High-sensitivity optical thermometry in Tb³⁺/Eu³⁺ co-doped Li₂Y₄(MoO₄)₇ phosphors synthesized via solid-state reaction

Abstract

Developing ratiometric optical thermometers with high sensitivity and signal stability remains a critical challenge in non-contact temperature sensing. Herein, a series of 5% Eu³⁺, 40% Tb³⁺ doped, and 40% Tb³⁺/5% Eu³⁺ co-doped Li₂Y₄(MoO₄)₇ (LiYMoO:Tb/Eu) phosphors were synthesized utilizing a conventional solid-state reaction route. Structural refinement and morphological inspections confirmed the phase purity and crystallinity of the resulting molybdate host. Under 325 nm ultraviolet excitation, the photoluminescence spectra revealed a multi-center emission profile, featuring characteristic transitions of Tb³⁺ (green) and Eu³⁺ (red) host framework, indicating an efficient energy transfer network among the dopants and the host. To evaluate the material's potential for thermometry, the fluorescence intensity ratio (FIR) technique was applied using the non-thermally coupled transitions of Tb³⁺ (⁵D₄ → ⁷F₅ at 542 nm) and Eu³⁺ (⁵D₀ → ⁷F₄ at 701 nm). The thermometric performance was investigated over the physiological and industrial temperature range of 300–403 K. Notably, the phosphor exhibited a superior maximum relative sensitivity (S_r) of 6.5% K⁻¹ at 300 K, significantly outperforming many existing molybdate-based sensors. These findings suggest that the 40% Tb³⁺/5% Eu³⁺ co-doped Li₂Y₄(MoO₄)₇ system is a promising candidate for precise optical temperature sensing applications.

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1. Introduction

In recent years, lanthanide-doped luminescent materials have attracted significant scientific interest due to their multifunctional capabilities in next-generation solid-state lighting (SSL), optical display technologies, and non-contact temperature sensing.^1–6 The development of high-performance phosphors requires a careful selection of the host matrix, as its crystal field environment and phonon energy critically influence the radiative and non-radiative relaxation pathways of the dopant ions. Among the various oxide hosts, molybdate compounds have emerged as exceptional candidates owing to their high chemical stability, excellent thermal durability, and moderate phonon energy, which minimizes quenching losses.^7,8

A strategic approach to optimizing luminescent performance involves the co-doping of rare-earth (RE³⁺) ions to exploit energy transfer (ET) mechanisms. Europium (Eu³⁺) is widely recognized as a superior activator for red emission, characterized by its intense ⁵D₀ to ⁷F₂ transition, which is essential for improving the color rendering index (CRI) and warm-light components in white light-emitting diodes (W-LEDs).^9,10 Conversely, terbium (Tb³⁺) provides a distinct green emission via the ⁵D₄ to ⁷F₅ transition at approximately 542 nm. While singly doped materials are limited to monochromatic outputs, the Tb³⁺/Eu³⁺ co-doped system has garnered considerable attention because it enables color tunability through efficient Tb³⁺ to Eu³⁺ energy transfer. This mechanism allows for the precise adjustment of emission chromaticity from green to red by modulating dopant concentrations, offering a versatile platform for tunable LED components.^11,12 Beyond illumination, these RE³⁺-doped phosphors are increasingly utilized for optical thermometry, a technique that overcomes the limitations of conventional contact sensors in harsh or electromagnetic environments. The Fluorescence Intensity Ratio (FIR) method, which relies on the intensity ratio of two distinct emission bands, is particularly favored for its immunity to fluctuations in excitation power and spectrum losses.¹³ Building upon our previous investigations into rare-earth-doped phosphors, our group has extensively explored the thermometric potential of various host matrices. We have previously demonstrated high-sensitivity sensing in systems such as Er³⁺/Yb³⁺ co-doped molybdates and vanadates, where energy transfer plays a pivotal role in signal stability.^14,15 For instance, our recent work on Na₃Y(VO₄)₂:Eu³⁺ highlighted the efficacy of Eu³⁺ transitions for temperature monitoring,¹⁶ while our studies on phosphate matrices elucidated the mechanisms of color tuning via Tb³⁺/Eu³⁺ interaction.¹⁷ However, the exploration of the Li₂Y₄(MoO₄)₇ host remains limited.

The present work does not aim to provide a comprehensive study of energy transfer dynamics between Tb³⁺ and Eu³⁺ ions, but rather to exploit the thermally distinct emission responses of their characteristic bands as the basis for high-sensitivity non-contact LIR thermometry in the novel Li₂Y₄(MoO₄)₇ host.

In this study, we report the synthesis of a novel series of Tb³⁺, Eu³⁺, and Tb³⁺/Eu³⁺ co-doped Li₂Y₄(MoO₄)₇ (LiYMoO) phosphors via a conventional solid-state reaction. We present a systematic analysis of their structural phase purity and morphological characteristics. Furthermore, we investigate the photoluminescence properties and the Tb³⁺ to Eu³⁺ energy transfer mechanism in detail. A key focus of this work is to evaluate the optical thermometry performance of the co-doped system using the FIR technique under 325 nm excitation. The results are compared with our previous findings to demonstrate the superior relative sensitivity and signal reproducibility of the Li₂Y₄(MoO₄)₇ matrix, establishing its suitability for sensing applications.

2. Experimental section

2.1. Sample synthesis

A series of LiYMoO phosphors, including singly doped (Eu³⁺, Tb³⁺) and co-doped (Tb³⁺/Eu³⁺) samples, were synthesized via a conventional high-temperature solid-state reaction method. The starting precursors employed were analytical grade lithium carbonate (Li₂CO₃, 99.99%), molybdenum trioxide (MoO₃, 99.99%), yttrium oxide (Y₂O₃, 99.99%), terbium oxide (Tb₄O₇, 99.99%), and europium oxide (Eu₂O₃, 99.99%). Stoichiometric amounts of the raw materials were accurately weighed according to the nominal composition of Li₂Y_{4−x−y}Tb_xEu_y(MoO₄)₇. The powders were thoroughly ground in an agate mortar with a small quantity of ethanol to facilitate homogenization. After drying, the mixtures were transferred into alumina crucibles and sintered in an electric furnace at 1273 K for 8 h in an air atmosphere. Finally, the obtained products were naturally cooled to room temperature and ground into fine powders for further characterization.

2.2. Characterization techniques

The phase purity and crystal structure of the as-prepared phosphors were identified by X-ray diffraction (XRD) using a Bruker AXS D8 diffractometer equipped with Cu K alpha radiation (lambda = 1.5406 Å). The surface morphology and particle size distribution were examined using a field-emission scanning electron microscope (FE-SEM, Hitachi SU-4800). Photoluminescence (PL) measurements were conducted at room temperature using a high-resolution Horiba-Jobin Yvon LabRAM HR 800 UV Raman spectrometer coupled with an Olympus BX41 confocal microscope. The excitation source was the 325 nm line of a He–Cd continuous-wave laser. The emitted signal was detected by a charge-coupled device (CCD) detector. Spectral resolution was controlled using holographic gratings with groove densities of 2400 lines per mm (for high-resolution scans) and 150 lines per mm (for broad surveys) and the spectral resolution was approximately ∼1 nm. To prevent thermal heating effects during measurement, the laser power incident on the sample surface was maintained below 1 mW, corresponding to an irradiance of less than 100 kW cm⁻², for which no line shifts were observed.

3. Results and discussion

3.1. Structural analysis and phase purity

The phase composition and crystal structure of the synthesized LiYMoO phosphors singly doped with 5% Eu³⁺ and 40% Tb³⁺, as well as co-doped with ion patterns are presented in Fig. 1. The detected diffraction peaks are in accordance well with the standard Na_0.5Gd_0.5MoO₄ (JCPDS 25-0829) which is of a single phase with tetragonal scheelite structure and a space group I4₁/a. The diffraction data are consistent with the general family of defect scheelite-type double molybdates with composition A₂Ln₄(MoO₄)₇ (where A = alkali metal, Ln = rare-earth element). No secondary phases or impurity peaks related to unreacted precursors were detected, confirming that the synthesis conditions (1273 K for 8 h) were sufficient to obtain a pure, single-phase solid solution. The sharp and intense diffraction peaks attest to the high crystallinity of the samples. Furthermore, the incorporation of Eu³⁺ and Tb³⁺ activators into the host lattice did not induce any significant shift in peak positions or observable changes in the diffraction profiles. This structural stability is attributed to the successful isovalent substitution of Y³⁺ ions by Eu³⁺ and Tb³⁺ ions at the same crystallographic site, which is facilitated by the close similarity of their ionic radii. According to Shannon's crystal ionic radii tables,¹⁸ all three ions share coordination number CN = 8 with the following values: Y³⁺ (1.019 Å), Tb³⁺ (1.040 Å), and Eu³⁺ (1.066 Å), corresponding to lattice mismatch values of only +2.1% and +4.6% for Tb³⁺ and Eu³⁺ with respect to Y³⁺, respectively. These marginal size differences are well within the tolerance range for solid-solution substitution without structural distortion. Consequently, the structural integrity of the Li₂Y₄(MoO₄)₇ host matrix is fully preserved across all doping compositions, as evidenced by the absence of systematic lattice parameter shifts and the conservation of the tetragonal scheelite-type structure throughout the entire series.

Fig. 1 XRD patterns of Li₂Y₄(MoO₄)₇:Tb³⁺ (40 mol%), Li₂Y₄(MoO₄)₇:Eu³⁺ (5 mol%), and co-doped Li₂Y₄(MoO₄)₇:Tb³⁺ (40 mol%)/Eu³⁺ (5 mol%) phosphors synthesized by solid-state reaction, together with the standard JCPDS reference pattern for the Li₂Y₄(MoO₄)₇ tetragonal scheelite-type structure. The absence of secondary phase reflections confirms the phase purity of all compositions.

To further confirm the phase purity and crystal structure of the synthesized LiYMoO:Tb³⁺, LiYMoO:Eu³⁺ and LiYMoO:5% Eu³⁺/40% Tb³⁺ phosphors, Rietveld refinement of the powder XRD data were performed using the tetragonal scheelite-type structural model with space group I4₁/a. The refined patterns are presented in Fig. 2. Rietveld refinement of the XRD patterns confirmed that both Tb³⁺, Eu³⁺ and Eu³⁺/Tb³⁺ doped LiYMoO phosphors crystallize in the tetragonal scheelite-type structure (space group I4₁/a) without secondary phases. The good agreement between experimental and calculated patterns indicates high crystallinity and phase purity. Eu³⁺, Tb³⁺ and Eu³⁺/Tb³⁺ ions enter into Y³⁺ sites without inducing structural distortion, thereby preserving the structural stability of the host lattice. Table 1 lists the crystals cell and structural properties.

Fig. 2 Refined powder X-ray diffraction pattern for Li₂Y₄(MoO₄)₇ samples (a) 40% Tb³⁺, (b) 5% Eu³⁺ and (c) 40% Tb³⁺/5% Eu³⁺.

Table 1 Rietveld refinement parameters from XRD analysis

Sample	α = β = γ	a (Å)	b (Å)	c (Å)	V (Å^3)	Rwp (%)	χ^2
Li2Y4(MoO4)7:40% Tb^3+	90°	5.26361	5.26361	11.56449	320.400	6.60	2.2
Li2Y4(MoO4)7:5% Eu^3+	90°	5.26353	5.26353	11.56436	320.387	5.00	2.4
Li2Y4(MoO4)7:40% Tb^3+/5% Eu^3+	90°	5.26361	5.26361	11.56458	320.404	9.4	2.1

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3.2. Morphological characterization

The surface morphology and particle microstructure were examined by Scanning Electron Microscopy (SEM). Fig. 3 displays representative SEM micrographs of the undoped, singly doped, and co-doped LiYMoO samples.

Fig. 3 SEM images of the (a) LiYMoO:40% Tb³⁺, (b) LiYMoO:5% Eu³⁺ and (c) LiYMoO:40% Tb³⁺/5% Eu³⁺.

The micrographs reveal that the samples consist of agglomerated particles with irregular shapes and distinct boundaries, a characteristic feature of ceramic phosphors prepared via the high-temperature solid-state reaction route. The particle sizes generally range from 3 to 10 µm. Comparative analysis of the images indicates that the introduction of Eu³⁺ and Tb³⁺ dopants had no significant impact on the overall morphology or grain size distribution. The microstructure remains uniform across all compositions, suggesting that the doping concentration does not alter the grain growth kinetics during the sintering process. This micron-scale particle size is favorable for phosphor applications, as it typically enables easier coating and better light extraction efficiency in packaged devices.

4. Luminescence properties

4.1. Photoluminescence properties and spectral analysis

The room-temperature photoluminescence (PL) spectra of the LiYMoO phosphors doped with Tb³, Eu³⁺, and codoped with Tb³⁺/Eu³⁺ under near-UV excitation (325 nm) are plotted in Fig. 4(a). The excitation at 325 nm is well-suited for this system for two complementary reasons: (i) it falls within the charge-transfer absorption band of the [MoO₄]²⁻ polyanionic groups, which act as sensitizers that transfer energy to both Tb³⁺ and Eu³⁺ ions; and (ii) it overlaps with 4f–4f absorption transitions of both lanthanide ions, enabling their direct excitation.²³

Fig. 4 (a) Photoluminescence (PL) emission spectra under 325 nm excitation of singly doped Li₂Y₄(MoO₄)₇:Tb³⁺ (40 mol%), singly doped Li₂Y₄(MoO₄)₇:Eu³⁺ (5 mol%), and co-doped Li₂Y₄(MoO₄)₇:Tb³⁺ (40 mol%)/Eu³⁺ (5 mol%) phosphors, illustrating the characteristic Tb³⁺ (⁵D₄ → ⁷F_J) and Eu³⁺ (⁵D₀ → ⁷F_J) emission bands and the effect of co-doping on their relative intensities. (b) Luminescence decay curve of the Tb^{3+ 5}D₄ emission (540 nm) in the singly doped Li₂Y₄(MoO₄)₇:Tb³⁺ (40 mol%) phosphor under 325 nm excitation. (c) Luminescence decay curve of the Tb^{3+ 5}D₄ emission (540 nm) in the co-doped Li₂Y₄(MoO₄)₇:Tb³⁺ (40 mol%)/Eu³⁺ (5 mol%) phosphor, showing the reduction in donor lifetime due to Tb³⁺ → Eu³⁺ energy transfer with an efficiency of η_ET ≈ 12%. (d) Schematic energy level diagram of Tb³⁺ and Eu³⁺ ions illustrating the excitation pathways, radiative transitions, and non-radiative Tb³⁺ → Eu³⁺ energy transfer process under 325 nm excitation.

The spectrum of the Tb³⁺ doped sample exhibits a series of sharp emission bands characteristic of the ⁵D₄ to ⁷F_{J (J=6,5,4,3)} transitions. The peaks centered at 488, 542, 585, and 621 nm are ascribed to the ⁵D₄ → ⁷F₆, ⁵D₄ → ⁷F₅, ⁵D₄ → ⁷F₄, and ⁵D₄ → ⁷F₃ transitions, respectively. Among these, the green emission at 542 nm (⁵D₄ → ⁷F₅) is the most dominant. The Eu³⁺ doped sample displays a distinct spectral profile featuring multiple narrow peaks arising from the intra-configurational 4f–4f transitions of Eu³⁺. These are identified as the ⁵D₀ → ⁷F₀ (578 nm), ⁵D₀ → ⁷F₁ (592 nm), ⁵D₀ → ⁷F₂ (612 nm), ⁵D₀ → ⁷F₃ (653 nm), and ⁵D₀ → ⁷F₄ (701 nm) transitions. The hypersensitive electric dipole transition (⁵D₀ → ⁷F₂) at 612 nm exhibits the highest intensity, resulting in a strong red emission for the Tb³⁺/Eu³⁺ co-doped sample, the PL spectrum reveals a dual-emission character, containing the characteristic peaks of both Tb³⁺ (green) and Eu³⁺ (red) ions, notably, the simultaneous presence of these bands under an excitation wavelength that primarily targets the host and Tb³⁺ levels suggest the existence of an efficient energy transfer pathway within the matrix.

4.2. Energy transfer mechanism

Under typical conditions, Tb³⁺ ions can act as sensitizers for Eu³⁺ ions, enhancing the Tb³⁺ emission. In this study, the doping concentrations of Tb³⁺ and Eu³⁺ ions were fixed at 40% and 5%, respectively.

Therefore, to carefully estimate the ET efficiency (η_ET) from Tb³⁺ to Eu³⁺ ions we measured the lifetimes of Tb³⁺ ion and Tb³⁺/Eu³⁺ as illustrated in Fig. 4(b) and (c), respectively and this can be expressed by the equation of Paulose:^19,20

Here, τ and τ₀ are the lifetimes of the sensitizer Tb³⁺ in the presence and absence of the activator Eu³⁺ ions, respectively. The energy transfer efficiency value is about 12%, which suggests that the energy transfer from Tb³⁺ to Eu³⁺ is performing efficiently.

To elucidate the interaction between the dopants, the energy transfer (ET) mechanism in the Tb³⁺/Eu³⁺ co-doped system is schematically illustrated in Fig. 4(d). The mechanism relies on Tb³⁺ acting as an effective sensitizer for Eu³⁺. Upon excitation at 325 nm, Tb³⁺ ions are promoted to high-energy excited states and subsequently relax non-radiatively to the metastable ⁵D₄ level. While a portion of this energy is released radiatively (green emission), a significant fraction is transferred non-radiatively to the neighboring Eu³⁺ ions via phonon-assisted energy transfer. This process is facilitated by the spectral overlap between the emission energy of the Tb³⁺ donor (⁵D₄ → ⁷F_{6,5}) and the absorption energy levels of the Eu³⁺ acceptor. Consequently, the Eu³⁺ ions are excited to the ⁵D₀ state and decay radiatively to the ground state manifold (⁷F_J), intensifying the red emission (⁵D₀ → ⁷F₂). This synergistic interaction not only enhances the overall luminescence efficiency, but also enables the tuning of the emission color by modulating the Tb/Eu ratio.

4.3. Chromaticity coordinates (CIE)

The influence of co-doping on the perceived emission color was quantified using Commission Internationale de l'Éclairage (CIE 1931) chromaticity coordinates, as depicted in Fig. 5.

Fig. 5 Commission Internationale de l'Éclairage (CIE) 1931 chromaticity diagram showing the color coordinates of the singly doped Li₂Y₄(MoO₄)₇:Tb³⁺ (40 mol%), singly doped Li₂Y₄(MoO₄)₇:Eu³⁺ (5 mol%), and co-doped Li₂Y₄(MoO₄)₇:Tb³⁺ (40 mol%)/Eu³⁺ (5 mol%) phosphors under 325 nm excitation, demonstrating the color tunability from green (Tb³⁺-dominated) to orange-red (Eu³⁺-dominated) emission achievable through lanthanide co-doping in the Li₂Y₄(MoO₄)₇ host matrix.

The chromaticity analysis reveals a clear evolution of the emission color depending on the dopant composition. In the Tb³⁺-doped sample, the coordinates are located in the deep green region of the CIE diagram, which is consistent with the dominant ⁵D₄ → ⁷F₅ transition centered at 542 nm. In contrast, the Eu³⁺-doped sample exhibits chromaticity coordinates situated in the reddish-orange region, mainly driven by the intense ⁵D₀ → ⁷F₂ electric-dipole transition at 612 nm. For the Tb³⁺/Eu³⁺ codoped sample, the chromaticity coordinates shift toward the orange to warm-white region. This shift represents the macroscopic manifestation of the microscopic energy transfer mechanism between Tb³⁺ and Eu³⁺ ions, where the effective mixing of the green emission from Tb³⁺ and the red emission from Eu³⁺ results in a tunable orange luminescence. By adjusting the [Tb³⁺]/[Eu³⁺] ratio, the emission color can be precisely controlled.

5. Temperature-dependent luminescence and optical thermometry

To assess the potential of the synthesized material for contactless temperature sensing, the photoluminescence properties of the Tb³⁺/Eu³⁺ co-doped LiYMoO phosphor were systematically investigated under 325 nm excitation over the temperature range of 300–403 K. The temperature-dependent PL spectra are illustrated in Fig. 6.

Fig. 6 Temperature-dependent photoluminescence (PL) emission spectra of the co-doped Li₂Y₄(MoO₄)₇:Tb³⁺ (40 mol%)/Eu³⁺ (5 mol%) phosphor under 325 nm (He–Cd laser) excitation, recorded over the temperature range 300–403 K, showing the simultaneous spectral evolution of the Tb³⁺ (⁵D₄ → ⁷F_J, 480–630 nm) and Eu³⁺ (⁵D₀ → ⁷F_J, 578–720 nm) emission bands with increasing temperature.

The Eu³⁺ emission (⁵D₀ → ⁷F₂, 615 nm) undergoes progressive thermal quenching with increasing temperature, while the Tb³⁺ emission (⁵D₄ → ⁷F₅, 545 nm) exhibits a thermally activated enhancement in the range 300–403 K. This asymmetric and opposite thermal behavior of the two emission centers constitutes the physical basis enabling high-sensitivity LIR thermometry in this system, as it maximizes the rate of change of the intensity ratio with temperature.

The temperature-dependent luminescence behavior of the Tb³⁺/Eu³⁺ co-doped Li₂Y₄(MoO₄)₇ phosphor reveals an asymmetric thermal response between the two emitting centers. While the Eu³⁺ emission (⁵D₀ → ⁷F₂, 612 nm) undergoes progressive thermal quenching with increasing temperature consistent with thermally activated non-radiative relaxation via the host lattice phonons: the Tb³⁺ emission (⁵D₄ → ⁷F₅, 542 nm) exhibits a thermally activated enhancement in the range 300–403 K. This behavior arises from the interplay of three physical mechanisms: (i) thermally enhanced multi-phonon relaxation from the higher-lying ⁵D₃ level of Tb³⁺ to the emitting ⁵D₄ level, which increases ⁵D₄ population at elevated temperatures; (ii) a temperature-dependent reduction in the Tb³⁺ → Eu³⁺ non-radiative energy transfer efficiency, which decreases with rising temperature due to the increased phonon-assisted back-transfer probability, thereby retaining more excitation energy in the Tb^{3+ 5}D₄ level; and (iii) the lower activation energy for non-radiative decay of Eu³⁺ relative to Tb³⁺ in the molybdate host, causing preferential quenching of Eu³⁺ emission. Taken together, these effects produce the observed asymmetric thermal behavior, where the Eu³⁺ emission weakens while the Tb³⁺ emission is temporarily enhanced, maximizing the rate of change of the LIR with temperature and thereby providing the physical basis for the high thermometric sensitivity of this system.

The diverse thermal response of the two emission centers (Tb³⁺ and Eu³⁺) allows for the application of the Fluorescence Intensity Ratio (FIR). This method mitigates measurement errors arising from fluctuations in excitation power or sample geometry. To identify the optimal thermometric signal, four distinct FIR modes were defined based on the ratio of integrated intensities of the non-thermally coupled levels of Tb³⁺ and Eu³⁺: FIR₁ = I₄₈₅/I₆₁₂, FIR₂ = I₄₈₅/I₇₀₁, FIR₃ = I₅₄₂/I₆₁₂, FIR₄ = I₅₄₂/I₇₀₁.

As shown in Fig. 7, all calculated FIR values exhibit a monotonic and non-linear evolution with temperature from 300 to 403 K.

Fig. 7 Temperature dependence of the fluorescence intensity ratio (FIR/LIR) for the co-doped Li₂Y₄(MoO₄)₇:Tb³⁺ (40 mol%)/Eu³⁺ (5 mol%) phosphor under 325 nm excitation, constructed from four independent emission band combinations: (a) I₄₈₅/I₆₁₂, corresponding to the Tb^{3+ 5}D₄ → ⁷F₃ (485 nm) and Eu^{3+ 5}D₀ → ⁷F₂ (612 nm) transitions; (b) I₄₈₅/I₇₀₁, corresponding to the Tb^{3+ 5}D₄ → ⁷F₃ (485 nm) and Eu^{3+ 5}D₀ → ⁷F₄ (701 nm) transitions; (c) I₅₄₂/I₆₁₂, corresponding to the Tb^{3+ 5}D₄ → ⁷F₅ (542 nm) and Eu^{3+ 5}D₀ → ⁷F₂ (612 nm) transitions; and (d) I₅₄₂/I₇₀₁, corresponding to the Tb^{3+ 5}D₄ → ⁷F₅ (542 nm) and Eu^{3+ 5}D₀ → ⁷F₄ (701 nm) transitions, in the temperature range 300–403 K.

To quantify this behavior, the experimental data for all FIR modes were fitted using a cubic polynomial function, given by:^21,22

FIR = A + B × T + C × T² + D × T³ (2)

where T is the absolute temperature, and A, B, C, and D are the fitting coefficients (Table 2).

Table 2 Transitions used for the different intensity ratio techniques, wavelengths, fitted equations, correlation coefficients and maxima values for the relative sensitivity

Transitions	λ (nm)	Equation	Sr (max) (% K^−1)
Tb^3+:^5D4 → ^7F6, Eu^3+:^5D0 → ^7F2	485/612	−12.729 + 0.125T − 4.518T^2 + 4.518T^3	2.5 (400 K)
Tb^3+:^5D4 → ^7F6, Eu^3+:^5D0 → ^7F4	485/701	−242.35 + 2.243T − 0.0069T^2 + 7.12 × 10^−6T^3	2.7 (300 K), 3 (400 K)
Tb^3+:^5D4 → ^7F5, Eu^3+:^5D0 → ^7F2	542/612	−165.158 + 1.553T − 0.004T^2 + 5.091T^3	3.4 (400 K)
Tb^3+:^5D4 → ^7F5, Eu^3+:^5D0 → ^7F4	542/701	−2580.518 + 23.64T − 0.072T^2 + 7.323 × 10^−5T^3	6.62 (300 K)

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The performance of an optical thermometer is critically evaluated using two figures of merit: absolute sensitivity (S_a) and relative sensitivity (S_r). These parameters are defined as:^23–25

Fig. 8 depicts the temperature dependence of S_a and S_r for the investigated FIR modes. Among the four studied ratios, the FIR₄(I₅₄₂/I₇₀₁) mode demonstrates the superior performance. It achieves a maximum absolute sensitivity (S_a) of 1.22 K⁻¹ at 403 K and a remarkable relative sensitivity (S_r) of 6.5% K⁻¹ at 300 K. This high relative sensitivity value significantly surpasses that of many previously reported rare-earth doped molybdate phosphors,²⁶ positioning the Tb³⁺/Eu³⁺ co-doped LiYMoO system as a highly competitive candidate for high-precision optical thermometry in the physiological and industrial temperature ranges.

Fig. 8 Temperature dependence of the absolute sensitivity S_a (K⁻¹, left axis) and relative sensitivity S_r (% K⁻¹, right axis) for the co-doped Li₂Y₄(MoO₄)₇:Tb³⁺ (40 mol%)/Eu³⁺ (5 mol%) phosphor under 325 nm excitation, calculated from four independent fluorescence intensity ratio (FIR/LIR) combinations: (a) FIR₁ = I₄₈₅/I₆₁₂, based on the Tb^{3+ 5}D₄ → ⁷F₃ (485 nm) and Eu^{3+ 5}D₀ → ⁷F₂ (612 nm) transitions; (b) FIR₂ = I₄₈₅/I₇₀₁, based on the Tb^{3+ 5}D₄ → ⁷F₃ (485 nm) and Eu^{3+ 5}D₀ → ⁷F₄ (701 nm) transitions; (c) FIR₃ = I₅₄₂/I₆₁₂, based on the Tb^{3+ 5}D₄ → ⁷F₅ (542 nm) and Eu^{3+ 5}D₀ → ⁷F₂ (612 nm) transitions; and (d) FIR₄ = I₅₄₂/I₇₀₁, based on the Tb^{3+ 5}D₄ → ⁷F₅ (542 nm) and Eu^{3+ 5}D₀ → ⁷F₄ (701 nm) transitions, over the temperature range 300–403 K.

To properly evaluate the thermometric performance of the synthesized phosphors, a comparative analysis was conducted against other recently reported Tb³⁺/Eu³⁺ co-doped hosts. Table 3 summarizes the host matrix, temperature range, FIR mode, and maximum relative sensitivity (S_r) for these materials. As evidenced by the data, the LiYMoO:Tb³⁺/Eu³⁺ phosphor investigated in this work exhibits a remarkably high maximum relative sensitivity of 6.5% K⁻¹ (300 K). This value significantly exceeds that of many established oxide, fluoride, and phosphate hosts, such as CaWO₄ (4.01% K⁻¹) and LiLaP₄O₁₂ (1.00% K⁻¹). The superior sensitivity in our system can likely be attributed to the efficient phonon-assisted energy transfer between the Tb³⁺ and Eu³⁺ ions within the molybdate host. This confirms that the prepared material is a top-tier candidate for high-sensitivity optical thermometry applications. The temperature range of 300–403 K was chosen to ensure reliable and reproducible measurements while avoiding potential thermal degradation of the phosphors. Within this range, the FIR exhibits a clear and monotonic increase, allowing accurate evaluation of the temperature sensitivity. At higher temperatures, although the FIR trend suggests that sensitivity may continue to increase, thermal quenching and possible lattice distortions could reduce emission intensity and measurement stability. Therefore, the selected range provides a practical balance between sensitivity and the structural and optical stability of the phosphor material, ensuring accurate temperature sensing while maintaining reproducible luminescence performance.

Table 3 Comparison of optical thermometric performance (relative sensitivity S_r) of various Tb³⁺/Eu³⁺ co-doped phosphors reported in the literature

Host matrix	Temp. range (K)	FIR transition pair	Max Sr (% K^−1)	Ref.
Li2Y4(MoO4)7 (LiYMoO)	300–403	Tb(^5D4)/Eu(^5D0)	6.5	This work
CaWO4	303–573	Tb(^5D4)/Eu(^5D0)	4.01	26
LiLaP4O12	303–753	Tb(^5D4)/Eu(^5D0)	1.00	27
NaCaPO4 (glass ceramic)	298–573	Tb(^5D4)/Eu(^5D0)	0.66	28
Sr8MgLa(PO4)7	298–523	Tb(^5D4)/Eu(^5D0)	0.16	29
NaGd(MoO4)2	300–500	Tb(^5D4)/Eu(^5D0)	1.25	30
Gd2O3	303–873	Tb(^5D4)/Eu(^5D0)	0.82	31

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To evaluate the reliability of the temperature sensing performance, the thermometric parameters (FIR values) were systematically measured over repeated heating–cooling cycles between 300 K and 403 K, as illustrated in Fig. 9. The repeatability (R) of the sensor was calculated using the following expression:²⁷

where M(T)_c represents the average FIR value at a given temperature over 10 cycles, and M_i(T)_c corresponds to the FIR value obtained during the ith cycle. The obtained results reveal that the FIR values exhibit reversible and stable behavior throughout the investigated temperature range. The calculated repeatability values for FIR₁(I₅₈₅/I₆₁₂), FIR₂(I₄₈₅/I₇₀₁), FIR₃(I₅₄₂/I₆₁₂) and FIR₄(I₅₄₂/I₇₀₁) are 97%, 96%, 94%, and 95%, respectively. These results confirm the excellent stability, reliability, and reproducibility of the proposed optical thermometry approach.^32,33

Fig. 9 Repeatability assessment (R) (a) FIR₁(I₅₈₅/I₆₁₂) (b) FIR₂(I₄₈₅/I₇₀₁), (c) FIR₃(I₅₄₂/I₆₁₂) and (d) FIR₄(I₅₄₂/I₇₀₁) for LiYMoO phosphors.

6. Conclusions

In summary, single-phase Li₂Y₄(MoO₄)₇ (LiYMoO) phosphors doped with 40% Tb³⁺, 5% Eu³⁺, and co-doped with 40% Tb³⁺/5% Eu³⁺ were successfully synthesized via a high-temperature solid-state reaction method. Structural and morphological characterizations confirmed that the incorporation of activator ions preserved the tetragonal crystal lattice and resulted in uniform microparticles suitable for luminescent applications. Under near-UV excitation (325 nm), the co-doped samples exhibited a tunable dual-emission profile, featuring the characteristic transitions of both Tb³⁺ (green) and Eu³⁺ (red) ions. The spectral evolution provided clear evidence of an efficient Tb³⁺ to Eu³⁺ non-radiative energy transfer mechanism. Crucially, the material demonstrated exceptional performance as a radiometric optical thermometer. By employing the Fluorescence Intensity Ratio (FIR) technique based on the non-thermally coupled transitions of Tb³⁺ (⁵D₄ → ⁷F₅ at 542 nm) and Eu³⁺ (⁵D₀ → ⁷F₄ at 701 nm), a reliable thermal response was established over the working range of 300–403 K. The sensor achieved a superior maximum relative sensitivity (S_r) of 6.5% K⁻¹ at 300 K, significantly outperforming many comparable molybdate and phosphate hosts. These findings position the Tb³⁺/Eu³⁺ codoped Li₂Y₄(MoO₄)₇ system as a highly promising candidate for high-sensitivity contactless temperature sensing devices.

Conflicts of interest

There are no conflicts to declare.

Data availability

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

References

1. F. Ayachi, K. Saidi, K. Soler-Carracedo, M. Dammak and I. R. Martín, J. Alloys Compd., 2023, 961, 171146.
2. F. Ayachi, K. Saidi, W. Chaabani and M. Dammak, J. Lumin., 2021, 240, 118451.
3. B. Koudsi, H. Refai and J. J. Sluss, J. Disp. Technol., 2015, 11, 266–272.
4. X. Zhang, J. Wang, L. Huang, F. Pan, Y. Chen, B. Lei, M. Peng and M. Wu, ACS Appl. Mater. Interfaces, 2015, 7, 10044–10054.
5. H. M. Yang, J. X. Shi and M. L. Gong, J. Solid State Chem., 2005, 178, 917–920.
6. E. Castellano-Hernández, X. Han, M. Rico, L. Roso, C. Cascales and C. Zaldo, Opt. Express, 2015, 23, 11135–11140.
7. B. Kukliński, D. Wileńska, S. Mahlik, K. Szczodrowski, M. Grinberg and A. M. Kłonkowski, Opt. Mater., 2014, 36, 633–638.
8. F. Ayachi, K. Saidi, M. Dammak, I. Mediavilla and J. Jiménez, RSC Adv., 2025, 15, 655–664.
9. Z. E. A. A. Taleb, I. Kachou, K. Saidi, M. Dammak, I. Mediavilla and J. Jiménez, Mater. Adv., 2025, 6, 2385–2396.
10. X. Zhang, L. Zhou, J. Shi and M. Gong, Mater. Lett., 2014, 137, 32–35.
11. Z. Yahiaoui, M. A. Hassairi, M. Dammak, E. Cavalli and F. Mezzadri, J. Lumin., 2018, 194, 96–101.
12. F. Ayachi, K. Saidi, M. Dammak, J. J. Carvajal and M. C. Pujol, RSC Adv., 2024, 14, 13494–13504.
13. K. Saidi, M. Dammak, K. Soler-Carracedo and I. R. Martín, Dalton Trans., 2022, 51, 5108–5117.
14. K. Saidi and M. Dammak, J. Solid State Chem., 2021, 300, 122214.
15. N. Ben Amar, K. Saidi, C. Hernández-Álvarez, M. Dammak and I. R. Martín, ACS Appl. Nano Mater., 2025, 8, 17133–17143.
16. I. Kachou, K. Saidi, R. Salhi and M. Dammak, RSC Adv., 2022, 12, 7529–7539.
17. K. Saidi and M. Dammak, RSC Adv., 2020, 10, 21867–21875.
18. C. Harriswangler, J. C. Frías, M. T. Albelda, L. Valencia, E. García-España, D. Esteban-Gómez and C. Platas-Iglesias, Inorg. Chem., 2023, 62, 17030–17040.
19. M. Fan, L. Zhao, X. Jin, W. Sun, W. Qi and Y. Li, Anal. Chim. Acta, 2022, 1221, 340026.
20. B. Wang, Q. Ren, O. Hai and X. Wu, RSC Adv., 2017, 7, 15222–15227.
21. Q. Han, H. Hao, J. Yang, Z. Sun, J. Sun, Y. Song, Y. Wang and X. Zhang, J. Alloys Compd., 2019, 786, 770–778.
22. S. Bhushan and M. V. Chukichev, J. Mater. Sci. Lett., 1988, 7, 319–321.
23. Y. Bahrouni, I. Kachou, K. Saidi, C. Hernández-Álvarez, M. Dammak and I. R. Martín, RSC Adv., 2026, 16, 4452–4460.
24. M. Fhoula, M. Khitouni and M. Dammak, RSC Adv., 2024, 14, 39373–39380.
25. Y. Jiang, Y. Tong, S. Chen, W. Zhang, F. Hu, R. Wei and H. Guo, Chem. Eng. J., 2021, 413, 127470.
26. M. D. Dramićanin, J. Appl. Phys., 2020, 128, 040902.
27. Y. Bahrouni, I. Kachou, K. Saidi, C. Hernández-Álvarez, M. Dammak and I. R. Martín, J. Alloys Compd., 2026, 1056, 186657.
28. Y. Zhang, W. Gong, J. Yu, Z. Cheng and G. Ning, RSC Adv., 2016, 6, 30886–30894.
29. L. Marciniak and A. Bednarkiewicz, Phys. Chem. Chem. Phys., 2016, 18, 15584–15592.
30. L.-Q. Yao, G.-H. Chen, T. Yang, S.-C. Cui, Z.-C. Li and Y. Yang, Ceram. Int., 2016, 42, 13086–13090.
31. W. Xia, L. Li, P. Yang, F. Ling, Y. Wang, Z. Cao, S. Jiang, G. Xiang, X. Zhou and Y. Hua, J. Lumin., 2021, 239, 118383.
32. L. Peng, Q. Meng and W. Sun, Ceram. Int., 2019, 45, 20656–20663.
33. I. E. Kolesnikov, D. V. Mamonova, M. A. Kurochkin, V. A. Medvedev, E. V. Borisov and E. Y. Kolesnikov, Ceram. Int., 2023, 49, 6899–6905.

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