Influence of Alkali Metal Ions on the Structural and Spectroscopic Properties of Sm³⁺-Doped Silicate Glasses

Abstract

In the present work, the influence of alkali ions (Li, Na, K) on the structural and spectroscopic properties of silica glasses doped with Sm³⁺ was investigated. Infrared and Raman spectroscopy techniques were used to investigate the structural properties of the alkali silicate glasses. The optical absorption showed bands characteristic of Sm³⁺ ions in alkali silicate glasses, and this was investigated. The Judd–Ofelt theory was applied to evaluate the phenomenological intensity parameters (Ω₂, Ω₄, and Ω₆) of the optical absorption measurements. The multi-channel visible and near infrared emission transitions originating from the ⁴G_5/2-emitting state of the Sm³⁺ in alkali silicate glasses with a maximum phonon energy of ~1050 cm⁻¹ were investigated. From the evaluated Judd–Ofelt parameters, radiative parameters such as spontaneous emission probabilities, radiative lifetimes, branching ratios, and stimulated emission cross-sections were calculated. The recorded luminescence spectra regions revealed intense green, orange, red, and near-infrared emission bands, providing new traces for developing tunable laser and optoelectronic devices.

1. Introduction

Due to their potential for use as lasers and phosphors in a variety of hosts, including glasses, crystals, and transparent vitro-ceramics that exhibit luminescent transitions in the VIS and NIR spectra regions, Ln³⁺-doped ions have attracted considerable technological interest [1]. According to research, the optical spectra of the rare earth ions can be used as structural probes to determine the local field parameters within a specific host glass because the energy levels, profiles, and intensities of the absorption and emission bands depend on how charges are distributed in the first coordination shells of the rare earth ions [2,3]. In oxide glasses, silicate glasses are one of the most popular glass hosts for making optical fiber lasers and amplifiers. Several papers have been published on the optical properties of rare earth ions in different glasses [4,5,6], but only a few of them have been concerned with Sm³⁺ [7,8,9,10]. The significant energy gap (less non-radiative decay) between the ⁴G_5/2 level and the next lower-lying energy level, ⁶F_11/2, which is roughly 7200 cm⁻¹, causes glasses doped with Sm³⁺ ions to have relatively high quantum efficiencies. It has been known for a long time that Sm³⁺ ion provide very strong luminescence in the orange and red spectral regions in a variety of lattices [11,12]. However, only a few attempts have been made to explore the possibility of using the orange-red luminescence of Sm³⁺ ions for the development of visible optical devices. The main reason for not carrying out spectral studies on Sm³⁺ ions doped in glasses is the complicated structure of the 4f⁶ configuration of this ion because these glasses show different channels that lead to luminescence-quenching and have strong phosphorescent intensities [13].

In the present work, the spectroscopic properties of Sm³⁺ ions were used to investigate the local structures of alkali (i.e., Li, Na, and K) oxides in silica. It has been known for a long time that the gradual replacement of one alkali oxide by another induces nonlinear changes in certain physical properties of glasses [14,15], and the explanation for this effect in terms of the atomic structure is not simply due to the amorphous nature of the glass.

2. Materials and Methods

Silicate glasses have the following composition: 33.0 R₂O + 66.0 SiO₂ + 1.0 Sm₂O₃ (mol%), where R = Li, Na, and K. The glasses were prepared using the traditional melting quenching technique. The laboratory chemicals used included purified sand Li₂CO₃, Na₂CO₃, K₂CO₃, SiO₂, and Sm₂O₃ (99.99% purity grade from Sigma-Aldrich (St. Louis, MO, USA)), and these were used for sample preparation. Since carbonates and silica are highly hygroscopic, they were dried at 200 °C for 1–2 h. The mixture was placed in a platinum crucible and heated at 700 to 800 °C to eliminate CO₂ [16], and then it was melted at 1550 °C for 2 h in air. The melted mixture was quickly poured into a preheated stainless steel mold and annealed at 350 °C for 4 h. After that, it was cooled down slowly to room temperature at a rate of 5 °C/min.

Table 1. Chemical compositions of the glass samples.

Sample Notation	Glass Compositions (mol%)	Glass Appearances	Melting Temperatures (°C)
LS-Sm	33.0 Li2O + 66.0 SiO2 + 1.0 Sm2O3	Transparent glass	1550
NS-Sm	33.0 Na2O + 66.0 SiO2 + 1.0 Sm2O3	Transparent glass	1550
KS-Sm	33.0 K2O + 66.0 SiO2 + 1.0 Sm2O3	Transparent glass	1550

shows the compositions, appearances, and melting temperatures of the prepared samples.

The densities of the prepared samples were measured using the Archimedes method (with an analytical balance Shimadzu AUW220D, 0.1 mg/0.01 mg) using distilled water as the immersion liquid. The refractive index was measured using a SOPRA GES-5E ellipsometer. The FTIR transmittance spectra were measured using a Bomem MB100 spectrometer using KBr pellets. The Raman spectra were measured using an HeNe laser as the excitation source. The absorption spectra in the UV-Vis-Nir region were recorded using a high-performance spectrometer (PerkinElmer model LAMBDA 1050) with a spectral resolution of 0.2 nm. The photoluminescence (PL) data were measured using a CryLas GmbH 488 nm CW laser as a pumping source. The PL signal was dispersed by an Acton SP 2300 monochromator and detected by a Pixis 256E CCD. The detected signal was fed to an SR 430 multichannel analyzer and transferred to a computer running acquisition software (OriginPro).

3. Results and Discussions

3.1. Structural Analysis

One of the most successful techniques for structural analysis is infrared spectroscopy, which makes it possible to identify structural features related to both the anionic sites containing the modifying cations and the local units that form the glass network [17]. Figure 1 shows the vibrational spectra of the alkali silicate glasses, which exhibit prominent bands at ~1037 cm⁻¹ for LS, ~1060 cm⁻¹ for NS, and ~1013 cm⁻¹ for KS due to Si–O–Si stretching within the tetrahedral by the presence of alkali ions. These bands are sharpened by the addition of rare earth ions. The introduction of rare earth ions in the matrix will alter the environment of the defect centers in the silica [18]. Also, the relative intensities of the peaks corresponding to the Si–O stretching from the tetrahedral and the non-bridged Si–O were nearly the same for the LS and NS samples, but for the KS sample, it was possible to observe a shift in the band at ~1013 cm⁻¹ to lower wavenumbers, suggesting that the effect of K on the Si–O stretching within the tetrahedral was slightly different from that of Li and Na. The bands at ~916 cm⁻¹ for LS, ~891 cm⁻¹ for NS, and 884 cm⁻¹ for KS were due to the stretching of the non-bridged terminal Si–O. The addition of alkali ions would decrease the local symmetry due to the formation of non-bridging oxygen bonds, giving rise to a stretching mode at ~900 cm⁻¹ [19]. Comparing the spectra shown in Figure 1 and described in

Table 2. Vibration mode FTIR spectra of the alkali silicate glasses.

Vibration Mode	LS-Sm Wavenumber (cm−1)	NS-Sm Wavenumber (cm−1)	KS-Sm Wavenumber (cm−1)
Si–O–Si bending vibration	472	468	447
O–Si–O bending vibration	774	752	745
Si–O–Si symmetric stretching	916	891	884
Si–O–Si asymmetric stretching	1037	1060	1013

, all of them presented shoulders at ~900 cm⁻¹, though they were more pronounced for LS and NS samples, indicating the formation of non-bridged terminal Si–O. The bands at ~774 cm⁻¹ for LS, ~752 cm⁻¹ for NS, and ~745 cm⁻¹ for KS were due to the stretching of the bridged Si–O–Si symmetric vibrations; more precisely, they were due to the bending mode of the bridging oxygen perpendicular to the Si–Si axis within the Si–O–Si plane and were at ~470 cm⁻¹ for LS and NS and ~450 cm⁻¹ for KS due to the Si–O rocking motion.

Figure 2 shows the Raman spectra of the samples. These spectra consisted of three intense peaks as representative cases, and the band assignments are presented in

Table 3. Vibration-mode Raman spectra of the alkali silicate glasses.

Vibration mode	LS-Sm Wavenumber (cm−1)	NS-Sm Wavenumber (cm−1)	KS-Sm Wavenumber (cm−1)
Si–O	591	589	587
Si–O–Si symmetric stretching	939	942	930
Si–O–Si antisymmetric stretching	1073	1094	1097

. Prominent features of the silica glass Raman spectrum were shown in Matson et al. [20]. The Raman spectrum of silica contains a large and asymmetric band near ~440 cm⁻¹ and a sharp band at ~492 cm⁻¹ related to the SiO₄ tetrahedra. A broad band at ~800 cm⁻¹ is related to the network of the SiO₂ glass, a band at ~600 cm⁻¹ is related to defects, bands at 1060 and 1190 cm⁻¹ are related to Si–O–Si vibrations, and a band at ~950 cm⁻¹ is related to crystalline metasilicates containing chains of SiO₄ tetrahedra. According to Matson et al. [20], the introduction of alkali ions in the silica network induces changes in the Raman spectrum depending on the ion type and concentration. The stretching of non-bonding oxygen on SiO₄ tetrahedra due to the alkali ion appeared at ~1100 cm⁻¹ [20]. The results shown in Figure 2 showed that intense bands were observed for all samples at close to 600 cm⁻¹, and they were related to defects in the silica network and were likely induced by the alkali ions (the creation of non-bonding oxygen atoms). Consistently, the bands close to ~1100 cm⁻¹ and ~950 cm⁻¹ were also observed for all samples while the band at ~800 cm⁻¹, related to the silica network, was not observed. This suggested that the incorporation of the rare earth oxides studied in the alkali silicate glasses caused more non-bridging oxygen to occur, and consequentially, it led to more polymerization in the network. It is expected that the presence of rare earth oxides as modifiers contribute three NBO (non-bridging-oxygen) atoms to a glass system while traditional modifiers (Li₂O, Na₂O, and K₂O) contribute only one NBO (non-bridging-oxygen) atom [21].

Physical parameters such as the densities, refractive indexes, concentrations of Sm³⁺ ions, polaron radii, inter-ionic distances, field strengths, reflection losses, optical dielectrics, and dielectric constants of the glasses were calculated using standard equations [22,23] and are presented in

Table 4. Physical properties of the Sm³⁺-doped alkali metal ion silicate samples.

Physical Properties	LS-Sm	NS-Sm	KS-Sm
Density, ρ (gm/cm3) (± 0.0005)	2.4552	2.5396	2.5598
Refractive index (n) (585 nm)	1.4930	1.4490	1.4310
Concentration of Sm3+, N (×1020 ions/cm3)	2.1896	1.8854	1.7370
Molar volume (Vm) (cm3/mol)	27.5024	31.9404	34.6697
Polaron radius, rp (Å)	6.6846	7.0264	7.2211
Interionic distance, ri (nm)	1.6591	1.7439	1.7923
Field strength, F (×1016 cm−2)	6.7139	6.0766	5.7533
Reflection losses (RL) (×10−2)	3.9107	3.3614	3.1433
Molar refraction (Rm) (cm3/mol)	7.9928	8.5671	8.9742
Optical dielectric constant (P$\partial t/\partial P$)	1.2290	1.0996	1.0478
Dielectric constant (ε)	2.2290	2.0996	2.0478
Molar polarizability (αm) × 10−22 cm3	3.1687	3.3964	3.5578

.

3.2. Optical Properties

The absorption bands of the RE ions in a host glass can be expressed as oscillator strengths, and the electronic transitions can be calculated using the expression:

$${\mathrm{f}}_{\mathrm{e}\mathrm{x}\mathrm{p}}=4.318\times {10}^{-9}\int \mathsf{\epsilon }\left(\mathsf{\nu }\right)\mathrm{d}\mathsf{\nu }$$

(1)

where the integral represents the area under the absorption curve and ε(ν) is the molar extinction coefficient. According to the Judd–Ofelt (JO) theory [24,25], which defines a set of three parameters (Ω₂, Ω₄, and Ω₆) susceptible to the environment of the RE ion, the parameter Ω₂ is related to the covalence of the metal-ligand bond while Ω₄ and Ω₆ are related to the rigidity of the host matrix. In the Judd–Ofelt theory, the oscillator strength, $f_{cal}$, of the spectral intensity of an electric dipole absorption transition from the initial state, aJ, to the final state, bJ’,

$$f_{cal}\left(aJ,b{J}^{\prime }\right)=\frac{8{\pi }^{2}mc\nu }{3h(2J+1)}\left[\frac{{(n^2+2)}^2}{9n}\right]x\sum _{\lambda =\mathrm{2,4},6}{\mathsf{\Omega }}_{\lambda }{⟨aJ|U^{\lambda }|bJ\prime ⟩}^2$$

(2)

where m is the mass of an electron, c is the velocity of the light, h is the Planck’s constant, v is the wavenumber of the absorption peak, n is the refractive index, and U^λ is the doubly reduced matrix element.

The optical absorption spectra of the Sm³⁺-doped different alkali silicate glasses (Li, Na, and K) recorded at temperature room are shown in Figure 3. The spectral behavior of the Sm³⁺ in the glasses was very similar to that found in fluorophosphate [26], borosulphate [27], and germanate [28] glasses. Seven absorption bands were shown for each sample, as described in

Table 5. Bands positions (cm⁻¹) and their assignments for the Sm³⁺-doped alkali silicates.

Transitions 6H5/2	LS-Sm	NS-Sm	KS-Sm
6F11/2	10,626.99	10,655.73	10,672.36
6F9/2	9325.81	9353.91	9369.21
6F7/2	8185.95	8183.33	8169.19
6F5/2	7316.94	7325.95	7322.03
6F3/2	6815.18	6821.16	6806.60
6H15/2	6558.67	6560.53	6549.88
6F1/2	6288.49	6306.04	6309.73
$\overline{\beta }$	1.01436	1.01569	1.01573
$\delta$	−0.01416	−0.01544	−0.01548

, which were associated with the absorption transitions from the ⁶H_5/2 ground state to the excited states, as labeled in Figure 3. The observed band positions of different absorption peaks of the samples were due to the interactions between the alkali ions and the crystal field. The band shapes of the ions in all three alkali silicate glasses were similar, with small differences in the half-band widths and peak positions. These results suggested that the Sm³⁺ ions resided in essentially the same sites as the three alkali ions. The calculated $\overline{\beta }$ and $\delta$ values of the glasses are presented in Table 5. The observed the nephelauxetic effect [29,30] was increased with the increase in the alkali ions as follows: K > Na > Li. The negative results demonstrated that the Sm³⁺ ions and their surrounding ligands were primarily ionic in character. Sm³⁺ has a 4f⁵ electron configuration which is characterized by ^2S+1L_J free-ion levels. With knowledge of the Sm³⁺ concentrations, sample thicknesses, peaks positions, and peak areas, the experimental oscillator strengths were obtained by using Equation (1). From the experimentally measured oscillator strengths and doubly reduced matrix elements [31], the J–O intensity parameters were obtained using Equation (2). The measured, as well calculated, oscillator strengths and J–O intensity parameters of the Sm³⁺ in the alkali silicate glasses are presented in

Table 6. Experimental ($f_{exp}$) and calculated ($f_{cal}$) oscillator strengths (×10⁻⁶) and rms (${\sigma }_{rms}$) for the Sm³⁺ alkali silicate glasses.

Transitions 6H5/2	LS-Sm		NS-Sm		KS-Sm
	${\mathit{f}}_{\mathit{e}\mathit{x}\mathit{p}}$	${\mathit{f}}_{\mathbf{c}\mathit{a}\mathit{l}}$	${\mathit{f}}_{\mathit{e}\mathit{x}\mathit{p}}$	${\mathit{f}}_{\mathbf{c}\mathit{a}\mathit{l}}$	${\mathit{f}}_{\mathit{e}\mathit{x}\mathit{p}}$	${\mathit{f}}_{\mathbf{c}\mathit{a}\mathit{l}}$
6F11/2	0.602	0.568	0.363	0.407	0.309	0.205
6F9/2	4.720	3.440	3.390	2.470	1.640	1.230
6F7/2	3.490	4.660	2.490	3.33	1.220	1.610
6F5/2	2.360	1.890	1.630	1.370	0.648	0.595
6F3/2	1.210	1.060	1.170	0.874	0.760	0.475
6H15/2 + 6F1/2	0.897	0.326	1.091	0.383	0.806	0.307
${\sigma }_{rms}$	0.9856		0.8310		0.5036

and Table 7, respectively. The magnitudes of J–O intensity parameters are important for investigations of glass structures and transition properties of rare earth ions. In general, the Ω₂ parameter is an indicator of the covalence of a metal–ligand bond (short-range effect) and the Ω₄ and Ω₆ parameters provide information about the rigidity of a host glass’s matrix (long-range effects) [32].

In the values for Ω₂, Ω₄, and Ω₆ in Table 7 for the Sm³⁺-doped alkali silicate glasses, we observed variations in the intensities of the parameters as follows: Ω₆ > Ω₄ > Ω₂. The lithium silicate glass matrix exhibited higher Ω₂ values and the potassium silicate glass matrix was minimal, indicating lower covalence values for these compositions. It has been proposed that in oxide glasses, a rare earth ion is surrounded by eight neighboring oxygen atoms belonging to the corners of SiO₄, forming a tetrahedral. Normally, the intensity parameter Ω₂, which indicates covalence, decreases with decreases in the intensity of the hypersensitive transition.

The hypersensitive transitions for the Sm³⁺ ions were ⁶H_5/2→⁶F_1/2 and ⁶F_3/2. The hypersensitivity of a transition is proportional to the nephelauxetic ratio, indicating the RE–O bond’s ionic nature [33]. The Ω₆ parameter was the largest in the glasses, showing that the Sm–O bond was more ionic in these glasses than in other glasses and indicating the higher rigidity of the glasses. The shift in the peak wavelengths of the hypersensitive transitions towards shorter wavelengths increased with the size of the alkali ions in the glass matrix, indicating decreases in the ionic nature of the RE–O bonds. Also, these changes were not the result of a large-scale structural rearrangement of the local glass network, but rather, they were primarily caused by the interactions between the alkali ions and the matrix.

Table 7. A comparison of the Judd–Ofelt parameters (Ω_λ) (×10⁻²⁰ cm²) in different hosts.

Host Matrix	${\mathsf{\Omega }}_2$	${\mathsf{\Omega }}_4$	${\mathsf{\Omega }}_6$	${\mathsf{\chi }=\mathsf{\Omega }}_4{/\mathsf{\Omega }}_6$	Order
LS-Sm3+ (present work)	1.28	3.14	4.53	0.69	${\mathsf{\Omega }}_6>{\mathsf{\Omega }}_4>{\mathsf{\Omega }}_2$
LCN borate [34]	0.84	4.00	5.02	0.79	${\mathsf{\Omega }}_6>{\mathsf{\Omega }}_4>{\mathsf{\Omega }}_2$
30Li2O:70B2O3-Pr3+ [35]	0.10	4.71	5.28	0.89	${\mathsf{\Omega }}_6>{\mathsf{\Omega }}_4>{\mathsf{\Omega }}_2$
NS-Sm3+ (present work)	1.27	2.69	3.15	0.85	${\mathsf{\Omega }}_6>{\mathsf{\Omega }}_4>{\mathsf{\Omega }}_2$
NaZnBS-Sm3+ [27]	0.55	9.68	9.77	0.99	${\mathsf{\Omega }}_6>{\mathsf{\Omega }}_4>{\mathsf{\Omega }}_2$
30Na2O:70B2O3-Pr3+ [35]	0.98	4.76	4.86	0.97	${\mathsf{\Omega }}_6>{\mathsf{\Omega }}_4>{\mathsf{\Omega }}_2$
KS-Sm3+ (present work)	1.04	1.12	1.61	0.69	${\mathsf{\Omega }}_6>{\mathsf{\Omega }}_4>{\mathsf{\Omega }}_2$
KZnBS-Sm3+ [27]	0.18	11.37	11.45	0.99	${\mathsf{\Omega }}_6>{\mathsf{\Omega }}_4>{\mathsf{\Omega }}_2$
LKG [11]	0.63	4.05	4.69	0.86	${\mathsf{\Omega }}_6>{\mathsf{\Omega }}_4>{\mathsf{\Omega }}_2$

Table 7. A comparison of the Judd–Ofelt parameters (Ω_λ) (×10⁻²⁰ cm²) in different hosts.

Host Matrix	${\mathsf{\Omega }}_2$	${\mathsf{\Omega }}_4$	${\mathsf{\Omega }}_6$	${\mathsf{\chi }=\mathsf{\Omega }}_4{/\mathsf{\Omega }}_6$	Order
LS-Sm3+ (present work)	1.28	3.14	4.53	0.69	${\mathsf{\Omega }}_6>{\mathsf{\Omega }}_4>{\mathsf{\Omega }}_2$
LCN borate [34]	0.84	4.00	5.02	0.79	${\mathsf{\Omega }}_6>{\mathsf{\Omega }}_4>{\mathsf{\Omega }}_2$
30Li2O:70B2O3-Pr3+ [35]	0.10	4.71	5.28	0.89	${\mathsf{\Omega }}_6>{\mathsf{\Omega }}_4>{\mathsf{\Omega }}_2$
NS-Sm3+ (present work)	1.27	2.69	3.15	0.85	${\mathsf{\Omega }}_6>{\mathsf{\Omega }}_4>{\mathsf{\Omega }}_2$
NaZnBS-Sm3+ [27]	0.55	9.68	9.77	0.99	${\mathsf{\Omega }}_6>{\mathsf{\Omega }}_4>{\mathsf{\Omega }}_2$
30Na2O:70B2O3-Pr3+ [35]	0.98	4.76	4.86	0.97	${\mathsf{\Omega }}_6>{\mathsf{\Omega }}_4>{\mathsf{\Omega }}_2$
KS-Sm3+ (present work)	1.04	1.12	1.61	0.69	${\mathsf{\Omega }}_6>{\mathsf{\Omega }}_4>{\mathsf{\Omega }}_2$
KZnBS-Sm3+ [27]	0.18	11.37	11.45	0.99	${\mathsf{\Omega }}_6>{\mathsf{\Omega }}_4>{\mathsf{\Omega }}_2$
LKG [11]	0.63	4.05	4.69	0.86	${\mathsf{\Omega }}_6>{\mathsf{\Omega }}_4>{\mathsf{\Omega }}_2$

3.3. Emission Spectra and Radiative Properties

The emission spectra under the 488 nm excitation wavelength consisted of potential green, yellow, and orange-reddish emissions at ~565 nm (⁴G_5/2→⁶H_5/2 magnetic dipole (MD) transition), ~600 nm (⁴G_5/2→⁶H_7/2 mixed magnetic-electric dipole transition), ~650 nm (⁴G_5/2→⁶H_9/2 electric dipole transition), and ~710 nm (⁴G_5/2→⁶H_11/2 electronic dipole transition), all of which were recorded at room temperature in the VIS regions, as shown in Figure 4. The peaks in the NIR region centered at ~915 nm (⁴G_5/2→⁶F_3/2), ~960 nm (⁴G_5/2→⁶F_5/2), ~1045 nm (⁴G_5/2→⁶F_7/2), ~1200 nm (⁴G_5/2→⁶F_9/2), and ~1300 nm (⁴G_5/2→⁶F_11/2) are presented in Figure 5. It is important to note the largest number of transitions observed, including the one at 1300 nm, indicated that the alkali silicate glasses were very efficient systems.

The radiative transition probability A_rad (aJ and bJ’) is given by the expression:

$$A_{rad}\left(aJ,b{J}^{\prime }\right)=\frac{64{\pi }^4{\upsilon }^3{e}^2}{3h(2J+1)}\left[\frac{n{(n^2+2)}^2}{9}{S}_{ed}\right]$$

(3)

where S_ed and S_md are electric and magnetic dipole line-strengths, respectively. From these values, it is possible to calculate the branching ratio and radiative lifetime of an emitting state, aJ, which is calculated from:

$${\beta }_{rad}\left(aJ,b{J}^{\prime }\right)=\frac{A_{rad}(aJ,bJ\prime )}{{\sum }_{bJ\prime }{A}_{rad}(aJ,bJ\prime )}$$

(4)

and

$$\frac{1}{{\tau }_{rad}}={\sum }_{bJ\prime }{A}_{rad}(aJ,bJ\prime )$$

(5)

In addition, another important radiative parameter property, the fluorescent integrated emission cross-section (σ_p) for stimulated emissions, is estimated from the following equation:

$${\sigma }_p\left(aJ,b{J}^{\prime }\right)=\frac{{\lambda }_p^4}{8\pi c{n}^2\mathsf{\Delta }{\lambda }_{eff}}{A}_{rad}(aJ,bJ\prime )$$

(6)

Due to the characteristic lack of long-range order in glasses, the local environment of RE ions is expected to be slightly different from one site to another, resulting in broad absorption bands. The crystal field splitting is also responsible for the broadening of these bands [26]. It has been assumed that in oxide glasses, an RE ion has eight neighboring oxygen atoms that are shared with the corners of the glass, forming a tetrahedral. In lanthanides, there are certain f–f transitions that are exceptionally sensitive to the local environment, and they are known as hypersensitive transitions (∆J ≤ 2, ∆L ≤ 2, and ∆S = 0). For Sm⁺³, they corresponded to the ⁶H_5/2→⁶F_3/2 and ⁶F_1/2 transitions, which were in the visible range. The addition of alkali ions as network modifiers in oxide glasses induced the formation of non-bridging oxygen atoms, changing the forming cation coordination number. This effect induced changes in the lanthanide–oxygen distances and, therefore, should affect the optical properties of the RE ion in the oxide glass [26].

Using the phenomenological intensity parameters, one can estimate the radiative transition probability values for A_rad(s⁻¹) (Equation (3)). From these values, it is possible to calculate the branching ratio β_R (Equation (4)) and radiative lifetime τ_rad (ms) (Equation (5)) from the ⁴G_5/2 excited level to all its lower-lying levels for alkali silicate glasses, as shown in

Table 8. Radiative transition probability values (A_rad (s⁻¹)), branching ratio values (β_R), and radiative lifetime values (τ_rad (ms)) for the Sm³⁺-doped alkali silicate glasses.

Transitions from 4G5/2 to	LS-Sm				NS-Sm				KS-Sm
	$\mathsf{\Delta }\mathit{E}\left({\mathbf{c}\mathbf{m}}^{-1}\right)$	${\mathit{A}}_{\mathit{r}\mathit{a}\mathit{d}}\left({\mathbf{s}}^{-1}\right)$	${\mathit{\beta }}_{\mathit{R}}$	${\mathit{\tau }}_{\mathit{r}\mathit{a}\mathit{d}}$	$\mathsf{\Delta }\mathit{E}\left({\mathbf{c}\mathbf{m}}^{-1}\right)$	${\mathit{A}}_{\mathit{r}\mathit{a}\mathit{d}}\left({\mathbf{s}}^{-1}\right)$	${\mathit{\beta }}_{\mathit{R}}$	${\mathit{\tau }}_{\mathit{r}\mathit{a}\mathit{d}}$	$\mathsf{\Delta }\mathit{E}\left({\mathbf{c}\mathbf{m}}^{-1}\right)$	${\mathit{A}}_{\mathit{r}\mathit{a}\mathit{d}}\left({\mathbf{s}}^{-1}\right)$	${\mathit{\beta }}_{\mathit{R}}$	${\mathit{\tau }}_{\mathit{r}\mathit{a}\mathit{d}}$
6H5/2	17,740	45.40	0.1812	3.99	17,731	36.28	0.1951	5.37	17,728	17.19	0.1906	11.08
6H7/2	16,608	110.618	0.4415		16,584	76.318	0.4104		16,568	34.372	0.3808
6H9/2	15,407	58.314	0.2327		15,408	46.000	0.2473		15,369	25.510	0.2827
6H11/2	14,101	26.256	0.1048		14,065	18.945	0.1019		14,025	8.074	0.0895
6F3/2	10,890	0.800	0.0032		10,905	0.705	0.0038		10,834	0.502	0.0056
6F5/2	10,424	5.925	0.0236		10,395	5.257	0.0283		10,373	3.405	0.0377
6F7/2	9578	1.965	0.0078		9606	1.505	0.0081		9533	0.606	0.0067
6F9/2	8324	0.860	0.0034		8326	0.701	0.0038		8321	0.458	0.0051
6F11/2	7686	0.419	0.0017		7669	0.271	0.0015		7645	0.129	0.0014

. The stimulated emission cross-section (Equation (6)) is an important parameter for predicting a better laser performance and high quantum efficiency of glass matrices.

Table 9. Stimulated emission cross-sections (σ(λ_p) × 10⁻²² cm²) for the emission peak wavelengths (λ_p) in the infrared with the effective line width values (λ_eff) for the Sm³⁺-doped alkali silicate glasses.

Transitions from 4G5/2 to	LS-Sm			NS-Sm			KS-Sm
	${\mathit{\lambda }}_{\mathit{p}}\left(\mathbf{n}\mathbf{m}\right)$	$\mathsf{\Delta }{\mathit{\lambda }}_{\mathit{e}\mathit{f}\mathit{f}}\left(\mathbf{n}\mathbf{m}\right)$	$\mathit{\sigma }\left({\mathit{\lambda }}_{\mathit{p}}\right)$	${\mathit{\lambda }}_{\mathit{p}}\left(\mathbf{n}\mathbf{m}\right)$	$\mathsf{\Delta }{\mathit{\lambda }}_{\mathit{e}\mathit{f}\mathit{f}}\left(\mathbf{n}\mathbf{m}\right)$	$\mathit{\sigma }\left({\mathit{\lambda }}_{\mathit{p}}\right)$	${\mathit{\lambda }}_{\mathit{p}}\left(\mathbf{n}\mathbf{m}\right)$	$\mathsf{\Delta }{\mathit{\lambda }}_{\mathit{e}\mathit{f}\mathit{f}}\left(\mathbf{n}\mathbf{m}\right)$	$\mathit{\sigma }\left({\mathit{\lambda }}_{\mathit{p}}\right)$
6H5/2	564	10.14	2.69	564	11.01	2.11	565	12.28	0.92
6H7/2	602	14.16	6.14	603	12.82	4.97	604	12.68	2.31
6H9/2	649	14.76	4.17	650	14.83	3.49	651	13.73	2.16
6H11/2	710	22.91	1.73	711	21.63	1.42	713	18.75	0.72
6F3/2	919	19.03	0.18	919	17.74	0.18	921	19.60	0.12
6F5/2	959	27.37	1.09	962	28.36	1.01	964	22.92	0.83
6F7/2	1044	27.25	0.51	1043	32.19	0.35	1052	24.68	0.19
6F9/2	1199	70.61	0.15	1201	60.99	0.15	1204	61.82	0.10
6F11/2	1301	39.59	0.18	1309	33.35	0.15	1307	41.09	0.06

reports the emission band positions, the effective bandwidths, and the values of emission cross-section $\sigma \left({\lambda }_p\right)$ for some of the transitions originating from the ⁴G_5/2 level of the Sm³⁺ ions in the silicate glasses. Even though the magnitudes of the intensity parameters obtained in the present work were low, the measured $\sigma \left({\lambda }_p\right)$ values for the ⁴G_5/2→⁶H_7/2, _9/2 transitions were relatively higher than those reported for other glasses [36,37,38,39]. The highest values were observed for the ⁴G_5/2-⁶H_7/2 transitions (6.14 × 10⁻²² cm², 4.97 × 10⁻²² cm², and 2.31 × 10⁻²² cm² for the LS, NS, and KS samples, respectively). Despite the values of $\sigma \left({\lambda }_p\right)$ for the NIR emissions being lower than those of the VIS emissions, they were of the same order of magnitude. Therefore, the novel NIR emissions of the alkali silicate glasses provided a new clue for developing potential NIR optoelectronic devices.

4. Conclusions

The present work detailed the influence of alkali ions on the structural and spectroscopic properties of Sm³⁺ ion-doped silicon glasses. The FTIR spectra of the doped silica showed that the Si–O–Si bending modes were sharpened with the introduction of rare earth ions. The addition of alkali ions decreased the local symmetry due to the formation of non-bridging oxygen bonds, giving rise to a stretching mode at ~900 cm⁻¹. The optical properties of the Sm³⁺ alkali silicates were measured and analyzed using the Judd–Ofelt theory. The Ω₆ parameter was the largest in the glasses, showing that the Sm–O bonds were more ionic in these glasses than in other glasses and indicating the higher rigidity of these glasses. The intensities of the emission bands in the VIS and NIR regions of the samples containing Li were relatively high compared to those of the samples containing Na and K. The emission spectra showed potential for green, yellow, and orange-reddish emissions. The branching ratios were larger for the ⁴G_5/2→⁶H_7/2 (VIS region) transition and decreased from the Li and Na to the K silicates, and the ratios for the ⁴G_5/2→⁶F_5/2 (NIR region) transition decreased as follows: Li > Na > K. The stimulated emission cross-sections obtained were similar to the values reported in the literature for Sm³⁺ in different glasses.