et al. 2020). In addition, the increase in the refractive index (n) may be explained by the role of yttrium oxide in altering the structural properties of the strontium phosphate glass system. The addition of trivalent yttrium leads to the modification of the glass matrix by converting the metaphosphate unit to the pyrophosphate unit, which was previously confirmed by FTIR spectroscopy. The conversion of a structural unit of a glass system leads to the creation of non-bridging oxygen, which increases the polarizability of the phosphate glass network. The high polarizability of the lone pair in non-bridging oxygen causes an increase in the refractive index. The formation of non-bridging oxygen leads to the creation of ionic bonds, which increase the number of refractive indices.

The molar refractive (Rm), molar polarizability \(\left({\alpha }_{m}\right)\) and reflection loss (RL) can be evaluated by the following relationship (Shaaban et al. 2020):

$${\text{R}}_{\text{m}} = {\text{V}}_{\text{m}} \left( {1 - \sqrt {{{\text{E}}_{\text{g}} /20}} } \right)$$
(18)

The molar polarizability \(\left( {{\alpha }_{\text{m}} } \right)\) is directly proportional to the molar refractivity \(\left( {{\text{R}}_{\text{m}} } \right)\) of the material by the following equation:

$$\alpha_m = \left( {\frac{3}{4\pi N_A }} \right)R_m$$
(19)

where NA is the number Avogadro.

The reflection loss (RL) can be determinate through the ratio between the molar volume (Vm) and the molar refractivity (Rm):

$${\text{R}}_{\text{L}} = \left( {\frac{{{\text{R}}_{\text{m}} }}{{{\text{V}}_{\text{m}} }}} \right)$$
(20)

The molar refractivity (Rm) of a substance is determined by its molar polarizability \(\left({\mathrm{\alpha }}_{\mathrm{m}}\right)\). The values obtained for the refractive molar (Rm), molar polarizability (αm), and reflection loss (RL) are given in Table 5. It was discovered that (see Table 5) the values of (Rm), \(\left({\mathrm{\alpha }}_{\mathrm{m}}\right)\) and reflection loss (RL) increase as the Y2O3 content increases. These values are increasing as a result of an increase in molar volume.

Electronic oxide ion polarizability, \({\alpha }_{{O}^{2-}}\) can be calculated using the obtained molar volume (Vm) and the refraction index (n) of the following relationship reported in (Moss 1985):

$$\alpha_{O^{2 - } } = {{\left[ {\left( {\frac{V_m }{{2.52}}} \right)\left( {\frac{n^2 - 1}{{n^2 + 2}}} \right) - \sum^\alpha_{cat} } \right]} / {N_{O^{2 - } } }}$$
(21)

where \({\alpha }_{{O}^{2-}}\) represents the electronic oxide ion polarizability, \(\sum {\alpha }_{cat}\) represents the molar cation polarizability, \({\mathrm{N}}_{{\mathrm{O}}^{2-}}\) is a symbol for the quantity of oxide ions, Vm is the molar volume and n n is the refractive index of the glass sample. For a glass with matrix xAaOb–yBcOd–zCeOf–tDgOh, \(\sum {\alpha }_{cat}\) will be \(xa{\alpha }_{A}+yc{\alpha }_{B}+ze{\alpha }_{C}+tg{\alpha }_{D}\) and \({N}_{{O}^{2-}}\) will be\(xb+yd+zf+th\). From the available literature, the values of αP, αSr, and αY are 0.021, 0.861 and 0.554 Å3, respectively (Dimitrov and Komatsu 1999). The values obtained for the electronic oxide ion polarizability for all glass samples are listed Table 5. As can be seen in Table 5, the electronic polarizability of oxide ions increases when the concentration of trivalent yttrium ions is added to the strontium phosphate glasses. The compositional dependence of the molar polarizability and oxide ion polarizability of prepared glasses on the concentrations of yttrium is presented in Fig. 13. As the concentration of trivalent yttrium increases, the increasing trend of molar polarizability of the glass samples is visible. The amount of non-bridging oxygen that is present in the phosphate glass affects the deformability of the electron cloud. Non-bridging oxygen is made up of lone pair free electrons that are less strongly coupled to the nuclear charge. Therefore, free electrons are readily polarized as the electrical field penetrates the material (Halimah et al. 2019). As the concentration of the yttrium trivalent (Y3+) increases, so does the quantity of non-bridging oxygen ions. An increase in the amount of non-bridging oxygen in the glass system causes an increase in the number of free electrons. As a result, as the concentration of Y3+ increases, so does the molar polarizability. As can be demonstrated in Fig. 13, we have observed the linear trend of the polarizability of the values of the oxide ions, which are found in the glass samples. The undoped strontium phosphate glasses present the lowest value of the electronic oxide ion polarizability. We have found that the values of oxide-ion polarizability increase when the concentration of yttrium increases from 0.0 to 1.5 mol.% in glass systems. In the past, we hypothesized that a rise in the refractive index causes a rise in the polarizability of oxide ions. This is because there is a direct correlation between the polarizability of oxide ions and molar refraction (Duffy and Ingram 1971).

Fig. 13
figure 13

Molar polarizability and oxide ion polarizability as a function of the yttrium content in the prepared glass

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The metallization criterion of glass samples can be evaluated using the following relationship:

$${\text{M}} = 1 - \frac{{{\text{R}}_{\text{m}} }}{{{\text{V}}_{\text{m}} }}$$
(22)

The values of the metallization criteria (M) are directly dependent on the reflection loss (RL); according to the data in Table 5 and plotted in Fig. 14, it can be seen that M decreases with the addition of yttrium content. This decrease in M shows the metallization of the strontium phosphate-based glass samples, which leads to a decrease in the optical band gap. Therefore, the energy band gap of the phosphate glass affects the metallization criteria. The decreasing value of the energy band gaps suggests that the valence and conduction bands have widened. The glass system will become metalized as a result of this process (Azlan et al. 2019).

Fig. 14
figure 14

Optical basicity (˄) and metallization criterion (M) of the synthesized glasses with different concentrations of yttrium

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The optical electronegativity (χ), the electron polarizability (∝ °), and optical basicity (⋀) can be estimated using the following equations:

$${\chi } = 0.2688 \times E_g$$
(23)
$$\propto^{\circ} = - 0.9 \chi + 3.5$$
(24)
$$\wedge = - 0.5\chi + 1.7$$
(25)

The electronegativity (χ) of the glass samples can be calculated using the values of (Eg) (see Table 5). Therefore, the values of electronegativity (χ) decrease with increasing yttrium concentration. Because electronic polarizability and optical basicity follow the opposite trend of electronegativity (χ), their values increase as the concentration of yttrium increases.

Figure 14 also shows the change in optical basicity according to the yttrium content in strontium phosphate glasses. The high value of the optical basicity of the glass series shows that the phosphate glass system is more basic. As a Lewis base in the glass system, oxygen ions lend electrons to the nearby cations. The high degree of donating capacity of oxide ions strongly influenced the property of acid–base. Moreover, the polarization state of the oxide ions in the glass system has been said to be connected to their optical basicity (Duffy 1989). The linear trend of optical basicity along with the concentration is found in the prepared glass. In addition to that, the values of optical basicity are shown to increase somewhat with increasing concentration of Y3+. This behavior can be explained by comparing the optical basicity value of a single element in the phosphate glass system. Former oxides should be less basic than modifier oxides in glass former and modifier conditions. When modifier oxide is added to the former oxide, the modification reaction becomes an acid–base reaction in which modifier oxide ions approach the acidic area of the former oxide in decreasing order of acidity (Sidek et al. 2004).

4 Conclusion

In summary, we successfully prepared pure and yttrium-doped strontium phosphate glasses using the conventional melt-quenching method. The structural properties of the strontium phosphate-based glasses were examined by X- ray diffraction (XRD) and Fourier transform infrared (FTIR) spectra. The X-ray diffraction (XRD) pattern revealed that all of the prepared glasses were amorphous. FTIR spectroscopy shows that the addition of the yttrium content leads to depolymerization of the structural composition of the glass system network, reducing the number of metaphosphate chains and increasing the number of pyrophosphate groups. When the amount of yttrium in the sample glass increased, it was discovered that the density increased from 2.108 to 2.099 g/cm3 and the molar volume reduced from 60.974 to 62.641 cm3/mol. The optical measurement confirmed that the energy band gap of synthesized glasses decreases with increasing yttrium content. The index of refractive of the sample glass is found to increase with Y2O3 supporting the creation of non-bridging oxygen, which increases the polarizability of the phosphate glass network. We have obtained that the molar polarizability values of strontium phosphate glasses increase with increasing concentration of Y3+ (1.921–2.108 Å3). The results obtained for the oxide ion polarizability of strontium phosphate glasses are found to be linear in trend along with the Y3+ concentration (2.881–3.173 Å). With increasing concentrations of Y3+, the linear trend of optical basicity is observed in the strontium phosphate glass system (0.823–0.927). In addition, the value of the metallization criterion of strontium phosphate glass decreased with the content of yttrium (0.571 to 0.540). All of the results of all of the structural, physical, and optical analyzes are in good relation to each other and suggest that these prepared glasses are promising materials for photoelectronic and optical filters.