High-precision optical thermometry using Pr³⁺-doped NaCaY(MoO₄)₃ luminophores: a multi-spectral and chromaticity-based approach to non-contact temperature sensing

Abstract

A series of NaCaY(MoO₄)₃ (NCYM) phosphors doped with Pr³⁺ ions was synthesized to develop advanced materials for optical temperature sensing. The structures, morphologies, and luminescent characteristics of these phosphors were thoroughly analyzed. X-ray diffraction (XRD) results confirm that all phosphors exhibit a tetragonal phase with a scheelite-type structure. Optical properties were characterized using UV-visible absorption and photoluminescence (PL) spectroscopy. Under 450 nm excitation, optimal luminescence intensity was achieved at a Pr³⁺ concentration of 30 mol%. Fluorescence intensity ratio (FIR) techniques, based on emissions from various excited states of Pr³⁺ (³P₁ → ³H₅ and ³P₀ → ³H₅; ³P₁ → ³H₅ and ³P₀ → ³F₂), were employed for thermometric characterization over the 298–498 K range. The results indicate excellent temperature detection performance, with maximum relative sensitivities of 0.69% per K and 1.2% per K at 298 K, respectively. Additionally, a study of temperature uncertainty (δT) demonstrated values below 0.06 K, with repeatability (R) exceeding 97%. The temperature-induced shift in chromaticity further improves the material's functionality, as the CIE coordinates change from (0.3806, 0.4278) at 298 K to (0.3772, 0.4229) at 498 K, demonstrating a stable transition towards yellow. These findings suggest that Pr³⁺-activated NCYM phosphors have significant potential for application in non-contact optical thermometry.

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1. Introduction

Temperature is a fundamental physical parameter that holds a pivotal role across diverse domains, including scientific research, engineering, and industrial applications. In recent decades, luminescent materials have emerged as an innovative solution to the inherent limitations of traditional contact thermometers.^1,2 These remote temperature measurement systems have been extensively explored and applied in numerous fields, offering distinct advantages. One key benefit of optical thermometry is its exceptional accuracy, which is crucial in precision-demanding applications such as semiconductor manufacturing, food safety monitoring, and medical diagnostics.^3,4 Moreover, optical thermometers are particularly advantageous in extreme environments where contact-based methods are impractical, such as high-temperature or corrosive conditions.⁵ Recent technological advancements have further refined the precision of optical thermometry, making it an increasingly preferred option for a wide range of applications.⁶ Among the various approaches, the fluorescence intensity ratio (FIR) technique in phosphor materials has garnered significant attention. This method is extensively studied for its advantages, including non-contact measurement, rapid response times, and high spatial and temporal resolution, enabling effective temperature detection in challenging conditions or for rapidly moving objects.^7,8 In FIR thermometry, two distinct emission peaks serve as reference signals. The efficacy of FIR-based thermometric materials is defined by three critical parameters: high absolute temperature sensitivity (S_a), high relative temperature sensitivity (S_r), and excellent signal discriminability.^9,10 Rare-earth ions are highly suitable for temperature sensing due to their unique optical properties, which include sharp emission bands, narrow linewidths, long lifetimes, and high quantum efficiencies. These features make them highly responsive to temperature variations. Most studies in this field focus on thermally coupled level pairs (TCLs) of rare-earth ions, such as the ²H_11/2 and ⁴S_3/2 levels of Er³⁺, the ³F_2,3 and ³H₄ levels of Tm³⁺, and the ⁴F_7/2 and ⁴F_3/2 levels of Nd³⁺.^11–15 In these systems, the populations of the upper and lower TCLs exhibit an inverse relationship as temperature increases, leading to variations in the fluorescence intensity ratio (FIR).

The energy gap between TCLs significantly impacts thermometric performance. A narrow gap enhances absolute temperature sensitivity (S_a) but compromises relative sensitivity (S_r) and signal discriminability due to overlapping emission peaks. Conversely, a wider energy gap improves Sr and signal discriminability but reduces thermal coupling, leading to a lower S_a.^16,17 Consequently, achieving simultaneous optimization of S_a, S_r, and signal discriminability remains a challenge in TCL-based optical thermometry. In addition, the selection of host materials is a critical factor in designing effective optical sensors, as it influences the overall performance and suitability of the thermometric system.

In this study, we propose a highly sensitive and precise method for monitoring elevated temperatures, up to approximately 498 K, utilizing a newly developed Pr³⁺-doped NCYM phosphor. Molybdates, represented by the general formula NaCaY(MoO₄)₃ (NCYM), stand out for several reasons. These materials exhibit broad, intense absorption bands in the near-UV region, mainly attributed to charge transfer (CT) transitions. Among the various molybdate compounds, NCYM was chosen for its stable physical and chemical properties, as well as its particularly low phonon energy, and can accommodate higher concentrations of rare earth ions.

This material is not only cost-effective but also straightforward to synthesize, exhibiting broad and intense absorption bands in the near-UV region, primarily attributed to charge transfer transitions (CTB). The molybdate host matrix serves as an excellent platform due to its stable chemical properties, low phonon energy, and the ability to accommodate high concentrations of rare-earth ions. These attributes, combined with the crystal structure and luminescent characteristics of Pr³⁺-activated NCYM phosphors, make them highly suitable for temperature sensing applications. To characterize its thermometric properties, we examined the thermal responses of Pr³⁺ emissions over the temperature range of 298–498 K, focusing on specific transitions: (³P₁ → ³H₅ and ³P₀ → ³H₅) and (³P₁ → ³H₄ and ³P₀ → ³F₂). The temperature sensor's performance was evaluated using three fluorescence intensity ratio (FIR)-based optical thermometry models, which demonstrated high sensitivity and consistent reliability in temperature measurement.

2. Experimental

2.1. Materials

In this study, all chemical reagents were obtained from Sigma-Aldrich and used without additional purification. NaCaY(MoO₄)₃:xPr³⁺ phosphor samples (x = 0.1, 0.2, 0.3, and 0.4) were synthesized using a citrate-based sol–gel method. The raw materials included sodium nitrate [(Na(NO₃)₃, 99.0%)], calcium nitrate tetrahydrate [(Ca(NO₃)₂·4H₂O, 99.0%)], yttrium(III) nitrate hexahydrate [(Y(NO₃)₃·6H₂O, 99.9%)], praseodymium(III) nitrate hexahydrate [(Pr(NO₃)₃·6H₂O, 99.9%)], ammonium molybdate tetrahydrate [((NH₄)₆Mo₇O₂₄·4H₂O, 99.9%)], and citric acid [C₆H₈O₇, 99.0%].

2.2. Synthesis procedure

At the outset of the synthesis process, stoichiometric amounts of sodium nitrate, calcium nitrate tetrahydrate, yttrium nitrate hexahydrate, praseodymium nitrate hexahydrate, and ammonium molybdate tetrahydrate were dissolved in 200 mL of deionized water. To this solution, 5 moles of citric acid were added as a chelating agent, ensuring a citric acid-to-metal ion ratio of 1 : 2. This process produced a total of 1.5 g of the final product. The solution was stirred at room temperature for one hour to ensure homogeneity. Upon heating, the initially transparent solution gradually turned blue, eventually forming a blue wet gel. The container was then left uncovered to facilitate liquid evaporation. The resultant blue gel was dried at 423 K for 12 hours in an oven, producing a xerogel—a stable, porous matrix. This xerogel was subsequently annealed at 673 K for 3 hours, yielding black particles. Finally, the sample underwent sintering in air at 973 K for 3 hours, resulting in pure-phase crystalline particles.

3. Characterizations techniques

The characterization of the synthesized samples involved multiple analytical techniques. X-ray powder diffraction (PXRD) patterns were recorded at room temperature using a Bruker D8-Advance diffractometer with CuKα₁ radiation (λ = 1.5406 Å) 20°–80° 2θ degrees. The morphology of the samples was examined using a Zeiss Supra55VP field-emission scanning electron microscope (FEG-SEM), equipped with a Bruker XFlash 5030 detector for enhanced imaging resolution. UV-vis-NIR absorption spectra were acquired using a PerkinElmer Lambda 365 spectrometer. For photoluminescence (PL) characterization, excitation spectra were measured with a Horiba Fluoromax 4P spectrometer, utilizing a xenon lamp as the excitation source at room temperature. The absolute quantum (QY) yield was determined by the integrating sphere method on the HORIBA JOBIN YVON Fluoromax-4 spectrofluorometer. An Easy Life-Horiba instrument was employed to measure the emission lifetime. The light source was a 450 nm diode laser. The optical thermometry study was conducted in a custom-built nanometric-heating chamber, ensuring temperature control accuracy within ±0.5 K. Emission spectra were recorded under excitation by a 450 nm diode laser operating at a constant power of 30 mW. A Horiba Jobin Yvon iHR320 monochromator spectrometer, equipped with an 1800 lines per mm grating optimized for 500 nm, was used for spectral analysis. A Hamamatsu R928 photomultiplier tube detected luminescence across the blue, green, and red emission bands. Sample temperatures were systematically varied from room temperature to 498 K in 10 K increments.

4. Results and discussion

4.1. X-ray diffraction analysis

X-ray diffraction (XRD) patterns were employed to examine the crystal structure and phase purity of the NSGM:xPr³⁺ microcrystals (where x = 0.1, 0.2, 0.3, and 0.4). Fig. 1(a) displays the XRD patterns of these samples in the 20–80° 2θ range. All observed diffraction peaks are indexed to a pure scheelite-type tetragonal structure with the space group I4₁/a, consistent with the Joint Committee on Powder Diffraction Standards (JCPDS) pattern no. 25-0828. The absence of impurity peaks confirms that the Pr³⁺ ions were successfully incorporated into the Y³⁺ sites, resulting in the intended pure molybdate structure. In order to confirm the crystal structure, the structural parameters of NCYM:xPr³⁺ were refined using the Rietveld method (Fig. S₁(a–d)†). The diffraction peaks show good agreement between the observed and calculated patterns. The refined lattice parameters, cell volume, and structural details are provided in Table S₁,† which further reinforce the structural integrity of the synthesized material. Fig. 1(b) illustrates the of NCYM doped with Pr³⁺ and the coordination of Na/Ca/Y/Pr and Mo coordination. Given the similar ionic radii of Y³⁺ (1.019 Å, C. N. = 8) and Pr³⁺ (1.126 Å, C. N. = 8),^18,19 the Pr³⁺ ions are most likely to replace Y³⁺ ions and occupy the Y positions. The radius percentage difference (D_r) between the potential substituted ions and dopants is calculated using the following equation:²⁰where R_S and R_D refer to the effective ionic radii of substituted cations and dopants, respectively. In our work, the D_r value between Y³⁺ and Pr³⁺ is about 9.5%, which is less than 30%, indicating that Pr³⁺ ions can be substituted for Y³⁺ ions.

Fig. 1 (a) XRD of the NCYM:xPr³⁺ (x = 0.1, 0.2, 0.3 and 0.4), (b) crystal structure of NCYM.

4.2. Morphology characterization

The scanning electron microscope (SEM) technique was utilized to explore the surface morphology and grain size distribution. The SEM images of NCYM:xPr³⁺ samples are shown in Fig. 2(a–d). It is evident that the irregular shapes result from the aggregation of grains and are on the micrometer scale, which is favorable for improving their photoluminescence (PL) properties.

Fig. 2 SEM microscopic morphology image of NCYM:xPr³⁺ (x = 0.1 (a), 0.2 (b), 0.3 (c) and 0.4 (d)).

4.3. Optical characterization

4.3.1. UV absorption spectroscopy. Fig. 3(a–d) presents the UV-vis DR spectra for the NCYM:xPr³⁺ samples (x = 0.1, 0.2, 0.3, and 0.4). A prominent absorption band is observed in the near-ultraviolet (NUV) region, extending from 200 to 800 nm, which is attributed to the charge transfer band (CTB) of Mo–O. Furthermore, additional absorption peaks at 452, 475, 490, and 600 nm correspond to the ³H₄ → ³P₂, ³P₁, ³P₀, and ³H₄ → ¹D₂ electronic transitions, respectively.^21,22

Fig. 3 (a–d) Diffuse reflectance spectra of the NCYM:xPr³⁺ (x = 0.1, 0.2, 0.3 and 0.4).

The plots of [F(R_∞)hν]² versus photon energy (hν) for NCYM:xPr³⁺ (x = 0.1, 0.2, 0.3 and 0.4) are shown in Fig. S₂(a–d),† the direct optical band gap values (E_g) can be obtained by using K–M function and Tauc formulae:^23,24

[F(R_∞)hν] = B(hv − E_g)ⁿ (3)

where R_∞ was the ratio of the light scattered from the sample and F(R_∞) was K–M function; S was scattering coefficient; K was absorption coefficient; hν was the photon energy, E_g was the bandgap energy and n is a parameter that defines the nature of the band transition. The value of n is 1/2 for direct allowed transitions and 2 for indirect allowed transitions. Previous studies have confirmed that NCYM is a direct bandgap material. The optical bandgap values for the NCYM:xPr³⁺ samples (x = 0.1, 0.2, 0.3, and 0.4) fall within the range of 3.36 to 3.84 eV, demonstrating that the incorporation of Pr³⁺ ions have minimal influence on the band gap values.

4.3.2. Photoluminescence properties. The room temperature PLE and PL spectra of NCYM:Pr³⁺ phosphor are shown in Fig. S₃† and 4(a), respectively. PLE spectrum, with an excitation wavelength of 650 nm exhibit a broad band in the UV range and a series of sharp lines in the visible range, which are in accordance with the results in UV-vis diffuse reflectance spectra (Fig. 3). The PL of the NCYM:0.3Pr³⁺ sample at the monitoring wavelength of 450 nm is shown in Fig. 4(a). Several prominent peaks are observed, including tow cyan emissions at 490 nm and 500 nm, two green emissions at 533 nm and 558 nm, as well as intense red emissions at 621 nm and 651 nm. Additionally, several weaker red emissions are detected at 602 nm, 628 nm, 692 nm, 712 nm, and 735 nm. The ³P₀ state is populated via optical excitation, leading to electronic transitions that are followed by a non-radiative mechanism. The radiative depopulation of the ³P₀ state gives rise to multiple emission bands in the red region of the spectrum, including transitions from ³P₀ → ³H₆ (with a maximum at 621 nm), ³P₀ → 3F₂ (at 651 nm), and ³P₀ → ³F₄ (at 736 nm). It is noteworthy that the emission characteristics of Pr³⁺ in NCYM are host-dependent; in certain hosts, Pr³⁺ favors a strong ¹D₂ → ³H₄ transition with weak or even negligible emission from the ³P₀ state.^25,26 On the other hand, when excitation by a laser diode by energy matched to the ³P₂ praseodymium level, non-radiative transition of the excitation energy to the ³P₀ and ¹D₂ emitting levels occurs. Then, the emission from these levels to the ground state ³H₄ and higher excited states take place. The observed luminescence is also quenched by a non-radiative cross-relaxation process that occurs in ion pairs when ions are close enough to each other according to the following schemes: [¹D₂, ³H₄] → [¹G₄, ³F₄]. When excited in the ³P₂ band, the energy is transferred non-radioactively to the nearby ³P₀ level, from which luminescence occurs. On the other hand, the emission from the ¹D₂ level can be populated from the ³P₀ level by multi-phonon relaxation due to the high energy of the phonons in the molybdate matrices of around 900 cm⁻¹.²⁷

Fig. 4 (a) PL spectrum of NCYM:0.3Pr³⁺ excited by 450 nm; (b) the schematic of the energy levels of Pr³⁺ ions and the possible transitions when excited by 450 nm.

The energy bandgap law states that if the difference between two energy levels is greater than five times the value of the highest energy phonon in the host medium, the probability of multiphonon relaxation will be negligible. The multiphonon non-radiative transition rates “W_NR” can be written using the formula:^28,29

W_NR = (1/t₀)exp(−ap) (4)

where p = ΔE/hv_max is the number of phonons needed to overcome the ΔE gap, h is the Planck constant, v_max is the highest phonon frequency in the host, and t₀ and a are the empirically fitted parameters. The absorption spectrum shows that the energy distance between the ³P₀ and ¹D₂ levels is less than 3000 cm⁻¹; therefore, in the case of this host as well as other molybdate, since p ≈ 3, the probability of multiphonon relaxation will be significant. The cross-relaxation process [³P₀, ³H₄] → [³H₆, ¹D₂] could also populate the ¹D₂ level but is much weaker than those from ¹D₂ because, according to the selection rule, they are spin-forbidden. The multiphonon relaxations taking place between ¹D₂ and ¹G₄ state can be ignored, as they need at least 7 to 8 phonons to bridge the large energy gap (9982 cm⁻¹). Therefore, the involvement of multiphonon relaxations cannot be completely neglected in case of emission from ³P₀. This suggests that, depopulation of ³P₀ state takes place very fast through multi phonon relaxations by feeding the ¹D₂ state, and also W_MPR has negligible influence on the depopulation of this state resulting in high emission efficiency at lower concentration (0.5 mol%). In other words non-radiative multiphonon relaxations from the ³P_0,1 states at lower concentrations (≤0.005) favours the emission from ¹D₂ state to be more dominant, while at higher concentrations (≥0.005 mol) the energy transfer process becomes efficient and fast quenching in emission intensity takes place through cross relaxations due to enhanced nonradiative coupling between ions. Based on this discussion, the multiphonon relaxation rates along with cross relaxations favours the depopulation of ³P₀ state while multiphonon relaxations can be neglected in case of ¹D₂ state. Hence, cross relaxations could play a major role in the trends observed in emission spectra emission from ¹D₂ state with respect to ion concentration.^30–33

The 602 nm emission associated with the ¹D₂ → ³H₄ transition is weak in the studied molybdate, whereas the ³P₀-related emissions at 490, 621, and 651 nm are dominant. Similar phenomena have been observed in other luminophores, such as PMN-PT:Pr³⁺ and La₂MgTiO₆:Pr³⁺.^34,35 A simplified diagram of the Pr³⁺ energy levels is shown in the Fig. 4(b).

The quantum yield provides valuable insights into the optical performance of phosphors. The quantum yield (QY) of the CDs was determined using the integrating sphere method, which allows for an absolute measurement by directly quantifying both the emitted and absorbed photons. The general equation is as follows:

QY = number of photons emitted/number of photons absorbed

Fig. S₄† presents the curve of the absolute quantum yield measured using an integrating sphere. A quantum yield of 75% was observed for the studied, demonstrating its exceptional optical efficiency. Furthermore, the quantum yield of our analyzed phosphor exceeds that of several other commercial phosphors, such as Ca₂GdSbO₆:Eu³⁺ (73%),³⁶ Pr³⁺-doped CAS glasses (37%),³⁷ GCs:Pr³⁺/Yb³⁺ (38.9%)³⁸ and Bi₄Si₃O₁₂:Eu³⁺ (14.5%).³⁹

In order to demonstrate ideal light qualities in the host under study findings, various concentrations of Pr³⁺ ions were doped into the host material, and their emission properties were examined. PL emission spectra at room temperature for different concentrations of Pr³⁺ ions doped into NCYM are shown in Fig. 5(a) NCYM:xPr³⁺ (x = 0.1, 0.2, 0.3 and 0.4), which shows only the emission bands of note for Pr³⁺ ions, and their intensities are controlled by the doping concentration at 450 nm excitation.

Fig. 5 (a) Emission spectrum of NCYM:xPr³⁺ (x = 0.1, 0.2, 0.3 and 0.4) excited at 450 nm, (b) decay curves of the 650 nm emission for the samples with different Pr³⁺ concentrations under 450 nm excitation.

The photoluminescence (PL) intensity is observed to increase to 0.3 and subsequently decrease, as depicted in Fig. 5(a), due to the process of concentration quenching. Typically, concentration quenching occurs through one of two mechanisms: exchange interaction or electric multipolar interaction. Exchange interaction occurs when the wave functions of acceptor and donor ions overlap within a defined critical distance (R_C). This overlap reduces the intensity of emission peaks and increases nonradiative energy transfer between the ions. The critical distance between the two Pr³⁺ ions present in the NCYM lattice is calculated.^40–42

The formula for the critical distance for Pr³⁺ ions in NCYM arrays is estimated as R_c = 7.86 Å, demonstrating the presence of the electric multipole interaction.

The emission decay curves of lanthanide-activated phosphors were also analyzed, and the correlation between activator concentration, luminescence characteristics, and decay duration could strongly support concentration quenching during the energy migration process. Fig. 5(b) shows the logarithmic intensity decay curves of red emissions at 650 nm in NCYM:xPr³⁺ phosphors (x = 0.1, 0.2, 0.3, and 0.4) under 450 nm excitation. The observed emission at 650 nm corresponds to the ³P₀ → ³F₂ transition of Pr³⁺, confirming that the initial excited state for this luminescence is 3P0^3P_03P0. All the decay curves can be fitted by the following:⁴³

where t is time, I(t) is afterglow emission, A₁ and A₂ is constant, τ₁ and τ₂ is the decay time of the exponential components, respectively. The average afterglow decay time (τ) of the bi-exponential function can be calculated by eqn (6). As shown in the figure, we measured the decay curves of a series of samples containing different concentrations of Pr³⁺ to illustrate the mechanism of cross-relaxation. As the Pr³⁺ doping concentration increases, the time required for the ³F₂-excited state to return to its ground state varies. This behavior is attributed to the cross-relaxation mechanism between Pr³⁺ ions. The results show that the decay time of the ³F₂ level increases with the doping concentration, rising from 0.54 μs for x = 0.1 to 0.62 μs for x = 0.3. However, at a higher concentration (x = 0.4), the decay time decreases to 0.50 μs, suggesting that excessive Pr³⁺ concentration may intensify ion–ion interactions, thereby reducing the decay time. The other NCYM:Pr³⁺ decay parameters were summarized, as shown in Table 1.

Table 1 Decay parameters of NCYM:xPr³⁺ (x = 0,1, 0.2, 0.3 and 0.4)

Sample	A1	τ1 (μs)	A2	τ2 (μs)	τ (μs)
NCYM:0.1Pr^3+	0.61	0.35	0.43	0.69	0.54
NCYM:0.2Pr^3+	0.75	0.45	0.27	0.81	0.59
NCYM:0.3Pr^3+	0.71	0.45	0.25	0.88	0.62
NCYM:0.4Pr^3+	0.63	0.28	0.41	0.64	0.50

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4.4. Temperature sensing

In order to determine the potential of the materials studied for optical temperature measurement, the high-temperature photoluminescence (PL) spectra of NCYM:0.3Pr³⁺ were recorded. Fig. 6 shows the PL spectra recorded from 298 K to 498 K under excitation at 450 nm. Thanks to the presence of different Pr³⁺ emission bands, especially those of the ³P₀ → ³H₄, ³P₁ → ³H₅, ³P₀ → ³H₅ and ³P₀ → ³F₂ transitions, the microcrystal obtained can act as optical temperature monitors, based on the light intensity ratio of these bands. The total intensity of luminescence decreases progressively with increasing temperature. Therefore, two fluorescence intensity ratio (FIR) patterns are used as temperature detection signals due to the marked differences in temperature dependence between the three emissions (³P₁ → ³H₅ and ³P₀ → ³H₅ (FIR1)), and (³P₁ → ³H₄ and ³P₀ → ³F₂ (FIR2)).

Fig. 6 Temperature dependence of the emission spectra of NCYM:0.3Pr³⁺ under 450 nm excitation.

The integrated intensities of the three emissions in Fig. 7(a and b) show that the intensities of FIR1 and FIR2 increase monotonically with increasing temperature. The FIR of the two models can be deduced and expressed as follows:^44,45

where K_B is the Boltzmann constant, T is the absolute temperature, A, and ΔE are parameters related to the curve of FIR.

Fig. 7 (a and b) Variations of FIR as a function of temperature for NCYM:0.3Pr³⁺ between (a) (³P₁–³H₅/³P₀–³H₅), and (b) (³P₁–³H₅/³P₀–³F₂).

In order to quantitatively determine the sensitivity of the two modes for optical thermometry, it is very important to analyze the relative and absolute sensitivity of the sensor, which is the ratio of the variation of the FIR with temperature and can be defined as follows:^46,47

The values of S_r and S_a, calculated by eqn (9) and (10), are shown in Fig. 8(a and b). The maximum S_r values for the models reach 0.69% per K and 1.2% per K at 298 K for FIR1 and FIR2, respectively. Similarly, the maximum S_a values reach 0.0054 per K (at 298 K) and 0.00085 per K (at 298 K) for FIR1 and FIR2, respectively. The NCYM:Pr³⁺ phosphors had three FIR models monitored. When one of the FIR patterns is in error, other patterns can be simultaneously monitored to correct the inaccuracy. In this way, the self-calibrating optical thermometer can be made by simultaneously monitoring two FIR patterns. To give the reader an idea of the current state of the art, a summary of the various luminescent thermometers based on the FIR technique is given in Table 2. It can be concluded that, compared with other works, our material offers superior performance.

Fig. 8 (a and b) S_a and S_r values corresponding to (a) FIR1 (³P₁–³H₅/³P₀–³H₅), and (b) FIR2 (³P₁–³H₅/³P₀–³F₂).

Table 2 FIR-based thermometric performances of several typical Ln³⁺ ions doped materials

Materials	Sr max (% per K)	T (K)	T-range (K)	Transitions	λ (nm)	Ref.
CaZnOS:Er^3+	0.33	603.15	303–603	^2H11/2/^4S3/2	520/539	48
PKAZLF glass:Er^3+	0.79	630	298–773	^2H11/2/^4S3/2	530/550	49
α-NaYF4:Nd^3+	0.12	273	273–423	^4F3/2–^4I9/2 (Stark)	863/870	50
LaF3:Nd^3+	0.26	296	296–345	^4F3/2–^4I9/2 (Stark)	865/885	51
β-NaYF4:Yb^3+, Ho^3+	0.9	300	300–500	^5F5–^5I8/^2S2, ^5F4–^5I8	545/650	52
SrF2:Yb^3+, Er^3+	1.20	298	298–383	^2H11/2/^4S3/2	525/545	53
β-NaLuF4:Yb^3+, Er^3+, Ho^3+	0.74	298	298–503	^2H11/2/^4S3/2	527/547	54
GdVO4:Er^3+/Yb^3+	0.7	300	300–453	^2H11/2/^4S3/2	530/550	55
LaMgTiO6:Pr^3+	1.28	350	77–500	^1D2, ^3P0–^3H4	608/499	35
PMN-PT-PYN:Er^3+	0.3	—	113–413	^2H11/2/^4S3/2	525/539	56
PMN-PT:Pr^3+	0.70	—	223–313	^3P1–^3H5/^3P0–^3H5	534/500	34
PMN-PT:Pr^3+	0.06	—	223–313	^3P1–^3H5/^3P0–^3F2	534/650	34
NCYM:Pr^3+	0.69	298	298–498	^3P1–^3H5/^3P0–^3H5	533/558	This work
NCYM:Pr^3+	1.2	298	298–498	^3P1–^3H5/^3P0–^3F2	533/651	This work

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Temperature resolution, also known as uncertainty (δT), is another key parameter that determines the smallest temperature change that can be detected by an optical thermometer and describes the possible error in temperature reading. The temperature uncertainty (δT) using luminescent thermometry can be calculated using the equation as follows:^57,58

In this study, the standard deviation of the fluorescence intensity ratio (δFIR) is defined as the variability of FIR values obtained from repeated measurements at a constant temperature. The FIR itself represents the average value derived from these measurements. To establish the detection limit, 40 independent measurements were performed at room temperature under controlled conditions. The data are summarized in the histograms displayed in Fig. S₅(a and b).† The uncertainty in FIR (δFIR) was computed based on the statistical spread of the data for the NCYM:0.3Pr³⁺ sample, yielding values of 0.016 and 0.003 for FIR1 and FIR2, respectively. Using eqn (11), the corresponding temperature uncertainties (δT) were calculated and found to be below 0.06 K. These results highlight the high precision and reliability of the FIR-based models across the investigated temperature range.

This analysis underscores the exceptional temperature resolution of the NCYM phosphor. Specifically, the NCYM:0.3Pr³⁺ sample exhibits high thermal sensitivity coupled with minimal temperature measurement errors, making it a promising candidate for optical thermometry using the FIR method. The δT values for FIR1 and FIR2 are shown in Fig. 9(a and b), further demonstrating the robustness of this material in temperature sensing applications.

Fig. 9 (a and b) Temperature resolution values δT, corresponding to (a) FIR₂ (³P₁–³H₅/³P₀–³H₅) and (b) FIR₃ (³P₁–³H₅/³P₀–³F₂) for NCYM:0.3Pr³⁺.

The repeatability (R) is another important way, to verify the accuracy of the temperature sensing techniques used. The thermometric parameters determined (FIR values) were measured repeatedly by moving the sample from a low temperature to a high temperature, as shown in Fig. 10(a and b). The calculation used to determine repeatability (R) was as follows:⁵⁹

where Mi(T)_c is the measured parameter (FIR or band centroid) in the i^th cycle and M(T)_c is the mean value of the M(T)_c over 10 cycles. The FIR values determined change reversibly with temperature, and the FIR values for all FIRs were above 97% over the temperature range measured, confirming the good repeatability and reliability of the thermometric methods applied. Based on the above results, NCYM:0.3Pr³⁺ phosphor is a promising candidate for high-performance temperature sensing materials, as all two modes exhibit high temperature sensitivity as well as good resolution and repeatability.

Fig. 10 Repeatability assessment (R) (a and b) for NCYM:0.3Pr³⁺.

4.5. Temperature-dependent color shifts and their implications for thermal sensing

The chromaticity diagram, along with the Commission Internationale de l'Éclairage (CIE) coordinates, offers a precise method for evaluating the emission color characteristics of luminescent materials. In this study, the impact of temperature on the emission color of NCYM:0.3Pr³⁺ was systematically analyzed through CIE chromaticity coordinates, measured under 450 nm excitation across a temperature range from 298 K to 498 K. As depicted in Fig. 11, the CIE coordinates of the sample shift from (x = 0.3806, y = 0.4278) at 298 K to (x = 0.3772, y = 0.4229) at 498 K, indicating a clear and stable transition in emission color toward the yellow region as the temperature increases. The change in the x and y values reflects the continuous and stable shift in emission towards yellow, which occurs as a result of the thermal activation of different electronic states in the Pr³⁺ ions.

Fig. 11 CIE (x,y) coordinate diagram of the emission color at different temperature for NCYM:0.3Pr³⁺.

This color shift provides critical insights into the temperature-dependent behavior of the material, suggesting that NCYM:0.3Pr³⁺ could be employed as a visual thermal sensor. The color change is stable and consistent over the studied temperature range, which is a crucial characteristic for practical thermal safety applications. Unlike traditional temperature sensors, which rely on electronic measurements, the color shift observed here offers an optical, non-contact alternative for temperature monitoring, making it suitable for environments where conventional sensors may be impractical.

The observed temperature-induced color shift is attributed to the thermal population of different electronic states in the Pr³⁺ ion, specifically the ³P₀ and ³P₁ states, which are responsible for the observed visible emissions. The correlation between temperature and emission color could thus be exploited for real-time monitoring of temperature fluctuations. This feature enhances the material's potential as a thermal safety indicator, where rapid visual detection of temperature variations could serve as an early warning system in sensitive applications, such as in electronics, automotive, or aerospace industries.

Additionally, the stable shift in CIE coordinates towards yellow over the entire temperature range suggests that NCYM:0.3Pr³⁺ can be a versatile material for applications requiring continuous monitoring or dynamic color changes with temperature. The gradual shift across the chromaticity diagram could be utilized in color-based thermal imaging systems, offering a straightforward method for visualizing temperature distributions. The precise CIE coordinates at room temperature (298 K) of (x = 0.3806, y = 0.4278) and at 498 K of (x = 0.3772, y = 0.4229) confirm that the material's emission color transitions in a controlled, repeatable manner, making it a reliable choice for real-time thermal imaging applications.

5. Conclusion

In summary, Pr³⁺-doped NCYM luminophores, synthesized using the citrate-based sol–gel method, demonstrate significant potential for advanced optical applications. The scheelite-type tetragonal structure with space group I4₁/a was confirmed by XRD and Rietveld refinement analysis. SEM analysis revealed an average particle size of approximately 3 μm. Absorption measurements revealed the molybdate host's characteristic absorption band in the visible region. The optical band gap (3.36–3.84 eV) showed minimal impact from Pr³⁺ doping on the host's electronic structure. Photoluminescence spectra under 450 nm excitation exhibited strong cyan, green, and red emissions, with optimal luminescence achieved at a 0.3 mol% doping concentration. These microparticles displayed excellent potential as optical, non-contact temperature sensors, with temperature-dependent emission intensity ratios yielding high sensitivity (0.69, and 1.2% per K for 533/558, and 533/651 nm ratios, respectively). Luminescence thermometry demonstrated impressive precision, with temperature uncertainty (δT) below 0.06 K and repeatability over 97%. Additionally, the temperature-induced chromaticity shift further enhances the material's functionality, as the CIE coordinates shifted from (0.3806, 0.4278) at 298 K to (0.3772, 0.4229) at 498 K, indicating a stable transition toward yellow. This highlights NCYM:0.3Pr³⁺ as a promising candidate for practical thermal indicators and sensors in high-precision, non-contact temperature measurements. Overall, the superior accuracy and reliability of Pr³⁺-doped NCYM as an optical thermometer position it for transformative applications in precision temperature monitoring, thermal management, and safety systems. This work lays the groundwork for future advancements in optical thermometry.

Data availability

All data underlying the results are available as part of the article and no additional source data are required.

Conflicts of interest

There are no conflicts to declare.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ra00093a

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