1. Introduction

In recent years, for Lead-based piezoelectric ceramics used as an important class of smart materials, the dominant one is still conventional Pb(Zr_xTi_1-x)O₃ (PZT) due to its excellent piezoelectric performance, which plays a crucial role in practical applications, such as transducers, actuators, sensors and other electro-mechanical devices and systems [1,2]. However, some countries have enacted restrictions prohibiting the consumption of in electronic devices containing lead due to their hazardous and volatile nature in the manufacturing operation, posing a significant risk to human health and the environment [3,4]. As a result of the growing awareness of environmental protection and long-term development, lead-free piezoelectric materials for replacing conventional lead-containing piezoelectric materials have received increased attention [5–8].

Na_0.5Bi_0.5TiO₃ is recognized as one of the most promising lead-free piezoelectric material possibilities. Raman results show that the perovskite ceramic with a highly crystalline-structure has a lower phonon energy [9]. In addition, the material Na_0.5Bi_0.5TiO₃ has a high Curie temperature (T_c= 320 °C) [10]. This property shows that Na_0.5Bi_0.5TiO₃ ceramic has a potential applicability in high temperature applications. Thus, Na_0.5Bi_0.5TiO₃-based ceramics can serve as an excellent host material for photoluminescence, which drops the probability of non-radiative transitions caused by phonon vibrations and increases the activators up-conversion emission luminous intensity. However, the piezoelectric properties of the material Na_0.5Bi_0.5TiO₃ are not ideal, limiting its practical application. To improve its piezoelectric properties, a new one or more solid solutions with perovskite structure are usually incorporated into Na_0.5Bi_0.5TiO₃, forming new binary or multiple solid solutions. Among them, 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃ has been extensively studied as a binary system with a composition near the morphological phase boundary (MPB) for the optimum piezoelectric performance [11].

It is of great interest that rare earth (RE) ion-activated 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃ ferroelectrics not only exhibit additional functionality of excellent photoluminescence, and improved intrinsic ferroelectric and piezoelectric properties [12], but their photoluminescence can also be efficiently modulated by an electric field based on a physical approach [13,14], which is usually reversible. There are numerous uses for these materials in the discipline of photonics, which can expand the applications of piezoelectric nanogenerators [15]. Where photoluminescence associated to RE ions serving as a straightforward yet effective method to probe their local structure of the MPB composition [16]. Furthermore, Na_0.5Bi_0.5TiO₃, BaTiO₃ and (1-y)Na_0.5Bi_0.5TiO₃-yBaTiO₃ consisting of both activated RE ions show high up-conversion emission, and the degree of their upconversion luminescence is significantly influenced by the ambient temperature [17–19].

Upconversion luminescence can generate short-wavelength higher-energy photons by absorbing two or more long-wavelength lesser-energy photons, a phenomenon known as the anti-Stokes shift [20,21]. The upconversion luminescence phenomenon is a very important and fundamental nonlinear optical process. Upconversion luminescence materials have attracted widespread interest because of their potential applications in medicinal chemistry, anti-counterfeiting, photovoltaic technologies, photocatalytic technique, displays, optical fibers, temperature sensing, and optoelectronic devices [21–25]. Furthermore, temperature sensing materials can be employed in luminescent probes and thermal sensors [26,27].

For upconversion luminescence materials, trivalent Er³⁺, Ho³⁺, but also Tm³⁺ are the most commonly utilized as activator ions because they have special ladder-like arranged metastable energy levels [28]. To improve the fluorescence intensity in upconversion luminescence process associated to Er³⁺, Ho³⁺ and Tm³⁺ ions, Yb³⁺ is frequently adopted as a sensitizer due to their intuitive energy level configuration and large absorption cross section in the near-infrared (NIR) region, which can enhance energy absorption [29]. Subsequently, the upconversion luminescence intensity and sensing sensitivity are enhanced by effectively transferring energy from Yb³⁺ to these RE³⁺ ions. Notably, Ho³⁺ is among the few RE³⁺ with a large amount of long-lived metastable energy levels capable of photoluminescence with intense emission across a broad spectral region spanning ultraviolet (UV)-to-mid-infrared (IR) [30]. For example, Ho³⁺ is one of the most efficient luminescent ions for its abundant energy level transition processes producing strong visible light, e.g., green and red emissions, accompanied by relatively weak NIR emission under 980 nm excitation. Over the past few years, some researches on lead free ferroelectrics doped with Ho³⁺ or Ho³⁺/Yb³⁺ have been carried out since their outstanding optical temperature sensing performance [31], for instance, Na_0.5Bi_0.5TiO₃: Ho³⁺ [17], (K_0.47Na_0.47Li_0.06)(Nb_0.94Bi_0.06)O₃: Ho³⁺ [32], (K, Na)NbO₃–SrTiO₃: Ho³⁺ [33] and BaTiO₃: Ho³⁺/Yb³⁺ [21], and so on.

As a method of studying optical temperature sensing, the fluorescence intensity ratio (FIR) technique can avert several external disturbances in the measurement process to achieve high measurement sensing accuracy and sensitivity [34,35]. In order to further enhance the sensitivity, the FIR originated from non-thermally coupled energy levels (non-TCLs) in RE³⁺ activated upconversion luminescence materials with respect to temperature sensing strategy has recently attracted more and more attention [36,37]. The significant reason for this is that the emission bands induced via the non-TCLs are separated in the spectra, i.e., they do not intersect compared with the TCLs.

As a result, Ho³⁺/Yb³⁺ codoped 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃ ceramics are speculated to be ideal materials with efficient upconversion luminescence for optical temperature sensing applications. In the present work, specimens of 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃: Ho³⁺/Yb³⁺ were prepared. The Ho³⁺ doping content on crystal structure as well as upconversion luminescence characteristics were investigated in detail. Additionally, based on upconversion luminescence spectra, the temperature from 303 K to 483 K independent FIR (I₆₅₆/I₅₄₇) generated by non-thermally coupled levels, i.e., the red (⁵F₅→⁵I₈) emission band to the green (⁵F₄/⁵S₂→⁵I₈), were studied. Meanwhile, the relative sensing sensitivity exists a highest value of 0.0042 K^-1 at 303 K, which successfully achieves a 20% improvement over Ho³⁺ mono-doped 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃.

2. Experimental

The details of the preparation process originated to the polycrystalline lead-free ceramics with the following chemical formula 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃: xHo³⁺/0.01Yb³⁺ (x = 0.0025, 0.005, 0.0075, 0.01, 0.0125, 0.015) can be found in Ref. [31]. The final sintered samples are all bulk ceramics. To assess the crystal structures of the sintered samples, a powder PANalytical X'pert3 X-ray diffractometer (Netherlands) with CuKα (λ=0.15406 nm) as the radiation resource was used to characterized X-ray diffraction (XRD) patterns. The surface morphology and element mapping were performed via a Quanta FEG450 field emission scanning electron microscope (FESEM, USA). The diffuse reflection spectra (DRS) were recorded using a Shimadzu UV-3600 UV/visible spectrophotometer (Japan). The content and surrounding temperature dependent emission spectrums of the as-prepared ceramics upon the excitation of 980 nm were collected in a Fluorlog-3 spectrofluorometer (France). The spectra measurement has a wavelength accuracy of 0.5 nm, the pulsed excitation was accomplished with a nanosecond laser diode (LD) with adjustable power. The sample was put on a heating device whose temperature could be adjusted from ambient temperature to 573 K, achieving upconversion luminescence spectra measurement at different temperatures.

3. Result and discussion

Based on room-temperature XRD diffraction patterns of 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃: xHo³⁺/0.01Yb³⁺ ceramics, a single-phase perovskite-structure is discovered as presented in Fig. 1(a). All observed characteristic peaks match with the pure rhombohedral NBT (PDF No.36-0340, R3c), free of impurity peak in the XRD detection resolution range. Figure 1(b)-(d) give the amplified XRD profiles with a 2θ in range of 31.75°-47.5° to further explain phase structure of the polycrystalline. The diffraction peaks shift observed from Fig. 1(b)-(d) are slightly displaced to higher angles when the x-value (content of Ho³⁺ ions) increases. Ho³⁺ and Yb³⁺ ions are most likely to substitute Bi³⁺ ions at the A-site, both in terms of ionic radii [38] and valence considerations, followed by the lattice shrinkage phenomenon described above. Similar to our earlier research [31], in the XRD profiles of 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃: xHo³⁺/0.01Yb³⁺ ceramics, as shown in Fig. 1(b) and (c), two prominent splitting peaks near 32.50° and 40.10° indicate that the rhombohedral phase is present in the as-prepared ceramics. Figure 1(d) depicts the variation of the (202) reflections that are split into doublets, suggesting the tetragonal phase is also present [39,40], which is similar to 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃ matrix. These results indicate the appearance of MPB in ceramics containing both rhombohedral and tetragonal phases.

 

Fig. 1. a) Room-temperature XRD diffraction patterns related to 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃: xHo³⁺/0.01Yb³⁺ ceramics. b)-d) are the magnified XRD patterns from 31.75° to 33.25°, 39.25° to 40.75° and 46° to 47.5° in the 2θ range, respectively.

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Figure 2(a) gives SEM images for the prepared ceramic with x = 0.0075. Apparently, the distribution of grain size is inhomogeneous. The present ceramics have a compact microstructure and good sinterability. Figure 2(b) shows EDS spectrum of 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃: 0.0075Ho³⁺/0.01Yb³⁺ ceramic and Fig. 2(c-i) present its EDS elemental mapping images. Figure 2(b) demonstrates that the presence of Na, Ba, Yb, Ho, Bi, Ti and O. As shown in Fig. 2(c-i), all constituent elements of this compound within a specific area can be determined using the EDS elemental mappings. It is confirmed that the 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃: 0.0075Ho³⁺/0.01Yb³⁺ ceramic include the all components, i.e., Na, Ba, Yb, Ho, Bi, Ti and O, and that these components are dispersed randomly all through the ceramic particles.

 

Fig. 2. a) Scanning electron microscope (SEM) images and b) EDS spectrum of 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃: 0.0075Ho³⁺/0.01Yb³⁺ ceramic. c)-i) corresponding EDS elemental mapping images of Na, Ba, Yb, Ho, Bi, Ti and O in 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃: 0.0075Ho³⁺/0.01Yb³⁺ ceramic, respectively.

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Figure 3 gives the UV-visible diffuse reflectance spectra (DRS, 200–800 nm) of the 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃: xHo³⁺/0.01Yb³⁺ ceramics including x = 0.0075 and 0.015. The DRS displays a high reflectance (>80%) between 450 and 800 nm. Three distinct absorption bands centered at 453, 540 and 644 nm are observed in the DRS related to those two typical samples, corresponding to 4f-4f transitions of Ho³⁺ ions, that is ⁵I₈→⁵G₆, ⁵I₈→⁵F₄/⁵S₂ and ⁵I₈→⁵F₅, respectively. In addition, at 485 nm, a somewhat faint absorption band is observed corresponding to the ⁵I₈→⁵F₃ transition [41]. A sharp drop near 360 nm is observed, corresponding to the absorption edge that determines its optical band gap. Below 340 nm, there is a wide absorption band induced by the electrons transition from the valence band (VB) to the conduction band (CB) [42].

 

Fig. 3. The UV–visible diffuse reflectance spectra (DRS) of typical 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃: xHo³⁺/0.01Yb³⁺ ceramics (x = 0.0075 and 0.015). The inset exhibits the corresponding absorption spectra.

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To experimentally determine the band gap energy (forbidden band width) of typical 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃: xHo³⁺/0.01Yb³⁺ ceramic, the absorption spectra are shown in the inset of Fig. 3. These spectra were derived from their UV–visible DRS using the Kubelka-Munk (K-M) function $F(R)$ [43], as provided in Eq. (1)

With regard to the function, R(=R_sample/R_standard) is the diffuse reflectance. S denote scattering coefficients, K is proportional to S. When the powder sample scatters in completely diffuse manner or when it is continuously illuminated at 60 ° incidence, then $K = 2\alpha $, $\alpha $ is linear absorption coefficient, S stands for a constant in relation to the wavelength of the incident light [31]. According to earlier publications [44,45], the Wood-Tauc relation can be used to describe the relationship of α and E_g for materials:

Figure 4(a) displays the upconversion emission spectra of Ho³⁺/Yb³⁺ codoped 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃ binary system when it is stimulated by a 980 nm laser diode fitted with a power of about 0.1W. As shown in Fig. 4(a), the sintered ceramics display strong upconversion emissions in the 450–800 nm range. In their upconversion emission spectra, there are three distinct bands of Ho³⁺ in the green (520-575 nm), red (625-690 nm) and NIR (730-775 nm) regions, are ascribed to the radiative transitions of Ho³⁺: ⁵F₄/⁵S₂→⁵I₈, ⁵F₅→⁵I₈ and ⁵F₄/⁵S₂→⁵I₇, respectively. When the content of Ho³⁺ ions increase, the peak positions of all emission bands remain almost unchanged, but the emission intensities change significantly. To further determine the dependence of the intensity related to the upconversion luminescence spectra on the amount of Ho³⁺ ions, the main peak intensities for different Ho³⁺ doping concentration located at different positions are shown in Fig. 4(b). The emission intensity increases initially, reaching the maximum when x = 0.0075, and then declines with increasing Ho³⁺ content. Therefore, the ideal Ho³⁺ doping concentration for the 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃: xHo³⁺/0.01Yb³⁺ system is 0.0075. The concentration quenching effect takes place as Ho³⁺ content is greater than 0.0075, which is due to the fact that the distance between two adjoining Ho³⁺ and Ho³⁺ or Ho³⁺ and Yb³⁺ decreases with increasing Ho³⁺ content, which facilitates the non-radiative energy transfer (ET) between them and thus results in a reduction in the upconversion luminescence intensity [46].

 

Fig. 4. a) The upconversion emission spectrums dependent on Ho³⁺ ion composition in the 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃: xHo³⁺/0.01Yb³⁺ ferroelectric system, b) The upconversion emission intensity of Ho³⁺ ions in various wavelength ranges with the different content of Ho³⁺.

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Figure 5(a) depicts the upconversion emission spectra of Ho³⁺ single-doped and Ho³⁺/Yb³⁺ co-doped 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃ ceramics. From Fig. 5(a), we found that the introduction of appropriate Yb³⁺ ions greatly enhance the luminous intensity. A similar upconversion emission spectral pattern to that the one containing only Ho³⁺ ions is observed, which is consistent with the description in the introduction.

 

Fig. 5. a) When x = 0.0075, upconversion emission spectra with and without Yb³⁺ at pump power 0.5 W. The inset shows enlarged upconversion emission spectrum in the Yb³⁺ free ceramic. b) The upconversion luminescence mechanism of Ho³⁺/Yb³⁺ codoped 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃ specimen.

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According to the upconversion luminescence spectrums of 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃: xHo³⁺/0.01Yb³⁺ previously presented in Fig. 4(a) above, Fig. 5(b) gives an energy level diagram about the potential ET between Ho³⁺ and Yb³⁺ ions, resulting in upconversion luminescence processes involving possible excitation and emission paths [47], which contributes to the understanding of the upconversion luminescence mechanism. Though ground state absorption (GSA), Yb³⁺ ions are first excited by a 980 nm laser diode from the ²F_7/2 ground state to the ²F_5/2 excited state. Subsequently, an ET occurs from the excited Yb³⁺ to the adjacent Ho³⁺, which is main reason for the population of Ho³⁺ ions in excited levels due to the absorption cross-section of Yb³⁺ acted as efficient sensitizers being larger than its activators Ho³⁺ for 980 nm photons. And then, the upward ⁵I₈→⁵I₆, ⁵I₇→⁵F₅ and ⁵I₆→⁵F₄/⁵S₂ transitions associated to Ho³⁺ ions are promoted. In which, the population on ⁵F₄/ ⁵S₂ level is raised through two successive ETs from Yb³⁺.

And then, the majority of the Ho³⁺ ions in the ⁵F₄/⁵S₂ levels radiatively transiton to the ⁵I₈ ground state energy level, generating a green emission peak with a 547 nm center. Simultaneously, a tiny proportion of Ho³⁺ ions relax to the ⁵I₇ level (⁵F₄/⁵S₂→⁵I₇), creating a 756 nm-centered NIR emission band. In addition, there are two mechanisms that account for the population of Ho³⁺ in ⁵F₅ level. The first one is the non-radiative multi-phonon relaxation (marked as ①) from ⁵F₄/⁵S₂ levels. The second one is the non-radiatively decay of Ho³⁺ ions from their ⁵I₆ excited state (marked as ②) to the lower level ⁵I₇. Then, through excited state absorption (ESA), Ho³⁺ ions in ⁵I₇ absorb the energy from Yb³⁺ or a 980 nm photon and transition to their ⁵F₅ level. The majority of Ho³⁺ ions in the ⁵F₅ level radiatively transition to the ground state ⁵I₈ level, resulting in a red emission. [47,48]

To study the mechanism and process of upconversion luminescence, it is necessary to determine the relationship between the pump power and its corresponding luminous intensity of the luminescent material. The upconversion luminescence spectra of the prepared sample with the best upconversion luminescence performance in the 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃: xHo³⁺/0.01Yb³⁺ system with 980 nm excitation working at different laser pumping powers (310-570 mW) are shown in Fig. 6(a). The intensity of the emissions increases significantly with increasing pump power. In upconversion luminescence mechanism, the following relationship formula can be used to express the intensity I as a function of the pump power P [49]:

 

Fig. 6. a) Pump power dependence of the upconversion luminescence spectra of the 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃: 0.0075Ho³⁺/0.01Yb³⁺ sample. b) Ln(Power) dependent Ln (Intensity) for 547, 656, and 756 nm.

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The upconversion luminescence performances of luminescent materials are usually sensitive to the temperature, for example color rendering and light yield, which make them to be applied to optical thermometry. To study the temperature characteristics of the upconversion luminescence in 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃: 0.0075Ho³⁺/0.01Yb³⁺ ceramics, Fig. 7 displays the measured upconversion luminescence spectra at different temperatures (303-483 K), with excitation at 980 nm. The luminous intensity drops monotonically with rising temperature. Because of the interaction between crystal-field and 4f energy levels of rare earth (RE) ions, the PL performance of RE³⁺-doped phosphors is hypersensitive to the local symmetry of ligand environment where the luminous active ions reside [50]. The maximum temperature is 483 K, which is below the Curie temperature reported [51]. Therefore, fluctuations in luminous intensity caused by structural phase changes are excluded during the test at variable temperatures.

 

Fig. 7. The upconversion luminescence spectra associated to the 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃: 0.075Ho³⁺/0.01Yb³⁺ ceramic at various temperatures. The inset displays the emission intensity normalized to the emission intensity at room temperature T = 303 K.

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The upconversion luminescence spectra consists of three bands, namely an intense green due to the ⁵F₄/⁵S₂→⁵I₈ transition, a red band attributed to ⁵F₅→⁵I₈ and a weaker NIR band assigned to ⁵F₄/⁵S₂→⁵I₇ transition. Their emission intensity at different temperature normalized by the intensity at T = 303 K are shown in the inset of Fig. 7. With increasing temperature (303–483 K), the decline evolutions of normalized intensity associated with these three emission bands are very similar, which is explained to thermal quenching effects. When the ceramic is heated, the lattice vibrations increase, increasing the likelihood of non-radiative multi-phonon relaxation (MPR) processes. Therefore, the upconversion luminescence intensity derived from the radiative transition weakens, and fluorescence quenching phenomenon occurs as temperature rises.

In contrast to red light emission (656 nm), the normalized emission intensity of green light emission (547 nm) and NIR emission (756 nm) decreases rapidly with rising temperature. This result can be explained by the fact that the probability of non-radiative relaxation of Ho³⁺ ions between the ⁵F₄/⁵S₂ and ⁵F₅ levels (①) may increase. This would result in a decrease in the number of electrons on the ⁵F₄/⁵S₂ levels of Ho³⁺ and an increase in the electronic populations on ⁵F₅ level. Therefore, the normalized intensity of red emission with a center at 656 nm declines more gradually with increasing temperature than the normalized intensity of the green emission with a center at 547 nm [52]. I₆₅₆ as well as I₅₄₇ stand for the integral intensities of luminous bands with centers at 656 nm and 547 nm, respectively. As shown in Fig. 8(a), the FIR (I₆₅₆/I₅₄₇) increases as temperature rises. At T = 483 K, the upconversion luminescence intensity of green, red and NIR light is only 11%, 20% and 12% of their initial intensities at room temperature T = 303 K, implying that the present prepared ceramic should be a promising candidate for temperature sensitive devices, i.e., non-contact optical temperature sensing.

 

Fig. 8. a) Temperature dependent I₆₅₆/I₅₄₇ related to Ho³⁺/Yb³⁺ codoped 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃ ceramic; b) The ceramic's relative sensitivity versus absolute temperature.

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Figure 8(a) presents the calculated fluorescence intensity ratios (FIR, i.e., I₆₅₆/I₅₄₇) of red to green emission peaks for 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃: 0.0075Ho³⁺/0.01Yb³⁺ ceramic to investigate the potential uses of as-prepared ceramics in optical temperature sensors. Previous researches have shown that these two emissions of the activators are non-TCLs when Ho³⁺ is in a matrix with low phonon energy [31,53]. As the temperature rises, the FIR value gradually improves. It reaches maximum value when the temperature T = 483 K. According to the experimental data, I₆₅₆/I₅₄₇= 0.00298*T-0.18733 is acquired by fitting, which is similar to the reported 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃: Ho³⁺ [31] and KBaYSi₂O₇: Yb³⁺-Er³⁺/Ho³⁺ phosphors [53].

It is critical to investigate the sensor sensitivity of the sintered ceramics. The calculation equations for sensitivity are as follows. [54,55]

Table 1. Comparative value table for the maximum S_R of different samples doped with Ho³⁺ ions.

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To reveal the precise upconversion luminescence color visually for 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃: 0.0075Ho³⁺/0.01Yb³⁺ ceramic at room temperature and better understanding of the temperature dependence of luminescent crystal colors, Fig. 9 provides the CIE 1931 chromaticity diagram and the calculated coordinates of 0.0075Ho³⁺/0.01Yb³⁺ solid solution based on the upconversion luminescence spectrums at different temperature captured in Fig. 7. The relevant data of chromaticity coordinates (x, y) of CIE are summarized in Table 2, indicating that the ceramic exhibits intense green emission with the color coordinates of (0.3426, 0.6477) at room temperature T = 303 K as observed in Fig. 9. Moreover, the luminous color turns from green to yellow-green when the specimen is heated from 303 to 483 K. These findings are very comparable to those of Ho³⁺-doped NBT ceramics [62], which suggests the upconversion luminescence color has a very significant temperature dependence. Since the human eye is extremely sensitive to green light, the current samples can also be utilized as a safety sign in low temperature conditions in addition to non-contact optical thermometry.

 

Fig. 9. The temperature-dependent chromaticity coordinates of the 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃: 0.0075Ho³⁺/0.01Yb³⁺ specimen in CIE-1931 chromaticity diagram.

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Table 2. The color coordinates and color purity of the 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃: 0.0075Ho³⁺/0.01Yb³⁺ ceramic.

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The color purity of materials plays a dominant role in the applications of emission display. Commonly, it is calculated using the following formula [62]

4. Conclusions

In summary, Ho³⁺/Yb³⁺ codoped 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃ compounds have been fabricated via the traditional solid-state reaction strategy. The XRD patterns validate that in the synthesized 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃: xHo³⁺/0.01Yb³⁺ ceramics, rhombohedral and tetragonal phases with perovskite structure coexist. The impacts of Ho³⁺ content on the performance of upconversion luminescence are thoroughly investigated, and the optimum doping concentration of 0.0075 for this system is demonstrated. The upconversion luminescence spectra exhibit an intense green emission with three typical emission bands upon NIR excitation at 980 nm, corresponding to ⁵F₄/⁵S₂→⁵I₈, ⁵F₅→⁵I₈ and ⁵F₄/⁵S₂→⁵I₇ characteristic transitions of Ho³⁺, respectively. The thermal quenching behavior of 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃: 0.0075Ho³⁺/0.01Yb³⁺ ceramic indicates that the upconversion luminescence intensity is extremely temperature sensitive in the 303-483 K range. The FIR values of I₆₅₆/I₅₄₇ gradually increases with elevating temperature and reaches a maximum at 483 K. At 303 K, the maximum relative sensitivity is around 0.0042 K^-1. The high upconversion luminescence intensity and thermal sensitivity of the current ferroelectric ceramics suggest that they could be as a promising candidate material for operation in non-contact optical temperature thermometry.