Improving the temperature-sensing performance of the SrZn_0.33Nb_0.67O₃:Pr³⁺ phosphor via Ga³⁺ doping

Abstract

Optical thermometry offers promising applications in the fields of microelectronics and biomedicine, as well as in fire pre-warning systems. In this research, Pr³⁺-activated SrZn_0.33Nb_0.67O₃ phosphors were successfully prepared via solid-state reactions. Under ultraviolet excitation, the SrZn_0.33Nb_0.67O₃:x%Pr³⁺ (x = 0.25/0.75/1.25/2) phosphors exhibit bright blue and red emissions, located at 491 nm (³P₀ → ³H₄), 619 nm (¹D₂ → ³H₄) and 651 nm (³P₀ → ³F₂). Specifically, two fluorescence intensity ratio (FIR) models of ¹D₂ → ³H₄versus³P₀ → ³H₄ (Red1/Blue) and ¹D₂ → ³H₄versus³P₀ → ³F₂ (Red1/Red2) in the temperature range of 300–500 K are adopted for temperature sensing. Our results show that the designed phosphor can serve as a novel potential self-calibrated optical thermometer. Moreover, the temperature sensitivity of the FIR thermometer is significantly enhanced when Ga³⁺ is incorporated into the SrZn_0.33Nb_0.67O₃. Specifically, the maximum absolute and relative sensitivity for the 6% Ga³⁺ co-doped sample are increased five- and two-fold, respectively, compared with the undoped sample. This work may provide useful inspiration for effectively improving the temperature-sensing performance of optical thermometers.

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Introduction

With rapid developments in the microelectronics industry, biomedical field and scientific research, accurate temperature information acquisition with high spatio-temporal resolution has been gaining greater importance. Compared with traditional mercury-in-glass thermometers and thermocouples, rare-earth-doped non-contact optical thermometers are becoming more and more popular for temperature measurements in harsh environments, such as in corrosive liquids and in environments with electromagnetic interference.^1–4 Generally, the emission intensity/bandwidth, peak position, fluorescence intensity ratio (FIR) and lifetime of the luminescence center can all be used as indexes for temperature monitoring.⁵ Among these, the FIR technique has wide-ranging advantages due to its ease of operation, fast response and high reliability.^6–9

Over the past decades, most of the studies based on FIR thermometry have focused on Er³⁺-doped up-conversion temperature-sensing materials that have two closely spaced temperature-sensitive thermally coupled levels (²H_11/2, and ⁴S_3/2).^10–12 However, the narrow energy gap of the thermally coupled levels results in poor signal discrimination. Other luminescent materials, such as fluorescein dye, carbon dots, quantum dots, etc., have also been investigated for FIR thermometry.^13–15 However, these materials are usually vulnerable to the physical and chemical environments, which restricts their practical application. Currently, the temperature-sensing strategy based on the diverse temperature responses of 4f–5d or 4f–4f transitions of Pr³⁺ is attracting much attention for its high signal discrimination and considerable temperature sensitivity.^16–20 Pr³⁺ has abundant emission spectral lines, where blue, green, red and deep red emissions are derived from the ³P₀ → ³H₄, ³P_1,0 → ³H₅, ¹D₂ → ³H₄ and ³P₀ → ³F₂ electronic transitions, respectively. Among these, the blue and red emissions that stem from the ³P₀ and ¹D₂ states are usually used as signal peaks for research in FIR thermometry. As is known, influences from phonon-assisted cross-relaxation, multiphonon relaxation and the intervalence charge transfer state (IVCT) mean that the intensity ratio of these transitions is very vulnerable to temperature in some transition metal oxides. For example, Pr³⁺-doped (K_0.5Na_0.5)NbO₃, La₂MgTiO₆, Na₂La₂Ti₃O₁₀ and LaMg_0.402Nb_0.598O₃ have been reported as potential optical thermometers.^21–24 It is worth noting that IVCT-affected thermal-quenching process is the key factor that contributes to the excellent temperature-sensing performance. Hence, choosing a suitable host and regulating the IVCT band are effective ways to achieve a high temperature-sensing performance.

The complex perovskite compound SrZn_0.33Nb_0.67O₃ has drawn our attention. This compound belongs to the A₃B′B′′₂O₉ family in which the A-sites are occupied by larger cations and the B-sites by smaller cations. The abundant lattice sites, wide spectral excitation and excellent chemical stability make it a suitable matrix for developing rare-earth luminescent materials.^25–27 To the best of our knowledge, the thermo-sensitive properties of the SrZn_0.33Nb_0.67O₃:Pr³⁺ phosphor have not been systematically investigated so far. Therefore, a study on the temperature-dependent luminescent properties of Pr³⁺ in SrZn_0.33Nb_0.67O₃ will broaden the family of potential Pr³⁺-activated optical thermometers. In addition, co-doping with other ions is an effective way to achieve enhancement of the temperature-sensing performance.^28–30 For example, the green up-conversion emission and thermal sensitivity of NaYF₄:Yb,Er³⁺ are both increased when Ga³⁺ is introduced into the host.³¹ The luminescent intensity of CaAl₁₂O₁₉:Mn⁴⁺ is enhanced via Ga³⁺ doping.³² The thermal stability and red emission of Mn⁴⁺ in Li₂MgZrO₄ are improved when Ga³⁺ ions are incorporated into the phosphor.³³ The above studies indicate a practicable strategy for us to promote the performance of Pr³⁺-activated self-calibrated optical thermometers.

Thus, in this work, we chose to study a novel Pr³⁺-activated SrZn_0.33Nb_0.67O₃ thermo-sensitive phosphor and investigate the significant impact of Ga³⁺ co-doping on the temperature-sensing performance of this material. Herein, the SrZn_0.33Nb_0.67O₃:x%Pr³⁺ (x = 0.25/0.75/1.25/2) phosphors were successfully prepared using a high-temperature solid-state method, and the impact of the Pr³⁺ concentration on the luminescence properties is discussed. Under ultraviolet excitation, the as-synthesized samples exhibit efficient blue and red emissions located at 491 nm (³P₀ → ³H₄), 619 nm (¹D₂ → ³H₄) and 651 nm (³P₀ → ³F₂), and the FIR of different 4f configuration transitions changes significantly with temperature, demonstrating that the studied material can serve as a self-calibrated optical thermometer with good signal discrimination. Specifically, two FIR models of ¹D₂ → ³H₄versus³P₀ → ³H₄ (Red1/Blue) and ¹D₂ → ³H₄versus³P₀ → ³F₂ (Red1/Red2) in the temperature range of 300–500 K are adopted for temperature sensing. Notably, the temperature-sensing performance of the thermometer is improved markedly when Ga³⁺ is co-doped into the SrZn_0.33Nb_0.67O₃:Pr³⁺ phosphor. It is hoped that the designed phosphor will be a potential candidate for applications in optical thermometry.

Experimental

Synthesis

The SrZn_0.33Nb_0.67O₃:x% Pr³⁺,y% Ga³⁺(x = 0.25/0.75/1.25/2; y = 4/6/8) samples were synthesized using the high-temperature solid-state method. The strontium carbonate (SrCO₃), zinc oxide (ZnO), niobium oxide (Nb₂O₅), praseodymium oxide (Pr₆O₁₁), and gallium oxide (Ga₂O₃) used in the experiments were purchased from the Aladdin Reagent Company. In a typical synthesis, stoichiometric amounts of SrCO₃, Nb₂O₅, ZnO, Pr₆O₁₁, and Ga₂O₃ were mixed thoroughly and ground with flux using ethanol as a wetting agent, placed in a corundum crucible and heated at 800 °C for 2 h in air. After cooling to room temperature, the powder was ground and calcined at 1350 °C for 8 h in air.

Characterization

The X-ray diffraction (XRD) patterns were collected using a powder diffractometer with Cu Kα radiation (λ = 1.5418 Å). A scanning electron microscope (SEM, JSM-6700F) was used to characterize the morphology, energy dispersive X-ray spectroscopy (EDS) and elemental mapping of the samples. Diffuse reflectance spectra were recorded using a UV-Vis-NIR spectrophotometer (Lambda 950, PerkinElmer). An FLS 980 fluorescence spectrometer with a 450 W xenon lamp as the excitation light source was used to measure the photoluminescence excitation (PLE) and photoluminescence emission (PL) spectra and the fluorescence decay curves of the samples. Temperature-dependent PL spectra were measured using the above-mentioned spectrophotometer, which was equipped with a temperature-controlling stage.

Results and discussion

Fig. 1(a) shows the XRD patterns of SrZn_0.33Nb_0.67O₃:x% Pr³⁺,y% Ga³⁺ (x = 0/0.25/0.75/1.25/2; y = 4/6/8). All the diffraction peaks can be well indexed to the standard PDF card (no. 39-1470) with no impurity peaks observed, which confirms that the synthesized samples are of pure phase. The doping of Pr³⁺ and Ga³⁺ ions did not change the crystal structure of SrZn_0.33Nb_0.67O₃. When Pr³⁺ and Ga³⁺ ions are incorporated into SrZn_0.33Nb_0.67O₃, the position of the diffraction peaks shifts to the larger-angle side, as exhibited in Fig. 1(a) for the strongest line located at 31.6°. Fig. 1(b) presents the crystal structure of SrZn_0.33Nb_0.67O₃, which belongs to the cubic perovskite structure in the space group Fm3̅m. Zn and Nb atoms are disorderly distributed at the B site, coordinated with 6 oxygen atoms to form [Zn/NbO₆] octahedra, while the Sr²⁺ ion is located at the A site with 12-fold coordinated oxygen. Due to the similar ionic radii of Pr³⁺ and Sr²⁺ (r_Pr³⁺ = 1.29 Å for CN = 12, r_Sr²⁺ = 1.44 Å for CN = 12), Pr³⁺ should substitute for the Sr²⁺ ion. By contrast, the ionic radius of Ga³⁺ (r_Ga³⁺ = 0.62 Å, CN = 6) is similar to that of the Nb⁵⁺ and Zn²⁺ ions (r_Zn²⁺ = 0.74 Å, r_Nb⁵⁺ = 0.64 Å for CN = 6), so the Ga³⁺ ion should occupy the Zn²⁺/Nb⁵⁺ site. SEM and elemental map observations, as displayed in Fig. 1(c–j), show that the as-prepared SrZn_0.33Nb_0.67O₃:6% Ga³⁺,1.25% Pr³⁺ powder has a small particle size below 5 μm, and the Sr, Zn, Nb, O and Ga elements are homogeneously distributed throughout the observation area. Moreover, the peaks of the Sr, Zn, Nb, O and Ga elements all appear in the EDS energy spectrum, further proving the successful incorporation of Ga³⁺ ions into niobate matrix.

Fig. 1 (a) XRD patterns of SrZn_0.33Nb_0.67O₃:x% Pr³⁺,y% Ga³⁺ (x = 0/0.25/0.75/1.25/2; y = 4/6/8), referenced to the SrZn_0.33Nb_0.67O₃ (PDF no. 39-1470) standard card. Magnification of patterns in the 31–32° range are on the right. (b) Schematic diagram of the SrZn_0.33Nb_0.67O₃ crystal structure. (c) SEM image, (d) EDS spectrum and (e–j) elemental mapping images of SrZn_0.33Nb_0.67O₃:1.25% Pr³⁺,6% Ga³⁺.

Fig. 2(a) shows the room-temperature PL spectra of the SrZn_0.33Nb_0.67O₃ phosphors doped with different Pr³⁺-ion concentrations. It is found that under the excitation of 290 nm UV light, the as-prepared SrZn_0.33Nb_0.67O₃:x% Pr³⁺ (x = 0.25/0.75/1.25/2) samples exhibit three main emission peaks at 491 nm, 619 nm and 651 nm, which correspond to the ³P₀ → ³H₄, ¹D₂ → ³H₄ and ³P₀ → ³F₂ transitions of Pr³⁺, respectively. The emission intensity of the three emission peaks (491 nm, 619 nm and 651 nm) have similar line trends with the Pr³⁺ doping concentration. When the Pr³⁺ doping concentration was increased from 0.25% to 2%, the emission intensities of the peaks were all gradually increased, and any further increase in the doping concentration resulted in a lowered emission intensity due to concentration quenching; therefore, the optimal Pr³⁺ doping concentration is 1.25%. In order to investigate the energy-transfer mechanism of Pr³⁺ in the SrZn_0.33Nb_0.67O₃ host, the critical distance R_c is calculated using the following equation,³⁴

where V is the unit cell volume, x_c is the critical doping concentration, and N represents the number of lattice sites in the unit that can be occupied by activator ions. For SrZn_0.33Nb_0.67O₃:Pr³⁺, V = 64.1 ų, x_c = 1.25, N = 2, and R_c was calculated to be 3.68 Å; thus, the electric exchange interaction is responsible for the non-radiative energy transfer in SrZn_0.33Nb_0.67O₃.

Fig. 2 (a) PL spectra of SrZn_0.33Nb_0.67O₃:x% Pr³⁺ (x = 0.25/0.75/1.25/2) at room temperature. The inset shows a comparison of the emission intensity for the 491 nm, 619 nm and 651 nm peaks for different Pr³⁺-ion concentrations. (b) Fluorescence decay curves and (c) excitation spectra (λ_em = 491 nm, 619 nm, and 651 nm) for SrZn_0.33Nb_0.67O₃:1.25% Pr³⁺. (d) ultraviolet visible diffuse reflectance spectra (UV vis DRS) of the SrZn_0.33Nb_0.67O₃:1.25% Pr³⁺, where the inset shows the relationship of [F(R)hν]²versus the energy hν.

Fig. 2(b) exhibits the fluorescence decay curves measured at the 491 nm (³P₀ → ³H₄), 619 nm (¹D₂ → ³H₄), and 651 nm (³P₀ → ³F₂) emissions for SrZn_0.33Nb_0.67O₃:1.25% Pr³⁺. All curves exhibit triple-exponential decay behaviour, and can be fitted using the following formula,³⁵

where I(t) is the fluorescence intensity of the emission peak monitored at time t; B₁, B₂ and B₃ are constants, and τ₁, τ₂ and τ₃ are the lifetimes of each exponential component. The deviation from single-exponential decay is due to the existence of several de-excitation pathways including the radiative transition, cross-relaxation (CR) and multiphonon relaxation (MPR) processes as well as the intervalence charge transfer state (IVCT). The average lifetime (τ) can be calculated:

Finally, the fluorescence decay lifetime of the ¹D₂ energy level by via monitoring at the 619 nm emission is 20.51 μs, and the decay lifetime of the ³P₀ level by monitoring at the 491 nm and 651 nm emissions are calculated as 9.29 μs and 8.47 μs, respectively. Due to its spin-allowed transition characteristics, the excited electrons at the ³P₀ energy level decay much faster than at the ¹D₂ energy level. Fig. 2(c) shows the normal temperature PLE spectra of SrZn_0.33Nb_0.67O₃:1.25% Pr³⁺ measured by monitoring the emission peaks at λ_em = 491 nm, 619 nm, and 651 nm. It is found that all the excitation spectra are similar, and the maximum excitation wavelength is around 290 nm. The inset of Fig. 2(c) shows the Gaussian fitting result of one typical PLE spectrum, where the three Gaussian deconvolution peaks at 267 nm, 289 nm and 300 nm are suggested to be assigned to the 4f² → 4f5d transition of Pr³⁺, the host absorption and Pr³⁺–Nb⁵⁺ IVCT. From the diffuse reflectance spectra of the SrZn_0.33Nb_0.67O₃ host and SrZn_0.33Nb_0.67O₃:1.25% Pr³⁺ displayed in Fig. S1 (ESI†) and Fig. 2(d), respectively, both show a strong host absorption in the 200–400 nm UV region. Besides the host fundamental absorption, there also exist several absorption dips in the 450–500 nm and 580–625 nm regions of SrZn_0.33Nb_0.67O₃:1.25% Pr³⁺, which originate from the 4f–4f intra-configuration transitions of the Pr³⁺ ion, i.e., the ³H₄–³P_0,1,2 and the ³H₄–¹D₂ transition, respectively. Due to the parity-forbidden transition nature, their absorptions are very weak. According to the Tauc theoretical equation, the optical band gap of SrZn_0.33Nb_0.67O₃ can be calculated using the following formula:

[F(R)hν]ⁿ = A(hν − E_g) (4)

[F(R)] = (1 − R)²/2R (5)

where h is the Planck constant, v is the incident photon frequency, A and E_g are the proportional constant and the optical band gap energy, n is related to the transition type, i.e., n = 1/2 for an indirect transition, and n = 2 for a direct transition, and R is the reflection coefficient. In the SrZn_0.33Nb_0.67O₃ host, the value of n should be 2 due to it is direct bandgap.²⁵ The inset of Fig. 2(d) shows the function relationship between [F(R)hv]² and hv for SrZn_0.33Nb_0.67O₃:1.25% Pr³⁺. The optical band gaps of the SrZn_0.33Nb_0.67O₃ host and SrZn_0.33Nb_0.67O₃:1.25% Pr³⁺ were calculated to be about 4.20 eV.

Next, temperature-dependent photoluminescence spectrum testing was performed on SrZn_0.33Nb_0.67O₃:1.25% Pr³⁺ to investigate its potential applications in temperature sensing. As shown in Fig. 3(a), with the temperature increasing from 300–500 K, the intensity of the blue emission at 491 nm (³P₀ → ³H₄) and the red emission at 651 nm (³P₀ → ³F₂) drop dramatically, whereas the red emission at 619 nm (¹D₂ → ³H₄) shows insignificant quenching (Fig. 3b–d). Significantly, the FIR of ¹D₂ → ³H₄ and ³P₀ → ³H₄ (Red1/Blue), as well as ¹D₂ → ³H₄ and ³P₀ → ³F₂ (Red1/Red2) could be used as detection signals for temperature monitoring. The FIR versus 1/T plots for SrZn_0.33Nb_0.67O₃:1.25% Pr³⁺ are presented in Fig. 4(a and b). It can be seen that the FIR for the two models changes clearly with increasing temperature. The FIR can be expressed as follows:³⁶

FIR ≈ C + A exp (−B/k_BT) (6)

where k_B is the Boltzmann constant, T is the temperature, and A, B and C are constants. Since sensitivity is a very important working parameter for temperature sensors, in order to quantitatively assess the thermo-sensitive properties of the phosphor, the absolute temperature sensitivity (S_a) and the relative temperature sensitivity (S_r) were calculated using the following equations:³⁶

Fig. 3 (a) Temperature-dependent PL spectra of the SrZn_0.33Nb_0.67O₃:1.25% Pr³⁺ sample, recorded from 300 K to 500 K. (b), (c) and (d) Histograms displaying the luminescence intensity of the 491 nm, 619 nm and 651 nm peaks at different temperatures for the SrZn_0.33Nb_0.67O₃:1.25% Pr³⁺ sample.

Fig. 4 Experimentally measured and eqn (6)-fitted plots of (a) FIR (I₆₁₉/I₄₉₁) and (b) FIR (I₆₁₉/I₆₅₁) versus temperature. The measured and calculated plots of S_a and S_rversus temperature based on (c) FIR (I₆₁₉/I₄₉₁) and (d) FIR (I₆₁₉/I₆₅₁) models.

The relationship between T and the absolute sensitivity S_a as well as the relative sensitivity S_r are shown in Fig. 4(c and d). The maximum values of S_a and S_r for SrZn_0.33Nb_0.67O₃:1.25% Pr³⁺ were calculated to be 0.007 K⁻¹ (500 K) and 0.37% K⁻¹ (400 K), respectively. The above results demonstrate that the as-prepared phosphor is a new potential thermo-sensitive material.

To further improve the temperature-sensing characteristics of SrZn_0.33Nb_0.67O₃:1.25% Pr³⁺,y% Ga³⁺ (y = 4/6/8) ions were incorporated into the SrZn_0.33Nb_0.67O₃ matrix. Fig. 5(a and c) shows the PL and PLE spectra of SrZn_0.33Nb_0.67O₃:1.25% Pr³⁺,6% Ga³⁺ (the PL and PLE spectra for Ga³⁺-ion-doping concentrations of 4% and 8% are presented in Fig. S2, ESI†). The emission peaks are similar to those of SrZn_0.33Nb_0.67O₃:1.25% Pr³⁺ while the excitation bands shift towards a low energy for the 6% Ga³⁺-doped sample monitored at 491 nm, 619 nm and 651 nm. The smaller band gap determined via the diffuse reflectance spectrum for SrZn_0.33Nb_0.67O₃:6% Ga³⁺,1.25% Pr³⁺ in Fig. 5(d) corroborates the changes mentioned above. The band gaps are reduced as well when different concentrations of Ga³⁺ ions were introduced into the matrix (Fig. S3, ESI†). In addition, the fluorescence intensity ratio of red light (¹D₂ → ³H₄) and blue light (³P₀ → ³H₄) increased after Ga³⁺ doping, as demonstrated in Fig. 5(a), and to further understand this, the excited dynamics of the ¹D₂ and ³P₀ energy levels were also investigated. Fig. 5(b) shows the fluorescence decay curves of SrZn_0.33Nb_0.67O₃:6% Ga³⁺,1.25% Pr³⁺. The calculated fluorescence decay lifetime for the ¹D₂ energy level is 23.96 μs (619 nm, ¹D₂ → ³H₄), which is longer than that measured in SrZn_0.33Nb_0.67O₃:1.25% Pr³⁺, while the fluorescence decay rate of the ³P₀ state is accelerated slightly, resulting in a shorter decay time of the ³P₀ energy level. These results suggest that Ga³⁺ doping can enhance the non-radiative transition possibility of the ³P₀ state. At ambient temperature, the effect of multiphonon relaxation (MPR) is negligible. Therefore, the decay deviation could be mainly caused by cross-relaxation (CR) [³P₀, ³H₄] → [¹D₂, ³H₆] and crossover to IVCT.

Fig. 5 (a) PL spectra, (b) fluorescence decay curves, (c) PLE spectra (λ_em = 491 nm, 619 nm, 651 nm) and (d) the UV-vis DRS of the SrZn_0.33Nb_0.67O₃:6% Ga³⁺,1.25% Pr³⁺, where the inset in (c) shows the Gaussian fitting performed on the excitation spectra, the inset in (d) shows the relationship of [F(R)hν]²versus energy hν.

Subsequently, the temperature-dependent spectra of the phosphors with different Ga³⁺ doping concentrations (4%, 6%, and 8%) were investigated. The temperature-dependent PL spectra of SrZn_0.33Nb_0.67O₃:y% Ga³⁺,1.25% Pr³⁺ (y = 4/6/8) are shown in Fig. 6(a, c and d). It is demonstrated in Fig. 6(b) that the intensity of the ³P₀ → ³H₄ and ³P₀ → ³H₂ emissions drop more with increasing temperature after co-doping with Ga³⁺ ions. As a consequence, compared with SrZn_0.33Nb_0.67O₃:1.25% Pr³⁺, the thermal response of Red1/Blue and Red1/Red2 for the Ga³⁺ co-doped samples behave more dramatically. With the increase in temperature, the FIR value of Red1/Blue varies over 1.13–2.04 for SrZn_0.33Nb_0.67O₃:1.25% Pr³⁺, while it is 1.36–4.23 for the 6% Ga³⁺-doped sample. Similarly, the FIR value of Red1/Red2 varies over 1.13–2.13 for SrZn_0.33Nb_0.67O₃:1.25% Pr³⁺, while it is 1.57–4.14 for the 6% Ga³⁺-doped sample (Fig. S4, ESI†).

Fig. 6 (a) Temperature-dependent PL spectra of the SrZn_0.33Nb_0.67O₃:6% Ga³⁺,1.25% Pr³⁺, recorded from 300 K to 500 K. (b) Histograms displaying the luminescence intensity of the 491 nm (Blue), 619 nm (Red1) and 651 nm (Red2) peaks at different temperatures for the SrZn_0.33Nb_0.67O₃:6% Ga³⁺,1.25% Pr³⁺. (c) Temperature dependent PL spectra of the SrZn_0.33Nb_0.67O₃:4% Ga³⁺,1.25% Pr³⁺ and (d) SrZn_0.33Nb_0.67O₃:8% Ga³⁺,1.25% Pr³⁺, recorded from 300 K to 500 K.

The absolute sensitivity S_a and the relative sensitivity S_r curves calculated using eqn (5) and (6) for the Ga³⁺,Pr³⁺ co-doped SrZn_0.33Nb_0.67O₃ phosphors are shown in Fig. 7, and the maximum S_a and S_r values are also shown in Table 1. Clearly, in comparison with the Pr³⁺ single-doped SrZn_0.33Nb_0.67O₃ phosphor, S_a and S_r are both significantly increased to some degree for the 4%, 6% and 8% Ga³⁺ co-doped phosphors. Remarkably, the maximal S_a and S_r values reach as high as 0.035 K⁻¹ (500K) and 0.83% K⁻¹ (500 K) for the 6% Ga³⁺ co-doped sample, which are increased by five- and two-fold, respectively, compared with the undoped sample. All these demonstrate that co-doping with Ga³⁺ ions effectively improves the performance of the SrZn_0.33Nb_0.67O₃:Pr³⁺ thermo-sensitive phosphor.

Fig. 7 (a) Absolute sensitivity S_a and (b) relative sensitivity S_rversus temperature for the Red1/Blue model for SrZn_0.33Nb_0.67O₃:y% Ga³⁺,1.25% Pr³⁺ (y = 4/6/8). (c) Absolute sensitivity S_a and (d) relative sensitivity S_rversus temperature for the Red1/Red2 model for SrZn_0.33Nb_0.67O₃:y% Ga³⁺,1.25% Pr³⁺ (y = 4/6/8).

Table 1 Calculated maximum S_a and S_r values of SrZn_0.33Nb_0.67O₃:y% Ga³⁺,1.25% Pr³⁺ (y = 4/6/8)

Transition	Material	Temperature range (K)	S a-max (K^−1)	S r-max (% K^−1)
Pr^3+: ^1D2 → ^3H4, ^3P0 → ^3H4	SrZn0.33Nb0.67O3	300–500	0.007 (500 K)	0.37 (400 K)
	SrZn0.33Nb0.67O3:4% Ga^3+	300–500	0.026 (500 K)	0.58 (500 K)
	SrZn0.33Nb0.67O3:6% Ga^3+	300–500	0.035 (500 K)	0.83 (500 K)
	SrZn0.33Nb0.67O3:8% Ga^3+	300–500	0.032 (500 K)	0.73 (500 K)
Pr^3+: ^1D2 → ^3H4, ^3P0 → ^3F2	SrZn0.33Nb0.67O3	300–500	0.007 (500 K)	0.35 (400 K)
	SrZn0.33Nb0.67O3:4% Ga^3+	300–500	0.020 (500 K)	0.57 (500 K)
	SrZn0.33Nb0.67O3:6% Ga^3+	300–500	0.029 (500 K)	0.70 (500 K)
	SrZn0.33Nb0.67O3:8% Ga^3+	300–500	0.026 (500 K)	0.70 (500 K)

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According to the above results, the markedly improved temperature-sensing performance is caused directly by the more significant thermal quenching of the ³P₀ state after Ga³⁺ co-doping. To further understand the fluorescence quenching dynamics, the possible depopulation process for the ³P₀ state was discussed. As observed in Fig. 8, three de-excitation modes may contribute to the thermal quenching of the ³P₀-related luminescence, namely, phonon-assisted cross-relaxation(CR), multiphonon relaxation (MPR) and crossover to a low-lying IVCT state. Considering that the CR processes of [³P₀, ³H₄] [¹G₄, ¹G₄] and [³P₀, ³H₄] [¹D₂, ³H₆] are non-resonant, whereas the [¹D₂, ³H₄] [¹G₄, ³F₄] is resonant, as a consequence, the probability of the phonon-involved former CR processes is more significantly accelerated by elevating the temperature than that of the latter. However, the MPR process has less impact on the depopulation probability of the ³P₀ state in niobate due to the large energy gap (ΔE ≈ 3500 cm⁻¹) between the ³P₀ and ¹D₂ states according to previous studies.²¹ Notably, the low-lying Pr³⁺–Nb⁵⁺ IVCT state is regarded as one of the most important depopulation pathways for the ³P₀ level in SrZn_0.33Nb_0.67O₃. As illustrated in in Fig. 8(b), both of the ³P₀ and ¹D₂ emissions can be de-excited from Pr³⁺–Nb⁵⁺ IVCT with phonons assisting. However, the electrons in the ¹D₂ state have a higher cross-over energy than in ³P₀ state, thus leading to the diverse thermal response of the ³P₀ and ¹D₂ emissions. The empirical formula for estimating the IVCT energy in Pr³⁺-doped niobite is as follows:³⁷

where χ_opt(Nb⁵⁺) represents the electronegativity of Nb⁵⁺, and d(Pr³⁺–Nb⁵⁺) represents the shortest distance between Pr³⁺ and Nb⁵⁺ in the host. When Ga³⁺ ions are incorporated, the unit cell of SrZn_0.33Nb_0.67O₃ shrinks, as confirmed by the XRD results in Fig. 1(a), which may reduce the distance between Pr³⁺ and Nb⁵⁺, consequently resulting in a lower-lying Pr³⁺–Nb⁵⁺ IVCT state. Therefore, the energy barrier ΔE′ for the ³P₀ state in Ga³⁺ co-doped SrZn_0.33Nb_0.67O₃:Pr³⁺ is smaller than ΔE in SrZn_0.33Nb_0.67O₃:Pr³⁺. This explains why the doping of Ga³⁺ accelerates the thermal quenching of the blue emission (³P₀ → ³H₄) and the red emission (³P₀ → ³F₂). Since the crossover energy from ¹D₂ to the Pr³⁺–Nb⁵⁺ IVCT state is much larger, a small quantity of Ga³⁺ doping has minor influence on the ¹D₂ emission. Moreover, some of the electrons depopulated from the ³P₀ level may relax to the ¹D₂ level through the Pr³⁺–Nb⁵⁺ IVCT bridge. Thus, all these factors together promote the temperature-sensing performance of the designed luminescent thermometer.

Fig. 8 (a) Schematic energy-transfer diagram for the cross-relaxation and multiphonon relaxation processes of Pr³⁺. (b) Configurational coordinate diagrams showing the temperature increase-induced non-radiative relaxation processes of Pr³⁺ in SrZn_0.33Nb_0.67O₃ and SrZn_0.33Nb_0.67O₃:Ga³⁺.

It is known that the relative sensitivity S_r is a critical working parameter since it allows the standardization of diverse luminescence-based temperature sensors regardless of the difference in intuitive observations.¹ To further evaluate the temperature-sensing performance of the Ga³⁺,Pr³⁺ co-doped SrZn_0.33Nb_0.67O₃ phosphor, typical Pr³⁺-activated niobate, zirconate and germanate thermometric phosphors^{14,15,21,35–37} reported in recent years are displayed in Table 2 for comparison. As illustrated in the table, the SrZn_0.33Nb_0.67O₃:Pr³⁺,6% Ga³⁺ in this work has a higher S_r than the zirconate and germanate thermometric phosphors. Moreover, except for LaMg_0.402Nb_0.598O₃:Pr³⁺, there are few investigations on temperature sensitivities based on two-FIR models. Clearly, compared with LaMg_0.402Nb_0.598O₃:Pr³⁺, the SrZn_0.33Nb_0.67O₃:Pr³⁺,6% Ga³⁺ phosphor has made some progress to a certain degree. The analysis above indicates that the obtained SrZn_0.33Nb_0.67O₃:Pr³⁺,Ga³⁺ phosphor could be employed as a promising candidate for self-calibrating temperature measurement.

Table 2 Maximum relative sensitivities of representative Pr³⁺-activated optical temperature-sensing materials in previous reports

Transition	Material	Temperature range (K)	S r-max (% K^−1)	Ref.
Pr^3+: ^1D2 → ^3H4, ^3P0 → ^3H4	YNbO4	303–543	1.56 (503 K)	17
	Sr2(Ge0.75Si0.25)O4	275–650	0.30 (460 K)	18
	LaMg0.402Nb0.598O3	298–533	0.71 (473 K)	24
	GdNbO4	300–700	0.70 (430 K)	38
	Sr2GeO4	300–600	0.60 (300 K)	39
	La0.4Gd1.6Zr2O7	15–650	0.81 (650 K)	40
	SrZn0.33Nb0.67O3:6% Ga^3+	300–500	0.83 (500 K)	This work
Pr^3+: ^1D2 → ^3H4, ^3P0 → ^3F2	LaMg0.402Nb0.598O3	298–533	0.73 (473 K)	24
	SrZn0.33Nb0.67O3:6% Ga^3+	300–500	0.70 (500 K)	This work

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Conclusion

In summary, SrZn_0.33Nb_0.67O₃ phosphors doped with Pr³⁺,Ga³⁺ were successfully synthetized via a high-temperature solid-state reaction route, and the temperature-dependent luminescence of Pr³⁺ is investigated for the first time. Under ultraviolet excitation, the SrZn_0.33Nb_0.67O₃:x%Pr (x = 0.25/0.75/1.25/2) phosphors exhibit bright blue and red emissions, which are located at 491 nm, 619 nm and 651 nm. Remarkably, the temperature sensitivities of the FIR thermometer are significantly enhanced when Ga³⁺ is incorporated. Specifically, the maximum absolute and relative sensitivities for the 6% Ga³⁺ co-doped sample are increased five- and two-fold, respectively, compared with the undoped sample. Analysis of the configurational coordinate diagram indicated that the changed thermo-sensitive properties are ascribed to the intervalence charge transfer state interfered Pr³⁺ luminescence. The results suggest the potential application of the SrZn_0.33Nb_0.67O₃:Pr³⁺,Ga³⁺ phosphor in optical thermometry. In addition, this study offers new thoughts for improving the performance of FIR-based luminescent thermometers.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (52102186), the Guangdong Basic and Applied Basic Research Foundation (2019A1515110801), the Science Foundation for High-level Talents of Wuyi University (2018AL016), the Wuyi University–Macau University Joint Research Fund (2019WGALH08), and the Special projects in key fields of Guangdong Universities (2021ZDZX1022).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ma01247a

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