Wide-Temperature-Range Optical Thermometry Based on Yb³⁺,Er³⁺:CaYAlO₄ Phosphor

Abstract

In order to meet the demand for new optical temperature-sensing materials with high sensitivity and a wide application temperature range, Yb³⁺/Er³⁺: CaYAlO₄ phosphor with excellent physical and chemical stability and thermal conductivity was studied for the first time. Yb³⁺/Er³⁺: CaYAlO₄ phosphors have been synthesized by the high-temperature solid-state method. Under 980 nm excitation, three characteristic emission bands peaking at 528, 549 and 665 nm were observed which are attributed to the transitions ²H_11/2, ⁴S_3/2 and ⁴F_9/2 to ⁴I_15/2, respectively. The temperature-sensing behaviors of the phosphor were investigated using the luminescence intensity ratio technique based on both the TCL (²H_11/2/⁴S_3/2) and NTCL (⁴F_9/2/⁴S_3/2, ²H_11/2/⁴F_9/2) model over a wide temperature range of 163–700 K. The maximum relative sensitivities of TCLs (²H_11/2/⁴S_3/2), NTCLs (⁴F_9/2/⁴S_3/2) and NTCLs (²H_11/2/⁴F_9/2) were 3.69% K⁻¹, 0.443% K⁻¹ and 3.86% K⁻¹ at 163 K, 275 K and 163 K, while the maximum absolute sensitivities were 4.04 × 10⁻³ K⁻¹, 15.2 × 10⁻³ K⁻¹ and 7.81 × 10⁻⁴ K⁻¹ at 499 K, 499 K and 247 K, respectively. Results suggest that Yb³⁺/Er³⁺: CaYAlO₄ phosphor is a promising temperature-measuring material with advanced optical sensing capabilities over a wide temperature range.

1. Introduction

Recently, The optical thermometer has been found to be a remote alternative to traditional thermometers for its non-contact feature, fast response, high spatial resolution, long-term stability and reliable performance even in harsh surroundings [1,2,3]. Among the technologies for achieving temperature measurement, the Fluorescence Intensity Ratio (FIR) technique, characterized by the intensity ratio of two specific emission bands, has garnered significant attention. FIR can minimize the effects of spectral loss and excitation power fluctuations, thereby achieving high accuracy in temperature measurement [4,5]. In particular, many efforts are currently being made on the upconversion (UC) luminescent temperature-sensing materials based on Er³⁺/Yb³⁺, Tm³⁺/Yb³⁺, Er³⁺/Ho³⁺/Yb³⁺, etc., systems [4,6,7]. This type of upconversion photoluminescence has many practical applications. For example, the excitation wavelength 980 nm lies in the biological window [8], thus it possesses strong tissue penetration and causes no damage to the tissue, which is especially attractive in biomedical fields [9,10]. It also can be applied in infrared detectors [11], fingerprint identification [12], anti-counterfeit technology [13], etc. However, temperature sensing which can cover a wide temperature range and retain relatively high sensitivity is still an unsolved problem [6].

The matrix that provides the crystal field environment for doping ions is the main factor determining the performance of temperature-sensing materials. A good matrix which can be used in a wide temperature range should have high physical and chemical stability and good thermal conductivity. CaYAlO₄ (CYA) belongs to the tetragonal K₂NiF₄ structure. It has good mechanical strength with a density of 4.64 g/cm³ and excellent thermal conductivity (3.3 W/m/K along the c-axis and 3.7 W/m/K along the a-axis [14]). As an oxide-based aluminate, CYA has good stability, anti-moisture property and radiation resistance which can endure harsh environments like high temperature and pressure. In particular, for the UC luminescence, CYA possesses special advantages due to its relatively low phonon energy [15] and strong upconversion luminescence. Low phonon energy can help to reduce the possibility of non-radiative transitions, thus minimizing quenching phenomena and enhancing upconversion emission. The previous literature reported that the non-radiative relaxations of the optical excitations of the rare earth ions in this crystal were relatively week [16] and intense UC luminescence was observed in CYA [17,18].

Among the lanthanide ions, Er³⁺ is a typically chosen ion, for it has abundant energy levels, which can provide various wavelength emissions. In this work, the Er³⁺ ion was chosen as a luminous center and Yb³⁺ ion as a sensitizer. By using a commercial 980 nm laser diode for pumping, the Yb³⁺ ion can enhance the upconversion luminescence of the Er³⁺ ion through efficient energy transfer [11]. The FIR technique is generally implemented through two thermally coupled energy levels (TCLs), which follow the Boltzmann distribution law with an energy gap of around 200–2000 cm⁻¹ [19]. The sensitivity is constrained by the energy level gap. Compared with TCLs, non-thermally coupled energy levels (NTCLs) are not confined by $\mathrm{\Delta }E$. By flexibly adopting energy levels which could have originated from single or double emission centers, NTCLs become an effective way to enhance sensitivity. Thus, both TCLs and NTCLs were studied here to investigate the relationship between sensitivity and the coupled energy levels chosen.

The optical thermometer based on this series of host (ABAlO₄) is rare [6,20]. Yuqiang Li et al. analyzed the temperature sensing of Tm³⁺/Yb³⁺CaGdAlO₄ from 173 K to 823 K based on FIR_693/801 and FWHM₈₀₁ [6]. Ana Martinovic built a thermometric Judd–Ofelt model for Dy³⁺:CaYAlO₄ and evaluated its sensing performances for luminescence thermometry [20]. R.V. Perrella et al. reported broadened band C-telecom and the intense upconversion emission of Er³⁺/Yb³⁺ co-doped CaYAlO₄ [18]. However, to the best of our knowledge, there is no report on the temperature-sensing performance of Yb³⁺/Er³⁺:CYA phosphor. Herein, the Yb³⁺/Er³⁺:CYA phosphor was synthesized by the classical solid-state reaction. The temperature-dependent emission spectra of Yb³⁺/Er³⁺:CYA were conducted in a wide temperature range from 163 K to 700 K excited by 980 nm. The FIRs of both TCLs (²H_11/2/⁴S_3/2) and NTCLs (⁴F_9/2 /⁴S_3/2, ²H_11/2/⁴F_9/2) were studied, and the absolute and relative sensitivities as a function of temperature were calculated. Yb³⁺/Er³⁺:CYA phosphor has been shown to have good temperature-sensing performance over a wide temperature range.

2. Experimental Section

10at%Yb³⁺,4at%Er³⁺:CYA phosphor was synthesized by the high-temperature solid-state reaction. It is known that the concentrations of activators (Er³⁺ and Yb³⁺) and their relative ratio significantly influence the UPC mechanisms in Er³⁺/Yb³⁺ co-doped systems, thus changing the values of FIRs and their sensing performance. The change in Er³⁺ concentration will induce a change inof the crystal field surrounding the Er³⁺ion, thus evoking the a variation inof the optical transition rate of Er³⁺. Too high a concentration leads to a serious decrease in the FIR_529/551 value and the concentration quenching phenomenon [21]. We utilized a dopant ion concentration that was found to be effective through our previous experience. By effective energy transfer, the Yb³⁺ ion can markedly enhance the upconversion luminescence and increase the maximum sensing sensitivity [22,23]. Typically, the concentrations of sensitizer reach 10at%, and for the activator, a level about 4at% works well [24,25]. The starting chemicals were Al₂O₃ (A.R. grade), Y₂O₃ (4 N purity), Er₂O₃ (4 N purity), Yb₂O₃ (4 N purity) and CaCO₃ (A.R. grade). They were weighed according to the stoichiometric composition.

Before calcination at 1400 °C for 24 h, all chemicals were thoroughly ground and converted into pellets under a mechanical pressure of 50 MPa. And then, the procedure of grinding and sintering was repeated. The phases of the as-prepared power were examined using a power X-ray diffractometer (XRD) (SCXmini) (Rigaku Corporation, Tokyo, Japan), which was operating at 3 kv and 15 mA with CuKa1 (1.5406 Å) radiation in the continuous scanning mode. The 2θ ranged from 10 to 70° in a step of 0.02° with a count time of 1 s. The upconversion photoluminescence spectra at different temperatures were recorded by a steady-state lifetime spectrofluorometer (FLS920) (Edinburgh Instruments Ltd., Livingston, UK) excited by a 980 nm laser diode with emission slit size of 0.5 nm, power of 1000 mw, detection sensitivity (≤50 cps), integration time of 0.1 s and step of 0.5 nm. The temperature of the sample was controlled in real time by temperature control components of the spectrofluorometer FLS920 (Edinburgh Instruments Ltd., Livingston, UK) with 0.1 °C accuracy.

3. Results and Discussions

The diffraction peaks of X-rays of the synthesized Yb³⁺/Er³⁺:CYA are presented in Figure 1. Compared with theoretical JCPDS chart no. 13-0493, no other obvious diffraction peaks were detected. The result shows that the obtained phase is CYA without other impurity phases. The structural refinement of Yb³⁺/Er³⁺:CYA was performed using the Fullprof software (version: January 2006) to further investigate its phase and structural information. The obtained lattice parameter was a = b = 3.6485 Å and c = 11.8768 Å, which is close to the literature value from the standard card. The Chi² factor was 4.96. The unicellular structure is also given in Figure 1. As it is shown, CYA crystallizes in the perovskite phase with a tetragonal K₂NiF₄ structure, belonging to the space group I4/mmm. The centered Al³⁺ is surrounded by six adjacent oxygen ions to form an AlO₆ octahedron, and the Ca²⁺ and Y³⁺ ions distribute randomly in the nine coordinated sites. When Yb³⁺ and Er³⁺ ions are doped in CYA, owing to the similar radius, Yb³⁺ (ionic radius = 0.985 Å) and Er³⁺ (ionic radius = 1.030 Å) can successfully be incorporated at the Y³⁺ sites. The diffraction peaks of the doped CYA are in good agreement with the standard pattern of JCPDS chart no. 13-0493, which indicates doping of Yb³⁺and Er³⁺ without changing the intrinsic crystal structure.

As our previous study reported, the diffraction intensity is stronger and the crystallinity is better when CYA is sintered by the solid-state (SS) reaction route than by the citrate sol–gel method. The CYA particles prepared by the SS method are presented as irregular and nonuniform agglomerated blocks, with size varying from around 927 nm to 9.51 µm [26]. The agglomeration of particles is beneficial to achieve intense luminescent properties while preventing the diffusion of incident light [21].

Excited by at 980 nm, the temperature-dependent upconversion spectra based on Yb³⁺/Er³⁺:CYA phosphor in a wide range of 163–700 K were tested and are given in Figure 2a. It is found that the phosphors have two visible green emissions, which are centered at 528 nm and 549 nm, and one red emission centered at 663 nm. The green emissions correspond to Er³⁺:²H_11/2 → ⁴I_15/2 (528 nm) and ⁴S_3/2 → ⁴I_15/2 (549 nm, respectively, and the red emission is caused by the transition Er³⁺:⁴F_9/2 → ⁴I_15/2. Figure 3 shows the mechanisms for these upconversion emissions.

Pumped at 980 nm, both Yb³⁺ and Er³⁺ ions can absorb incident energy and be excited to the upper level. But due to the large absorption cross-section and much higher doping concentration, the absorption of the Yb³⁺ ion dominates. Then the process of energy transfer from the Yb³⁺ to Er³⁺ ions plays a significant role in the UC emission. There are three ET (energy transfer) processes occur, ET1: ²F_5/2 (Yb³⁺) + ⁴I_15/2 (Er³⁺) → ²F_7/2 (Yb³⁺) + ⁴I_11/2 (Er³⁺), ET2: ²F_5/2 (Yb³⁺) + ⁴I_13/2 (Er³⁺) → ²F_7/2 (Yb³⁺) + ⁴F_9/2 (Er³⁺), and ET3: ²F_5/2 (Yb³⁺) + ⁴ I_11/2 (Er³⁺) → ²F_7/2 (Yb³⁺) + ⁴F_7/2 (Er³⁺), as shown in Figure 3.

First, the population accumulates in the Er³⁺:⁴I_11/2 level by ground- state absorption (GSA) and ET1. Some ions in this level gets non-radiative relaxation and transition to the ⁴I_13/2 level; the others help in accumulating the ⁴F_7/2 level by excited -state absorption (ESA) and ET3. The population of the ⁴F_7/2 level rapidly non-radiatively decays to the ²H_11/2 and ⁴S_3/2 levels. Then the ²H_11/2 and ⁴S_3/2 levels are partially depopulated radiatively and generate green emissions corresponding to ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions. The rest relax to the lower ⁴F_9/2 level. The ions in the ⁴F_9/2 level can also be accumulated by ET2 and ESA from the ⁴I_13/2 level. Finally, the population of the ⁴F_9/2 level radiatively decay to the ⁴I_15/2 level and emit red light.

It is noticed from Figure 2a that the spectral position of the three emission bands negligibly changes with temperature in the range of 163–700 K. But temperature has a considerable influence on the intensity of these emission bands. To make it clearer, the integrated intensities of the three emissions centered at 528 nm, 549 nm and 663 nm were calculated and are presented in Figure 2b. When the temperature is increased from 163 K to 415 K, the intensity of the 549 nm emission decreases, whereas the intensity of the 528 nm emission increases. This is due to the thermalization processes occurring between the ²H_11/2 and ⁴S_3/2 levels. As the temperature rises, through phonon-assisted thermal excitation, more ions are accumulated in the ²H_11/2 level, which results in the increase of in the 528 nm emission (²H_11/2 → ⁴I_15/2) and the decrease of in 549 nm (⁴S_3/2 → ⁴I_15/2). When the temperature exceeds 415 K, the intensity of the 528 nm emission starts to decrease;, this is probably because of the increasing possibility of non-radiative relaxations. When the temperature goes up, ions in the upper level are more likely to relax to lower levels. We can find this phenomenon in Figure 2a, where the emission around 791 nm begins to appear when the temperature exceeds 400 K. This non-radiative relaxation and competition mechanism results in the change inof the intensity of the 528 nm emission trend at around 415 K. For the 663 nm emission (⁴F_9/2 → ⁴I_15/2), it generally shows a decreasing trend with increasing temperature, which is also due to non-radiative transition. The slight increase in the initial small part of the curve probably results from population accumulation from the upper ⁴S_3/2 level when the temperature is increased from 163 K to 247 K.

First, the FIRs between the emissions of ²H_11/2 and ⁴S_3/2 were studied for analyzing the temperature-sensing performance of Yb³⁺,Er³⁺: CaYAlO₄ phosphor. ²H_11/2 and ⁴S_3/2 are two typical thermally coupled levels with a small energy gap, thus the population of the two levels follow Boltzmann-type population distribution. The intensity of the ratio can be expressed by the following formula [27]:

$$FIR={\displaystyle \frac{I_H}{I_S}}={\displaystyle \frac{g_H{\sigma }_H{\omega }_H}{g_S{\sigma }_S{\omega }_S}}\exp ({\displaystyle \frac{-\mathrm{\Delta }E}{k_{B}T}})=C\exp ({\displaystyle \frac{-\mathrm{\Delta }E}{k_{B}T}})$$

(1)

where $I_H$ (higher level) and $I_S$ (lower level) represent the integrated intensities of two thermally coupled levels; $g$, $\sigma$ and $\omega$ are the radiative transition probability, the degeneracy and the angular frequency of relevant transition, respectively; $\mathrm{\Delta }E$ is the energy difference between the two TCLs; $C$ is a proportionality constant; and $k_B$ and $T$ are Boltzmann’s constant and absolute temperature, respectively. The calculated FIR values of 528 nm to 549 nm are shown in Figure 4a, and the logarithmic plot for FIR_528/549 versus inverse of temperature is also given in Figure 4b. It is observed that in the temperature range 163–700 K, the experiment values fit very well with Equation (1). The fitting value of $\mathrm{\Delta }E/k_B$ is −980.32, and the proportionality constant $C$ is 7.185. The energy gap $\mathrm{\Delta }E$ was deduced to be 681.3 cm⁻¹,which aligns with the experiment-determined value 724 cm⁻¹.

The sensor sensitivity $S$ is of vital importance to evaluate the practical application of temperature-sensing materials. It contains two sensitivities: absolute sensitivity $S_a$ and relative sensitivity $S_r$. The absolute sensitivity demonstrates the change in the FIR with temperature change, and the relative sensitivity is the normalized change in the FIR with temperature variation, as shown in the following equations [6,18].

$$S_a={\displaystyle \frac{dFIR}{dT}}$$

(2)

$$S_r={\displaystyle \frac{1}{FIR}}{\displaystyle \frac{dFIR}{dT}}\times 100\%$$

(3)

Figure 5 illustrates the $S_a$ and $S_r$ values for FIR_528/549 within the range of 163–700 K. It is observed that $S_a$ and $S_r$ exhibit contrasting trends with the increase in temperature. When the temperature is increased from 163 K to 415 K, the $S_a$ value first increases and then maintains a relatively high value in the range of 3.76 × 10⁻³ K⁻¹ to 4.04 × 10⁻³ K⁻¹. The maximal value of $S_a$ is at 499 K. The maximum $S_a$ can also be obtained by theoretical calculation. When ${\displaystyle \frac{d{S}_a}{dT}}=0$, $T={\displaystyle \frac{\mathrm{\Delta }E}{2k_B}}$, the temperature corresponding to the maximum $S_a$ can be deduced to be 490 K, which is consistent with the experiment data. The highest value of $S_r$ is 3.69%.K⁻¹ at 163 K.

The temperature- sensing performance using FIR_528/663 and FIR_663/549 have also been investigated. The 528 nm (²H_11/2→ ⁴I_15/2), 549 nm (⁴S_3/2 → ⁴I_15/2) and 663 nm (⁴F_9/2 → ⁴I_15/2) emissions correspond to the transition from the ²H_11/2, ⁴S_3/2 and ⁴F_9/2 levels. Due to the large energy difference, the ²H_11/2 and ⁴F_9/2 levels are non-thermally coupled and so are the ⁴F_9/2 and ⁴S_3/2 levels. For example, as shown in Figure 6b, the logarithm of FIR_663/549 as a function of temperature apparently no longer exhibits a linear relationship like TCLs. The FIR values of NTCLs can be fitted by the following equation [28]:

$$FIR={\displaystyle \frac{I_1}{I_2}}=A+B_{1}T+B_2{T}^2+B_3{T}^3+B_4{T}^4$$

(4)

where $A$, $B_1$, $B_2$, $B_3$ and $B_4$ are fitting parameters of the polynomial. Then the values of $S_a$ and $S_r$ can be obtained by Equations (2) and (3) as mentioned before. Figure 6a exhibits the temperature dependence of FIR_663/549 from 163 K to 700 K. The fitting parameters are also displayed in Figure 6a with a fitting degree higher than 0.999. Figure 7 presents the calculated $S_a$ and $S_r$ values as a function of temperature. As the temperature goes up, both $S_a$ and $S_r$ increase first and then decline. The maximum of $S_a$ and $S_r$ are 15.2 $\times$ 10⁻³ K⁻¹ at 499 K and 0.443% K⁻¹ at 275 K, respectively.

As for FIR_528/663, the luminescence intensity ratio values were also calculated and are shown in Figure 8a, together with the fitting parameters. The $S_a$ and $S_r$ curves of FIR_528/663 are present in Figure 8b. It is observed that the value of $S_a$ first increases and reaches a maximum value of 7.81 × 10⁻⁴ K⁻¹ at 247 K and then decreases to a minimum value of 1.44 × 10⁻⁴ K⁻¹. $S_r$ decreases with increasing temperature and its maximum value is 3.83% K⁻¹ at 163 K.

Table 1. Comparison of optical parameters based on TCLs of various UC temperature-sensing materials.

Compounds	Transitions Studied for Temperature Sensing	Temperature Range (K)	Sr-Max (% per K)	Sa-Max (% per K)	References
NaY2F7:Yb3+,Er3+	Er3+:2H11/2 → 4I15/2, 4S3/2 → 4I15/2	323–563	1.00 (323 K)	0.36 (563 K)	[29]
Al2O3:Yb3+,Er3+	Er3+:2H11/2 → 4I15/2, 4S3/2 → 4I15/2	295–973	—	0.51 (770 K)	[30]
PLZT:Yb3+,Er3+	Er3+:2H11/2 → 4I15/2, 4S3/2 → 4I15/2	140–320	—	0.059 (320 K)	[31]
YAG:Yb3+,Er3+	Er3+:2H11/2 → 4I15/2, 4S3/2 →4I15/2	298–573	—	0.17 (404 K)	[32]
NaYF4:Er3+	Er3+:2H11/2 → 4I15/2, 4S3/2 → 4I15/2	300–540	0.92 (300 K)	0.348 (420 K)	[8]
α-SiAlON ceramic:Yb3+,Er3+,Ho3+	Er3+:2H11/2 → 4I15/2, 4S3/2 → 4I15/2	298–1023	1.1 (298 K)	0.592 (298 K)	[7]
CYA:Yb3+,Er3+,Ho3+	Er3+:2H11/2 → 4I15/2, 4S3/2 → 4I15/2,Ho3+:5S2,5F4 → 5I8	163–700	4.49 (163 K)	0.24 (611 K)	[17]
CYA:Yb3+,Er3+	Er3+:2H11/2 → 4I15/2, 4S3/2 → 4I15/2	163–700	3.69 (163 K)	0.404 (415 K)	this work

and

Table 2. Comparison of optical parameters based on NTCLs of various UC temperature-sensing materials.

Compounds	Transitions Studied for Temperature Sensing	Temperature Range (K)	Sr-Max (% per K)	Sa-Max (% per K)	References
NaYF4:Er3+	Er3+:2H11/2 → 4I15/2, 4F9/2 → 4I15/2	300–540	0.584 (440 K)	0.262 (540 K)	[8]
YVO4:Yb3+,Er3+,Ho3+	Ho3+:5F5 → 5I8, Er3+: 2H11/2 → 4I15/2, 4S3/2 → 4I15/2	373–573	1.47 (373 K)	0.50 (373 K)	[13]
PLZT:Yb3+,Er3+	Er3+:2H11/2 → 4I15/2, 4F9/2 → 4I15/2	140–320	—	0.2184 (320 K)	[31]
LiYF4:Yb3+,Er3+	Er3+:2H11/2 → 4I15/2, 4F9/2 → 4I15/2	300–500	0.90	—	[32]
α-SiAlON ceramic:Yb3+,Er3+,Ho3+	Er3+:4F9/2 → 4I15/2, 4S3/2 → 4I15/2	298–873	0.345 (329 K)	1.08 (385 K)	[7]
CYA:Yb3+,Er3+,Ho3+	Er3+:4F9/2 → 4I15/2, Ho3+:5F5 → 5I8 Er3+:4S3/2 → 4I15/2	163–700	2.72 (191 K)	2.34 (583 K)	[17]
CYA:Yb3+,Er3+,Ho3+	Er3+:2H11/2 → 4I15/2, 4F9/2 → 4I15/2, Ho3+:5F5 → 5I8	163–700	1.77 (191 K)	0.031 (331 K)	[17]
CYA:Yb3+,Er3+	Er3+:4F9/2 → 4I15/2, 4S3/2 → 4I15/2	163–700	0.443 (275 K)	1.52 (499 K)	this work
CYA:Yb3+,Er3+	Er3+:2H11/2 → 4I15/2, 4F9/2 → 4I15/2	163–700	3.86 (163 K)	0.08 (247 K)	this work

provide a comparison of the thermometric performance of representative phosphors based on TCLs and NTCLs from the relevant literature, respectively. From Table 1, it is observed that the $S_a$ and $S_r$ values of Yb³⁺,Er³⁺:CYA are comparable to or even higher than many UC temperature-sensing materials based on TCLs with similar doping systems. Compared with the same series of Yb³⁺,Er³⁺,Ho³⁺:CYA in our previous work [17], Yb³⁺,Er³⁺:CYA also has higher $S_r$ and $S_a$ values. This is because when triple-doping a Ho³⁺ ion, as the Ho³⁺:⁵S₂,⁵F₄ is slightly lower than the Er³⁺:²H_11/2 level, energy transfers from the ²H_11/2 to the ⁵S₂,⁵F₄ level, which leads to a decrease of in the 528 nm emission. In addition, the 550 nm emission also comes from Ho³⁺:⁵S₂,⁵F₄ → ⁵I₈. These This finally results in the change inof sensitivity. To meet the requirements of wide-range temperature sensing, especially at high temperatures, Table 1 also lists several temperature-sensing materials working at high temperatures. As it is shown, Yb³⁺,Er³⁺:CYA has comparable $S_a$ value to that of traditional Yb³⁺,Er³⁺:Al₂O₃ and close $S_a$, higher $S_r$ values than the sensitivities of α-SiAlON ceramic.

From Table 2, it can be seen that the FIR_528/663 of Yb³⁺,Er³⁺:CYA has a high $S_r$ value but a low $S_a$ value, which probably comes from the large difference in intensity between the 528 nm and 663 nm emissions. In contrast, the FIR_663/549 of Yb³⁺,Er³⁺:CYA has a more superior $S_a$ value than FIR_528/663 and many other reported temperature-sensing materials listed in Table 2. Compared with Yb³⁺,Er³⁺,Ho³⁺:CYA, the maximum $S_r$ and $S_a$ values for FIR_663/549 are lower in Yb³⁺,Er³⁺:CYA, while the $S_a$ values for FIR_528/663 are slightly higher. This is due to the changing upconversion mechanisms by triple-doping Ho³⁺ion. The 550 nm emission is mainly contributed by Ho³⁺ in this case. Compared with the $S_a$ value of TCLs in Yb³⁺,Er³⁺:CYA, the $S_a$ value of NTCLs (⁴F_9/2 /⁴S_3/2) is higher, which is due to the increase in sensitivity resulting from a larger $\mathrm{\Delta }E$. It is worth mentioning that in the wide temperature range around 350–650 K, both the FIR_528/549 and FIR_663/549 of Yb³⁺,Er³⁺:CYA have relative high $S_a$ values. There is even a high sensitivity value platform for FIR_528/549. To summarize, FIR technology based on TCLs (²H_11/2/⁴S_3/2) and NTCLs (⁴F_9/2/⁴S_3/2) of Yb³⁺, Er³⁺: CaYAlO₄ provides a favorable possibility for temperature sensing which can work in a wide temperature range with high relative and absolute sensitivities.

4. Conclusions

The Yb³⁺,Er³⁺: CaYAlO₄ phosphor was successfully synthesized by the conventional solid-phase method. Excited by at 980 nm, the upconversion luminescence spectra over a wide temperature range of 163–700 K were investigated. Three emission bands centered at 528, 549 and 663 nm were observed, which were associated with the transitions of ²H_11/2 → ⁴I_15/2,⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2, respectively. The temperature-sensing characteristics of Yb³⁺,Er³⁺:CaYAlO₄ were explored using both TCL and NTCL methods. The maximum relative sensitivities of TCLs (²H_11/2/⁴S_3/2), NTCLs (⁴F_9/2/⁴S_3/2) and NTCLs (²H_11/2/⁴F_9/2) were 3.69% K⁻¹, 0.443% K⁻¹ and 3.86% K⁻¹ at 163 K, 275 K and 163 K, while the maximum absolute sensitivities were 4.04 × 10⁻³ K⁻¹, 15.2 × 10⁻³ K⁻¹ and 7.81 × 10⁻⁴ K⁻¹ at 499 K, 499 K and 247 K respectively. Results show that FIR technology based on TCLs (²H_11/2/⁴S_3/2) and NTCLs (⁴F_9/2/⁴S_3/2) of Yb³⁺,Er³⁺: CaYAlO₄ is promising for temperature sensing over a wide temperature range. The excellent thermal stability, high sensitivity value and especially the wide temperature resistance make Yb³⁺,Er³⁺:CYA a promising optical thermometry material for practical application.