1. Introduction

2. Materials and Methods

3. Experimental results

3.1. Glass samples

Pb-based glasses have a pale-yellow color, which is attributed to the presence of lead oxide (Figure 2). Pb-based glasses with a matrix composition of 40–40 have a more intense yellow color, due to an increase in the content of lead oxide shifts the short-wavelength absorption edge to the long-wavelength region of the spectrum [27].

Bi-based glasses had a ruby-red color due to the absorption shoulder in the region of 500 nm attributed with bismuth active centers [28], which becomes more saturated at bismuth oxide content increase.

Figure 2. The photographs of B₂O₃–GeO₂–PbO and B₂O₃–GeO₂–Bi₂O₃ glasses co-doped by Sm³⁺/Gd³⁺.

Figure 2. The photographs of B₂O₃–GeO₂–PbO and B₂O₃–GeO₂–Bi₂O₃ glasses co-doped by Sm³⁺/Gd³⁺.

The results of the elemental analysis (XFS-EDS) of the glasses showed that all samples were characterized by homogeneous composition and all of them contained an aluminum impurity due to the synthesis in corundum crucibles, which was explained by the chemical action of fluid PbO or Bi₂O₃ with the crucible material (for details see Figure S31-S42 and Table S1).

3.2. Glass structure characterisation

X-ray diffraction patterns of all synthesized glasses demonstrated a wide halo in 21-36 degree 2θ range (Figure S1) which proved the amorphous nature of all glass samples.

Samples of the two systems under consideration have common vibron modes in all frequency regions of Raman spectra (Figure S2-S3) and the corresponding interpretations are presented in Table 2, which correlates with various literature data.

In the low-frequency range (300–700 cm ^{̶ 1}) we observed a band in the 460-465 cm ^{̶ 1} range which was referred to the symmetrical stretching of Ge–O–Ge bonds in 4- or 6-membered Ge⁴⁺ rings in a tetrahedral arrangement [29,30,31]. The 800 cm ^{̶ 1} band referred to vibrations of oxygen atoms in the boroxol ring with the participation of a very small movement of boron atoms [32,33]. The presence of broad bands in the high-frequency region (1250–1500 cm ^{̶ 1}) were associated with stretching of the B–O^– terminal bonds in borate structures containing metaborate triangles [33,34,35,36].

We observed four additional bands: at 390, 650, 715, and 890 cm ^{̶ 1} for Bi-based glasses. The 390 cm^–1 band was associated with the bending modes of the Ge–O–Ge [37]. The 650 cm ^{̶ 1} band was referred as vibrations of the 6-coordinated germanium atom in the glass structure [30,38]. The 890 cm ^{̶ 1} band is generally attributed with a double degenerate vibration [39]. According to the Ref. [31] the 715 cm ^{̶ 1} band characterized tensile vibrations of the B-sublattice relative to the O-sublattice and out-of-plane ring B–O–B vibration.

Table 2. Interpretation of Raman bands in Pb-based and Bi-based glasses.

Table 2. Interpretation of Raman bands in Pb-based and Bi-based glasses.

Raman shift, cm–1		Interpretation	Reference
Pb-based	Bi-based
	390	Bending modes of Ge–O–Ge bonds	[37]
465	460	Symmetrical stretching of Ge−O−Ge bonds in 4- or 6-membered rings of Ge4+ in a tetrahedral arrangement	[29,30,31]
	650	Vibrations of 6-coordinated atom Ge (VI)	[30,38]
	715	Stretching vibration of the B-sublattice against the O-sublattice, a B–O–B out-of-plane ring vibration	[31]
800	800	Vibration of the oxygen atoms in the boroxol ring	[32,33]
	890	Ge–O–Ge double degenerate vibration	[39]
1315	1240	B−O− stretching mode in metaborate structures	[33,34,35]
1500		Overlap of following modes: B–O– in BØ2O-, BØ2O- triangle linked to BØ4 unit, and stretching in BØ3 triangles	[36]

The Fourier transform infrared spectra were interpreted based on the assumption that the B–O and O–Ge–O bonds play a key role in the glass structure. The FT-IR spectra of bismuth/lead borogermanate glasses doped with Sm³⁺ and Gd³⁺ ions were investigated in the 500–2000 cm ^{̶ 1} range (Figure S4-S5). All analyzed glass samples demonstrated active vibrational modes, indicating the presence of typical characteristics for a borogermanate glass network.

The 540 and 544 cm ^{̶ 1} bands referred to the deforming vibrations of the Ge–O bond [40]. The 773 cm ^{̶ 1} band was related to the Ge–O stretching vibrations in the structural units [GeO₄] [40,41]. Bands in the mid-frequency region (>800 cm ^{̶ 1}) at 805, 1170, 1185, 1450, and 1547 cm ^{̶ 1} described the stretching of the B–O bond. The 805 cm ^{̶ 1} band is attributed with [BO₄] unit stretching [34,41], while the bands at 1170, 1185, 1450, 1547 cm ^{̶ 1} are responsible for [BO₃] unit stretching [41,42].

An analysis of IR spectra obtained by FT-IR and Raman spectroscopies did not reveal any contradictions. Hypotheses regarding the structure of glass presented earlier [29,30,31,32,33,34,35,36,37,38,39,40,41,42] were confirmed.

3.4. Spectral-luminescent properties

We observed that the absorption spectrum of Pb-based glasses (Figure 3) included 11 main bands associated with f–f transitions from the ground state (⁶H_5/2) to various excited states of Sm³⁺. The transition to the ⁶P_3/2 state at 402 nm is the most intense, since the 4f electrons are screened by the 5s and 5p shells [43]. The remaining absorption bands are observed in the IR region. The most intense band corresponds to the ⁶H_5/2 → ⁶F_7/2 transition. The intensity of all absorption bands increased with increasing content of Sm³⁺ in the obtained Pb-based glasses.

The absorption spectra of Bi-based samples similarly included the above 11 bands specific for Sm³⁺ (Figure 4). The bands attributed to the ⁴I_9/2, ⁴I_11/2 and ⁴I_13/2 transitions (~475 nm) are invisible, since the absorption in the region of 500 nm of bismuth active centers (BACs) overlaps these bands [28,45]. An increase in the Bi₂O₃ concentration resulted in the intensity growth of 475 nm band, and in a shift of the short-wavelength absorption edge to the long-wavelength region, which led to a narrowing of the transparency window of the Bi-based glasses.

The intensive excitation band for all glasses was observed around 400 nm (Figure 5, S18). The intensity of the ⁶H_5/2 → ⁴K_11/2 transition at 404 nm is the highest regardless of the composition.

The photoluminescence (PL) spectra of glasses (Figure 6) revealed the Sm³⁺ bands at 562 nm (⁴G_5/2 → ⁶H_5/2), 597 nm (⁴G_5/2 → ⁶H_7/2), 645 nm (⁴G_5/2 → ⁶H_9/2) and 704 nm (⁴G_5/2 → ⁶H_11/2) resulted from the transitions between 4f–4f levels. The most intense transition is ⁴G_5/2 → ⁶H_7/2, which corresponds to the orange luminescence. An increase in the Sm³⁺ ion concentration led to concentration quenching. In the obtained glasses Gd³⁺ played a role of a sensitizer with respect to Sm³⁺ [46]. The presence of Gd³⁺ increased the PL intensity in times. The most intense luminescence was obtained for the (12.75)Pb(1.5)Sm(0.75)Gd-42.5 and (12.75)Bi(1.5)Sm(0.75)Gd-42.5 glass samples.

An analysis of PL kinetics (Figure 7, for details see Figure S19-S30) showed that the decay curves could be described by bi-exponential decay kinetics. The fast PL centers had the lifetime of 90-100 ns while the slow PL centers had the lifetime of 590-620 ns (Table 5).

The (12.75)Pb(1.5)Sm(0.75)Gd-42.5 and (12.75)Bi(1.5)Sm(0.75)Gd-42.5 glass samples demonstrated the most intensive PL (Figure 7, 8) and they were chosen for the testing of LT properties in the 298–673 K temperature range.

At temperature rise, the (12.75)Pb(1.5)Sm(0.75)Gd-42.5 sample demonstrated an appearance of 525–542 nm band starting with 423 K and its further growth up to 673 K, which could be associated with an increase in the population of the overlying ⁴F_3/2 state due to transitions from the ⁴G_5/2 level in samarium ions under the influence of temperature [47]. Simultaneously, the most intensive 597 and 645 nm bands gradually reduced at temperature rise from 423 to 673 K in 1.4 times maximum (Figure 8).

It is of common use that FIR is calculated using the ratio of normalized intensities of the band (Figure 9a). So, calculate FIR in the 423 to 673 K temperature range we used PL intensity of the ⁴F_3/2 → ⁶H_5/2 transition (${\lambda }_{PL}^{max}=$530 nm) as a constant value of numerator while for a denominator the PL intensities of the ⁴G_5/2 → ⁶H_5/2 (${\lambda }_{PL}^{max}=$ 562 nm); ⁴G_5/2 → ⁶H_7/2 (${\lambda }_{PL}^{max}=$597 nm); ⁴G_5/2 → ⁶H_9/2 (${\lambda }_{PL}^{max}=$645 nm) transitions were used (Figure 9b). At temperature increase starting from 423 K the PL peak at 535 nm attributed with ⁴F_3/2 → ⁶H_5/2 transition has been growing up. Using this thermosensitive transition for the numerator and thermostable transition ⁴G_5/2 → ⁶H_7/2 as the denominator we calculated FIR according to the equation (7).

$$FIR=\frac{I_{{}^{4}F_{3/2}}\to {}_{{}^{6}H_{5/2}}}{I_{{}^{4}G_{5/2}}\to {}_{{}^{6}H_{7/2}}},$$

(7)

The temperature dependence of FIR was approximated by a linear function (FIR = $a+b\times T$) (Figure 10a) and an exponential Boltzmann function (FIR= $a\times {exp}^{-∆E/kT}+b$) (Figure 10b). Regardless of the function the best descriptions of the obtained experimental data of FIR was calculated for the transitions I(⁴F_3/2 → ⁶H_5/2)/I(⁴G_5/2 → ⁶H_7/2) FIR=I₅₃₅/I₆₄₅ . We obtained Adj. R²=0.90644 for linear function and Adj. R²=0.97815 for the Boltzmann function.

The relative thermal sensitivity (S_r) for the parameter under consideration was obtained S_r ~0.20 %×K ^{̶ 1} for the temperature range 423–673 K.

We tried to increase FIR sensitivity by manipulation of different peak intensities. Indeed, when calculating FIR as a ratio

$$FIR=Q=\frac{I_1}{I_2},$$

(8)

to increase the sensitivity S_r we need to reduce the denominator at temperature growth.

In the case of (12.75)Pb(1.5)Sm(0.75)Gd-42.5 sample the generalized picture of relative PL intensities redistribution with temperature are presented in Figure 11b. The PL intensity of ⁴F_3/2 → ⁶H_5/2 transition is growing up in 2 times. And this resulted to increase of the nominator in Eq. 7. But the denominator associated with PL intensity of ⁴G_5/2 → ⁶H_7/2 transition stayed unchangeable. We need to reduce the denominator. To do this we can use the difference of the constant PL intensity of ⁴G_5/2 → ⁶H_7/2 transition and growing up PL intensity of ⁴G_5/2 → ⁶H_9/2 or ⁴G_5/2 → ⁶H_5/2. Both of the latters are temperature sensitive transitions. In this case new efficient FIR_eff could be calculated from the expression

$${FIR}_{eff}(1)=\frac{I_{{}^{4}F_{3/2}}\to {}_{{}^{6}H_{5/2}}}{I_{{}^{4}G_{5/2}}\to {}_{{}^{6}H_{7/2}}-I_{{}^{4}G_{5/2}}\to {}_{{}^{6}H_{5/2}}},$$

(9)

$${FIR}_{eff}(2)=\frac{I_{{}^{4}F_{3/2}}\to {}_{{}^{6}H_{5/2}}}{I_{{}^{4}G_{5/2}}\to {}_{{}^{6}H_{7/2}}-I_{{}^{4}G_{5/2}}\to {}_{{}^{6}H_{9/2}}},$$

(10)

Figure 11 demonstrated the privilege of such approach when using the same raw data one can increase S_r in several times by manipulations of the PL intensities from different T-sensitive and non-sensitive transitions.

Figure 11. The temperature dependence of FIR for (12.75)Pb(1.5)Sm(0.75)Gd-42.5 sample calculated using the Eq.(7) red dots (1); Eq.(9) green dots (2), Eq.(10) blue dots (3).

Figure 11. The temperature dependence of FIR for (12.75)Pb(1.5)Sm(0.75)Gd-42.5 sample calculated using the Eq.(7) red dots (1); Eq.(9) green dots (2), Eq.(10) blue dots (3).

For the (12.75)Pb(1.5)Sm(0.75)Gd-42.5 sample the corresponding sensitivities for different FIRs occurred to be S_r(1) = 0.20 %×K^–1; S_r(2) = 0.68 %×K^–1; S_r(3) = 0.61 %×K^–1. The better achieved sensitivity was more than in 3 times higher than that determined by the classical expression (7).

In the series of Bi-based samples, the most interesting was the (17)Bi(2)Sm(1)Gd-40 glass. We detected a redistribution of the intensities of the peaks corresponding to the ⁴G_5/2 → ⁶H_7/2 and ⁴G_5/2 → ⁶H_9/2 transitions (Figure 12, 13a). This redistribution is a temperature-sensitive parameter, which could be used for temperature determination. Such redistribution is usually observed for dyes [48], but we found out it for the glass sample for the first time.

So, calculating FIR in 298–673 K temperature range we tried to choose the most sensitive parameter based on PL intensities of the bands which had been redistributed: ⁴G_5/2 → ⁶H_9/2 (${\lambda }_{PL}^{max}=$ 645 nm) or ⁴G_5/2 → ⁶H_7/2 (${\lambda }_{PL}^{max}=$ 597 nm) (Figure 13b).

We varied the PL intensities ratio by changing nominator and denominator vice versa. The better approximation was derived for linear function when the I(⁴G_5/2 → ⁶H_7/2)/I(⁴G_5/2 → ⁶H_9/2) ratio was used (Figure 14a). Contrary, for the Boltzmann function the better result was achieved for the I(⁴G_5/2 → ⁶H_9/2)/I(⁴G_5/2 → ⁶H_7/2) ratio (Figure 14b). But taking into consideration the accuracy of the experimental data both these functions could be used in the case of (17)Bi(2)Sm(1)Gd-40 glass for luminescent thermometry. The relative thermal sensitivity for the considered parameter was ~0.105 %×K ^{̶ 1} for 298-673 K temperature range.

4. Discussion

5. Conclusions

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