Facile modulation the sensitivity of Eu2+/Eu3+-coactivated Li2CaSiO4 phosphors through adjusting spatial mode and doping concentration

Series of Eu2+/Eu3+-coactivated Li2CaSiO4 phosphors were prepared by solid-state reaction technique. All the samples emitted the unique emissions of Eu2+ and Eu3+ ions when excited by 395 nm, while the strongest emission intensity was received when x = 0.03. On the basis of theoretical discussion, it is evident that crossover relaxation should be responsible for the thermal quenching mechanism which was further proved by the unchanged lifetime at elevated temperature. Besides, through analyzing the inconsistent responses of the emission intensities of the Eu2+ and Eu3+ ions to the temperature, the optical thermometric properties of the designed phosphors were studied. By selecting different emissions of Eu3+ ions and combining with that of the Eu2+ ions, adjustable sensitivities were realized in the resultant phosphors. Furthermore, the sensitivities of the studied compound were also found to be greatly affected by the doping concentration. The maximum absolute and relative sensitivities of the synthesized compounds were 0.0025 K−1 and 0.289% K−1, respectively. These achieved results implied that the Eu2+/Eu3+-coactivated Li2CaSiO4 phosphors were promising candidates for optical thermometry. Additionally, this work also provided promising methods to modulate the sensitivities of the luminescent compounds by adjusting spatial mode and doping concentration.

Temperature, acts as a fundamental thermodynamic constant, has drawn considerable interest since it plays a significant role in our daily life, industrial manufacture, medical treatment and scientific research. Thus, its accurate measurement with high resolution is required. Unfortunately, the widely used traditional thermometers (e.g., liquid-in-glass and thermocouple thermometers) suffer from shortages of low accurate, unsatisfied resolution and contact characteristics. To fulfill these drawbacks, contactless optical thermometer, which shows the advantages of high spatial resolution, high accurate, fast response, remote monitoring, etc., had been developed and attracted intensive attention [1][2][3] . Generally, the contactless optical thermometers are realized by employing the fluorescence intensity rate (FIR) technique to study the diverse responses of the emission intensities of the thermally coupled levels (TCLs) to the temperature 4,5 . In order to achieve an ideal optical temperature sensing materials by using the FIR technique, the luminescent materials should exhibit two distinct emission peaks, which must have different changing tendency to the temperature, as the monitoring signals. Currently, the thermometric behaviors of the rare-earth ions, such as Er 3+ , Tm 3+ , Nd 3+ , etc., have been widely studied since they have pairs of TCLs [6][7][8][9] . Note that, the energy separation of the TCLs is narrow (200-2000 cm −1 ) which hinders its maximum relative sensitivity (S r ) value as well as induces the deviation of the experimental FIR value from the real value, resulting in large error. To overcome these kinds of intrinsic imperfections, novel optical thermometry based on dual-emitting centers was proposed. Up to date, some results have been reported in the dual-emitting centers based optical thermometers, such as Eu 3+ /Tb 3+ , Eu 3+ /Mn 2+ , Ce 3+ /Tb 3+ , Eu 2+ /Eu 3+ and Bi 3+ /Eu 3+ coactivated luminescent materials [10][11][12][13][14] . Notably, for practical applications, the developed optical thermometers should exhibit large sensitivities, whereas the currently reported sensitivities of the dual-emitting centers based optical thermometers were still not high enough. Thus, some available routes should be carried out to further improve the thermometric performance of the dual-emitting centers based optical thermometers.

Results and discussion
The X-ray diffraction (XRD) profiles of the Li 2 CaSiO 4 :xEu 2+ /Eu 3+ phosphors were monitored so as to identify their phase components. As demonstrated in Fig. S1, when the dopant content was less than 2 mol%, the recorded diffraction peaks of the designed phosphors were the same as those of the standard tetragonal Li 2 CaSiO 4 (JCPDS#27-0290), suggesting that the prepared phosphors had tetragonal phase and the Ca 2+ ions can be replaced by the dopants. Nevertheless, several tinny impurity peaks originating from Eu 3 SiO 7 (JCPDS#20-0404) occurred when the doping concentration was further increased (see Fig. S1). These results suggested that the dopants (i.e., Eu 2+ and Eu 3+ ) had a limited solid solubility in the Li 2 CaSiO 4 host lattices.
For the sake of disclosing the morphological information of synthesized samples, the FE-SEM images of the representative Li 2 CaSiO 4 :0.005Eu 2+ /Eu 3+ and Li 2 CaSiO 4 :0.03Eu 2+ /Eu 3+ phosphors were detected, as shown in Fig. 1a,b, respectively. It can be seen that the studied compounds were made up of anomalous particles and their sizes were in micron level. Notably, although the doping content was changed, the morphology (i.e., size and shape) of the resultant products changed little, revealing that the introduction of the Eu 2+ /Eu 3+ ions exhibited scarcely impact on the microstructure of the studied samples. Furthermore, according to the EDX spectrum (see Fig. 1c), one knows that that the synthesized samples contained the elements of Ca, Si, O and Eu. With aid of the EDX technique, we can not detect Li since it belongs to the light element with small Kα energy. Additionally, the observation of the C peak in the EDX spectrum was assigned to the conductive tape, while the detection of the Pt peak in the EDX spectrum was attributed to the platinum electrode that was used for FE-SEM operation. Ultimately, it was also found that these detected elements (i.e., Ca, Si, O, Eu) were equally distributed throughout the particles, as presented in Fig. 1d-g.  www.nature.com/scientificreports/ As discussed above, the components of the studied samples can not be successfully confirmed by utilizing the EDX technique due to its limitation. For the purpose of getting deeper insight into the compositions and elemental states of the studied samples, the X-ray photoelectron spectroscopy (XPS) measurement was performed and the typical results of the Li 2 CaSiO 4 :0.03Eu 2+ /Eu 3+ phosphors are depicted in Fig. S2. As described in Fig. S2a, only one peak centered at 57.8 eV, which was attributed to the Li + 1 s, was observed in the high-resolution XPS spectrum 22 . The XPS spectrum presented in Fig. S2b was dominated by an intense peak with the binding energy of 348.7 eV which was ascribed to the Ca 2+ 2p 3/2 22 . The existence of the Si 4+ 2p 3/2 in the resultant compounds was confirmed by the binding energy at approximately 104.8 eV, as displayed in Fig. S2c 23 . Furthermore, the peak with the binding energy of around 532.4 eV was assigned to the O 2− 1 s (see Fig. S2d) 5 . In addition, the XPS spectrum consisted of two bands with the binding energies of 1134.5 and 1164.7 eV which were associated with the Eu 3+ 3d 5/2 and Eu 2+ 3d 5/2 , respectively, as shown in Fig. S2e 24 . The XPS results did not only prove that the synthesized samples were composed of Li, Ca, Si, O and Eu elements, but also revealed that the Eu 3+ ions were partly transferred into Eu 2+ ions.
The diffuse reflectance spectrum of the Li 2 CaSiO 4 :0.03Eu 2+ /Eu 3+ phosphors was measured to examine the absorption ability of the studied samples. As demonstrated in Fig. S3a, both the broad absorption band originating from the Eu 2+ ions and a sharp peaks centered at 395 nm ( 7 F 0 → 5 L 6 ) arising from the Eu 3+ ions were observed in the diffuse reflectance spectrum 21,25 , which further verified the coexistence of Eu 3+ and Eu 2+ ions in the resultant products. Furthermore, it has been confirmed that the relation between the absorption constant (i.e., α) and energy band gap (i.e., E g ) keeps to the following function 26,27 : where hv refers to the energy, A is related to the coefficient, while the value of n can be 1/2, 2, 3/2 and 3 which corresponds to direct, indirect, forbidden direct, and forbidden indirect electronic transitions, respectively. Furthermore, the absorption spectrum (i.e., F(R)) is able to be achieved from the diffuse reflectance spectrum with the aid of the Kubelka-Munk expression, as defined below 28 : here R is assigned to the reflectivity of the compounds. Through combining Eqs. (1) and (2), the following function is obtained: For Li 2 CaSiO 4 , the n value is 1/2 29 . Thus, the E g value of the Li 2 CaSiO 4 :0.03Eu 2+ /Eu 3+ phosphors was demonstrated to be 4.54 eV through extrapolating the linear fitted region to [hvF(R)] 2 = 0, as displayed in Fig. S3b.
The optical performance of the resultant phosphors was investigated by measuring their emission and excitation spectra. Figure 2a depicts the excitation spectra of the Li 2 CaSiO 4 :0.03Eu 2+ /Eu 3+ phosphors monitored at 480 and 702 nm. As displayed, when the monitoring wavelength was 480 nm, the excitation spectrum consisted of two broad bands arising from the 4f.-5d transition of Eu 2+ ions 30,31 . In comparison, when the monitoring wavelength was switched to 702 nm, only several sharp peaks were seen in the excitation spectrum and the broad intense bands vanished. Specially, these narrow peaks located at 319, 362, 376, 384, 395 and 416 nm pertained to the intra-4f transitions of Eu 3+ ions from the 7 F 0 level to 5 H 6 , 5 D 4 , 5 G 2 , 5 G 3 , 5 L 6 and 5 D 3 levels, respectively 32,33 . Note that, these two excitation spectra exhibited an overlap at the wavelength of 395 nm (see Fig. 2a). Thus, to allow the studied samples present superior optical performance, we selected it as the excitation wavelength. The emission spectrum of the Li 2 CaSiO 4 :0.03Eu 2+ /Eu 3+ phosphors excited by 395 nm is illustrated in Fig. 2b. Evidently, the emission profile was composed of an intense broad band and four sharp peaks. Among them, the intense broad band centered at 480 nm was attributed to the featured emissions of Eu 2+ ions, whereas these narrow peaks located at 594 ( 5 D 0 → 7 F 1 ), 617 ( 5 D 0 → 7 F 2 ), 654 ( 5 D 0 → 7 F 3 ) and 702 ( 5 D 0 → 7 F 4 ) nm all pertained to the characteristic emissions of Eu 3+ ions 33,34 . The simultaneous observation of the featured emissions of Eu 3+ and Eu 2+ ions in the prepared samples further uncovered that the Eu 3+ ions were partially transferred to the Eu 2+ ions. The energy level diagram of Eu 3+ and Eu 2+ ions was constructed and shown in Fig. 2c so as to describe the near-ultraviolet light triggered the visible emission mechanism in the Li 2 CaSiO 4 :xEu 2+ /Eu 3+ system.
It has been verified that the luminescent properties of the rare-earth ions based materials were sensitivity to the doping content. For the aim of digging out the optimal doping concentration, the concentration dependent photoluminescent properties of the Li 2 CaSiO 4 :xEu 2+ /Eu 3+ phosphors excited by 395 nm was studied and the corresponding results are presented in Fig. 2d. It is shown in Fig. 2d that the emission profiles did not vary with the addition of Eu 2+ /Eu 3+ ions, whereas the emission intensities of Eu 2+ and Eu 3+ ions were determined to be relied on the dopant content. In particular, the emission intensities of Eu 2+ and Eu 3+ ions were all elevated with the increment of the doping concentration and their maximum values were gained when x = 0.03. However, the quenched emission intensity, which was caused by the concentration quenching effect, arose when the dopant content was over 3 mol%, as described in Fig. 2f. The colorific behaviors of the resultant phosphors were investigated and their CIE coordinates, which were estimated from the emission spectra, are presented in Fig. 2g and Table S1. Significantly, with changing the doping content in the range of 0.5-4 mol%, we found that the emitting color of the resultant compounds was varied and their CIE coordinates were changed from (0.394,0.326) to (0.293,0.274) (see Fig. 2g and Table S1). It is shown in Fig. S4 that FIR values of the Eu 2+ to Eu 3+ ions in the designed samples varied from each other and it can be responsible for the obtained multicolor emissions.
The room temperature decay curves of the Li 2 CaSiO 4 :xEu 2+ /Eu 3+ phosphors with the doping concentration of 3 mol% excited at 395 nm and monitored at different wavelengths of 480 and 702 nm were tested, as illustrated in Fig. S5a,b, respectively. As presented in Fig. S5a, the decay curve of Eu 2+ ions (λ ex = 395 nm, λ em = 480 nm) www.nature.com/scientificreports/ was able to be fitted through utilizing a second-order exponential decay mode, which may be assigned to the nonradiative energy transfer process involving Eu 2+ ions and defects 21 , as described below: In this equation, I 0 and I(t) denote the emission intensities at time t = 0 and t, respectively, A i (i = 1, 2) is constant, τ 1 and τ 2 are attributed to the decay time for exponential components, respectively. Besides, the following function can be applied to estimate the average lifetime (i.e., τ avg ): Consequently, the lifetime of the Eu 2+ ions was found to be around 11.46 μs. In comparison, the decay curve of the Eu 3+ ions (λ ex = 395 nm, λ em = 702 nm) can be fitted by employing a single exponential decay mode (see Fig. S5b), as demonstrated below: where I 0 and I(t) denote the emission intensities at time t = 0 and t, respectively, A refers to the coefficient, τ is related to the decay time. Obviously, the decay time of the Eu 3+ ions in the studied samples was 1898.68 μs. For purpose of exploring the feasibility of the Eu 2+ /Eu 3+ -coactivated Li 2 CaSiO 4 phosphors for contactless optical thermometry, their temperature-dependent emission spectra in the range of 303-583 K were examined. Figure 3a illustrates the temperature-dependent emission spectra of the Li 2 CaSiO 4 :0.03Eu 2+ /Eu 3+ phosphors excited by 395 nm. It is significant that the emission profiles were not affected by the temperature, while the emission intensities of the Eu 2+ and Eu 3+ ions were dependent on temperature. Moreover, the temperaturedependent color coordinates of the studied samples, which were evaluated from the measured emission spectra, are listed in Table S2. Obviously, the CIE coordinates were sensitive to the temperature and their values were shifted from (0.293, 0.274) to (0.276, 0.260) when the surrounding temperature was changed in the range of 303-583 K, resulting in the tunable emissions at high temperature (see Fig. 3b). Furthermore, the temperaturedependent integrated emission intensities of Eu 2+ ions, 594 nm ( 5 D 0 → 7 F 1 ) transition, 617 nm ( 5 D 0 → 7 F 2 ), 654 nm ( 5 D 0 → 7 F 4 ) and total 5 D 0 → 7 F J (J = 1, 2 and 4) transitions of Eu 3+ ions were investigated and demonstrated in Fig. 3c-g, respectively. As disclosed, although all the emission intensities decreased, which was induced by the thermal quenching effect, with rising the temperature in the range of 303-583 K, the emission intensities of Eu 2+ and Eu 3+ ions exhibited diverse decline rates. Consequently, the accurate optical temperature measurement is able to be realized through analyzing the temperature-dependent FIR value of the emission intensities between the Eu 2+ and Eu 3+ ions.
As pointed out above, the thermal quenching effect occurred at elevated temperature, resulting in the declined emission intensities of Eu 2+ and Eu 3+ ions. In order to explain thermal quenching mechanism as well as clarify (4) I(t) = I 0 + A 1 exp(−t/τ 1 ) + A 2 exp(−t/τ 2 ),   Fig. 4a. As for the Eu 2+ ions, the crossover relaxation between the excited level of 5d and ground state of 4f can contribute to the quenched emission intensity at elevated temperature. Specially, with rising the temperature, electrons populated at the bottom of the excited level would shift to the intersection between the parabolas of the 4f ground state and the 5d excited level (see Fig. 4a). After that, these populated electrons will nonradiatively decay to the ground state, leading to the declined emission intensity at high temperature. On the other hand, unlike the Eu 2+ ions, the Eu 3+ ions show totally different thermal quenching mode since its excited levels and ground states do not have any crossover points 20 . As presented, electrons can be pumped from the ground state to the 5 L 6 excited level excited at 395 nm, and then the nonradiative transition occurs, leading to the formation of 5 D 0 level. Subsequently, the emissions originating from the 5 D 0 to 7 F J (J = 1, 2, 3, 4) levels appeared. Note that, the charge transfer (CT) band of O 2− → Eu 3+ is located at a relatively low energy which makes it possible to supply a pathway for the electrons located at the 5 D 0 level to nonradiatively return to the ground states 20,35 . Therefore, the thermal quenching mode of Eu 3+ ions is the thermal activation of the electrons from the 5 D 0 excited level to the CT band of O 2− → Eu 3+ , as demonstrated in Fig. 4a. Finally, these generated electrons will return to the ground state through a nonradiative pathway and the quenched emission intensities are observed at elevated temperature. In order to confirm the aforementioned guess, the temperature-dependent decay curves of the Eu 2+ and Eu 3+ ions were examined, as shown in Fig. 4b,c, respectively. As is known to all, the thermal quenching effect can be realized by three diverse pathways of energy transfer from the luminescent states, cascade multiphonon relaxation and crossover process at high temperature 36,37 . Notably, these three different channels are able to be discriminated by utilizing the temperature-dependent lifetime. It is shown in Fig. 4b,c that the decay carves of the Eu 2+ and Eu 3+ ions hardly changed with the increment of temperature, indicating that the thermal quenching effect can not be contributed by the cascade multiphonon relaxation. Additionally, we also found that the both the decay time of the Eu 2+ and Eu 3+ ions were insensitive to the temperature (see Fig. 4b,c), implying that the energy transfer from the luminescent levels was not the potential route for the thermal quenching effect. As a consequence, it is reasonable www.nature.com/scientificreports/ for us to conclude that the thermal quenching mechanisms of the Eu 2+ and Eu 3+ ions in the Li 2 CaSiO 4 host were all dominated by the crossover relation process. Based on the recorded emission spectra shown in Fig. 3a, the temperature-dependent FIR values of Eu 2+ ions to 5 D 0 → 7 F 1 transition (i.e., Eu 2+ / 7 F 1 ), Eu 2+ ions to 5 D 0 → 7 F 2 transition (i.e., Eu 2+ / 7 F 2 ), Eu 2+ ions to 5 D 0 → 7 F 4 transition (i.e., Eu 2+ / 7 F 4 ) and Eu 2+ ions to 5 D 0 → 7 F J (J = 1, 2, 4) transition (i.e., total Eu 2+ / 7 F J ) were evaluated and the corresponding results are depicted in Fig. 5a-d, respectively. Significantly, with elevating the temperature in the range of 303-583 K, all of the calculated FIR values increased gradually. Nevertheless, these calculated FIR values based on the different emission combinations exhibited various values which indicated that the sensitivities of the studied samples may be tuned through choosing diverse emission combination. From previously reports, it is clear that the relation between the FIR value of the non-thermally coupled levels and temperature satisfies the following function 38,39 : In this expression, the parameters of A, B and C are all constants. With the help of Eq. (7), these obtained temperature-dependent FIR values were fitted well, as shown in Fig. 5a-d Fig. 5a-d, the S a and S r values of the Li 2 CaSiO 4 :0.03Eu 2+ /Eu 3+ phosphors as a function of temperature were achieved, as demonstrated in Fig. 5e,f, respectively. As disclosed in Fig. 5e, the S a values showed an upward tendency with the temperature, reaching their maximum values when the temperature was 583 K. Specially, for the combinations of the Eu 2+ / 7 F 1 , Eu 2+ / 7 F 2 , Eu 2+ / 7 F 4 and total Eu 2+ / 7 F J , their maximum S a values were around 0.0020 K −1 , 0.0009 K −1 , 0.0008 K −1 and 0.0003 K −1 , respectively. Furthermore, it is shown in Fig. 5f that the S r values firstly increased with the temperature, achieving their maximum values, and then they started to decease with further increasing the temperature. The maximum S r values of the Eu 2+ / 7 F 1 , Eu 2+ / 7 F 2 , Eu 2+ / 7 F 4 and Eu 2+ / 7 F J combinations were 0.110% K −1 , 0.098% K −1 , 0.113% K −1 and 0.105% K −1 , respectively. Evidently, the S a and S r values of the studied samples can be facilely modulated through utilizing diverse emission combinations (i.e., spatial mode). The reversibility and thermal stability of the studied samples were investigated through analyzing the temperature-caused switching of FIR values in the range of 303-583 K. Figure S6 shows the temperature-caused switching of FIR values of the Li 2 CaSiO 4 :0.03Eu 2+ /Eu 3+ phosphors. It is clear that the FIR values were reversible and repeatable even after six heating-cooling processes. Note that, the phase structure of the resultant compounds also did not change after six heating-cooling processes, as shown in Fig. S7. These results implied that the resultant compounds had good reversibility and stability. As disclosed in previous reports, the temperature uncertainty of the optical temperature sensor based on the luminescent compounds is able to be determined by means of following functions 42-44 : where I 1 refers to the emission intensity of Eu 2+ ions, I 2 stands for the emission intensities of the Eu 3+ ions originating from the 5 D 0 → 7 F J transitions, δI 1 and δI 2 are ascribed to the errors of I 1 and I 2 , respectively, and δT is the temperature uncertainty. Via these above expressions, the δT values were estimated to be 0.139-0.248 K (303-583 K), 0.146-0.352 K (303-583 K), 0.116-0.433 K (303-583 K), 0.102-0.383 K (303-583 K), respectively, when the combinations of Eu 2+ / 7 F 1 , Eu 2+ / 7 F 2 , Eu 2+ / 7 F 4 and Eu 2+ / 7 F J were employed. From the room temperature emission spectra (Fig. 2d), one knows that the relative emission intensities of the Eu 2+ and Eu 3+ ions were impacted by the doping content. Besides, the room temperature FIR values of Eu 2+ to Eu 3+ ions were dependent on the doping concentration (Fig. S4), suggesting that the sensitivities of the studied samples may be affected by the dopant content. For the sake of verifying this speculation, the thermometric properties of the Li 2 CaSiO 4 :xEu 2+ /Eu 3+ phosphors with different doping contents of 0.5 and 4 mol% were explored. Upon the irradiation of 395 nm, the emission spectra of the Li 2 CaSiO 4 :0.005Eu 2+ /Eu 3+ and Li 2 CaSiO 4 :0.04Eu 2+ / Eu 3+ phosphors as a function of temperature in the range of 303-583 K were measured, as illustrated in Fig. S8a,b, respectively. It can be seen that the emission profiles of these two compounds varied little with boosting the temperature, whereas the emission intensities of the Eu 2+ and Eu 3+ ions were affected by the temperature. Similar as those of in the resultant phosphors with optimum doping content, the emission intensities of the Eu 2+ ions, 594 nm ( 5 D 0 → 7 F 1 ) transition, 617 nm ( 5 D 0 → 7 F 2 ), 654 nm ( 5 D 0 → 7 F 4 ) and total 5 D 0 → 7 F J (J = 1, 2 and 4) transitions of Eu 3+ ions in the Li 2 CaSiO 4 :0.005Eu 2+ /Eu 3+ and Li 2 CaSiO 4 :0.04Eu 2+ /Eu 3+ phosphors all decreased monotonously with the temperature (see Fig. S9). Nevertheless, these emissions exhibited various decreasing rates which make them were suitable for contactless optical thermometry. Figure S10 presents the FIR values of these two compounds as a function of temperature. Clearly, the whole FIR values increased gradually with the temperature and they can be perfectly fitted by Eq. (7), as shown in Fig. S8. Based on the fitted results as well as Eqs. (8) and (9), the temperature-dependent S a and S r values of the Li 2 CaSiO 4 :0.005Eu 2+ /Eu 3+ and Li 2 CaSiO 4 :0.04Eu 2+ / Eu 3+ phosphors were calculated and their corresponding results are presented in Fig. 6. As uncovered, both the S a and S r values of these two compounds were sensitive to the emission combinations of Eu 2+ /Eu 3+ ions which further confirmed that the temperature sensing ability of the synthesized phosphors can be manipulated through adjusting the spatial mode. As shown in Fig. 6a,b, the maximum S a values of the Li 2 CaSiO 4 :0.005Eu 2+ /Eu 3+ and Li 2 CaSiO 4 :0.04Eu 2+ /Eu 3+ phosphors were revealed to be about 0.0018 K −1 and 0.0025 K −1 , respectively, at 303 K when the Eu 2+ / 7 F 1 combination was used. Furthermore, the maximum S r value of the Li 2 CaSiO 4 :0.005Eu 2+ /Eu 3+ phosphors was 0.289% K −1 when the combination of total Eu 2+ / 7 F J was employed (see Fig. 6c), whereas that of the Li 2 CaSiO 4 :0.04Eu 2+ /Eu 3+ phosphors reached up to 0.219% K −1 when the Eu 2+ / 7 F J mode was adopted (see Fig. 6d). According to these aforementioned results, it is obvious that the S a and S r values of the prepared phosphors were totally different which were greatly impacted by the doping concentration (see Table 1), implying that the optical thermometric performance of the Eu 2+ /Eu 3+ -coactivated Li 2 CaSiO 4 phosphors was able to modified by adjusting the doping content aside from selecting different spatial mode. Additionally, compared previously  with were purchased and used as the raw materials to synthesize these above compounds. On the basis of the designed stoichiometric proportion, the starting materials including Li 2 CO 3 , CaCO 3 , SiO 2 and Eu 2 O 3 were firstly weighted, and then thoroughly mixed by an agate mortar. Subsequently, these powders were kept in crucibles and heat at 900 °C for 5 h with the heating rate of 3 °C/min. After cooling down to the room temperature, the white powders were collected and ground. For the sake of making part of Eu 3+ ions change to Eu 2+ ions, the obtained white powders were heat at 600 °C for 20 min under a reducing atmosphere (N 2 :H 2 = 95%:5%). Finally, the Li 2 CaSiO 4 :xEu 2+ /Eu 3+ powders were achieved and they can be used for further characterization.
Materials characterization. The phase component, elemental compositions and morphological information of the final compounds was examined by means of an X-ray diffractometer with Cu Kα radiation (Bruker D8 Advance), PHI 5000 VersaProbe spectrometer and field-emission scanning electron microscope (FE-SEM; HITACHI SU3500) equipped with an energy dispersive X-ray (EDX) spectroscopy. The diffuse reflectance spectrum of the studied samples was recorded by utilizing the Cary 5000 UV-Vis spectrophotometer. The Edinburgh FS5 spectrofluorometer was adopted to detect the emission and excitation spectra of the prepared samples. The decay curves of the resultant phosphors were measured by utilizing a FLS920 fluorescence spectrophotometer. Through contacting a temperature control system (Linkam HFS600E-PB2), the emission spectra of the studied samples as a function of temperature were monitored by the Edinburgh FS5 spectrofluorometer.