Optical properties of borate glasses containing chromium and erbium oxide

Abstract

Borate glass samples containing chromium and erbium were prepared. According to the density and molar volume, the Cr₂O₃-free glass sample had an expanded glass structure. Cr has three distinct absorption bands, and according to the measured optical absorption characteristics, a band was observed at 688 nm due to the ⁴A_2g(F) → ²E_g(G) transition, indicating the presence of Cr⁶⁺. Cr³⁺ was observed in the bands at 446 and 620 nm, which were attributed to ⁴A_2g (F) → ⁴T_2g (F) and ⁴A_2g (F) → ⁴T_1g (F). Using the absorption spectra of the glass samples, the Judd–Ofelt theory was used to calculate the three parameters for glass: Ω2, Ω4, and Ω6. The slow transformation of chromium ions in these glasses from Cr⁶⁺ to Cr³⁺ disturbs the local symmetry and adds coordinated bond defects, which affect the surroundings of Er³⁺ ions.

1 Introduction

The world is concentrating on finding ways to conserve energy while lowering pollution. Solid-state lighting is popular because of its eco-friendly attributes, compact size, high efficiency, low-temperature performance, extended lifespan, and low energy consumption. This could help reduce carbon emissions in places where it is utilized. A material with transparency, ease of preparation, and homogeneous structural characterization that emphasizes the amorphous aspect of glass preparation is desirable from the perspective of material manufacturing [1,2,3,4].

Owing to its superior mechanical qualities, low melting point, high thermal and chemical stability, and good solubility of rare earth elements, borate glass is often used to fabricate transparent glasses.

As the T.M. (Transition metal) has several valence states and the d-d transition, while the R.E. (rare earth) has greater emission efficiencies because of its 4f–4f and 4f–5d electronic transitions, both rare earth and transition metal-doped glass are employed to increase the optical and electrical properties [3, 5]. Er exhibits three luminescence transitions in the visible region: violet (²H_9/2 → ⁴I_15/2), green (⁴S_3/2 → ⁴I_15/2), and red (⁴F_9/2 → ⁴I_15/2). This makes it a suitable candidate for the rare-earth element category. The transition in near-IR 1.5 µm [6] can be applicable, especially for eye-safe lasers [1, 5], and in mid-IR emission at 2.7 µm (⁴I_11/2 → ⁴I_13/2) in the visible range [7].

Among other transition metals, Cr ions were chosen because of their multiple oxidation states and their ability to behave in small amounts as paramagnetic substances [8]. Cr ions can be found in glass in three different states: Cr³⁺ with CrO6 structural units functioning as modifiers, Cr⁵⁺ and Cr⁶⁺ with and structural units acting as glass formers [2, 9,10,11].

The Cr³⁺ luminescence is a superposition of a broadband and sharp emission created from the absorption transition ⁴A₂ → ⁴T₂ in the visible range [2, 12] when Cr³⁺ ions occupy low- and high-field sites in glasses [13].

In their investigation of the physical and optical characteristics of soda-lime silicate glass doped with low concentrations of Cr₂O₃, Meejitpaisan et al. [14] found that the Cr₂O₃ concentration primarily affected the density and molar volume, while the molar electron polarizability and molar refraction decreased as the refractive index increased. The presence of Cr in the hexavalent phase is shown by the peaks at 330, 350, and 370 nm, and by the minuscule peaks in trivalentat 641, 660, and 688 nm.

Pisarski et al. [15] studied composition dependent lead borate glasses doped with transition metal and rare earth ions using absorption and luminescence spectroscopy, and the presence of trivalent Cr³⁺, Eu³⁺, and Dy³⁺ ions as spectroscopic probe in glass samples. Spectral analysis indicated that Cr³⁺ ions occupy intermediate field sites; both sites coexist and are emitted from the ⁴T₂ (low-field) and ²E (high-field) states.

Using Cr₂O₃ as the nucleating agent, Suresh et al. [16] investigated the effects of local structural disorders on the spectroscopic characteristics of multicomponent CaF₂–Bi₂O₃–P₂O₅–B₂O₃ glass ceramics and the partial conversion of Cr³⁺ ions into Cr⁶⁺ ions with an increase in the concentration of the nucleating agent. Glass ceramics become more compact or polymerized as the Cr₂O₃ content increases. BO₄ and PO₄³⁻ structural components alternate in the presence of Cr⁶⁺ ions.

Vijayalakshmi et al. [17] investigated different Er³⁺ doping concentrations in lithium fluoro zinc borate glasses (0.3, 0.5, 1.0, and 1.5 mol%). The intensity parameters were examined using the Judd Ofelt hypothesis. It was noted that the glass produced would be a good option for white-light photonic applications, optical telecommunication windows, and amplifiers.

KNbO₃ phosphors that were Er-doped and Er/Cr-co-doped were investigated by Kim et al. [18]. Emissions appeared in both systems at 555 nm and 405 nm excitation. Up-conversion emissions from the ⁴S_3/2 and ⁴F_9/2 states of Er³⁺ and the ⁴T₂ state of Cr³⁺ ions were observed in the green, red, and infrared spectra of the co-doped phosphors.

I. Arul Rayappan and K. Marimuthu [19] examined the luminescence spectra and structure of Er³⁺ doped alkali borate and fluoroborate glasses and found that Er is an ionic species with a surrounding ligand, indicating a higher degree of asymmetry in the ion site with two parameter value.

Under 378 nm excitation, the title glasses showed strong green emission associated with the ⁴S_3/2 → ⁴I_15/2 transition. Sample glass, which has a composition of 49.5 Na₂O, 49.5 B₂O₃, and 1 Er₂O₃, is suitable for use with green lasers.

The optical absorption, excitation, fluorescence properties, and emission lifetimes of chromium (III) in various glass formers were studied by Lachheb et al. [10]. The weak crystal field strengths were calculated for all glasses. The increased fluorescence intensity of Cr³⁺ is affected mainly by the low optical basicity, spectral position of the Cr³⁺ absorption, and emission. The optical absorption spectra showed the existence of Cr in various oxidation stages, depending on the former. The Cr³⁺ concentration affected the maximum fluorescence emission intensity.

Li₂B₄O₇-doped glasses with Er³⁺ have also been studied [20]. Conventional glass synthesis was used to create high-quality glass samples from the matching polycrystalline compounds that contained 0.5 and 1.0 mol% Er₂O₃. The Er³⁺ luminescence center at the Li sites of the glass network had a local structure. The Judd–Ofelt theory was used to calculate the oscillator strength and experimental oscillator strength of Li₂B₄O₇: Er glass.

The aim of this study was to understand the oxidation state of Cr when replacing boron and the effect of different oxidation states of Cr ions on the emissions from glass samples containing Cr and Er ions.

2 Experimental work

Glass samples with the formula (70–x) B₂O₃–30Li₂O–xCr₂O₃ doped with 1Er₂O₃ mol% were made, with x = 0, 0.1, 0.5, and 1 mol%. The mixture of raw materials was taken into a platinum crucible. The samples were maintained at 400 °C for 30 min, followed by an hour at 1000 °C. At normal temperature, the molten substance was poured between the two copper plates and dispersed. The glassy samples were verified by XRD analysis carried out using a Philips Analytical X-ray diffractometer (PW3710). The generator current was 30 mA, the generator tension was 40 kV. 10° was the start angle (2θ) and 70° was the end angle.

To measure the optical transmission spectra, a computerized recording spectrophotometer was used to measure the wavelengths between 190 and 2500 nm (JASCO, V-570). According to the formula ρ (g/cm³) = [a/(a – b)] X (density of the toluene), the density is estimated using the Archimedes technique, where a is the weight of the glass sample in air and b is the weight in toluene.

The room temperature emission spectra recorded by a JASCO FP-8300, Japan spectrofluorometer were produced by a 150 W Xenon arc lamp.

3 Results and discussion

Figure 1 displays the X-ray diffraction patterns of samples examined to identify the type of samples material. In Fig. 1, the amorphous structure is indicated by the absence of sharp peaks.

Fig. 1

X-ray diffraction patterns of all samples

Changes in geometry, hole focus, and number coordination can have a considerable impact on density, which is a crucial attribute for any alteration induced by the glass structure [8, 21, 22]. Figure 2A and B depict the relationship between the density, molar volume, and Cr₂O₃ concentration.

Fig. 2

A and B depicts the relationship between the density, molar volume, and Cr₂O₃ concentration

An increased density was observed in comparison to Cr₂O₃ because boron was swapped out for an element with a lower molecular weight [23, 24]. The glass becomes more compact when Cr₂O₃ is added because the molar volume changes from BO4 with a radius of 0.25^oA to BO3 with a radius of 0.15^oA [21]. Glass compactness can be assessed using OPD (oxygen packing density) and d_B–B (distance separation between B–B) techniques.

According to the OPD values listed in Table 1, the value of [22] OPD changes slightly, even with an increase in oxygen in the samples under study. In addition, for d_B–B, the substitution by the oxygen number did not change, resulting in values with a slight change.

Table 1 Density, Molar volume, OPD, d_B–B, optical band gap and band tail

An optical absorption analysis can be used to determine the oxidation status of TM [25] and any splitting in the d–d transition of TM and the F–F transition of RE. Figure 3A and B shows the optical absorption spectra of the glass samples containing both TM and RE. After the initial addition of Er, peaks corresponding to the transition from the ground state to the states ²G_7/2, ²K_15/2, ²G_9/2, ⁴F_3/2–⁴F_5/2, ⁴F_7/2, ²H_11/2, ⁴S_3/2, ⁴F_9/2, ⁴I_9/2, ⁴I_11/2, and ⁴I_13/2 were seen at 356, 378, 390, 448, 486, 518, 532, 684, 784, 960, and 1514 nm [26].

Fig. 3

A and B shows the optical absorption spectra of the glass samples containing both TM and RE

When examining the growth of Cr₂O₃ in Er-doped glass samples, the creation of peaks at 350 nm [27, 28], which represent the synthesis of Cr⁶⁺ acts as a former, and an increase in Cr₂O₃ concentration up to 0.5 mol percent were noted. A broad signal at 610 nm [29] indicates that Cr³⁺ acts as a modifier (⁴A_2g(f)–⁴T_2g (F)).

Additionally, it exhibits the peaks at 669 and 697 nm [16, 30], which are attributed to the transitions ⁴A_2g → ²T_1g (2G) and ⁴A_2g → ²E (2G) [31], which are both spin-forbidden. The Cr⁶⁺/Cr³⁺ ratio in the sample was high, up to 5/0.7, and the higher the ratio, the more Cr was added up to 1 mol percent. Cr⁶⁺ sharp peaks convert predominantly to Cr³⁺ in an intermediate state of Cr⁵⁺ formation [29], as shown by the broad band formation, and the formed peak at 457 nm represents ⁴A_2g → ⁴T_1g(F) [16]. Peaks in the UV range and a small range in the visible range overcome the Er peak, as shown in the diagram.

The structural modifications that occur based on stoichiometry [9, 22] are attributed to the differences in the Eg values. In real-world applications, the optical bandgap (Eg) is calculated by measuring the optical absorption coefficient close to the absorption edge [32]. According to a previous report, powerful optical fields are favored by glasses with high refractive indices and minimal energy gaps.

One of the primary aspects that can be assessed by altering absorbance is optical band gap analysis, which may be useful in assessing the electronic band structure in the created material [22]. The optical bandgap was investigated using the absorption coefficient of the relationship.

$$\alpha = \frac{B (hv-Eg{)}^{n}}{hv}$$

(1)

Figure 4 shows the relationship between (αhv)^2/3 and hv (eV). The optical bandgap values at (hv) = 0 were generated via extrapolation, as displayed in Table 1. The Eg values for glass containing Er were similar to those reported in other studies [22, 32,33,34]. Alternatively, it can be thought of as the insertion of a Cr sublevel above the Fermi level [24, 35], as Cr lowers the Eg values [9, 36]. At this level, the BOS increases, whereas the valance-conduction distances decrease [9, 37]. Cr₂O₃ enters the glass network in two valence states, Cr³⁺ and Cr⁶⁺,which significantly affect the insulating quality and optical transmission [20, 33, 36], with CrO6 and CrO4 structural units, respectively. In the octahedral symmetry, the optical absorption spectrum of Cr³⁺ displays two broad visible bands.

Fig. 4

Shows the relationship between (αhv)^2/3 and hv (eV).

The transition ⁴A_2g(F) → ⁴T_1g (P) is also associated with two wide UV absorption bands, although it is less intense because of the double electron jump t32 g t12g e2g and one of the transitions ⁴A_2g(F) → ⁴T_2g(F) and ⁴A₂g(F) → ⁴T_1g (F).

The intensity of peaks increased as the increase of chromium oxide, and several bands in the spectrum are difficult to identify because they overlap with Er³⁺ and Cr³⁺ absorption bands.

A faint, weak band at 370 nm in the spectra of the glass sample with 0.1 mol% Cr₂O₃ shows that these glasses contain some chromium ions in the Cr⁶⁺ form [36]. Additionally, a 465 nm absorption band was predicted for Cr⁵⁺ ions [38]. The spectra of the glass sample showed a decrease in the strength of the narrow, weak band at 370 nm, which indicates a decrease in the concentration of Cr⁶⁺ ions at the expense of Cr³⁺ ions. These ions play a role in the glass network, causing an increase in the disorder of the glass network. The amount of non-bridging oxygen (NBO) in the glass matrix increases as the octahedral Cr³⁺ ion concentration increases.

Consequently, the degree of electron localization increases, which reduces the number of donor centers in the glass matrix. As demonstrated in Table 1 and Fig. 2, the increased donor centers increase the optical band tail and move the absorption edge towards lower wavelengths.

For Cr³⁺ doped lithium tetraborate glass, the crystal field Dq and the Racach parameters (B) were calculated from the relation using the measured energies of the absorption bands [39].

$${\text{Dq}} = {\text{E }}(^{{4}} {\text{A}}_{{\mathbf{2}}} ) - {\text{E }}(^{{4}} {\text{T}}_{{\mathbf{2}}} )/{1}0$$

(2)

$${\text{Dq}}/{\text{B}} = {15}\left( {{\text{X}} - {8}} \right)/{\text{X}}\left( {{\text{X}} - {1}0} \right)$$

(3)

where

$${\text{X}} = [{\text{E }}(^{{4}} {\text{A}}_{{{\mathbf{2g}}}} -^{{2}} {\text{T}}_{{{\mathbf{1g}}}} ) - {\text{E }}(^{{4}} {\text{A}}_{{{\mathbf{2g}}}} -^{{4}} {\text{T}}_{{{\mathbf{2g}}}} )]/{\text{Dq}}$$

(4)

The Dq was calculated to be 1612.9 cm⁻¹. B was estimated to have a value of 625 cm⁻¹, which is much less than the value of the free ion Cr³⁺, where B_free = 918 cm⁻¹.

When examining the ionic/covalent characteristics of the bonding between the Cr–O ligands, the value of B, which measures the interelectronic repulsion in the d-shell, is helpful. The value of Dq/B is greater than 2.3 which represents the presence of Cr³⁺ in a high-field site [40]. As a result, Cr³⁺ Centers in the high-field sites dominate the luminescence spectra of the glasses. These centers have narrow emission bands because of the ²E_g → ⁴A_2g or ²T_2g → ⁴A_2g spin-forbidden transitions, which produce conventional laser action.

The Judd–Ofelt theory is helpful in calculating the oscillator strength in the theoretical manner, and the root mean square deviation [41].

The spectral strength (f_exp) of absorption bands of glass composition doped with rare earth calculated from the integrated area under the peaks according to the relation:

$${\text{f}}_{{{\mathbf{exp}}}} = {4}.{32} \times {1}0^{{ - {\mathbf{9}}}} \smallint \alpha \left( {\text{v}} \right){\text{dv}}$$

(5)

With the Judd–Ofelt theory, the spectral strength calculated theatrically according to the relation:

$${\text{f}}_{{{\text{cal}}}} = [8\, {\text{mcv}}/3{\text{hc}}^{2} (2{\text{J}} + 1)][({\text{n}}^{2} + 2)^{2} /9{\text{n}}]\left[ {\sum\nolimits_{(\lambda = 2,4,6)} {\Omega_{\lambda } \left| {\left\langle {aj\left\| {U^{\lambda } } \right\|bj\prime } \right\rangle } \right|} } \right]$$

(6)

where Ω is the Judd–Ofelt intensity parameters and h is Plank constant, n is the refractive index, m is the mass of electron, c is the speed of light and v is frequency of transition. U are the reduced matrix elements due to the J–J transition of Er³⁺. The quality fit between the results obtained experimentally and theoretically obtained from the root mean square equation according to the relation:

$$\delta_{{{\mathbf{rms}}}} = \left[ {\left( {{\text{f}}_{{{\mathbf{exp}}}} - {\text{ f}}_{{{\text{cal}}}} } \right)^{{\mathbf{2}}} /({\text{N}}{-}{ 3}}) \right]^{{{\mathbf{1}}/{\mathbf{2}}}}$$

(7)

where N is the number energy level.

The root mean square deviation can be used to determine the correctness of the experiment, which is determined by the area of the absorption peak and the calculated oscillator strength.

The data are summarized in Table 2. The transitions ⁴I_15/2 → ²H_11/2 and ⁴G_11/2, which are classified as hypersensitive transitions, obey the selection |S|= 0 and |∆L|≤ 2 and |∆J|≤ 2 [33, 34], have high values. The RMS values were smaller, which contradicts the validity of applying the Judd–Ofelt theory.

Table 2 The experimental and the calculated oscillator strength, root mean square and Judd–Ofelt intensity parameters.

The Judd–Ofelt parameters that were created using the least-squares method are also included in Table 2. These outcomes could be contrasted with those of earlier investigations [33, 34, 38].

These features can be used to describe the radiative properties of RE, as well as the structural changes, covalency, and rigidity of the glass samples. While Ω4 is utilized to assess the stiffness of a created glass sample and has an opposing view of covalency, Ω2 is crucial for describing the covalency and asymmetry of the RE with the surrounding ligand. The Ω 6 parameter is primarily responsible for controlling the electron density between the 5d and 4f levels and is utilized to calculate the stiffness and viscosity of a sample [42]. Because each glass exhibits solitary behavior, the obtained value demonstrates the high effect of adding Cr.

The behavior of is dictated by the covalency of the samples containing 0 or 0.5 mol% Cr₂O₃, which allows for luminescence. The stiffness of the sample containing 0.1Cr₂O₃ was very high. Based on the acquired spectral values and absorption curves, the values were directly related to the Cr³⁺/Cr⁶⁺ ratio. The glass sample has a higher Cr³⁺ content, which may result in maximum fluorescence emission intensity [10].

To explore the optical qualities, the excitation and emission characteristics of the manufactured glass samples can be used. The glass sample excitation spectra recorded at 550 nm emission wavelength are shown in Fig. 4.

In the two broad bands centered at 396 nm, bands (⁴I_15/2 → ⁴G_9/2(336) and ⁴G_11/2 (378) overlap, whereas bands (⁴I_15/2 → ⁴F_3/2(443), ⁴F_5/2(451), and ⁴F_7/2 (488) overlap in the broad band centered at 430 nm [43]. The band at 378 nm was used as the pump wavelength to evaluate the visible-luminescence properties of the glass samples.

Figure 5 illustrates the development of bands at 468, 541, 570, and 667 nm, which reflect the emission from the excited states ⁴F_5/2, ²H_11/2 (green), ⁴S_3/2 (green), and ⁴F_9/2 (red) to the ground state ⁴I_15/2 with various emission colors.

Fig. 5

Shows the glass sample excitation spectra recorded at 550 nm emission wavelength.

The addition of Cr³⁺ reduces the peak intensity in the broad version [18] because Cr³⁺ ions absorb some of the excitation energy and produce near-infrared light.

The band at 450 nm disappears from the sample containing the 0.5 mol% Cr2O3 emission curve, and peaks at 410 and 430 nm arise instead. This is explained by the appearance of the emission peak ²H_9/2 → ⁴I_15/2(violet) [19] for Er, or it could be attributed to the reduction of Cr⁶⁺ to Cr³⁺, which is consistent with the absorption curve.

The CIE1931 chromaticity diagram's monochromatic (x, y) color coordinates were established using the fluorescence emission spectra produced by excitation at 378 nm. The results shown in Fig. 6 and the calculations in Table 3 indicate that the glass emits light that is almost entirely white. It was compared with other data [1, 7, 17, 44,45,46] and other results. The related color temperature was calculated using the chromaticity coordinates (x, y) of samples tabulated in Table 3 and [5, 17].

$${\text{CCT}} = - {\text{449n3}} + {\text{3525n2}} - {6823}.{\text{3n}} + {552}0.{33}$$

(8)

where n = (x – 0.332)/(y – 0.186).

Fig. 6

Illustrates the emission from the excited at 378 nm.

Table 3 The chromaticity coordinates (x, y), color temperature and Kelvin ranges indicate

The Kelvin ranges indicate that the glass samples under investigation were used as follows:

The optimum sample for use in display areas and workplaces that require extremely bright illumination is that with 1 mol% Cr₂O₃ and 1 mol% Er₂O₃. This sample emitted a bright amount of blue-white light, similar to that of daylight.

A strong bluish color of light is produced by samples with 1 mol% Er₂O₃ and 0, 0.1, or 0.5 mol% Cr₂O₃, which is frequently seen in commercial settings and is ideal for bright work lighting [7, 44] (Fig. 7).

Fig. 7

Shows the CIE1931 chromaticity diagram's monochromatic (x, y) color coordinates.

4 Conclusion

Lithium borate glass containing x Cr₂O₃—1 mol% Er₂O₃ (where x = 0, 0.1, 0.50, and 1 mol%) were prepared. The increase in density, decrease in molar volume, and values of the optical band gap with the addition of chromium oxide indicate the compaction of the glass samples under study. The Racach parameters and crystal field Dq were established. The number Dq/B represents Cr³⁺ in a high field. The Judd–Ofelt parameters (Ω2, Ω4, and Ω6) are strongly influenced by the symmetry of the Eu³⁺ site, the lithium borate composition containing Cr₂O₃, and the surrounding Eu³⁺ions (Cr³⁺, O–B groups). Except for samples containing 0.5 mol% Cr³⁺ doped glass matrix, the trend of the JO intensity parameters is Ω2 > Ω6 > Ω4.