High sensitive optical thermometry based on redshift of charge transfer band in Ba2CaWO6:Eu3+ phosphors

Optical thermometry has emerged as a promising technology to overcome the limitations of traditional thermometry techniques. Herein, we report an Eu3+-activated tungstate Ba2CaWO6 phosphors for luminescent thermometry with a maximum relative sensitivity of 4.84% based on a single-band ratiometric technique. By monitoring the 5D0 → 7F1 (595 nm) emission of Eu3+, excitation spectrum of the as-prepared sample was measured, showing a dominant broad band ranging from 240 to 345 nm corresponding to the O2−-W6+ charge transfer band (CTB). The intensity of CTB gradually decreases and the CTB edge shows an appreciable red shift when the temperature increases due to thermal population of the [WO6]6− moiety at higher vibrational levels. The opposite temperature dependences of the two specific excitations, one at the peak of CTB and the other from the edge of CTB, were employed to design a luminescence intensity ratio thermometry scheme in the tungstate. When compared to other related schemes, our scheme showed a higher relative sensitivity.


Introduction
Optical temperature sensing has become a revolutionary measurement technique benefiting from the temperature dependence of the phosphor's optical response to light stimuli, and has been widely adopted in various scientific and industrial fields in recent years [1,2].Due to its many advantages, such as non-contact measurement, fast response, high adaptability, and high spatial temperature resolution, it has surpassed traditional temperature measurement methods [3].To date, luminescence thermometers that rely on temperature-dependent emission intensity (or intensity ratio), spectral positions (emission bands or peaks), band width, and fluorescence lifetime (rise or decay) have already demonstrated excellent performance [4][5][6].Among all these techniques, luminescence intensity ratio (LIR) is one of the most imposing resort in that circumventing the detrimental deviations arising from the volatility of the excitation power and the scattering [7].At present, the LIR method, which is based on the ratio between two thermally coupled energy levels (TCELs), is typically limited by the requirement that the gap between the levels be within the range of 200-2000 cm −1 to ensure thermal coupling.Rare-earth trivalent ions, for instance Er 3+ ( 2 H 11 , 4 S 3/2 ), Dy 3+ ( 4 I 15/2 , 4 F 9/2 ), Nd 3+ ( 4 F 5/2 , 4 F 3/2 ), Tm 3+ ( 3 F 2,3 , 3 H 4 ) and Ho 3+ ( 5 F 2,3 , 3 K 8 or 5 G 6 , 4 F 1 ), are prevailing among those reported schemes [8][9][10][11][12][13].However, the relative sensibility (S r ) is limited by the restriction of the energy difference between two thermally coupled states.To overcome this limitation, various modified strategies have been proposed in recent years, including fluorescence thermometry based on intervalence charge transfer state (IVCTs) or metal-to-metal charge transfer state (MMCTs) in some Pr 3+ or Tb 3+ -doped oxide host materials such as vanadate, niobate, titanate, and tungstate [14], the dual-emitting centers in singular or multiphase crystals [15], the thermal population of low-lying excited energy states [16], etc.In 2016, Cao et al reported a strategy taking advantage of the infra-( f-f ) and inter-( f-d) transitions of Sm 2+ ion in SrB 4 O 7 (S r reaches to 2.16% K −1 at 500 K) of late [17].In 2019, Ruan et al successfully developed Ca 8 ZrMg(PO 4 ) 6 (SiO 4 ) self-reducible host with the co-existence of Eu 2+ and Eu 3+ ions [18].High S r of which the zenith up to 5.94% K −1 was obtained.More recently, Suo et al achieved a novel near-infrared (NIR) emission luminescence thermometry that is greatly enhanced by phonon-assisted energy transfer in LaPO 4 :Nd 3+ phosphor [19].It was tested at biological window and achieved a maximum S r up to 3.51% K −1 at 280 K.It is expiring that these works have significantly improved the performance of optical temperature probes.It is anticipated that these new temperature sensing strategies will lead to the development of optical temperature sensors with enhanced performance.
Recently, a new-stratagem luminescent thermometry based on single-band ratiometric technique was proposed by Zhou et al They utilizing the redshift at the edge of the charge-transfer band (CTB) in YVO 4 :Eu 3+ to achieve sensitive luminescence thermometry [20].Strikingly different but monotonous temperature-dependent behaviors were observed under the excitations of 358 nm and 394.8 nm lights and employed for temperature sensing.An S r = 3923/T 2 was achieved in the range of 300 to 480 K. Analogous phenomena have been discovered in other vanadate such as LuVO 4 and GdVO 4 with excellent temperature-sensing properties [21,22].The strategy of using CTB redshift for optical thermometry has shown great success.However, the majority of previous studies have focused on vanadates.To broaden the scope of this technique, it would be beneficial to explore other materials.
Ba 2 CaWO 6 is a double-perovskite system AA'BO 6 with stable physical and chemical properties.Its WO 6 octahedral group can enhance the emission intensity of dopants through efficient energy transfer [23].This makes it possible to tailor its temperature sensing properties.In addition, many investigations about Ba 2 CaWO 6 , such as the designs of white light emission Ba 2 CaWO 6 :Dy 3+ phosphor and yellowish-green emission Ba 2 CaWO 6 :Er 3+ [24,25].Although Ba 2 CaWO 6 has exceptional optical properties, there is still limited research on its use for temperature sensing.Due to its potential excellent temperature-dependent fluorescent properties, Eu 3+ is widely used for LIR-based temperature sensing [26,27].In this study, Eu 3+ -doped Ba 2 CaWO 6 phosphors were synthesized and the structure, photoluminescence and temperature quenching luminescence properties were investigated in detail.Ultimately, our attempt to develop a new temperature-measuring strategy using the tungstate Ba 2 CaWO 6 :0.1Eu 3+ was successful.The results showed that it has the potential to be an excellent ratio-metric thermometer.

Experimental
The yellowish tungstate Ba 2 CaWO 6 :0.1Eu 3+ sample was fabricated via a standard high-temperature solid state route successfully.Raw ingredients contained of WO 3 (Analytical Reagent, AR), CaCO 3 (AR), BaCO 3 (AR) as well as Eu 2 O 3 (99.99%)were weighed scrupulously adhering to the stoichiometric ratio without further purification and ground in agate mortar for 30 min to achieve homogeneousness.The powder was ground and then placed in a crucible.It was preheated at 950 °C for 6 h in a muffle furnace.After cooling to room temperature, the precursor was reground for 15 min and then sintered at 1250 °C for 4 h.The final sample was obtained after natural cooling and another round of grinding.The as-prepared Ba 2 CaWO 6 :0.1Eu 3+ phosphor was then ready for further characterization.

Characterizations
The x-ray diffractometer (XRD) Smartlab (Rigaku, Japan) equipped by Cu-kα radiation (λ = 1.5406Å) was implemented to identify the phase of Ba 2 CaWO 6 :0.1 Eu 3+ sample, and the scanning rate used for XRD was 1°min −1 in the 2θ range of 5°to 90°.The morphology of the as-synthesized phosphor was investigated using Quanta FEG 250 scanning electron microscope (FEI inc., USA).The photoluminescence excitation (PLE) and photoluminescence (PL) spectra were recorded by a photoluminescence spectrometer FLS-1000 (Edinburgh Instrument, Britain) with a 450 W xenon lamp.To obtain and investigate the temperature-dependent properties of the sample in the range of 225 to 425 K, a temperature controller X-1AL (ARS inc., USA) cryostat equipped with a powder chamber was used and the interval for the measurements was not less than 15 min in order to achieve thermal equilibrium.

Phase and crystal structure
The Rietveld refinement XRD pattern of the as-synthesized Ba 2 CaWO 6 :0.1Eu 3+ phosphor is shown in figure 1(a) with reliable R wp , R p , and χ 2 factors illustrate that the crystal structure of Ba 2 CaWO 6 :0.1Eu 3+ matches well with Ba 2 CaWO 6 host and is free from any impurity phase.The detailed lattice parameters and the crystallographic data listed in table 1.The SEM image of Ba 2 CaWO 6 :0.1Eu 3+ depicted in figure1(b) reveals the presence of micrometer-scale particles.
The crystal structure of Ba 2 CaWO 6 sample is presented in figure1(c), manifesting a typical double perovskite structure with the space group Fm 3̅ m [26].From the schematic crystal structure, the Ca 2+ and W 6+ form [CaO 6 ] and [WO 6 ] regular octahedron with six nearest oxygen atoms, while larger Ba 2+ ion is in occupation of  .It is noted that the charge compensation can be fulfilled by replacing every three Ca 2+ with two Eu 3+ , leaving a Ca 2+ vacancy in this structure.This results in several distinctive sites.Interestingly, it appears that Eu 3+ ions only enter the inversion symmetry site, as indicated by the strong 595 nm emission compared to the weak 615 nm emission.

Properties and mechanisms of photoluminescence
Figure 2(a) depicts the photoluminescence spectra at 225 and 300 K.As is seen, under the 308 nm excitation, it is overt that the 595 nm ( 5 D 0 → 7 F 1 ) is prevailing over the 615 nm electric dipole transition ( 5 D 0 → 7 F 2 ), demonstrating the forementioned conclusion that Eu 3+ ion only occupies the Ca 2+ site with an inversion surrounding in those two temperatures [26].The excitation spectra monitoring the 595 nm emission shows a predominant band from 240 to 345 nm, which is due to the CTB resulting from the electron transition from O 2− to W 6+ in [WO 6 ] 6− ligand.There are several small peaks, including 394 nm ( 7 F 0 → 5 L 6 ), 464 nm ( 7 F 0 → 5 D 2 ) and 528 nm ( 7 F 0 → 5 D 1 ), corresponding to the characteristic 4 f intra-transition of Eu 3+ ion.The small intensity is due to the inversion symmetry environment of the Eu 3+ ions, which greatly weakens the 4f-4f parityforbidden transitions.From figure 2(a), we can also find that the positions of the CTB edges for 225 and 300 K are different.In order to study the influence of temperature in detail, temperature-dependent excitation spectra were recorded at a series of temperatures from 225 to 425 K as presented in figure 2(b).As the temperature increases from 225 to 425 K, the CTB undergoes a noticeable red shift, with its peak moves from 308 nm at 225 K to 317 nm at 425 K.The characteristic transitions of Eu 3+ remain unchanged as the temperature increases.To clarify this interesting phenomenon, the schematic diagram to the entire luminescence process is plotted in figure 2(c).First and foremost, [WO 6 ] 6− absorbs a UV photon and is excited to higher energy level, followed by several lattice relaxation processes and energy transfer process to Eu 3+ ion, eventually the typical emission of Eu 3+ ion is observable including the dominant 595 nm emission.Additionally, an emission peak at 698 nm was detected and ascribed to the charge transition of W 6+ -O 2− within the [WO 6 ] 6− moiety.This finding is in agreement with prior studies reported in reference [30].The thermal population of [WO 6 ] 6− at higher vibration level ensue by heating leading to relatively smaller energy gaps with the excitation state than low temperature which is the main reason for the observation of the CTB red-shift [23].Interestingly, the redshift of the CTB edge with increasing temperature leads to an increase in excitation intensity around the tail of the CTB.This suggests that ratiometric temperature sensing can be implemented by using the redshift of the CTB.
3.3.Temperature-dependent luminescence in Ba 2 CaWO 6 : 0.1Eu 3+ phosphor As mentioned earlier, the shift of the CTB provides a ratiometric strategy for temperature sensing.In this study, we specifically investigated the temperature-dependent photoluminescence under excitation within the CTB.
The emission spectra from 225 to 425 K, excited by 345 and 308 nm, which are both part of the charge transfer process from O 2− to W 6+ , are shown in figure 3. The 595 nm emission in the spectra under different excitations shows a clear temperature-dependent relationship.Specifically, the emission from 345 nm excitation increases, while the emission from 308 nm excitation decreases slightly due to thermal quenching.Therefore, both 595 nm emission peaks excited by different wavelengths can be used for ratiometric temperature sensing.Figure 4(a) shows the temperature-dependent behavior of Eu 3+ emission (integrated from 585 to 603 nm) under 345 and 308 nm excitations, and figure 4(b) shows the temperature-dependent LIR.The experimental data fits well with an exponential curve of LIR = 22.44 × exp(−2451.92/T),where LIR is emission intensity ratio under two different excitations and T is the absolute temperature in units of K.
To better understand the temperature sensing ability of the sample, It is important to investigate the relative and absolute sensitivity (S r and S a ), two key parameters for quantifying the sensitivity of optical thermometry, which are defined by the following formulae [3]: general, S r is considered to be more versatile when comparing the performances of different types of optical thermometer materials.In accordance with the expressions (1) and (2), the S r (blue line) and S a (red line) are calculated and plotted in figure 4(c), which shows that S r reaches 4.84% K −1 at 225 K.
For the practical application, the thermal stability is a rather critical variable for temperature sensing.In order to have a clue about the thermal performance of the Ba 2 CaWO 6 :0.1Eu 3+ sample, the thermal cycles of the LIR (I ex345 /I ex308 ) were carried out and studied in the temperature range from 225 to 425 K then the result was given in figure 4(d).As shown in the diagram, the change in LIR during heating-cooling cycles is not significant, indicating good thermal stability of the sample.Table 2 shows the optical thermometry properties of various Eu 3+ activated materials.The experiments and results demonstrate that this tungstate-based phosphor performs well in temperature sensing and is a promising candidate for use in optical thermometers.

Conclusions
In conclusion, we successfully synthesized a tungstate-based, Eu 3+ doped Ba 2 CaWO 6 phosphor for temperature sensing using a high-temperature solid-state method.XRD analysis confirmed the successful preparation of the sample without any impurities or structural changes.Excitation spectroscopy was performed at different temperatures and the redshift of the CTB edge was observed.The temperature-dependent luminescence properties of the tungstate were investigated in detail and the results indicate that luminescence thermometry based on single-band ratiometric showed good performance overall.A maximum S r value of 4.84% K −1 was achieved at 225 K with Ba 2 CaWO 6 :0.1Eu 3+ , which together with the thermal cycling experiments showed its potential as an excellent optical temperature sensing material.

Figure 4 .
Figure 4. (a) The emission intensities versus temperature under 345 and 308 nm excitation, respectively; (b) The temperaturedependent LIR at 595 nm excited by 345 nm to that excited by 308 nm; (c) the calculated S a and S r versus temperature; (d) The thermal cycles of LIR(I 308ex /I 345ex ) versus temperature.

Table 2 .
The optical thermometry properties of various Eu 3+ -activated matrixes.