CaTiO₃: Eu³⁺ red phosphors with high thermal stability and temperature sensitivity were synthesized by KCl flux assisted solid phase reaction method and they had orthorhombic perovskite phase. CaTiO₃:15%Eu³⁺ had the irregular particle structure, with mean particle size of about 1.58 μm, while CaTiO₃:15%Eu³⁺ +6%KCl also had the irregular particle structure, with mean particle size of about 2.25 μm. When Eu³⁺ ion concentration was 15mol%, concentration quenching phenomenon took place, which could be explained by electric dipole-dipole interaction due to the critical Eu³⁺- Eu³⁺ distance of ~ 8.93 Å. We found that KCl flux could increase the particle size and crystallinity, which was beneficial to luminescence property. Based on the PL spectra excited at 398 nm, the color coordinates of CaTiO₃:15%Eu³⁺ were (0.5911, 0.4008) and the color purity was 97.7%, while the color coordinates of sample CaTiO₃:15%Eu³⁺+ 6%KCl were (0.6042, 0.3895) and the color purity was 98.4%. The relative intensity I_R1 and I_R2 at 423 K is 87. 29% and 83. 22%, indicating the sample had high thermal stability. Based on TCLs FIR, S_r of CaTiO₃:15%Eu³⁺ is 2322.67/T². CaTiO₃:Eu³⁺ red phosphor was expected to be a promising inorganic material for possible applications in temperature sensing

Keywords: Flux, optical thermometry, solid state reaction, calcium titanate, europium

Recently, rare earth doped luminescent materials have attracted much attention because of their wide application in various fields of phosphor-converted white light-emitting diodes (pc-WLEDs) [1, 2], photocatalysis [3], temperature sensing [4]. Recently, Eu³⁺ of the numerous rare earth ions is considered as a promising red luminescent activator because of their red ⁵D₀→⁷F₂ emission at about 616 nm [5, 6]. In addition, a selection of host material is also crucial for the luminescence properties of Eu³⁺ ion, depending on the phonon energy and bandgap value of the host material. As is known to all, CaTiO₃ with low phonon energy and excellent physical and chemical properties is one of the most important host materials for luminescent materials [7].
Recently, the red emitting Eu³⁺ doped CaTiO₃ phosphors prepared by polymerizing precursors [8, 9], microwave-assisted hydrothermal method [10], solvothermal method [11], sol-gel method [12, 13], solid-phase method [14-17]. Among many synthesis methods, although the solid-phase method is often accompanied by a small amount of impurities, its luminous efficiency and intensity are higher than other methods, and it is considered to be the best method for synthesizing phosphors [18]. In order to increase the luminescence intensity, charge compensation is usually carried out by adding ions, such as Li⁺ [16], Zn²⁺ [17], Mg²⁺ [14, 19], Sr²⁺ [20], Al³⁺ [13], Bi³⁺ [19], Sn⁴⁺ [20] and Nb⁵⁺ [21]. In addition, flux assisted solid-phase method, such as H₃BO₃ flux [21-23], NaF flux [24, 25] can not only improve the ion migration rate of reactants, but also improve the crystallinity of products [21, 22], thus enhancing the intensity of photoluminescence. However, the effect of KCl flux on the PL intensity of CaTiO₃:Eu³⁺ red phosphors hasn't been reported yet.
In recent years, a great deal of research has focused on luminescent materials for optical thermometry [24, 26-32]. It is suitable for thermometry in corrosive en- vironments, strong electromagnetic field environment or other dangerous and harsh environments [35]. Phosphor thermometry are usually used in gas centrifuges, gas turbine, jet turbines, and rocket engines [28]. In particular, the fluorescence intensity ratio (FIR) technology, which is not affected by fluorescence loss and excitation light fluctuation and can achieve non-contact and accurate thermometry, has attracted much attention of researchers [23, 24]. At present, the research on the optical therm- ometer phosphors based on FIR technology mostly uses two thermally coupled energy levels (TCLs) related to temperature. However, relative sensitivity, as an important parameter of thermometry, is limited by the effective energy interval between TCLs. In order to prevent the population of the upper thermal coupling level from being too low, the energy interval is usually lower than 2,000 cm^-1. In other words, the highest relative sensitivity value is lower than 2878/T² [24]. For instance, Zhou et al. [34] has reported the temperature sensing of an upconversion (UC) NaYF₄:Yb³⁺/Er³⁺ nanothermometer based on the TCLs of ²H_11/2 and ⁴S_3/2. However, FIR involved in the UC process is often affected by the thermal effect of the 980 nm excitation light, which will affect the measurement accuracy, not to mention the low luminous efficiency of the UC process. Therefore, a highly efficient FIR method free of UC process should be pursued, for which emissions from Eu³⁺ offer a possibility [34]. In 2017, Nanda et al. [27] reported that, based on the ⁵D₁→⁷F₂ /⁵D₀→⁷F₂ FIR value of Eu³⁺, the relative sensitivity of Eu³⁺ doped ZnO was as high as 3013/T², higher than the theoretical relative sensitivity of 2878/T². High relative sensitivity depends on the types, concentrations of doped ions, and host material. In general, for Eu³⁺ ions, the difference between the TCLs ⁵D₁ and ⁵D₀ is about 1,700 cm^-1 [35]. At a certain temperature, they can achieve thermal equilibrium. For the YBO₃:Eu³⁺ reported by Zhao et. al., the temperature required for the thermal coupling is above 450 K [35]. Otherwise, the electron could not cross the energy interval without enough energy, and so the upper thermally coupled level will not be filled. Therefore, CaTiO₃:Eu³⁺ phosphors for optical therm- ometry are interesting by the FIR values of TCLs ⁵D₁ and ⁵D₀. At present, the commercial optical thermometry is carried out by using optical pyrometer and radiation thermometer. When the measured signal is converted to the real temperature, the emissivity of the measured object needs to be corrected. However, the emissivity of a substance is related to its properties, surface roughness, pollution and oxidation of surface, and temperature. Therefore, the emissivity of many non-blackbody materials is not accurately known, so the accuracy of thermometry is bound to be limited. If the infrared radiometer is used to measure the temperature, it is also affected by the infrared transmittance of the medium, Therefore, optical thermometry will encounter a lot of troubles. It is an effective method to use temperature sensitive phosphor for optical thermometry in some specific environment and certain temperature range, which is not affected by fluorescence loss and excitation light fluctuation and can achieve non-contact and accurate thermometry and can make up for the above shortcomings.
In this paper, Eu³⁺ doped CaTiO₃ phosphors were prepared by KCl flux assisted solid state method. The Eu³⁺ ion concentration was optimized and the type of non-radiative energy transfer was determined. The influence of KCl flux doping amount on luminescence intensity of Eu³⁺ doped CaTiO₃ phosphors was also discussed. For Eu³⁺ doped CaTiO₃ phosphors, the energy gap of TCLs ⁵D₁ and ⁵D₀ is about 1,800 ± 22 cm^-1. The FIR value is calculated by measuring the emitting intensity of ⁵D₁ and ⁵D₀ for the temperature sensing.

Preparation of samples
CaTiO₃:xEu³⁺ phosphors (Eu³⁺/CaTiO₃ molar ratio x is 1%, 5%, 10%, 15%, 20%, 25% and 30%) and CaTiO₃:15%Eu³⁺+yKCl with different amounts of KCl (molar ratio y of KCl/CaTiO₃ is 0.5%, 1.0%, 1.5%, 2.0%, 3.0%, 4%, 5% and 6%) was prepared by traditional solid state reaction method. Reagent grade CaCO₃ and TiO₂ were used as raw materials, high purity Eu₂O₃ (99.99%) was used as activator, reagent grade KCl was used as flux and anhydrous ethanol was used as grinding agent and the mixture was ground for 45 min. It was then dried for 1 h. And then, it was transferred to the resistance furnace. It rose initially to 800 ^oC after 100 min, then rose to 1,300 ^oC in the air after 125 min and kept it for 2 h.

Characterization
PANalytical X 'pert PRO X-ray diffractometer was used for the X-ray diffraction (XRD) pattern. The surface morphologies were obtained by COXEM EM-30 PLUS scanning electron microscope (SEM), and the element distribution was analyzed by energy dispersive spectrometer (EDS, Oxford). The UV-visible diffuse reflectance (UV-vis DR) spectra were from Shimazu UV-2700 UV-vis spectrophotometer with an integral sphere. The photoluminescence (PL) and photolumine- scence excitation (PLE) spectra were from Hitachi F-7000 fluorescence spectrophotometer. Quantum efficiency was measured by a FLS1000 photoluminescence spec- trometer (Edinburgh Instruments Ltd.) with an integral sphere at room temperature. The standard TAP-02 fitting was used for controlling temperature (Orient KOJI instrument Co., Ltd) in order to obtain temperature-dependent photoluminescence spectra.

XRD analysis
Fig. 1(a) shows the XRD pattern of CaTiO₃:xEu³⁺ (x = 1%, 5%, 10%, 15%, 20%, 25% and 30%). It was observed that the typical peak of samples were basically consistent with stand card JCPDS#22-0153, attributed to orthorhombic perovskite structure and space group Pnma(62). However, when the Eu³⁺ concentration is not less than 10%, a small amount of impurity phase Eu₂Ti₂O₇ (JCPDS#23-1072) appeared in the samples. In general, to form a novel solid solution, the radius percentage difference (D_r) of the substituted ion and the doped ion is lower than 30%, which can be determined by Eq. (1): [36]



where R_S is the radius of substituted ion and R_D is the radius of doped ion, respectively. When the coordination number is 6, the radii of Ca²⁺, Ti⁴⁺ and Eu³⁺ is 1. 00, 0. 605 and 0. 947 Å, respectively [37]. Herein, D_r value of Ca²⁺/Eu³⁺ was calculated to be about 5.3%. It indicates that Eu³⁺ ions are easier to replace Ca²⁺ ions due to the similar radius of Ca²⁺ and Eu³⁺ ions, compared with Ti⁴⁺ ions.
Fig. 1(b) shows the XRD patterns of sample CaTiO₃: 15%Eu³⁺+6%KCl. It could be seen that it had a small amount of impurity phase Eu₂Ti₂O₇. In Table 1, we could see that the full width at half maximum (FWHM = 0.184^o) of CaTiO₃:15%Eu³⁺+6%KCl was less than that of CaTiO₃:15%Eu³⁺ (FWHM = 0.204^o) for the typical peak (040), indicating that the addition of KCl flux can improve their crystallinity.

EDS and SEM analysis
Fig. 2(a) and (b) show particle size distribution and SEM image of CaTiO₃:15%Eu³⁺, indicating that the sample has an irregular particle structure with mean particle size of about 1.58 μm. Fig. 2(c) and (d) show particle size distribution and SEM image of CaTiO₃: 15%Eu³⁺+6%KCl, indicating that the sample also has a similar irregular particle structure with mean particle size of about 2.25 μm. EDS energy spectra and element mappings of CaTiO₃:15%Eu³⁺ and CaTiO₃:15%Eu³⁺+ 6%KCl are also shown in Fig. 2(e, f, g) and (h). It was indicated that the samples not only contained elements of Ca, Ti, O, and Eu, but also contained Au and Mg, which were derived from the gold injection layer, which may be associated with the substrate material. Moreover, the Eu³⁺ ions were dispersed more evenly into the host material. However, for CaTiO₃:15%Eu³⁺+ 6%KCl, no K or Cl elements was found, which could be caused by the low sensitivity of the instrument and element content of KCl flux. In addition, it is found that the flux KCl can enhanced the diffusion coefficient and improve the crystallinity of host materials, beneficial to luminescence property.

UV-Vis DR spectra
Fig. 3(a) and (b-h) are UV-vis DR spectra and the plots of [F(R_¥)hn]² vs photon energy (hn) of CaTiO₃: xEu³⁺(x=1%, 5%, 10%, 15%, 20%, 25% and 30%). From Fig. 3(a), the band of 200-330nm was from the host absorption band of CaTiO₃. The absorption peak at ~396 nm (⁷F₀→⁵L₆), ~466 nm (⁷F₀→⁵D₂), ~526 nm (⁷F₀→⁵D₁), ~536 nm (⁷F₁→⁵D₁), ~592 nm (⁷F₁→⁵D₀), ~614 nm (⁷F₂→⁵D₀) and ~696 nm (⁷F₄→⁵D₀) is from in-layer 4f-4f transitions of Eu³⁺ ions. In Fig. 3(b-h), the direct optical bandgap values (E_g) are calculated according to Kubelka-Munk (K-M) and Tauc function [38]:



Where R_¥ is the reflectivity ratio of the sample to BaSO₄, F(R_¥) is the K-M function, S is the scattering coefficient, K is the absorption coefficient, hn is the photon energy and E_g is the optical bandgap value. For the samples CaTiO₃:xEu³⁺ (x = 1%, 5%, 10%, 15%, 20%, 25% and 30%), the optical bandgap values are in the range of ~3. 59-3.67 eV. Fig. 4(a) and (b) shows UV-vis DR spectra and the plots of [F(R_¥)hn]² versus hn of the sample CaTiO₃:15%Eu³⁺+6%KCl. Similarly, the band of 200-330 nm belongs to the absorption band of CaTiO₃ host material. The absorption peak at ~396 nm (⁷F₀→⁵L₆), ~466 nm (⁷F₀→⁵D₂), ~526 nm (⁷F₀→⁵D₁), ~536 nm (⁷F₁→⁵D₁), ~592 nm (⁷F₁→⁵D₀), ~614 nm (⁷F₂→⁵D₀) and ~696 nm (⁷F₄→⁵D₀) is attributed to the Eu³⁺ in-layer 4f-4f transitions. The sample CaTiO₃: 15%Eu³⁺+6%KCl has an optical bandgap value of 3. 64 eV, which is more than that of CaTiO₃:15%Eu³⁺ (3.60 eV), indicating that KCl flux can also promote to substitute Eu³⁺ ions for Ca²⁺ ions.

PLE and PL spectra
Fig. 5(a) is the PLE spectra of sample CaTiO₃:xEu³⁺ (x = 1%, 5%, 10%, 15%, 20%, 25% and 30%) at the detection wavelength of 617nm. The band of 200-350 nm is from Eu³⁺−O²⁻ charge transfer state (CTs), corresponding to 2p(O²⁻)→4f(Eu³⁺) [39]. The band of 350-560 nm are caused by the Eu³⁺ 4f-4f transitions [38]. The emitting peak at ~364 nm was from ⁷F₀→⁵D₄. In addition, we also observed that the emitting peak at ~380 nm, ~398 nm, ~418 nm, ~466 nm, ~528 nm and ~536 nm was from ⁷F₀→⁵L₇, ⁷F₀→⁵L₆, ⁷F₀→⁵D₃, ⁷F₀→⁵D₂, ⁷F₀→⁵D₁ and ⁷F₁→⁵D₁, respectively. The strength of emitting peak at ~398 nm from 4f-4f ⁷F₀→⁵L₆ transition is the strongest, which is basically consistent with the above UV-vis results, and matches with the GaN based near UV LED chip, suitable for red phosphors for WLED. Fig. 5(b) is the PL spectra of sample CaTiO₃:xEu³⁺ (x = 1%, 5%, 10%, 15%, 20%, 25% and 30%) with an excitation wavelength of 398 nm. When the samples were excited by 398 nm, the main emitting peak of Eu³⁺ ions (~539 nm, ~556 nm, ~578 nm, ~594 nm, ~617 nm, ~656 nm and ~698 nm) is from ⁵D₁→⁷F₁, ⁵D₁→⁷F₂, ⁵D₀→⁷F₀, ⁵D₀→⁷F₁, ⁵D₀→⁷F₂, ⁵D₀→⁷F₃ and ⁵D₀→⁷F₄ transition. As known, the ⁵D₀→⁷F₁ magnetic dipole transition at ~594 nm is not limited by the strength of the Eu³⁺ crystal field, but strongest ⁵D₀→⁷F₂ emitting peak at ~617 nm results from extremely sensitive electric dipole transition, which is controlled by the local electric field. So Eu³⁺ ions mainly occupy non-inversion symmetric lattice site. Therefore, it is believed that the asymmetric ratio of ⁵D₀→⁷F₂/⁵D₀→⁷F₁ represents distortion degree of inversion symmetry [40]. As can be seen from Fig. 5(b), the Eu³⁺ concentration is 1%, 5%, 10%, 15%, 20%, 25% and 30%, the ratio of ⁵D₀→⁷F₂/⁵D₀→⁷F₁ is 1.54, 1.89, 2.02, 2.09, 2.20, 2.20 and 2.24, respectively. The large value indicates the asymmetric crystal field environment around Eu³⁺ ions. From Fig. 6(a), we can also observe that with the increase of Eu³⁺ ion concentration, the peak strength first increased and then decreased. When Eu³⁺ ion concentration is 15%, the sample excited at 398 nm has the strongest PL intensity, and then the PL intensity begins to decrease, which is called concentration quenching effect, from the reduction distance of Eu³⁺-Eu³⁺ in CaTiO₃ host material. Critical distance (R_c) of Eu³⁺-Eu³⁺ is obtained from eq. (3) [41]:



Where, V is unit cell volume, Z is the number of formula units per unit cell and x_c is critical concentration. As mentioned above, V and Z of pure CaTiO₃ is 223.25 ų and 4, x_c of CaTiO₃:Eu³⁺ is 15%. Therefore, Eu³⁺-Eu³⁺ critical distance is ~8. 93 Å.
As known, non-radiative energy transfer can refer to electrical multipole interaction, exchange interaction and radiation reabsorption. It is considered as radiation reabsorption only when the excitation and emission spectra exist a lot of overlap [42] and that's unlikely to happen in our work. Moreover, for exchange interaction, critical distance is usually less than 5 Å, but the above calculated value of ~8.93 Å is greater than 5 Å. Therefore, it can also be excluded for exchange interactions, and in our work, electric multipole interaction can be used for explaining non-radiative energy transfer.
According to the relation of log(I/x) versus log(x) of CaTiO₃:xEu³⁺ (x > 15%) in Fig. 6(b). the specific type from electrical multipole interaction can be obtained by eq. (4) [43, 44]:



Where β is constant, θ is electrical multipole characteristic, I is PL intensity and x > 15%. The q value of 6 is electric dipole-dipole (d-d), 8 is from dipole-quadrupole (d-q), and 10 is from quadrupole-quadrupole (q-q) interactions [43]. For CaTiO₃:xEu³⁺, through linear fitting, its slope is −1.57386 = −θ/3, and θ is 4.72158 and approaches 6. Therefore, it is most appropriate to explain the concentration quenching by the type of electric dipole-dipole interaction.
Fig. 7(a) is the PLE spectra of the samples CaTiO₃: 15% Eu³⁺+ y KCl (y = 0, 0.5%, 1%, 1.5%, 2%, 3%, 4%, 5% and 6%) at the detection wavelength of 617 nm. As before, the band at 200 ~350 nm is Eu³⁺−O²⁻ CTs, from 2p(O²⁻)→4f(Eu³⁺) [39]. The characteristic peaks of 350-560 nm are from the Eu³⁺ 4f-4f transition [38]. We can observe that the emitting peak intensity at ~ 398 nm is the strongest, attributed to ⁷F₀→⁵L₆ 4f-4f transition. Fig. 7(b) is the PL spectrum of sample CaTiO₃:15% Eu³⁺+ y KCl (y = 0, 0.5%, 1%, 1.5%, 2%, 3%, 4%, 5% and 6% excited at 398 nm. The several major emitting peaks (~617 nm, ~594 nm and~ 578 nm) are transitions of ⁵D₀→⁷F₂, ⁵D₀→⁷F₁ and ⁵D₀→⁷F₀, respectively. The emitting peak of ~ 594nm is from magnetic dipole transition of ⁵D₀→⁷F₁ and the peak at ~617 nm from ⁵D₀→⁷F₂ electric dipole transition is more sensitive and depends on local electric field. This peak at ~617 nm is the strongest, indicating that Eu³⁺ ion is mainly from the non-inversion symmetric lattice. The intensity ratio ⁵D₀→⁷F₂/⁵D₀→⁷F₁ reflects the degree of asymmetry. From Fig. 7(b), the ratio of ⁵D₀→⁷F₂/⁵D₀→⁷F₁ is 2.17, 2.21, 2.33, 2. 37, 2.38, 2.33, 2.38 and 2.41, when the KCl concentration is 0. 5%, 1%, 1. 5%, 2%, 3%, 4%, 5% and 6%, respectively. With the increase of KCl concentration, this value has an overall trend of increasing, indicating that the asymmetry degree has increased. with increasing KCl concentration, the overall intensity trend increases. The emitting peak is strongest when the concentration of KCl is 6%.



At high temperature, Schottky defects (V_Ca'' and V_O^••) concentrations are very higher [45]. The intrinsic Schottky defects are shown as Eq. (5a). As known, Eu³⁺ ions could replace Ca²⁺ ions due to the similar ionic radius of Ca²⁺ and Eu³⁺ ions as Eq. (5b). However, some undesirable defects such as V_Ca'' and V_O^•• are close to Eu³⁺, which can contribute to the quenching of Eu³⁺ emission. In our work, KCl could play the role of flux and compensator. On the one hand, the flux KCl could enhance the diffusion coefficient. Moreover, flux could be beneficial to improve the crystallinity, giving rise to higher oscillating strengths and enhancing optical property. On the other hand, K⁺ ion is usually incorporated into host materials to replace Ca²⁺ and Cl^- ion substitutes for O^2- as Eq. (5c). PL emission is enhanced by the charge compensation of K⁺, resulting in elimination of the concentration of V_Ca'' near Eu³⁺. The substitution of O^2- by the Cl^- ion originates the lower V_O^•• near Eu³⁺. Therefore, the reduced point defects near Eu³⁺ contribute to improvement of PL intensity.

Quantum yield and CIE chromaticity diagram
Quantum yield of the samples with or without KCl flux were calculated according to the method described by the reference [46]. It allows determining the sample quantum yield (h) using the following Eq. (6):



Where L_S is the emission spectrum of sample; E_S is the spectrum of the light used for exciting the sample; E_R is the spectrum of the excitation light without the sample in the sphere; and all the spectra were collected using the sphere. Fig. 8(a) and (b) are excitation and emission spectra of samples CaTiO₃:15%Eu³⁺+6%KCl and CaTiO₃:15%Eu³⁺ collected by using an integrating sphere. The quantum yield of CaTiO₃:15%Eu³⁺ and CaTiO₃:15%Eu³⁺+6%KCl excited by 398 nm are listed in Table 2. Compared with the two samples, the sample CaTiO₃:15%Eu³⁺+6%KCl exhibits quantum yield of 66.04%, while that of the CaTiO₃:15%Eu³⁺ is 55.01% under the same excitation wavelength of 398 nm. It is indicated that the quantum yield of the sample with flux is higher than that of the sample without flux.
Fig. 8(c) shows Commission Internationale De I'eclairage (CIE) chromaticity diagram of samples CaTiO₃:15%Eu³⁺ and CaTiO₃:15%Eu³⁺+6%KCl excited at 398 nm. The color coordinate (x, y) of sample CaTiO₃:15%Eu³⁺ is (0.5911, 0.4008) and the color coordinate (x, y) of sample CaTiO₃:15%Eu³⁺+6%KCl is (0.6042, 0.3895). As far as we know, the intensity and position of the main emitting peak affect the position of the coloring coordinate. The KCl flux facilitate the improvement of crystallinity of the phosphor CaTiO₃: 15%Eu³⁺+6%KCl, which leads to the enhancement of luminous intensity. It's what causes the color coordinates to move. The color purity of the samples can be calculated by the following formula (7):



Where (x, y) is the color coordinate of the typical sample, (x_i, y_i) is the CIE coordinate of white illumination, and (x_d, y_d) is the CIE coordinate of the dominant wavelength. The color purity of sample CaTiO₃: 15%Eu³⁺ is up to 97.7%, and the color purity of sample CaTiO₃:15%Eu³⁺+6%KCl is up to 98. 4%.

Temperature sensing
Fig. 9(a) is the temperature dependent PL spectra of sample CaTiO₃:15%Eu³⁺ excited at 398 nm in the range of 298-573 K (temperature interval 25 K). The results show that the main emitting peaks at ~539 nm, ~556 nm, ~594 nm, ~617 nm, ~656 nm and ~698 nm were from 4f transitions of Eu³⁺( ⁵D₁→⁷F₁, ⁵D₁→⁷F₂, ⁵D₀→⁷F₁, ⁵D₀→⁷F₂, ⁵D₀→⁷F₃ and ⁵D₀→⁷F₄). The relative intensity I_R1 or I_R2 is the ratio of peak intensity at ~594 nm or ~617 nm at different temperatures to 298 K. In Fig. 9(b), when temperature is increasing, the relative luminescence intensity (I_R1 and I_R2) gradually decreases, resulting in the phenomenon of thermal quenching. As is known, phosphors in pc-WLEDs need to work at ~423 K for a long time, so intensity ratio (423 K/298 K) as an important parameter is used for evaluating its thermal stability. I_R1 at 423 K of sample CaTiO₃:15%Eu³⁺ is 87.29%, and I_R2 at 423 K is 83. 22%. The results showed that the thermal stability of CaTiO₃:15%Eu³⁺ for pc-WLEDs was acceptable. Moreover, the thermal quenching activation energy as another parameter is also used for evaluating thermal stability. The higher the activation energy value, the better the thermal stability of phosphors. Furthermore, in Fig. 10, the thermal quenching process can be explained by configuration coordinate diagram. When the excited state parabola of Eu³⁺ and the charge-transfer state parabola intersect at one point, because of thermal perturbation the electrons can reach the ground state via non-radiative transition. The energy gap from the bottom of the excited parabola to its intersection with the charge transfer parabola is activation energy. According to formula (8) [44], the data from ln(I₀/I-1) vs 1/kT are fitted linearly, and E_a can be obtained, as shown in Fig. 9(c) and (d).



Where, I₀ (298 K) is the initial PL intensity, and I is the intensity of different temperatures. A is a constant, the activation energy E_a is energy gap from bottom of ⁵D₀ state to its intersection with the charge transfer state, and k is boltzmann constant (8.61733×10⁻⁵ eV∙K^-1). Based on the peak intensity at ~594 nm, the slope is -0.524 = −E_a. Therefore, E_a is ~0.524 eV (~4,226 cm^-1). Based on the peak intensity at ~617 nm, the slope is -0.406 = −E_a and therefore, E_a is ~0.406 eV (~3,274 cm^-1). In addition, FIR₁ and FIR₂ values of ⁵D₁→⁷F₁/⁵D₀→⁷F₁, ⁵D₁→⁷F₂/⁵D₀→⁷F₂ have temperature-dependent characteristics. FIR values can be expressed by formula (9) as follows:



Here, A is a constant, B = ΔE/k, here ΔE is the difference between the TCLs ⁵D₁ and ⁵D₀, k is boltzmann constant of 8.61733×10⁻⁵ eV∙K^-1∙. Fig. 9(e) and (f) are nonlinear exponential function fitting plots based on the values of FIR₁ and FIR₂ and temperatures, respectively. The B values can be obtained as -1,973.64 and -2,322. 67 K, thus, DE are ~0.170 eV (~1,371 cm^-1) and 0.200 eV (~1,613 cm^-1). However, absolute sensitivity (S_a) and relative sensitivity (S_r) as important parameters are used for optical thermometry and can be expressed by equations (10a and b).



Based on the values of FIR₁ and FIR₂, the functional relationship between S_a and S_r to temperature is shown in Fig. 9(g) and (h). As shown in Fig. 9(g), based on FIR₁, the maximum value of S_a1 is ~2.4×10^-3 K^-1 at 573 K. As shown in Fig. 9(h), based on FIR₂, the maximum value of S_a2 is ~1.23×10^-3 K^-1 at 573 K. As shown in Fig. 9(g), based on FIR₁, S_r1max is ~0.983% K^-1 at 448 K (1973.64/T²). As shown in Fig. 9(h), based on FIR₂, S_r2max is ~1.157% K^-1 at 448 K (2322.67/T²).
Fig. 11(a) is the temperature dependent PL spectra of sample CaTiO₃:15%Eu³⁺+6%KCl excited at 398 nm of 298-573 K at an interval of 25 K. The results shows that the main emitting peaks at ~539 nm, ~556 nm, ~594 nm, ~617 nm, ~656 nm and ~698 nm were from Eu³⁺ 4f transitions of ⁵D₁→⁷F₁, ⁵D₁→⁷F₂, ⁵D₀→⁷F₁, ⁵D₀→⁷F₂, ⁵D₀→⁷F₃ and ⁵D₀→⁷F₄. In Fig. 11(b), as temperature is increasing, the I_R1 and I_R2 value gradually decreases, resulting in the thermal quenching phenomenon. For sample CaTiO₃:15%Eu³⁺+6%KCl, I_R1 at 423 K is 78.95%, and I_R2 at 423 K is 75.01%. Compared with the sample CaTiO₃:15%Eu³⁺, the thermal stability of CaTiO₃:15%Eu³⁺+6%KCl decreased, but for application for pc-WLEDs, its thermal stability is also acceptable. According to formula (8) [44], the data from ln(I₀/I-1) versus 1/kT are also fitted linearly, and the values E_a can be also obtained, as shown in Fig. 11(c) and (d). Based on the peak intensity at ~594 nm, the slope is -0.245 = −E_a. Therefore, E_a is ~0. 245 eV (~1,976 cm^-1). Based on the peak intensity at ~617 nm, the slope is -0.242 = −E_a and therefore, E_a is ~0.242 eV (~1,952 cm^-1). In addition, FIR₁ and FIR₂ values of ⁵D₁→⁷F₁/⁵D₀→⁷F₁, ⁵D₁→⁷F₂/⁵D₀→⁷F₂ have temperature-dependent characteristics. FIR values versus temperature can be expressed by formula (9). Fig. 11(e) and (f) are nonlinear exponential function fitting plots based on the values of FIR₁ and FIR₂ versus temperatures, respectively. The B values can be obtained as -1,913.07 and -2,016.73 K. Therefore, ΔE are ~0.165 eV (~1,331 cm^-1) and 0.174 eV (~1,403 cm^-1). Based on the values of FIR₁ and FIR₂, the functional relationship between absolute sensitivity (S_a) and relative sensitivity (S_r) to tem- perature is shown in Fig. 11(g) and (h). As shown in Fig. 11(g) and (h), based on FIR₁, S_a1max is ~2.08×10^-3 K^-1 at 573 K and based on FIR₂, S_a2max is ~0.782×10^-3 K^-1 at 573 K. As shown in Fig. 11(g) and (h), based on FIR₁, S_r1max is ~0.953%K^-1 at 448 K (1913.07/T²) and based on FIR₂, S_r2max is ~1.005%K^-1 at 448 K (2016.73/T²), which is close to that of YBO₃:Eu³⁺ phosphor reported by Zhao et al. (2017/T²) [35]. At present, the temperature sensing characteristics of Eu³⁺ doped luminescent materials are shown in Table 3. In conclusion, it can be seen that the 2322. 67 /T² of sample CaTiO₃: 15%Eu³⁺ is close to the 2350/T² of (Y_0.75Gd_0.25)₂O₃: Eu³⁺:Eu³⁺ reported by Nikoli'c et al. [47]. It can be seen that the value is not the best, but pretty prominent [48-51]. In addition, by comparing the above samples CaTiO₃:15%Eu³⁺ and CaTiO₃:15%Eu³⁺+6%KCl, it can be concluded that increasing its PL intensity does not necessarily improve its relative sensitivity. Therefore, CaTiO₃:Eu³⁺ phosphors have great potential for optical thermometry. Our samples based on FIR have the advantages of high precision, long-distance controllability, high signal resolution and in-situ temperature measurement.

KCl flux assisted CaTiO₃:Eu³⁺ red phosphors were synthesized by solid phase reaction method. The typical samples had rhombic perovskite phase. Sample CaTiO₃: 15%Eu³⁺ had the irregular particle structure, with mean particle size of about 1.58 μm, while CaTiO₃:15%Eu³⁺ +6%KCl also had the irregular particle structure, with mean particle size of about 2.25 μm. We found that KCl flux could increase the particle size and crystallinity, which was beneficial to luminescence property. When Eu³⁺ ion concentration was 15mol%, concentration quenching took place. The critical Eu³⁺-Eu³⁺ distance is ~8.93 Å, and types of non-radiative energy transfer can be dipole-dipole interaction. We found that a certain amount of KCl flux could improve its intensity of photo- luminescence. The sample CaTiO₃:15%Eu³⁺+6%KCl exhibits quantum yield of 66.04%, while that of the CaTiO₃:15%Eu³⁺ is 55.01% under the same excitation wavelength of 398 nm. It is indicated that the quantum yield of the sample with flux is higher than that of the sample without flux. Based on the PL spectra excited at 398 nm, the color coordinates of CaTiO₃:15%Eu³⁺ were (0.5911, 0.4008) and the color purity was 97.7%, while the color coordinates of sample CaTiO₃:15%Eu³⁺+ 6%KCl were (0.6042, 0.3895) and the color purity was 98.4%. Based on the thermally coupled energy level fluorescence intensity ratio FIR₂, the relative sensitivity of sample CaTiO₃:15%Eu³⁺ was 2322.67/T², which was expected to be used for optical temperature sensing.

This work was supported by Scientific Research Foundation of Hunan Provincial Education Department of China (Grant Nos. 20A274 and 18B450); Natural Science Foundation of Hunan Province of China (Grant No. 2019JJ50282); the Double First-Class Discipline Construction Program of Hunan Province of China (Grant No. Xiang Jiao Tong [2018]469); Undergraduate Innovation and Entrepreneurship Training Program of Hunan Province of China (Grant No.S202110553020) and the Planned Science and Technology Project of Hunan Province of China (Grant No.2016TP1028).