Novel phosphor GdY₂SbO₇ co-dope with Eu³⁺ and Bi³⁺ for optical thermometer

Abstract

A series of GdY₂SbO₇:Bi³⁺, Eu³⁺ phosphors were prepared using the conventional solid-state reaction. In this study, the photoluminescence properties and temperature sensitivity of the samples were investigated. When Bi³⁺ and Eu³⁺ were codoped into GdY₂SbO₇, the intensity of Bi³⁺ decreased with increasing Eu³⁺ concentration, indicating a potential energy transfer from Bi³⁺ to Eu³⁺. To examine the temperature sensitivity of the sample, its emission spectrum was investigated in the range of 300–500 K. Based on different temperature dependences of Bi³⁺ and Eu³⁺, the relative sensitivity (S_r) and absolute sensitivity (S_a) of the samples were calculated using the fluorescence intensity ratio (FIR) and thermochromic methods. In FIR modes, S_r based on I_Eu³⁺/I_Bi³⁺ reached 1.26 % K⁻¹ at 500 K, while S_r on double excitation method reached 1.36 % K⁻¹ at 340 K. In addition, according to the thermochromic properties of GdY₂SbO₇:Bi³⁺, Eu³⁺ phosphor, the temperature-sensing ability of the sample was investigated, and S_r reached a maximum value of 0.5996 % K⁻¹ at 300 K.

1. Introduction

Temperature is a crucial physical parameter in numerous industrial operations and scientific research. Because of their accuracy, sensitivity, and applicability to complex environments, non-contact fluorescence thermometers have attracted considerable research interest in recent years and are used for temperature measurements in medicine, chemistry, and other industries [1,2]. Two main temperature measurement strategies are generally practiced: one is temperature measurement using a pair of thermally coupled levels (TCL), where the relative sensitivity (S_r) is directly proportional to the energy gap of the relevant TCLs [3]. The second is based on the fluorescence intensity ratio (FIR) of the dual luminescence center. The FIR temperature-sensing material usually exhibits two or more distinct emission peaks as the monitoring signals, using which the temperature can be calibrated by analyzing the variation of the ratio of emission peak integral intensity with the varying temperatures of samples [4]. For the TCL mode, lanthanide elements with rich energy levels are widely used to implement this strategy [5,6]. However, owing to the small gap (200–2000 cm⁻¹) between the thermally coupled levels, the relative sensitivity of this mode to the sample is limited [7]. Therefore, in this study, we used dual-activated luminescence temperature probes to modify and improve sensitivity [8]. The FIR mode has received considerable attention because its relative sensitivity is not limited by the energy level gap. Several studies have been reported on dual-mode optical thermometers, such as SrMgAl₁₀O₇:Mn²⁺/Mn⁴⁺ [9], NaZr₂(PO₄)₃:Eu²⁺/Eu³⁺ [10], and YNbO₄:Pr³⁺/Tb³⁺ [11].

Because the principle of the dual luminescence center temperature measurement mode is based on the thermal quenching effect of ion fluorescence [12]. The exploitation of the emissions of Bi³⁺ and single trivalent rare earth (Re³⁺) ions can perform the temperature measurement, because they have completely different electronic structures. The fluorescence intensity of Bi³⁺ is readily affected by the environment because of its naked 6s electrons, whereas the Re³⁺ ions are less affected by temperature due to the outer electronic configuration of 5s²5p⁶ [13,14]. On the other hand, Bi³⁺ ions were used as sensitizer co-doped with Re³⁺ in some host materials due to the energy transfer from Bi³⁺ to Re³⁺ ions. Several studies have been reported such as LaVO₄:Bi³⁺/Eu³⁺ [15], LaNbO4:Bi³⁺/Ln³⁺ (Ln = Eu/Tb/Dy/Sm) [14] and Ca₃Sc₂Si₃O₁₂:Bi³⁺/Tb³⁺/Eu³⁺ [16]. Among them, the exploitation of the emissions of Bi³⁺and Eu³⁺ ions are a facile approach to achieve higher efficient energy transfer. Meanwhile, the fluorescence emission spectrum of Eu³⁺ hardly overlaps with that of Bi³⁺. Therefore, phosphors codoped with Bi³⁺ and Eu³⁺ are excellent temperature-sensing materials [[17], [18], [19], [20], [21], [22]]. In addition, antimonates are often used as fluorescent material substrates because of their chemical stability and low synthesis costs [3,23,24]. To the best of our knowledge, GdY₂SbO₇ has not been reported as a phosphor host. Therefore, in the current study, GdY₂SbO₇ was chosen as the host material for phosphors.

Herein, Gd_0.99-yY₂SbO₇:0.01Bi³⁺, yEu³⁺ (y = 0.01, 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30) phosphors were prepared using conventional solid-state reactions. The crystal structure and luminescence properties of the phosphors, along with their temperature-dependent sensing properties, were investigated. The absolute and relative sensitivities of the phosphor samples were calculated in FIR and thermochromic methods, respectively. Two kinds of FIR methods were adopted in our works. One was FIR based on I_Eu³⁺/I_Bi³⁺, the other was on double excitation method. Our findings revealed that Bi³⁺/Eu³⁺ codoped phosphors exhibit energy transfer, outstanding thermal properties, and excellent temperature sensitivity. The details of the study are discussed in the following sections.

2. Experimental

A series of phosphor samples, Gd_0.99−yY₂SbO₇:0.01Bi³⁺, yEu³⁺ (y = 0.01, 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30), were prepared using conventional solid-state reactions. The raw materials were high-purity Gd₂O₃ (99.99 %, Aladdin), Y₂O₃ (99.99 %, Aladdin), Sb₂O₃ (99.99 %, Aladdin), Eu₂O₃ (99.99 %, Aladdin), and Bi₂O₃ (99.99 %, Aladdin), weighed according to the stoichiometric ratio. All raw materials were ground in an agate mortar for 30 min, then loaded into a crucible, placed in a muffle furnace, and sintered at 1350 °C for 40 h, respectively. After the phosphor had naturally cooled to room temperature, the sample was ground again for subsequent testing.

To analyze the crystal structure of the samples, X-ray diffraction (XRD) patterns were recorded using a powder diffractometer (D/max-2200/PC, Rigaku, Japan) with Cu Kα radiation (λ = 1.5406 Å) in the 2θ range of 10°–90°. Scanning electron microscopy (SEM), energy dispersive spectroscopy, and the element distribution maps were measured using a field-emission scanning electron microscope (SU-8010, Hitachi, Japan). The F-7000 spectrometer (Hitachi, Japan) and 450 W xenon lamp were used to test the photoluminescence and photoluminescence excitation spectra. The aforementioned experiments were performed at room temperature. The decay curves and temperature-dependent luminescence spectra were obtained using an FLS-980 spectrometer (Edinburgh Instruments, England).

3. Results and discussion

The crystal structure of the GYSO host is shown in Fig. 1. GYSO exhibits an orthogonal structure with space group C222₁. The crystal parameters are a = 10.5172 Å, b = 7.4527 Å, c = 7.4806 Å, and V = 586.3409 Å³. In this structure, Gd³⁺ and Y³⁺ occupy two types of sites: the 4b site with an eight coordination and the 8c site with a seven coordination. The presence of six oxygen ions and two vacant anion sites around the ion at the Gd (2) position leads to the octa coordination of Gd³⁺ at this position. The six-coordinated Sb ions are staggered with the eight-coordinated Gd (2) position ions, forming planes parallel to the a direction, and between these planes are arranged the seven-coordinated Gd (1) position ions. In the phosphor samples, Bi³⁺ (r = 1.17 Å, CN = 8) and Eu³⁺ (r = 1.066 Å, CN = 8) replaced ions at position Gd (2) (r = 1.053 Å, CN = 8) owing to the similar ionic radii and following the Judd–Ofelt theory [25].

Fig. 1.

Crystal structure diagram of GYSO host and coordination environment of Gd, Y, and Sb cations in the lattice.

The XRD patterns of the synthesized Gd_0.99-yY₂SbO₇:0.01Bi³⁺, yEu³⁺ (y = 0.01, 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30) phosphors are displayed in Fig. 2. It can be seen that the most diffraction peaks of the samples match the GYSO standard card (PDF#04-010-2974) considerably well. Apart from this, the small amounts of Gd₂O₃ impurity were detected in the samples. However, the luminescence of impurity is too weak to influence the luminescence characteristics of GYSO:Bi³⁺,Eu³⁺. The inset shows a magnified XRD pattern in the range of 28°–31°. The slight shift of the diffraction peaks indicates that Eu³⁺ and Bi³⁺ were successfully doped into the hosts. As shown in Fig. 2(b) and (c), the XRD data of GYSO and GYSO:0.01Bi³⁺/0.15Eu³⁺ phosphors were refined by the Rietveld method using the GSAS software. The refinement results showed that the values of R_wp, R_p, and χ² were 1.1 %, 0.84 %, 1.323 and 1.18 %, 0.88 %, 1.535, respectively. It indicated that the refinement results are reasonable and reliable.

Fig. 2.

(a) X-ray diffraction spectra of G_0.99-yYSO:0.01Bi³⁺, yEu³⁺(y = 0.01, 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30) and standard GYSO data (PDF#04–010–2974). The inset shows a magnified XRD pattern in the range of 28–31°. The Rietveld refinement of (b) GYSO and (c) GYSO:0.01Bi³⁺/0.15Eu³⁺.

The SEM image of GYSO:0.01Bi³⁺/0.15Eu³⁺ is shown in Fig. 3, which indicates that the sample comprises irregular particles with a size of approximately 0.3 μm. The elemental mapping result shows that Gd, Y, Sb, Eu, and Bi are uniformly distributed throughout the range of phosphor particles.

Fig. 3.

SEM image and EDS pattern of GYSO:0.01Bi³⁺/0.15Eu³⁺.

Fig. 4 (a) shows the excitation spectrum of GYSO:0.15Eu³⁺ monitored at 580 nm. The spectrum comprises two parts. The broadband absorption centered at 274 nm is attributed to the O²⁻ → Eu³⁺ charge transfer. The subsequent weak absorption peak at 393 nm corresponds to the ⁷F₀ → ⁵L₆ transition of Eu³⁺ [18,19]. Fig. 4 (b) shows the PL of G_1-xYSO:xEu³⁺ (x = 0.01, 0.05, 0.10, 0.15, and 0.20) samples. Under the excitation wavelength of 274 nm, a series of emission peaks appear in the range of 580–713 nm, owing to the ⁵D₀ → ⁷F_0,1,2,3,4 transition of Eu³⁺ [[26], [27], [28], [29]]. In the current host, Gd³⁺ contains two types of sites: the 4b site with eight coordination and the 8c site with seven coordination. According to the Judd–Ofelt theory, the emission caused by the ⁵D₀ → ⁷F₂ electric dipole transition dominates when Eu³⁺ occupies the noncentrosymmetric site. However, the ⁵D₀ → ⁷F₁ magnetic dipole transition dominates when Eu³⁺ occupies the centrosymmetric site. The fluorescence emission of Eu³⁺ at 587 nm (⁵D₀ → ⁷F₁) is stronger than that at 627 nm (⁵D₀ → ⁷F₂), as shown in Fig. 4 (b), which proves that Eu³⁺ is more likely to occupy the Gd (2) centrosymmetric site with the eight coordination [30,31]. As the concentration of Eu³⁺ increases, the emission intensity of the host (446 nm) becomes weaker, while that of Eu³⁺ becomes stronger, indicating the possibility of energy transfer from the host to Eu³⁺ [32]. The emission intensity of Eu³⁺ is strongest at x = 0.15 and then decreases because of the concentration quenching effect.

Fig. 4.

(a) PLE spectrum of GYSO:0.15Eu³⁺ phosphor. (b) The PL spectra of a series of G_1-xYSO:xEu³⁺ (x = 0.01, 0.05, 0.10, 0.15, and 0.20). The inset shows the variation of the maximum value of fluorescence intensity of the samples with Eu³⁺ concentration.

The excitation and emission spectra of GYSO:0.01Bi³⁺, GYSO:0.15Eu³⁺, and GYSO:0.01Bi³⁺/0.15Eu³⁺ are shown in Fig. 5. Under 301 nm excitation, GYSO:0.01Bi³⁺ shows a broad emission band in the 350–500 nm range with the maximum at 426 nm, corresponding to the ¹S₀ → ³P₁ transition of Bi³⁺, as shown in Fig. 5 (a). As monitored at 426 nm, the obtained excitation spectrum shows two bands. One of broad absorption band is at 270–350 nm originating from the Bi³⁺ transition (¹S₀ → ³P₁) and the other located at about 250 nm belongs to (¹S₀ → ¹P₁) transition [33]. To illustrate the possibility of energy transfer between Bi³⁺ and Eu³⁺, the inset of Fig. 5 (b) shows an image of the excitation spectrum with magnified vertical coordinates in the range of 350–500 nm. The emission spectrum of GYSO:0.01Bi³⁺ overlaps with the excitation spectrum of GYSO:0.15Eu³⁺, indicating a probable energy transfer occurring between Eu³⁺ and Bi³⁺. For optical thermometer applications, we investigated the photoluminescence properties of Bi³⁺/Eu³⁺ that were codoped into the GdY₂SbO₇ phosphors. As shown in Fig. 5 (c), the absorption bands of GYSO:0.01Bi³/0.15Eu³⁺ primarily comprise Bi³⁺ ion ¹S₀ → ³P₁ (∼301 nm) transition and O²⁻ → Eu³⁺ charge transfer (274 nm) under 580 nm monitoring. The emission peaks of the codoped samples comprise Bi³⁺ ion ¹S₀ → ³P₁ (426 nm) and Eu³⁺ ion ⁵D₀ → ⁷F_0,1,2,3,4 (580–713 nm) transitions (λ_ex = 301 nm) [34].

Fig. 5.

PL and PLE spectra at room temperature of (a) GYSO: 0.01Bi³⁺, (b) GYSO: 0.15Eu³⁺, and (c) GYSO: 0.01Bi³⁺/0.15Eu³⁺. The inset shows the PLE spectrum of GYSO: 0.15Eu³⁺ in the 350 nm–500 nm range magnified in the vertical coordinate.

The excitation and photoluminescence spectra of G_0.99−yYSO:0.01Bi³⁺, yEu³⁺ (y = 0.01, 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30) are shown in Fig. 6 (a) and (b), respectively. The shape of the excitation spectra of the samples did not change considerably. When the concentration of Eu³⁺ was less than 0.2, the intensity of the emission band of Bi³⁺ decreased, while that of Eu³⁺ increased with increasing Eu³⁺ concentration. This indicates an energy transfer from Bi³⁺ to Eu³⁺. The intensity of the Eu³⁺ emission band decreased when the Eu³⁺ concentration was continuously increased, owing to the concentration quenching effect.

Fig. 6.

PL and PLE spectra of G_0.99-yYSO:0.01Bi³⁺, yEu³⁺ (y = 0.01, 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30). The inset shows the variation of the maximum value of fluorescence intensity of Bi³⁺ and Eu³⁺ with the concentration of Eu³⁺.

To further prove the energy transfer, the lifetime decay curves of G_0.99-yYSO:0.01Bi³⁺, yEu³⁺ (y = 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30) monitored at 420 nm were measured and described in Fig. 7. The attenuation curve fits a double exponential equation (1) well [33]:

$$I\left(\mathrm{t}\right)=A_1\exp \left(-\frac{t}{{\tau }_1}\right)+A_2\exp \left(\frac{t}{{\tau }_2}\right)+I_0\text{,}$$ (1)

where I(t) represents the emission intensity, τ₁ and τ₂ are defined as decay times, and A₁, A₂, and I₀ are fitting constants. The following equation (2) is used to calculate the average decay time [33]:

$$\bar{\tau }=\frac{A_1{{\tau }_1}^2+A_2{{\tau }_2}^2}{A_1{\tau }_1+A_2{\tau }_2}\text{.}$$ (2)

Fig. 7.

Decay curves of Bi³⁺ in G_0.99-yYSO:0.01Bi³⁺, yEu³⁺ (y = 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30).

The average lifetimes of Bi³⁺ in G_0.99−yYSO:0.01Bi³⁺, yEu³⁺ (y = 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30) phosphors are 15.867, 12.998, 11.992, 6.630, 4.940, and 4.173 ns, respectively. When the concentration of Eu³⁺ increases, the fluorescence lifetime of Bi³⁺ progressively decreases. This further proves that an energy transfer occurs from Bi³⁺ to Eu³⁺.

To determine the temperature sensitivity of the samples, the temperature dependence of GYSO:0.01Bi³⁺, 0.05Eu³⁺ was studied in the range of 300–500 K with 274 and 301 nm as the excitation wavelengths respectively as shown in Fig. 8 (a) and (b), respectively. The lattice vibration of the host strongly depend on the temperature of the phosphor. Therefore at higher temperature, the enhanced lattice vibration would cause the intense nonradiative transition processes which is usually characterized by the decrease in luminescence intensity [35,36]. Besides, due to the different electronic configuration of Bi³⁺ and Eu³⁺ ions, Bi³⁺ ions are more susceptible to the influence of surrounding environment than Eu³⁺ ions. Moreover, the probability of energy transfer from Bi³⁺ to Eu³⁺ increases with the rise of the temperature. It results the luminescence intensities of Bi³⁺ and Eu³⁺ are decreased by different degrees with rising the temperature. Therefore, the emission peaks of Bi³⁺ and Eu³⁺ can be selected for non-contact fluorescence temperature measurement.

Fig. 8.

Temperature-dependent photoluminescence spectra of GYSO:0.01Bi³⁺, 0.05Eu³⁺ at 300–500 K with the excitation wavelengths of (a) 274 nm (b) 301 nm.

According to the intensity histogram shown in Fig. 9, the fluorescence intensities of Bi³⁺ and Eu³⁺ do not change remarkably in the range of 300–340 K. However, when the temperature is higher than 340 K, the luminescence decreases considerably and the quenching of Bi³⁺ is faster than that of Eu³⁺. Based on this phenomenon shown in Fig. 10 (a) and (b), the FIR of Eu³⁺ and Bi³⁺ can be used as a temperature measurement parameter. FIR with these two ions is expressed by the following equation (3) [18]:

$$FIR=\frac{I_{E{u}^{3+}}\left(T\right)}{I_{B{i}^{3+}}\left(T\right)}=\frac{I_{E{u}^{3+}}}{I_{B{i}^{3+}}}\frac{1+A_{E{u}^{3+}}\exp \left(-\frac{E_{E{u}^{3+}}}{k_{B}T}\right)}{1+A_{B{i}^{3+}}\exp \left(-\frac{E_{B{i}^{3+}}}{k_{B}T}\right)}\approx C+B\exp \left(-\frac{\Delta E}{k_{B}T}\right)\text{,}$$ (3)

where C and B are constants, ΔE is the thermal quenching activation energy, k_B is the Boltzmann constant, and T is the temperature. The ratio of fluorescence intensities at I₆₀₇/I₄₂₆ and I₆₂₇/I₄₂₆ were used as the reference values.

Fig. 9.

Histogram of the intensity of Bi³⁺ and Eu³⁺ at varying temperatures under 301 nm excitation.

Fig. 10.

The fitting curves of FIR(I₆₀₇/I₄₂₆) (a) and FIR(I₆₂₇/I₄₂₆) (b) on temperature for GYSO:0.01Bi³⁺, 0.05Eu³⁺ phosphor.

The absolute sensitivity (S_a) and relative sensitivity (S_r) can be expressed by following equations (4), (5) [14]:

$$S_{\mathrm{a}}=\left|\frac{\partial FIR}{\partial T}\right|\approx B\exp \left(-\frac{\Delta E}{k_{B}T}\right)\times \frac{\Delta E}{k_B{T}^2}$$ (4)

$$S_r=100\%\times \left|\frac{1}{FIR}\frac{\partial FIR}{\partial T}\right|=100\%\times \frac{B\exp \left(-\frac{\Delta E}{k_{B}T}\right)}{C+B\exp \left(-\frac{\Delta E}{k_{B}T}\right)}\times \frac{\Delta E}{k_B{T}^2}\text{.}$$ (5)

The calculated results are shown in Fig. 11 (a) and (b) for FIR_{I607 /I426} and FIR_{I627 / I426}, from which it can be seen that both the S_a and S_r reach their maximum values of 0.01 K⁻¹ and 1.26 % K⁻¹ at 500 K when I_607/426 is used as the reference signal. When the reference signal is I_627/426, the maximum values of S_a and S_r are 0.0135 K⁻¹ and 1.19 % K⁻¹ at 500 K. Some materials reported for optical temperature measurement are shown in Table 1. Compared to the materials in Table 1, the phosphors developed herein demonstrate relatively outstanding performance.

Fig. 11.

The corresponding absolute sensitivity (S_a) and relative sensitivity (S_r) versus temperature for FIR_(I607/I426) (a) and FIR_(I627/I426) (b) as reference signals under excitation at 301 nm.

Table 1.

Comparison with the sensitivity of some reported temperature sensing materials.

Compounds	Max. Sa (K−1)	Max. Sr (% K−1)	Temperature range (K)	Ref.
Ca2YZr2Al3O12: Bi3+/Eu3+	0.0082	0.66	297–573	[39]
Ca2Y8(SiO4)6O2: Bi3+, Eu3+	0.0717	0.95	298–523	[4]
Ca3Sc2Si3O12: Bi3+/Tb3+/Eu3+	0.094	1.0	300–500	[16]
CaLaMgTaO6: Bi3+/Eu3+	0.0127	1.33	303–573	[18]
LaNbO4: Bi3+/Eu3+	0.044	1.89	300–480	[14]
GdY2SbO7: Bi3+, Eu3+	0.01	1.26	300–500	This word

In addition to the FIR-mode temperature measurement method, a double excitation method was considered [37]. The method is based on the fact that the Eu³⁺ to O²⁻ charge transfer band has a small energy gap (∼3300 cm⁻¹) with the ³P₁ energy level of Bi³⁺. The variation of fluorescence emission intensity with temperature was measured using 274 nm (CTB) and 301 nm (¹S₀ → ³P₁) as excitation wavelengths, respectively. The selected emission positions were 580, 587, and 627 nm.

Fig. 12 (a)-(c) show the intensity histogram of the fluorescence emission intensity with temperature for emission position at 580, 587, and 627 nm, respectively, under different excitation conditions. It can be seen from the figure that the intensity of the emission peaks corresponding to the two excitation positions in the range of 340–500 K exhibits a notable tendency to decrease in different degrees. Therefore, an exponential function image of the fluorescence intensity ratio under the dual excitation conditions can be fitted accordingly, and the results are shown in Fig. 13 (a)-c).

Fig. 12.

Histograms of I_580(274/301) (a), I_587(274/301) (b), and I_627(274/301) (c) with temperature for the double excitation method.

Fig. 13.

Exponential fits in the double excitation mode (a), (b), (c).

The FIR_2ex fitting equation for the dual excitation mode is equation (6) [37]:

$$FI{R}^{2ex}=\frac{I_{274\text{nm}}\left(T\right)}{I_{301\text{nm}}\left(T\right)}=\frac{I_{274\text{nm}}}{I_{301\text{nm}}}\frac{1+A_{274nm}\exp \left(-\frac{E_{274nm}}{k_{B}T}\right)}{1+A_{301nm}\exp \left(-\frac{E_{274nm}}{k_{B}T}\right)}\approx C+B\exp \left(-\frac{\Delta E^{2ex}}{k_{B}T}\right)\text{,}$$ (6)

where ΔE_2ex is the energy gap between the two thermally coupled energy levels, and the equation for S_a and S_r, in this case, can be expressed by equations (7), (8) [37]:

$$S_a=\left|\frac{\partial FI{R}^{2ex}}{\partial T}\right|\approx B\exp \left(-\frac{\Delta E^{2ex}}{k_{B}T}\right)\times \frac{\Delta E^{2ex}}{k_B{T}^2}$$ (7)

$$S_r=100\%\times \left|\frac{1}{\partial FI{R}^{2ex}}\frac{\partial FI{R}^{2ex}}{\partial T}\right|\approx 100\%\times \frac{B\exp \left(-\frac{\Delta E^{2ex}}{k_{B}T}\right)}{C+B\exp \left(-\frac{\Delta E^{2ex}}{k_{B}T}\right)}\times \frac{\Delta E^{2ex}}{k_B{T}^2}\text{.}$$ (8)

The results of the calculations are shown in Fig. 14 (a)-(c), where the maximum values of S_a and S_r are 0.0099 K⁻¹ and 1.33 % K⁻¹, respectively, when I_{580 (274/301)} is selected as the reference signal. If I_{587 (274/301)} is selected as the monitoring signal, the maximum values of S_a and S_r are 0.0094 K⁻¹ and 1.36 % K⁻¹, respectively. When I_{627 (274/301)} is chosen as the reference signal, the maximum values of S_a and S_r are 0.0061 K⁻¹ and 1.03 % K⁻¹, respectively.

Fig. 14.

The calculated results for S_a and S_r (a), (b), (c).

The thermoluminescence schematic and energy level diagram are shown in Fig. 15, which can fully explain the luminescence mechanism of the GYSO:Bi³⁺, Eu³⁺ phosphors. As shown in Fig. 15, the excited electron jumps from the valence band to the conduction band (process ①) and generates an electron and a hole. Owing to the defects in the crystal structure of GYSO, the trap energy level is below the conduction band. The electrons that jump to the conduction band partly fall into the trap energy level (process ②). The rest of the electrons emit light, which is captured by the luminescence center (processes ④ and ⑤).

Fig. 15.

The thermoluminescence schematic and the energy level diagrams of Bi³⁺ and Eu³⁺ in GYSO.

In the GYSO:Bi³⁺, Eu³⁺ phosphors, the ¹P₁ excited state of Bi³⁺ was very close to the ⁵D₄ energy levels of Eu³⁺; therefore, it was easy to transfer the energy to the adjacent Eu³⁺ ions through resonance nonradiation. Afterward, Eu³⁺ ions transited from the ground state of ⁷F₀ to the excited state, relaxing to the ⁵D₀ level through the nonradiative path, and then returning to the ground state. The radiative transition to the ⁷F_J (J = 0–4) levels produces red emissions from Eu³⁺ ions [4,38].

When the temperature rises, an increasing number of electrons in the trap energy level overcome the energy difference to leap to the conduction band (process ③) and produce fluorescence emission (processes ⑥ and ⑦). Hence, the fluorescence emission intensities of Bi³⁺ and Eu³⁺ do not decrease at 300–340 K, which is attributed to the compensation of the thermal burst by thermoluminescence.

To determine the variation of the chromaticity characteristics of the studied phosphors with temperature, the corresponding CIE chromaticity coordinates were calculated from the emission spectra of GYSO:0.01Bi³⁺, 0.05Eu³⁺ in the range of 300–500 K and are shown in Fig. 16. When the temperature increased from 300 to 500 K under excitation of 301 nm, the chromaticity coordinates of the sample moved from (0.2776, 0.1363) to (0.3368, 0.1857). This phenomenon indicates the possibility of applying the studied sample to visual temperature measurements.

Fig. 16.

CIE chromaticity diagram of GYSO:0.01Bi³⁺,0.05Eu³⁺phosphor in the range of 300–500 K with 301 nm as the excitation wavelength.

In addition, under the excitation of 301 nm, the change of the x and y values of the chromaticity coordinate with temperature is shown in Fig. 17 (a) and (b), rescpectively. As shown in the figure, with the x or y values as the ordinate coordinate and temperature as the abscissa, which can be well fitted to the exponential function, the values of S_a and S_r can be calculated using equations (2), (3). The results are shown in Fig. 18 (a) and (b). When the excitation wavelength is 301 nm, the S_r of the sample reaches maximum values of 0.4539 % K⁻¹ and 0.5996 % K⁻¹ at 500 K when x and y values are used as reference signals, respectively. This implies that the change of chromaticity coordinates with temperature can also be a supplementary strategy for non-contact temperature measurement.

Fig. 17.

The exponential fit plots with the x-value or y-value of the chromaticity coordinates as the reference signal under 301 nm excitation condition.

Fig. 18.

The values of S_a and S_r with the x-value or y-value of the chromaticity coordinate as the reference signal under 301 nm excitation condition in the range of 300–500 K.

4. Conclusion

Novel GdY₂SbO₇:Bi³⁺, Eu³⁺ phosphors with double emission were prepared using the high-temperature solid-phase method. The blue emission of Bi³⁺ (³P₁ → ¹S₀) and the orange emission of Eu³⁺ (⁵D₀ → ⁷F_{0, 1, 2, 3, and 4}) occurred under excitation at 301 nm. The temperature dependence of the fluorescence emission intensity of the samples was tested in the range of 300–500 K. The S_a and S_r of the samples were calculated using FIR and thermochromic modes, respectively. The best mode for GdY₂SbO₇:Bi³⁺, Eu³⁺ was the FIR based on double excitation method which S_r reached 1.36 % K⁻¹ at 340 K. It indicates that GdY₂SbO₇:Bi³⁺, Eu³⁺ phosphor exhibits potential applications in the field of optical temperature measurement.

Additional information

No additional information is available for this paper.

Data availability statement

Data included in article/supplementary material/referenced in article.

CRediT authorship contribution statement

Yanru Yin: Writing – original draft. Mengmeng Jiang: Writing – original draft, Formal analysis, Data curation. Lianhua Tian: Writing – review & editing, Supervision, Resources, Investigation, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.