Preparation and Photoluminescent Properties of Tb³⁺-Doped Lu₂W₃O₁₂ and Lu₂Mo₃O₁₂ Green Phosphors

Abstract

Tungstate and molybdate phosphors have received great attention for their excellent photoluminescent properties and thermal stabilities. In the article, Tb³⁺-activated tungstate and molybdate green phosphors were prepared by a solid-state reaction method at different caline temperatures and were compared and studied. The crystal structures and the morphologies of samples were characterized by X-ray diffraction (XRD) patterns and field emission scanning electron microscopy (FE-SEM) images. The energy-dispersive spectra (EDS) proved the compositions of the prepared samples. The photoluminescence (PL) spectra showed that the PL excitation spectra of Tb³⁺-doped Lu₂W₃O₁₂ and Lu₂Mo₃O₁₂ green phosphors consisted of a broad and strong charge transfer band (CTB) and 4f–5d transitions of Tb³⁺ in the ultraviolet (UV) wavelength range and some narrowed excitation peaks from the 4f–4f transition of Tb³⁺ in the near ultraviolet (NUV) wavelength region. The PL emission spectra of the phosphors exhibited the characteristic green emissions owing to the ⁵D₄→⁷F₅ transition of Tb³⁺ located at about 547 nm. The values of energy gap E_g were calculated based on the diffuse reflection spectra (DRS). The measuring temperature-dependent PL spectra illustrated the thermal stabilities of phosphors. The Tb³⁺-doped Lu₂Mo₃O₁₂ phosphor presented normal thermal quenching phenomena and the values of the thermal activation energy E_a were calculated based on the measuring temperature dependent PL emission spectra. The Tb³⁺-doped Lu₂W₃O₁₂ phosphor exhibited abnormal thermal enhancing CTB excitation intensity at about 170 °C. Furthermore, the PL decay curves suggested that the lifetime corresponding to the ⁵D₄ level of Tb³⁺ in the Lu₂W₃O₁₂ host lattice was longer than that in the Lu₂Mo₃O₁₂ host lattice. Compared the Tb³⁺-doped Lu₂Mo₃O₁₂ phosphor, the Tb³⁺-doped Lu₂W₃O₁₂ phosphor has shown potential as an application in temperature sensors.

1. Introduction

In recent years, “double carbon” has once again become a hot topic, and many have made contributions to energy conservation and carbon reduction, saving electric energy more effectively and helping to achieve the goal of “double carbon”. As far as illumination is concerned, the key lies in the correct selection of high-efficiency phosphors that can emit good visible light. This requires the luminous efficiency and working stability of fluorescent phosphors. Phosphors have strong practical and scientific interest for scientists. Much of the literature concerning rare earth phosphors has reported on its wide applications [1,2,3,4]. Phosphors are classified by matrixes such as oxides, nitrides, phosphates, fluorides, borates, sulfates, aluminates, silicates, sulfides, tungstates, vanadates, molybdates, etc. Host materials with the values of energy band gaps E_g between 3 eV and 5 eV are considered to be the best host matrix for phosphors [5]. Among these phosphors, tungstate phosphors and the molybdate phosphors are considered to be promising phosphors, owing to their high physical and chemical stability as well as thermal stabilities [6,7]. In addition, the WO₄²⁻, WO₆⁶⁻, MoO₄²⁻, and MoO₆⁶⁻ groups can efficiently absorb ultraviolet (UV) light through the excitation of the O²⁻−W⁶⁺ and O²⁻−Mo⁶⁺ charge transfer absorption [8,9,10,11]. The excitation energy can be transferred to luminescent ions, such as Eu³⁺ and Sm³⁺, and then emit red light. There are some reports on rare earth molybdate phosphors and tungstate phosphors [12,13,14].

Few reports on Tb³⁺-doped molybdate phosphors or tungstate phosphors were reported [15,16]. The Tb³⁺-doped materials are usually used as green phosphors because of their bright ⁵D₄→⁷F₅ emissions, which peak at about 544 nm in the green spectral region. To the best of our knowledge, the investigations on the Tb³⁺-doped Lu₂W₃O₁₂ and Lu₂Mo₃O₁₂ phosphors have been rarely reported. For phosphors, thermal quenching is a normal phenomenon for nonradiative transitions at higher temperatures, which is also a drawback for practical applications such as display, lighting, and photovoltaics [17]. Recently, thermally stable phosphors that conquered or were weak were reported as drawbacks [18,19]. Further, thermal enhancement up-conversion emissions in Yb₂W₃O₁₂ and Sc₂Mo₃O₁₂ materials were reported and the enhancement was attributed to the negative thermal expansion (NTE) [20,21]. When the up-conversion materials are integrated into biological sensors or optoelectronic devices, the thermally enhanced up-conversion luminescence can provide enhanced detection sensitivities and improved energy conversion efficiencies. Enhanced Eu³⁺ emissions were also reported in Lu₂(MoO₄)₃ phosphors [22,23]. Above 450 K, the Lu₂(MoO₄)₃ shows a very intense NTE phenomenon [24]. What about the Tb³⁺ emission in Lu₂W₃O₁₂ and Lu₂Mo₃O₁₂ host matrices?

In this work, the Tb³⁺-doped and undoped Lu₂W₃O₁₂ and Lu₂Mo₃O₁₂ green phosphors were prepared through a solid-state reaction method. The crystalline structures, the morphologies, the compositions, the diffuse reflectance spectra, the room temperature and high temperature photoluminescence spectra, the thermal stabilities, and the luminescent decay curves of Tb³⁺-doped Lu₂W₃O₁₂ and Lu₂Mo₃O₁₂ green phosphors were compared and studied.

2. Experimental Section

2.1. Materials

Lu₂O₃ (99.99%, Aladdin), Tb₄O₇ (99.99%, Aladdin), WO₃ (99.99%, Aladdin), and MoO₃ (99.9%, Aladdin) were used as raw materials without further purification to prepare the Tb³⁺-doped and undoped Lu₂W₃O₁₂ and Lu₂Mo₃O₁₂ materials.

2.2. Preparation and Characterization

The Tb³⁺-doped and undoped Lu₂W₃O₁₂ and Lu₂Mo₃O₁₂ materials were prepared through a solid-state reaction method as reported [22,23]. In a typical procedure, for the preparation of 10 mmol of 5 mol% Tb³⁺-doped Lu₂W₃O₁₂, 9.5 mmol of Lu₂O₃, 0.25 mmol of Tb₄O₇, and 30 mmol of WO₃ were weighed and ground thoroughly, placed into a corundum crucible, and calcined at 1000 °C for 4 h in a muffle furnace with some carbon powder in the muffle furnace. When the muffle furnace cooled down to room temperature, the obtained white powder was collected and the phosphor was obtained. For comparison, the 5 mol% Tb³⁺-doped Lu₂Mo₃O₁₂ and undoped Lu₂W₃O₁₂ and Lu₂Mo₃O₁₂ samples were prepared with the similar procedure except for the raw materials. Calcining temperatures of 800, 900, 1000, 1100, 1200, and 1300 °C were applied to prepare the materials. Under 254 nm or 365 nm UV light, the Tb³⁺-doped materials emit a bright green light seen by the naked eye, suggesting green phosphors were prepared.

To determine the crystal structure, the X-ray diffraction (XRD) patterns of the prepared phosphors were characterized with CuKα (λ = 1.5406 Å) radiation at 40 kV and 40 mA using a Rigaku Smart lab X-ray diffractometer. The 2θ ranges of the data were all set from 10° to 70° with a step size of 0.02°. A Zeiss field emission scanning electron microscope (FE-SEM) was used to study the morphologies and the particle size. The compositions of the materials were characterized by energy-dispersive spectra (EDS) with the attachment EDS equipment in the FE-SEM. The diffuse reflection spectra (DRS) were obtained on a visible-near infrared (Vis-NIR) spectrophotometer (UV-2700) with BaSO₄ as the standard reference. A Scientific Lumina fluorescence spectrophotometer with a power of 150 W Xe lamp as an excitation source was used to record the photoluminescence (PL) excitation and emission spectra at room temperature. The high temperature dependent PL spectra of obtained phosphors were recorded by the same fluorescence spectrophotometer with a heating equipment TAP-02 (Tian Jin Oriente KOJI Instrument Co., Ltd., Tianjin, China) high-temperature fluorescence attachment. The luminescent decay curves were recorded by a Horiba Scientific FluroMax-4 spectrophotometer with a pulsed 150 W xenon lamp as an excitation source.

3. Results

3.1. Characterization of XRD and Crystal Structure

Figure 1 shows XRD patterns of Tb³⁺-doped Lu₂W₃O₁₂ phosphors prepared at 800, 900, 1000, 1100, 1200, 1300 °C, and reference data. The XRD patterns can be indexed to the Yb₂Mo₃O₁₂ with JCPDS card No. 33-1451. Because Lu³⁺ is isostructural to the Yb³⁺ and the W⁶⁺ has similar electronic structure to that of the Mo⁶⁺, it can be inferred that the Lu₂W₃O₁₂ phosphor was prepared. When the preparation temperature was 800 °C, the XRD pattern of the obtained material could be indexed to the reference data of Yb₂Mo₃O₁₂ well. When the preparation temperature increased with 100 °C intervals up to 1300 °C, all the XRD patterns could be indexed to the reference data. The full width at half maximum (FWHM) of the diffraction peak became narrower and the diffraction intensity became stronger with the increase of preparation temperature. The XRD measurement results suggested that the Tb³⁺-doped Lu₂W₃O₁₂ phosphors can be prepared from 800 °C to 1300 °C.

The preparation of temperature-dependent XRD patterns of Tb³⁺-doped Lu₂Mo₃O₁₂ phosphors and the reference data are shown in Figure 2. Based on the analysis of [23] and the measurement results, the XRD patterns of Tb³⁺-doped Lu₂Mo₃O₁₂ phosphors prepared at different preparation temperatures can all be well indexed to the reference data of Yb₂Mo₃O₁₂ with JCPDS card No. 33-1451. Similar to those of the Tb³⁺-doped Lu₂W₃O₁₂ phosphors, with the increase of preparation temperature, the full width at half maximum of the XRD diffraction peak became narrower and the diffraction intensity gradually increased. The results show that the Tb³⁺-doped Lu₂Mo₃O₁₂ phosphors were obtained when the preparation temperature varied from 800 °C to 1300 °C.

Figure 3 shows the XRD patterns of 5 mol% Tb³⁺-doped and undoped Lu₂W₃O₁₂ and Lu₂Mo₃O₁₂ powders prepared at 1000 °C and the reference data. Referring to the preparation of the temperature-dependent XRD patterns of Tb³⁺-doped Lu₂W₃O₁₂ and Lu₂Mo₃O₁₂ phosphors, the XRD patterns of the 5 mol% Tb³⁺-doped and undoped Lu₂W₃O₁₂ and Lu₂Mo₃O₁₂ samples can be well indexed to Yb₂Mo₃O₁₂ with JCPDS card No. 33-1451. Comparing the Tb³⁺-doped and undoped XRD patterns of the Lu₂W₃O₁₂ and Lu₂Mo₃O₁₂ powders, the Tb³⁺ doping did not cause apparent variations on the patterns, with the exception of the diffraction intensities.

Figure 4a,b display schematic crystal structure diagrams of Lu₂W₃O₁₂ and Lu₂Mo₃O₁₂, respectively. As is illustrated, Lu³⁺ is bonded to six O²⁻ to form LuO₆ octahedra and W⁶⁺ or Mo⁶⁺ is bonded to four O²⁻ to form WO₄²⁻ or MoO₄²⁻ tetrahedra. The Lu₂W₃O₁₂ or Lu₂Mo₃O₁₂ crystal is composed of corner-sharing LuO₆⁶⁻ octahedra and WO₄²⁻ or MoO₄²⁻ tetrahedra. One LuO₆ octahedra shares corners with six WO₄ tetrahedra. With the temperature increasing, the Lu-O-W and Lu-O-Mo will rotate, resulting in the shrinkage of the Lu₂W₃O₁₂ and Lu₂Mo₃O₁₂ crystal lattice, which present the NTE phenomena. After doping 5 mol% Tb³⁺, the Tb³⁺ ions replace the Lu³⁺ ions in the Lu₂W₃O₁₂ and Lu₂Mo₃O₁₂ crystals for the similarity of Tb³⁺ to Lu³⁺. The contraction of the crystal lattice will affect the emissions of Tb³⁺ at high temperatures.

3.2. Composition and Morphology Characterization

Figure 5 presents the FE-SEM images and EDS spectra of an undoped Lu₂Mo₃O₁₂ sample (a,b), a 5 mol% Tb³⁺-doped Lu₂Mo₃O₁₂ phosphor (c,d), undoped Lu₂W₃O₁₂ material (e,f), and a 5 mol% Tb³⁺-doped Lu₂W₃O₁₂ phosphor (g,h) prepared at 1000 °C, respectively. For convenient comparison, the scales bars were all 2 μm in the FE-SEM images, and all the FE-SEM images were captured under the same magnifications. The undoped Lu₂Mo₃O₁₂ micron particles in Figure 5a were well dispersed with good homogeneity, non-agglomeration, uniformity in size distribution, and with a sphere-like morphology. Through a Nano Measurer software dealing, the average size of the particles was about 2.14 μm. The corresponding EDS spectrum in Figure 5b indicates that the material contains Lu, Mo, and O. After some Tb³⁺ was introduced into Lu₂Mo₃O₁₂ lattice, the morphology of the micro-particles (Figure 5c) did not change much and the average size of the particles was about 2.17 μm. Tb³⁺ was detected in the corresponding EDS pattern (Figure 5d). The Lu₂W₃O₁₂ was composed of some undefined and unsmoothed particles and some irregular agglomeration nanoparticles appeared on the surface of the particles (Figure 5e). Compared with Lu₂Mo₃O₁₂ material, the Lu₂W₃O₁₂ sample possessed smaller particles and the average size of the particles was about 1.42 μm. The corresponding EDS pattern in Figure 5f indicates that the material contains Lu, W, and O. After 5 mol%, Tb³⁺ was introduced in Lu₂W₃O₁₂ (Figure 5g), and the morphology of the sample did not change significantly, and the average size of the particles was about 1.19 μm. The Tb element was also detected in addition to Lu, W, and O in the corresponding EDS pattern, as shown in Figure 5h. The FE-SEM images suggest that the Lu₂Mo₃O₁₂ samples have bigger size particles than those Lu₂W₃O₁₂ samples. Tb³⁺ doping did not lead to apparent changes in the particle morphologies nor in their the sizes. The EDS patterns proved the compositions of the samples. The Tb³⁺ doping did affect the PL properties greatly.

3.3. PL Properties

The preparation of temperature-dependent PL excitation spectra of Tb³⁺-doped Lu₂W₃O₁₂ monitored at ⁵D₄→⁷F₅ emission are shown in Figure 6. With the preparation temperature increased from 800 °C to 1300 °C, all PL excitation spectra show broad excitation bands in the wavelength range of 200 nm to 330 nm, which can be attributed to the O²⁻-W⁶⁺ charge transfer band (CTB) transition from host [WO₄]²⁻ and the 4f–5d transition of Tb³⁺ [25,26]. The excitation intensities of CTB and 4f–5d transition of Tb³⁺ increased firstly, and reached the maxima at 1200 °C, and then decreased when the temperature increased to 1300 °C. The narrow weak excitation peaks in the wavelength range of 330 nm to 500 nm originated from the 4f–4f transition absorption of Tb³⁺. Comparing the samples prepared at 900 °C and higher temperatures, the 4f–4f transition absorptions of Tb³⁺ prepared at 800 °C were very weak, suggesting that the Tb³⁺ was not successfully doped into the host lattice. The phosphor prepared at 900 °C presented apparent Tb³⁺ 4f–4f transition absorptions. In addition, the intensities of CTB and 4f–5d of Tb³⁺ transitions were much stronger than those of the 4f–4f transition of Tb³⁺. The PL excitation spectra suggests that the optimum preparation temperature is 1200 °C for Tb³⁺-doped Lu₂W₃O₁₂ phosphors.

Figure 7 shows the preparation of the temperature-dependent PL excitation spectra of Tb³⁺-doped Lu₂Mo₃O₁₂. By monitoring the ⁵D₄→⁷F₅ emission at 547 nm, the PL excitation spectra of phosphor prepared at different preparation temperature shows a strong and broad excitation band in the wavelength range from 200 nm to 330 nm, which can be attributed to O²⁻-Mo⁶⁺ CTB transition from host [MoO₄]²⁻ at about 260 nm and the 4f–5d transition of Tb³⁺ located at about 282 nm. With the preparation temperature increasing, the relative CTB excitation intensities of the O²⁻-Mo⁶⁺ and 4f–5d transition of Tb³⁺ changed. The O²⁻-Mo⁶⁺ CTB was superior in the phosphors prepared at 800, 900, and 1000 °C, whereas for the phosphors prepared at 1100, 1200, and 1300 °C, the 4f–5d transition of Tb³⁺ prevailed. In addition, with the preparation temperature increased from 800 °C to 1300 °C, the excitation intensity of CTB increased firstly and then decreased, reaching the maxima at 1100 °C. For the excitation peaks originating from the 4f–4f transitions of Tb³⁺ located in the wavelength range of 330 nm to 500 nm, the intensities of the excitations increased with the calcine temperature increasing. Unlike the PL excitation intensities of Tb³⁺-doped Lu₂W₃O₁₂, the intensities of CTB and the 4f–5d transitions of Tb³⁺ were similar to those of the 4f–4f transitions of Tb³⁺.

As is shown in Figure 7, the CTB excitation intensity of the Tb³⁺-doped Lu₂Mo₃O₁₂ phosphors prepared at 1000 °C and 1100 °C were similar. Considering the principle of energy saving and the safety in lab, 1000 °C was selected as the preparation temperature to prepare and compare the Tb³⁺-doped Lu₂Mo₃O₁₂ and Lu₂W₃O₁₂ phosphors.

Figure 8 shows the PL excitation and emission spectra of 5 mol% Tb³⁺-doped Lu₂W₃O₁₂ and Lu₂Mo₃O₁₂ phosphors. By monitoring the 547 nm emission corresponding to the ⁵D₄→⁷F₅ transition of Tb³⁺, the PL excitation spectra are composed of two parts. One part is a strong and broad excitation band in the wavelength range of 200–330 nm. There is one peak located at about 260 nm, which is generated by the O²⁻ (2p)–W⁶⁺ (5d) or O²⁻ (2p)–Mo⁶⁺ (4d) CTB transition from host [WO₄]²⁻ or [MoO₄]²⁻ groups. In addition, there is a peak at about 275 nm (or 282 nm), which originates from the Tb³⁺ 4f–5d transition [25,26]. The existence of the CTBs and the spectral overlap of the excitation spectra suggest the energy transfer between the [WO₄]²⁻or [MoO₄]²⁻ groups and Tb³⁺. In the wavelength of 330 nm to 500 nm, there are characteristic 4f–4f transitions of Tb³⁺ peaked at 321, 344, 354, 361, 372, and 380 nm, corresponding to the transitions of the ⁷F₆→⁵D₀, ⁷F₆→⁵G₂, ⁷F₆→⁵D₂, ⁷F₆→⁵G₅, ⁷F₆→⁵G₆, and ⁷F₆→⁵D₃ transitions [27], respectively. The strongest one peaked at 380 nm in the near-ultraviolet (NUV) region, which suggests that the Lu₂W₃O₁₂ and Lu₂Mo₃O₁₂ phosphors can be excited by NUV LED chips. For the Tb³⁺-doped Lu₂W₃O₁₂ and Lu₂Mo₃O₁₂ phosphors, with 260 nm and 380 nm excitation, four emission peaks at 492, 547, 586, and 622 nm originating from the ⁵D₄→⁷F₆, ⁵D₄→⁷F₅, ⁵D₄→⁷F₄, ⁵D₄→⁷F₃ transitions of Tb³⁺ are recorded. The ⁵D₄→⁷F₅ transition that peaked at 547 nm is the strongest emission, and the phosphors yielded a bright green light. The insets are the corresponding photographs captured under the illumination of 254 nm light in a camera obscura, which indicate the successful preparation of green phosphors.

Under 254 nm light, the Lu₂W₃O₁₂ phosphor produced a stronger bright green light than that of the Lu₂Mo₃O₁₂ phosphor seen by the naked eye. To confirm that, Figure 9 displays the PL excitation spectra of the two phosphors which were recorded under the same measurement conditions. By monitoring at 547 nm emission of the transition ⁵D₄→⁷F₅ of Tb³⁺, the Lu₂W₃O₁₂ phosphor presented about 1.5 times the excitation intensity of CTBs and Tb³⁺ 4f–5d transitions than those of the Lu₂Mo₃O₁₂ phosphor. Yet, the two phosphors illustrated the similar intensities of Tb³⁺ 4f–4f transitions. When the preparation temperature was 1000 °C, Lu₂W₃O₁₂ phosphor was a better choice than Lu₂Mo₃O₁₂ phosphor for CTB excitation.

3.4. DRS

The DRS of 5 mol% Tb³⁺-doped and undoped Lu₂W₃O₁₂ and Lu₂Mo₃O₁₂ materials are shown in Figure 10. All the materials have low reflectance in the wavelength range of 200–350 nm, which mean the materials have strong absorptions in the wavelength range. The absorptions can be attributed to the CTBs of O²⁻-W⁶⁺ or O²⁻-Mo⁶⁺ and Tb³⁺ 4f–5d transitions. As can be seen from Figure 10, the incorporation of Tb³⁺ caused the movements of the absorptions to short wavelength range. The energy gap value E_g of the materials can be estimated from DRS by using equation [28]:

$${(\alpha h\nu )}^n=c(h\nu -E_g)$$

(1)

where α, h, υ, and c are the absorption coefficient, the Planck’s constant, the incident light frequency and a constant, respectively. The value of n is 2 or 1/2, depending on the transition mode of the material, and n = 2 is a direct transition, whereas n = 1/2 is an indirect transition. The absorption coefficient α can be solved by using the Kubelka–Munk equation [29]:

$$F(R)=\frac{(1-R{)}^2}{2R}=\frac{K}{S}$$

(2)

In this function, the reflection coefficient R is recorded by DRS, S is the scattering coefficient and independent on the wavelength of the incident light, and the value of K is proportional to α in Equation (1). In this case, n equals 1/2 and Lu₂W₃O₁₂ [30] and Lu₂Mo₃O₁₂ [31] are both indirect bandgap materials. By plotting [F(R)hν]^1/2 versus hν, the value of E_g can be obtained. The insert in Figure 10 presents the [F(R)hν]^1/2-hν plots of undoped Lu₂W₃O₁₂ and Lu₂Mo₃O₁₂ samples. By extrapolating F(R) to 0, the values of E_g of 4.02 eV and 3.94 eV for undoped Lu₂W₃O₁₂ and Lu₂Mo₃O₁₂ samples were obtained, respectively. For 5 mol% Tb³⁺-doped Lu₂W₃O₁₂ and Lu₂Mo₃O₁₂ phosphors, the obtained values of E_g were 4.06 eV and 3.95 eV. The value E_g of the Lu₂W₃O₁₂ material was larger than that of the Lu₂Mo₃O₁₂ material, and the value E_g of the doped material was larger than that of corresponding undoped material. The wide band gap is good for luminescent materials [32].

3.5. Measuring Temperature Dependent Spectra, Activation Energy, and Decay Curves

Thermal stability is usually used to evaluate phosphors [33,34]. Figure 11 shows the measuring of temperature-dependent PL excitation spectra of Tb³⁺-doped Lu₂Mo₃O₁₂ phosphors by monitoring at a 547 nm emission of Tb³⁺. With the measuring temperature increasing from 50 °C to 250 °C, all the intensities of the excitation peaks became weaker. Correspondingly, the emission intensity decreased as well. The thermal luminescence quenching is usually caused by non-radiative transitions at high working temperature [35,36]. At a normal working temperature about 150 °C, the Lu₂Mo₃O₁₂ phosphor kept 86% and 76% intensity of that at 50 °C with 260 nm and 380 nm excitations, respectively. Put another way, the emission intensity reduces to 50% of the initial intensity defined as quenching temperature T_0.5 [37]. In the Lu₂Mo₃O₁₂ phosphor, the T_0.5 is 215 °C and 192 °C with 260 nm and 380 nm excitations.

The activation energy (E_a) of the Lu₂Mo₃O₁₂ phosphor was also evaluated by using the Arrhenius formula [38]:

$$I_{\mathrm{T}}=\frac{I_0}{1+c\exp (-E_{\mathrm{a}}/kT)}$$

(3)

where I₀ is the initial emission intensity of Tb³⁺, I_T is the emission intensity of Tb³⁺ at temperature T, c is the quenching frequency factor, and k is the Boltzmann constant and equals 8.617 × 10⁻⁵ eV/K [39]. Evolving the formular to ln[(I₀/I_T) − 1] = (−E_a/k)(1/T) + c₁, plotting ln[(I₀/I_T) − 1] corresponding 1/T, and linear fitting, the −E_a/k can be determined. Further, the E_a value can be obtained. As shown in Figure 12, the data obtained at high temperatures of 220 °C and 250 °C decayed rapidly and the data were not used in the fitting procedure. The values of −E_a/k were −2340 and −4656 with 260 nm and 380 nm excitations, respectively. Correspondingly, the values of E_a were 0.202 eV and 0.401 eV with 260 nm and 380 nm excitations. The results suggest that the Tb³⁺-doped Lu₂Mo₃O₁₂ phosphor has wonderful thermal stabilities.

Unlike the Tb³⁺-doped Lu₂Mo₃O₁₂, the Tb³⁺-doped Lu₂W₃O₁₂ presents a different thermal behavior. For clarity, the measuring temperature and the monitoring wavelength were placed in the temperature-axis with the front number representing the measuring temperature and the latter number representing the monitoring wavelength. As shown in Figure 13, with the measuring temperature increasing, the PL excitation intensity varied differently by monitoring at the ⁵D₄→⁷F₅ transition. The intensities of Tb³⁺ 4f–4f transitions always decreased with the temperature increasing from 50 °C to 300 °C. Yet, the intensity of the CTB transition decreased with the temperature increasing to 150 °C. When the temperature increased further, the CTB and the Tb³⁺ 4f–5d transitions increased suddenly at about 170 °C and were hardly unchanged, even when the temperature was increased to 300 °C. Furthermore, the peak wavelength of the ⁵D₄→⁷F₅ transition varied from 546 nm to 552 nm with the measuring temperature increasing. Similar phenomena have been reported in Eu³⁺-doped Lu₂(MoO₄)₃ [22,23]. The enhancing luminescence of Eu³⁺ in Lu₂(MoO₄)₃ was recorded at about 250 °C, yet the enhancing luminescence of Tb³⁺ in Lu₂W₃O₁₂ was recorded at about 170 °C. The enhancing temperature was lowered to about 80 °C, which was closer to practical applications. The enhancing luminescence has a relationship with the NTE property of the Lu₂W₃O₁₂ crystal lattice.

The fluorescent decay curves of the Tb³⁺-doped Lu₂W₃O₁₂ and Lu₂Mo₃O₁₂ phosphors by monitoring at 547 nm with 260 nm excitation are shown in Figure 14. The two decay curves can both be fitted by a single exponential function:

$$I(t)=I_0\exp (-t/\tau )+c$$

(4)

where I₀, I(t), τ, and c are emission intensity at initial time, emission intensity at t time, the corresponding fluorescent lifetime, and the background or zero offset, respectively. The fitted lifetimes were 0.724 ms and 0.689 ms for the 5 mol% Tb³⁺-doped Lu₂W₃O₁₂ and Lu₂Mo₃O₁₂ phosphors, respectively. The results suggest that the lifetime corresponding to the ⁵D₄ level of Tb³⁺ was longer in the Lu₂W₃O₁₂ host lattice than in the Lu₂Mo₃O₁₂ host lattice, which is similar to those reported.

4. Conclusions

In conclusion, by using a solid-state reaction method, the Tb³⁺-doped and undoped Lu₂W₃O₁₂ and Lu₂Mo₃O₁₂ materials were prepared under different calcine temperatures. The XRD results suggest that the 5 mol% Tb³⁺-doping did not cause apparent variations on the XRD patterns besides the diffraction intensities. The FE-SEM pictures and the particle size distribution statistics illustrate that the materials were all composed of micrometer particles and the Lu₂W₃O₁₂ samples had smaller particles than that of the Lu₂Mo₃O₁₂ samples. The EDS patterns confirmed the compositions of Lu, Mo or W, O, and/or Tb in the materials. The preparing of temperature-dependent PL excitation spectra suggested that the optimum preparation temperature was 1200 °C for Tb³⁺-doped Lu₂W₃O₁₂ phosphors and 1100 °C for Tb³⁺-doped Lu₂Mo₃O₁₂ phosphors, respectively. The Tb³⁺-doped Lu₂Mo₃O₁₂ phosphors prepared at 1000 °C and 1100 °C presented equivalent CTB excitation intensities, and 1000 °C was selected as the calcine temperature to prepare and compare the Tb³⁺-doped and undoped Lu₂Mo₃O₁₂ and Lu₂W₃O₁₂ materials. The Tb³⁺-doped Lu₂W₃O₁₂ green phosphor showed a stronger PL excitation intensity of CTB than that of the Tb³⁺-doped Lu₂Mo₃O₁₂ green phosphor. For undoped and doped Lu₂W₃O₁₂ and Lu₂Mo₃O₁₂ materials, the values of E_g calculated from the DRS were 4.02, 4.06, 3.94, and 3.95 eV, respectively. The values of E_g of the Lu₂W₃O₁₂ materials were larger than that of the Lu₂Mo₃O₁₂ materials. The measuring temperature dependent spectra indicated that the values of activation energy E_a of the Tb³⁺-doped Lu₂Mo₃O₁₂ phosphors were 0.202 eV and 0.401 eV with 260 nm and 380 nm excitation, respectively, whereas the Tb³⁺-doped Lu₂W₃O₁₂ phosphor illustrated a thermal enhancing CTB excitation intensity at 170 °C. The PL decay curves suggested that the lifetime corresponding to the ⁵D₄ level of Tb³⁺ was longer in the Lu₂W₃O₁₂ host lattice than in the Lu₂Mo₃O₁₂ host lattice. The Tb³⁺-doped Lu₂W₃O₁₂ showed potential as an application in temperature sensors.