Enhancing upconversion luminescence via intermediate state in double perovskite phosphor: three-mode optical thermometry with python-assisted validation

Abstract

High luminescence intensity, multiple modes, and high sensitivity are critical to achieving high measurement accuracy for optical thermometry in microelectronic devices and biological systems. The double perovskite phosphor, Ca₂Sc_0.63Mg_0.07SbO₆:Yb³⁺,Er³⁺, proves to be promising in overcoming these challenges. A simple high-temperature solid-phase method was used to prepare this sample which was found to exhibit red anti-Stokes luminescence under 980 nm excitation. Heterovalent substitution of Mg²⁺ for Sc³⁺ leads to lattice shrinkage and oxygen vacancy content enhancement. The induced generation of the intermediate state by the oxygen vacancy is significantly increased. This consequently enhances the upconversion luminescence intensity. The Ca₂Sc_0.63Mg_0.07SbO₆:Yb³⁺,Er³⁺ phosphor is capable of three-mode optical thermometry by thermally coupled energy states (TCES), non-thermally coupled energy states (NTCES), and CIE chromaticity shift. The NTCES-based mode has a notable relative sensitivity of S_r-max = 4.8% K⁻¹ and superior signal resolution δT = 0.016 K. Furthermore, the NTCES-based model was tested for practical applications, and the difference between the predicted theoretical temperature and the actual test temperature was kept within 6 K after about 100 000 evaluations via Python assistance.

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1. Introduction

High-sensitivity and reliable temperature measurement plays an important role in life, production, and technology.^1–3 Conventional contact thermometers are limited in their capabilities. They do not work properly in humid, highly corrosive, and other extreme environments because they direct contact with the objects whose temperatures are to be measured.^4,5 This has led to the emergence of various non-contact thermometry methods. In light of this, optical thermometry, which relies on the thermal response spectral properties of luminescent materials, has aroused immense interest due to its advantages of real-time feedback, high spatial, and thermal resolutions.^6–8 Particularly, the concept of self-calibration has recently become a hot research topic. The method is highly resistant to external disturbances, such as fluctuations in sample concentration, fluctuations in excitation power, and other factors including those affecting the absolute intensity.^9,10 Optical thermometry relies on temperature-sensitive properties such as luminescence intensity ratio (LIR), excitation intensity ratio, lifetime, Commission Internationale de L'Eclairage (CIE), and bandwidth.^11–15 For self-calibration and multimode operation, the material needs to possess all these temperature-sensitive properties. This is necessary to avoid temperature measurement inaccuracies that may be caused by a single mode.

Upconversion (UC) luminescent materials are well known for their potential applications in biomedical imaging, solar cells, and optical anti-counterfeiting.^16–19 Also, due to the unique optical properties of visible light generated by near-infrared excitation, UC luminescent materials are promising candidates for optical thermometry. Abundant energy states and suitable thermally coupled energy states (TCES) in Er³⁺ make it a desirable UC luminescent center for optical thermometry. Although optical thermometry in different Er³⁺-doped materials based on TCES or/and non-thermally coupled energy states (NTCES) have been extensively studied,^20–22 it is of utmost interest to explore Er³⁺-activated materials utilizing multifunctional thermometry techniques. The 4f–4f transition prohibited for Er³⁺ ions leads to the small absorption cross-section at 980 nm and low UC luminescence intensity. Yb³⁺ ion has been utilized as a sensitizer due to its large absorption cross-section (980 nm), resulting in effective enhancement of luminescence intensity.^23,24 However, further, enhancing UC luminescence intensity based on Yb³⁺/Er³⁺ co-doping remains a significant challenge. Yet, improving UC luminescence intensity is important as it affects optical thermometry directly. Enhancement of UC luminescence intensity is commonly achieved by doping with alkali metal ions, alkaline earth metal ions, or transition metal ions through mechanisms like change in lattice parameters, concentration quenching, or oxygen vacancy.^25–30 Amongst these mechanisms, though the presence of oxygen vacancy is known to enhance UC luminescence, its exact role played remains ambiguous. Oxygen vacancy plays a very important role in enhancing the photocatalytic performance in the photocatalytic field.^31–33 Liang et al. achieved NIR-driven CO₂ overall splitting by introducing the intermediate band to collect near-infrared light through the oxygen vacancy.³⁴ Li et al. proposed the introduction of an intermediate band to enhance the UC luminescence and achieved good photocatalytic performance.³⁵ However, the origin and mechanism of enhancement of UC luminescence by oxygen vacancy remains yet to be solved with direct experimental evidence.

It is noteworthy that in the field of luminescence, A₂B′B′′O₆ double perovskite is commonly regarded as an ideal host material due to the rich octahedral environment and high chemical stability.^36–38 Again, to realize improved luminescence properties, appropriate crystal field environment is required as well as suitable sites to accommodate the luminescent centers.

Herein, the double perovskite phosphor Ca₂Sc_0.63Mg_0.07SbO₆:Yb³⁺,Er³⁺ is designed to enhance UC luminescence intensity and employed in three-mode optical thermometry. Heterovalent substitution of Mg²⁺ for Sc³⁺ leads to lattice shrinkage and increase in oxygen vacancy content. We show that the oxygen vacancy-induced generation of intermediate state results in the UC luminescence intensity enhancement and provide direct evidence for the existence of the intermediate state. The intrinsic process mechanism is explained. Self-calibration and multimode optical thermometry are realized by three thermometry modes: TCES, NTCES, and CIE chromaticity shift. The NTCES-based mode shows outstanding relative sensitivity and superior signal resolution. In addition, the reliability of this phosphor in practical applications is further evaluated by Python assistance.

2. Experimental

2.1. Preparation

The raw materials include Yb₂O₃ (99.99%), Er₂O₃ (99.99%), Sc₂O₃ (99.9%), CaCO₃ (99%), Sb₂O₅ (99%) and MgO (99%). The above weighed raw materials were mixed in an agate mortar and ground for 30 minutes and then sintered in a muffle furnace at 1500 °C for 6 h and cooled naturally to room temperature. Finally, the products were obtained after grinding into homogeneous particles. The reducing atmosphere is a mixture of 95% N₂ and 5% H₂ that is kept at 1000 °C for 200 minutes and then naturally cooled to room temperature.

2.2. Characterization

A DX-2700 diffractometer was used to collect X-ray diffraction (XRD) patterns for all samples. The Rietveld refinement was performed with the General Structure Analysis System (GSAS) software. A Nova NanoSEM 450 scanning electron microscope (SEM) was used to characterize the morphology of the sample with the accelerating voltage of 20 kV, and an energy dispersive spectrometer (EDS) was used to measure the elemental compositions. The powder samples were first uniformly dispersed on a conductive adhesive and then coated with a gold film on the surface. The diffuse reflectance spectra of the samples were measured by a Hitachi 4100 spectrometer. The optical properties of the samples including UC spectra were measured at different temperatures using an Edinburgh FLS1000 spectrometer with a 980 nm excitation source. For the upconversion spectra test, we used a suitable filter (850 nm) to separate the upconversion light from the diffraction grating with higher-order peaks. To ensure the accuracy of our experimental results, we carefully control external factors by using a stable excitation light source to maintain consistent excitation intensity and calibrating the detection system to minimize instrumental errors. In addition, we ensured that all measurement samples were prepared and tested under identical conditions to eliminate errors due to sample differences. Electron paramagnetic resonance (EPR) curves were recorded using a Bruker EMX Plus spectrometer and all other conditions were constant for two samples of undoped/doped Mg²⁺. The actual temperature is obtained using an infrared thermometer DLX-HC2508. Python is used as the computer programming language.

3. Results and discussion

3.1. Structural and morphological analysis

The XRD patterns of Ca₂Sc_0.9−xSbO₆:0.1Yb³⁺,xEr³⁺ (0.01 ≤ x ≤ 0.09) and Ca₂Sc_0.95−ySbO₆:0.05Er³⁺,yYb³⁺ (0.05 ≤ y ≤ 0.3) phosphors are shown in Fig. S1a and b.† The reflection peaks are consistent with those of the XRD pattern of Ca₂ScSbO₆ (ICSD#262995). The Yb³⁺/Er³⁺ doping thus maintains the Ca₂ScSbO₆ structure without introducing a new impurity phase. Since Yb³⁺/Er³⁺ are close to Sc³⁺ in valence and ionic radius [CN = 6, r(Sc³⁺) = 0.745 Å, r(Yb³⁺) = 0.868 Å, r(Er³⁺) = 0.89 Å],³⁹ they are likely to displace Sc³⁺ site. Table S1† provides detailed results of the structure refinement, and the concentration of Yb³⁺ (0.2243) is very close to the designed doping concentration (0.25), which confirms that they replace the Sc³⁺ site. Fig. S2† shows the refinement result for optimal Yb³⁺/Er³⁺ doping. The low residuals indicate that the compound indeed crystallizes in a pure phase single phase. Fig. 1a exhibits the XRD patterns of Ca₂Sc_0.7−zMg_zSbO₆:0.25Yb³⁺,0.05Er³⁺ with z in the interval of 0.01 ≤ z ≤ 0.15. It can be observed that all the samples remain in pure single phase after doping with Mg²⁺. Generally, Mg²⁺ would be expected to replace the alkaline earth metal Ca²⁺ of the same main group. However, considering the significant difference in ionic radii, [e.g., CN = 6, r(Ca²⁺) = 1 Å, r(Sc³⁺) = 0.745 Å, r(Mg²⁺) = 0.72 Å],³⁹ the Mg²⁺ is rather more inclined to displace Sc³⁺ with comparable ionic radius. In Fig. 1b, the diffraction peak located at 45.9° shifts towards the right with increasing Mg²⁺ doping, which proves the replacement of Mg²⁺ for Sc³⁺. The ionic radius of Mg²⁺ being smaller than that of Sc³⁺, results in the diffraction peak showing a shift to higher angles. Moreover, even when the doping concentration of Mg²⁺ reaches 0.07, the shift of the diffraction peak remains insignificant, a phenomenon that coincides with the fact that the ionic radii of Mg²⁺ and Sc³⁺ are very close, and thus Mg²⁺ prefers to substitute for Sc³⁺ rather than Ca²⁺. As the Mg²⁺ doping concentration continues to increase, the diffraction peak shift becomes more obvious. Tables S1 and S2† provide detailed structural refinement results for the Mg²⁺ undoped and the representative Mg²⁺ doped Ca₂Sc_0.63Mg_0.07SbO₆:0.25Yb³⁺,0.05Er³⁺ samples, respectively. The Rietveld refinement of the Ca₂Sc_0.63Mg_0.07SbO₆:0.25Yb³⁺,0.05Er³⁺ is shown in Fig. 1c. Results in Tables S1 and S2† show shrinkage of the unit cell after substitution by the Mg²⁺ ions. The cell parameters a, b, c, and volume V decrease for the Mg²⁺ ions doped sample. This is consistent with the shift in peak position in Fig. 1b. In addition, if Mg²⁺ is set to replace the Ca²⁺ position, the occupation of the refinement result is negative, which is wrong, indicating that Mg²⁺ cannot replace the Ca²⁺ position. While setting Mg²⁺ to replace the Sc³⁺ position, the occupation of the refined result (0.0608) is very close to the design doping (0.07) (Table S2†). Therefore, as a consequence of the Mg²⁺ ion doping, Mg²⁺ occupies the Sc³⁺ 2d site. Moreover, although oxygen vacancy is present in the Mg²⁺ undoped sample, it increases in content after doping. Structural alterations often cause changes in the optical properties of materials. The substitution of Mg²⁺ for Sc³⁺ is usually accompanied by the formation of oxygen vacancies (V_O) in order to maintain charge neutrality due to the lower valence state of Mg²⁺ compared to Sc³⁺. Therefore, the content of oxygen vacancy increases after doping Mg²⁺. The SEM image, EDS, and element mapping of the Ca₂Sc_0.63Mg_0.07SbO₆:0.25Yb³⁺,0.05Er³⁺ phosphor are shown in Fig. S3a–c.† The sample exhibits agglomeration, which may be attributed to high-temperature sintering.⁴⁰ In addition, some holes can be observed, caused by escaping gas during sintering.⁴¹ The seven elements Ca, Sc, Sb, O, Mg, Yb, and Er are homogeneously distributed in the synthesized sample, as shown in the EDS and elemental mapping images.

Fig. 1 (a) XRD patterns and (b) the magnified diffraction peak at the 45.9° of the Ca₂Sc_0.7−zMg_zSbO₆:0.25Yb³⁺,0.05Er³⁺. (c) Rietveld refinement of Ca₂Sc_0.63Mg_0.07SbO₆:0.25Yb³⁺,0.05Er³⁺.

3.2. Luminescent properties

To obtain the optimal UC luminescence performance, Ca₂ScSbO₆ samples doped with different concentrations of Yb³⁺ and Er³⁺ were prepared. Under 980 nm excitation, all the samples show a distinct red emission band and two weak green bands, including an intense red UC belonging to the ⁴F_9/2 → ⁴I_15/2 transition at 662 nm as shown in Fig. S4a and b.† The two weak green ones are located at 528 and 548 nm for the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions, respectively. The variation of Er³⁺ and Yb³⁺ doping concentrations hardly affects the position of the emission bands. However, with the increase in Er³⁺ and Yb³⁺ concentrations, the UC luminescence intensity changed remarkably to reach maxima at x = 0.05 and y = 0.25, respectively, and then decreased due to concentration quenching.^42–44 Therefore, the optimal composition in terms of the UC luminescence intensity is Ca₂Sc_0.7SbO₆:0.25Yb³⁺,0.05Er³⁺. To further enhance the UC luminescence intensity, Mg²⁺ ions are introduced for co-doping based on Yb³⁺/Er³⁺ doping. As depicted in Fig. 2a, the UC luminescence intensity reaches its strongest (4.32 times higher than that of the undoped) with increased Mg²⁺ doping. However, the UC luminescence intensity shows a decreasing trend with Mg doping greater than 0.07. Moreover, to further verify the effect of oxygen vacancy content, we annealed the optimal sample under reductive atmosphere treatment. As shown in Fig. S5,† the UC luminescence intensity of the optimal sample after annealing under reducing atmosphere is substantially weakened and the color of the sample changes from white to gray. Since doping Mg²⁺ to replace Sc³⁺ leads to charge imbalance due to valence difference, oxygen vacancy (V_O) is formed in the sample to maintain the electrical neutrality of the lattice. Therefore, this suggests that a moderate amount of oxygen vacancies can enhance the UC luminescence intensity, while an excess of oxygen vacancies can weaken it. This is because oxygen vacancies, as a type of defect, when present in excess, act as quenching centers that continuously absorb photons, which then leads to an increase in the harmful nonradiative relaxation of the Er³⁺ excited state, thus quenching upconversion luminescence. In addition, the calculated red-to-green integrated intensity ratio in Fig. 2b indicates a significant increase in the ratio from 10.7 to 18.9 for Mg²⁺ doping of z = 0.07. The UC luminescence spectra were further analyzed under different laser power pumping. This is shown in Fig. S6a and c.† The linear fitting of the bi-logarithmic graph of UC intensity and pumping power, according to eqn (S1)† gives the value of the photon number n. From Fig. S6b,† the slopes (n) of undoped Mg²⁺ at 528, 548, and 662 nm are 1.80, 1.70, and 1.49, respectively, indicating that the green UC luminescence is both two-photon processes, whereas the red UC emission may be a mix of single-photon and two-photon processes. However, from Fig. S6d,† upon doping with Mg²⁺, the increased slopes (n) of 2.15, 2.13, and 1.85 are obtained for the same respective pumping powers of 528, 548, and 662 nm. The slopes of the 528 and 548 nm pump powers are both slightly greater than 2, which means that they are still two-photon processes, whereas the red UC process completely changes to a two-photon process. Therefore, by doping with Mg²⁺ ions, the red UC luminescence intensity is enhanced more obviously, which corresponds well to the red–green ratio enhancement in Fig. 2(b).

Fig. 2 (a) UC spectra of Ca₂Sc_0.7−zMg_zSbO₆:0.25Yb³⁺,0.05Er³⁺ (z = 0–0.15) under 980 nm laser excitation. (b) Red–green integrated intensity ratio of Ca₂Sc_0.7SbO₆:0.25Yb³⁺,0.05Er³⁺ and Ca₂Sc_0.63Mg_0.07SbO₆:0.25Yb³⁺,0.05Er³⁺. Crystal structure of (c) Ca₂Sc_0.7SbO₆:0.25Yb³⁺,0.05Er³⁺ and (d) Ca₂Sc_0.63Mg_0.07SbO₆:0.25Yb³⁺,0.05Er³⁺. Schematic diagram of sublattice distortion caused by (e) undoped and (f) doped Mg²⁺.

Coupled with the previous Rietveld refinement result analysis above for the Mg²⁺ doped sample, the oxygen vacancy content is significantly increased after doping with Mg²⁺. In addition, shrinkage of the cell parameters (a, b, c, and V) after the Mg²⁺ doping indicates a change in the crystal field. Fig. 2c and d show schematic diagrams of the crystal structures of the Mg²⁺ undoped and Mg²⁺ doped samples. The Mg²⁺ undoped and Mg²⁺ doped polyhedra are drawn separately in Fig. 2e and f. The structure undergoes a slight distortion (see red dashed parallelogram in Fig. 2e and f) after doping with Mg²⁺. The substitution of Mg²⁺ for Sc³⁺ leads to changes in the lengths of the Sc–O bonds. The bond lengths of Sc–O1, O2, and O3 are 2.044, 2.067, and 2.180 Å, respectively for the Mg²⁺ undoped sample, while the bond lengths of Sc–O1 and O2 are shortened to 2.034 Å and 2.051 Å, respectively with that of Sc–O3 increased to 2.206 Å for the Mg²⁺ doped sample. Consequently, the polyhedra are contracted horizontally, and stretched vertically to become slender. Therefore, the lattice contraction and oxygen vacancy may be responsible for the enhanced UC luminescence intensity and the larger n value.

The diffuse reflectance spectra, bandgap, and EPR curves of both Mg²⁺ undoped and doped samples were tested. The results are shown in Fig. 3a–d. In Fig. 3a, many characteristic absorption peaks of Er³⁺ and Yb³⁺ occur. Peaks located at 379 (⁴I_15/2 → ⁴G_11/2), 491 (⁴I_15/2 → ⁴F_7/2), 522 (⁴I_15/2 → ²H_11/2), 546 (⁴I_15/2 → ⁴S_3/2), 658 (⁴I_15/2 → ⁴F_9/2), and 801 (⁴I_15/2 → ⁴I_9/2) nm, respectively, belong to Er³⁺ and the broad absorption band located at 980 nm is attributed to the transitions of Er³⁺ (⁴I_15/2 → ⁴I_11/2) and Yb³⁺ (²F_7/2 → ²F_5/2). The absorption peaks are enhanced after doping with Mg²⁺. Surprisingly, two new absorption peaks at 1116 and 1160 nm appear after Mg²⁺ doping. For comparison, we also tested the diffuse reflectance spectrum of Ca₂ScSbO₆, as shown in Fig. S7,† which does not show any absorption peaks at these two positions. They are new energy states (intermediate states) induced by the formation of oxygen vacancy generated by the heterovalent substitution of Mg²⁺ for Sc³⁺. In 2018, Liang et al. revealed through theoretical calculations that oxygen vacancy reaching a critical density results in the formation of an intermediate band,³⁴ but direct experimental evidence on the intermediate band is not yet directly available. So, this could be the reason for the enhanced UC luminescence intensity. The higher oxygen vacancy content after Mg²⁺ doping, confirmed by Rietveld refinement (Tables S1 and S2†), strongly confirms this. The EPR curve is a well-known method for detecting the presence of oxygen vacancy.⁴⁵ To further demonstrate the increase in oxygen vacancy content, the EPR curves of the Mg²⁺ undoped and Mg²⁺ doped samples were obtained under the same conditions, as shown in Fig. 3c. The signal intensity of the Mg²⁺ doped sample is much stronger than that of the non-Mg²⁺ doped (g = 2.004). This reflects their difference in oxygen vacancy content and is consistent with the Rietveld refinement results and diffuse reflectance spectra. In addition, the bandgaps of the undoped and doped Mg²⁺ samples were calculated (using eqn (S2) and (S3)†) to be 4.30 and 4.26 eV, respectively, as shown in Fig. 3b and d. It is evident that the Mg²⁺ doping decreases the bandgap from 4.30 eV to 4.26 eV. Also, the presence of oxygen vacancy shifts the oxide Fermi energy level upward, leading to the emergence of a defect energy state (the intermediate state) in the bandgap, thus reducing the width of the energy band.^46,47 This establishes that indeed oxygen vacancy induces the intermediate state that drives the appearance of new absorption peaks in the diffuse reflectance spectra.

Fig. 3 (a) Diffuse reflectance spectra and (c) EPR curves of Ca₂Sc_0.7SbO₆:0.25Yb³⁺,0.05Er³⁺ and Ca₂Sc_0.63Mg_0.07SbO₆:0.25Yb³⁺,0.05Er³⁺. Bandgap plots of (b) Ca₂Sc_0.7SbO₆:0.25Yb³⁺,0.05Er³⁺ and (d) Ca₂Sc_0.63Mg_0.07SbO₆:0.25Yb³⁺,0.05Er³⁺. (e) Schematic diagram and the possible UC mechanism.

On the basis of these results, a mechanism is proposed as shown in Fig. 3e. Under the excitation of 980 nm laser, the Yb³⁺ ions in the ground state (²F_7/2) are pumped to the excited state (²F_5/2). The excited Yb³⁺ ions relax back to the ground state by non-radiative relaxation. This transfers energy to the nearby Er³⁺ ions, exciting them from the ⁴I_15/2 ground state to ⁴I_11/2 excited state by ground state absorption (GSA). Subsequently, the ⁴I_11/2 state excited Er³⁺ ions, through energy transfer (ET) from the ²F_5/2 excited Yb³⁺ ions state, are further excited to the higher ⁴F_7/2 state via excited state absorption 1 (ESA1). Otherwise, the Er³⁺ ions at the ⁴I_11/2 state relaxes to the ⁴I_13/2 energy state via multi-phonon relaxation (MPR). Through MPR, the Er³⁺ ions in the excited ⁴F_7/2 state quickly fall to the ²H_11/2 and then to the ⁴S_3/2 states. Eventually, the Er³⁺ ions in the ²H_11/2 and ⁴S_3/2 states return to the ground ⁴I_15/2 state by radiative transition, producing two green emissions at 528 and 548 nm respectively. This is consistent with the power-dependent results of the 528, 548 nm in Fig. S6(d),† which the n values are 2.15 and 2.13, respectively, and both are two-photon processes, i.e., both green emissions go through both GSA and ESA1 processes. On the other hand, two pathways exist for red emission. The Er³⁺ ions at the ⁴S_3/2 state undergo MPR to the ⁴F_9/2 state before returning to the ground ⁴I_15/2 state by the radiative transition to produce red emission. Alternatively, Er³⁺ ions located in the ⁴I_13/2 state, are excited to the ⁴F_9/2 through ESA2. This is then followed by deexcitation to the ground ⁴I_15/2 state by the radiative transition to produce red emission. This is also consistent with the power-dependent test result of 662 nm in Fig. S6(d),† which has an n value of 1.85 for a two-photon process, i.e., the red emission undergoes both GSA and ESA2 processes. It is worth noting that the intermediate state contributes significantly to the enhancement of UC luminescence intensity. Since the intermediate states are at lower energies (1116 and 1160 nm) than the 980 nm laser, the 980 nm laser can excite electrons from the valence band to the intermediate state directly, thus resulting in two ET pathways. In addition, due to the generation of localized lattice distortions that enhance electroacoustic coupling and promote phonon-assisted processes, which enables energy transfer from the intermediate state to the Yb^{3+ 2}F_5/2 excited state and the Er^{3+ 4}I_11/2 excited state. One path is the transfer of the electrons at the intermediate state to the Yb^{3+ 2}F_5/2 excited state. The second path constitutes the transfer of electrons at the intermediate state to the Er^{3+ 4}I_11/2 excited state. The two pathways result in an increase in the populations of the Er³⁺ ion ²H_11/2, ⁴S_3/2, and ⁴F_9/2 states causing enhancement of the green and red UC luminescence.

3.3. Three-mode optical thermometry

Two- and three-dimensional temperature-dependent emission spectra of Ca₂Sc_0.63Mg_0.07SbO₆:0.25Yb³⁺,0.05Er³⁺ sample for the temperature range from 298 to 573 K are shown in Fig. 4a. The intensity of the peak located at 528 nm increases with temperature in the range from 298 to 498 K but decreases with temperature in the range from 498 to 573 K. In Fig. 4b, the intensity decreases monotonically with temperature for the peak located at 548 nm. This phenomenon can be attributed to the narrow energy gap between the ⁴S_3/2 and ²H_11/2 energy states, which belong to the TCES. The proximity of the two states allows the ²H_11/2 state to easily be occupied by electrons from the ⁴S_3/2 state through thermal excitation.⁴⁸ The different responses of the two green UC emissions to temperature are useful in optical thermometry. The plot of LIR (I₅₂₈/I₅₄₈) against the temperature curve shown in Fig. 4c was fitted using eqn (S4)† (the two energy states obey the Boltzmann distribution). The R² value of 0.999 for the curve fitting, indicates good agreement of the data with eqn (S4).† The absolute sensitivity (S_a) and relative sensitivity (S_r) are important metrics for evaluating the performance of optical thermometry given by the eqn (S5) and (S6).† As depicted in Fig. 4d, the maximum S_a-max at 573 K is 0.63% K⁻¹, and the maximum S_r-max at 298 K is 1.33% K⁻¹. To determine the smallest temperature change that can be resolved by the LIR sensor, the temperature resolution (δT) is evaluated. It is strongly dependent on the LIR deviation, in accordance with eqn (S7).† The optimal value of δT was determined from Fig. 4e to be 1.64 K at 298 K under the same conditions for all twenty spectral measurements. The repeatability (R) was calculated with eqn (S8)† to assess the temperature cycling behavior. From the LIR values for five consecutive cycles (Fig. 4f), the R values for 298 and 573 K were calculated to be 99.1% and 99.2%, respectively.

Fig. 4 (a) Two- and three-dimensional temperature-dependent emission spectra of Ca₂Sc_0.63Mg_0.07SbO₆:0.25Yb³⁺,0.05Er³⁺. (b) Intensities of 528 nm and 548 nm versus temperature. (c) LIR (I₅₂₈/I₅₄₈) versus temperature. (d) S_a and S_rversus temperature. (e) Twenty measurements of LIR (I₅₂₈/I₅₄₈) and calculated temperature resolution δT at 298 K. (f) Repeatability of LIR (I₅₂₈/I₅₄₈) for 298 and 573 K.

In Fig. S8,† the red UC intensity of the sample is significant over a wide temperature range and decreases rapidly with increasing temperature. These properties make the sample suitable for optical thermometry. Since the energy difference between the ²H_11/2 and ⁴F_9/2 energy states is relatively large, they are not TCESs. Boltzmann distribution can not be used to describe NCTES due to the high energy difference in the involved states. In the absence of a physical model, we use a numerical approach and fit to a polynomial. The FIR values of the NTCES can be fitted by the polynomial equation eqn (S9)† as shown in Fig. 5a. The experimental data fit well with a goodness of fit of 0.998. The corresponding relative and absolute sensitivities were calculated using eqn (S10) and (S11),† and the maximum values of both are S_a-max = 0.17% K⁻¹ and S_r-max = 4.8% K⁻¹, respectively, as shown in Fig. 5b. In addition, its δT was evaluated according to eqn (S7),† and the optimal value of δT at 298 K was determined to be 0.016 K under the same conditions for 20 spectral measurements (Fig. 5c). It is worth mentioning that the δT value (0.016 K) is two magnitudes lower than that of 0.7 K for the cubic phase LiLuF₄:18%Yb³⁺/2%Er³⁺,⁴⁹ which is also an intensity-based thermometry. Its R was also calculated using eqn (S8)† and based on the LIR values of five consecutive cycles (Fig. 5d), the R values at 298 K and 573 K were calculated to be 99.3% and 98.6%, respectively. Fig. 5e compares the S_r-max of other Yb³⁺/Er³⁺ co-doped LIR-based optical thermometry.^50–63 The maximum S_r-max value of Ca₂Sc_0.63Mg_0.07SbO₆:0.25Yb³⁺,0.05Er³⁺ is much superior to those of most other materials. The Ca₂Sc_0.63Mg_0.07SbO₆:0.25Yb³⁺,0.05Er³⁺ compound is therefore an ideal LIR optical thermometry with high S_r and low δT.

Fig. 5 (a) LIR (I₅₂₈/I₆₆₂) versus temperature. (b) S_a and S_rversus temperature. (c) Twenty measurements of LIR (I₅₂₈/I₅₄₈) and calculated temperature resolution δT at 298 K. (d) Repeatability of LIR (I₅₂₈/I₅₄₈) between 298 and 573 K. (e) Comparison of the maximum relative sensitivity of Ca₂Sc_0.63Mg_0.07SbO₆:0.25Yb³⁺,0.05Er³⁺ with those of some reported Yb³⁺,Er³⁺ co-doped samples based on LIR. Numbers in parentheses indicate references.

To further explore the potential of using Ca₂Sc_0.63Mg_0.07SbO₆:0.25Yb³⁺,0.05Er³⁺ for temperature sensing in the NTCES mode, we developed a straightforward temperature measurement platform, as illustrated in Fig. 6a. The Ca₂Sc_0.63Mg_0.07SbO₆:0.25Yb³⁺,0.05Er³⁺ sample was placed on a heating stage and excited with a 980 nm laser. The resulting spectral data were transmitted through optical fiber to a computer, which employed the Python programming language to calculate and predict the LIR and the corresponding temperature value (T). Subsequently, the real-time temperature measured by an infrared thermometer was compared with the predicted theoretical temperature values to further evaluate the reliability of this phosphor in practical application. Fig. 6b–g presents the theoretically predicted temperatures and the actual test temperatures. After approximately 100 000 evaluations, the difference between the predicted theoretical temperature and the actual test temperature remains within 6 K. This finding indicates that the Ca₂Sc_0.63Mg_0.07SbO₆:0.25Yb³⁺,0.05Er³⁺ phosphor is a reliable candidate for temperature sensing. Consequently, Ca₂Sc_0.63Mg_0.07SbO₆:0.25Yb³⁺,0.05Er³⁺ phosphor emerges as a feasible, dependable, and promising optical thermometry for UC applications, providing a solid foundation for further advancements in the field of optical temperature sensors.

Fig. 6 (a) Experimental arrangement of temperature sensing system. (b, d and f) Theoretical temperature values calculated using spectral data and computer programming language. (c, e and g) Actual temperature values measured by infrared thermometer.

Owing to the different changes in red and green emissions with increasing temperature, the color of the sample changes from red to yellow, showing a significant shift in the color coordinates as shown in Fig. S9.† The color change is quantified by chromaticity shift (ΔE_s), calculated with eqn (S13).† The results are given in Table S3.† With its ΔE_s as high as 0.147 the sample has remarkable thermochromic performance and has the potential in high-temperature safety signs. Moreover, the high ΔE_s value also implies its potential application in optical thermometry. Fig. S10a† presents the fitting curve of CIEy versus temperature. The R² = 0.994 value indicates a very good agreement of the data with eqn (S12).† The S_a and S_r were also determined from eqn (S5) and (S6)† and plotted in Fig. S10b.† The results give S_a-max and S_r-max values of 0.229% K⁻¹ (298 K) and 0.608% K⁻¹ (298 K), respectively. The S_r-max value of Ca₂Sc_0.63Mg_0.07SbO₆:0.25Yb³⁺,0.05Er³⁺ sample is superior to most optical thermometry based on CIE coordinates (see Table 1). Clearly, the Ca₂Sc_0.63Mg_0.07SbO₆:0.25Yb³⁺,0.05Er³⁺ phosphor possesses impressive UC emission characteristics and multimode temperature sensitivity based on TCES, NTCES, and CIE chromaticity shift, and that the combination of these multimode thermometry techniques can be used for self-calibrating thermometry to improve accuracy.

Table 1 Chromaticity shift (ΔE_s) of Ca₂Sc_0.63Mg_0.07SbO₆:0.25Yb³⁺,0.05Er³⁺ in response to temperature

Phosphor	Temperature (K)	CIE coordinates	S r-max (% K^−1)	Ref.
Al2Mo3O12:Ho^3+,Yb^3+	323–543	x	0.112	64
Al2Mo3O12:Ho^3+,Yb^3+	323–543	x	0.102	64
Sr4Al14O25:Mn^4+,Tb^3+	123–573	x	0.6	65
Sr4Al14O25:Mn^4+,Tb^3+	123–573	y	0.4	65
LaNbO4:Bi^3+,Eu^3+	303–483	x	0.47	66
LaNbO4:Bi^3+,Sm^3+	303–483	x	0.36	66
Ca3LiZnV3O12:Sm^3+	303–483	x	0.567	15
Ca2Sc0.63Mg0.07SbO6:Yb^3+,Er^3+	298–573	y	0.608	This work

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4. Conclusion

In summary, a red emission double perovskite phosphor usable for three-mode optical thermometry is proposed. Heterovalent substitution of Sc³⁺ by Mg²⁺ causes contraction of the lattice to result in a significant increase in the oxygen vacancy content. The oxygen vacancy in turn induces intermediate states that enhance the upconversion luminescence intensity up to 4.32 times the original. The different responses of the red and two green emissions to temperature suits application for three-mode optical thermometry, based on the NTCES (²H_11/2 and ⁴F_9/2) with S_r-max up to 4.8% K⁻¹ and δT as low as 0.016 K. The sample exhibits impressive relative sensitivity and superior signal resolution with good repeatability. Moreover, after approximately 100 000 evaluations with Python assistance, the difference between the predicted theoretical temperature and the actual test temperature remains within 6 K, which further demonstrates the reliability of this phosphor in practical applications. This work provides new insights for further enhancing UC luminescence intensity and the fabrication of high-performance multimode self-calibrating optical thermometry.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article [and/or its ESI†].

Conflicts of interest

The authors declare no competing interests.

Acknowledgements

This work was supported by the Natural Science Foundation of Sichuan Province (Grant No. 2022NSFSC0362), and the Radiation Oncology Key Laboratory of Sichuan Province Open Fund (Grant No. 2023ROKF05). The authors would like to thank Ceshigo Research Service (https://www.ceshigo.com) for supporting the EPR test.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5qi00572h

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