Site Occupancies, Luminescence, and Thermometric Properties of

Oct 4, 2016 - In this work, we report the tunable emission properties of Ce3+ in an apatite-type LiY9(SiO4)6O2 compound via adjusting the doping conce...
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Site Occupancies, Luminescence, and Thermometric Properties of LiY9(SiO4)6O2:Ce3+ Phosphors Weijie Zhou,† Fengjuan Pan,† Lei Zhou,† Dejian Hou,† Yan Huang,‡ Ye Tao,‡ and Hongbin Liang*,† †

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of Environment and Energy Chemistry, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, People’s Republic of China ‡ Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, People’s Republic of China S Supporting Information *

ABSTRACT: In this work, we report the tunable emission properties of Ce3+ in an apatite-type LiY9(SiO4)6O2 compound via adjusting the doping concentration or temperature. The occupancies of Ce3+ ions at two different sites (Wyckoff 6h and 4f sites) in LiY9(SiO4)6O2 have been determined by Rietveld refinements. Two kinds of Ce3+ f−d transitions have been studied in detail and then assigned to certain sites. The effects of temperature and doping concentration on Ce3+ luminescence properties have been systematically investigated. It is found that the Ce3+ ions prefer occupying Wyckoff 6h sites and the energy transfer between Ce3+ at two sites becomes more efficient with an increase in doping concentration. In addition, the charge-transfer vibronic exciton (CTVE) induced by the existence of free oxygen ion plays an important role in the thermal quenching of Ce3+ at 6h sites. Because of the tunable emissions from cyan to blue with increasing temperature, the phosphors LiY9(SiO4)6O2:Ce3+ are endowed with possible thermometric applications. P, V, Si, ...; B = F, Cl, Br, OH, ...), which has two types of Y3+ sites (Wyckoff 4f and 6h).20 To the best of our knowledge, the luminescence of lanthanide ions in LiY9(SiO4)6O2 has not been reported in detail so far. Due to possible different emission wavelengths of Ce3+ at these two sites, and the collaborative influences of site occupancies, energy transfer, and thermal quenching of Ce3+ at two sites, the tunable emission is expected to be achieved by either adjusting the doping concentrations of Ce3+ or varying the temperature of materials. In this paper, the site occupancies of Ce3+ ions in LiY9(SiO4)6O2 have been determined on the basis of Rietveld refinements. The influences of temperature and doping concentrations on the luminescence of Ce3+ at two sites are systematically discussed. Since the tunable emission can be realized through changing the temperature, the thermometric properties have been demonstrated by using the phosphor LiY8.50Ce0.50(SiO4)6O2.

1. INTRODUCTION Ce3+-activated inorganic luminescent materials have attracted considerable attention due to their various applications. The fast decay of parity-allowed 4f−5d emission makes Ce3+ a suitable activator in scintillators for detecting techniques.1,2 Meanwhile, the tunable f−d emissions of Ce3+ in different host compounds have gained applications in phosphors for lighting and displays.3−8 So far, several methods have been reported to tune the emissions of phosphors incorporating Ce3+ ions. Due to the large influence of coordination environments on the outer 5d state energies, different emissions of Ce3+ can be achieved through appropriate selection of suitable host compounds with ideal coordination environments of Ce3+ or gradual change of Ce3+ coordination environments via chemical unit substitution.9−11 In addition, the tunable emissions have been realized through energy transfer between Ce3+ sensitizer and other activators such as Eu2+, Tb3+, Sm3+, and so on in a specific host compound.12−15 On some occasions, the tunable emissions can also be fulfilled by a convenient way: that is, singly doping Ce3+ in a specific host compound which contains multication sites that Ce3+ can enter.16 Compounds with an apatite structure (space group P63/m) have been extensively studied as effective hosts for luminescence materials due to their excellent chemical stability and rigid crystal structure.17−19 LiY9(SiO4)6O2 belongs to the apatite compounds M10(AO4)6B2 (M = Ca, Sr, Ba, La, Y, ...; A = © XXXX American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials and Synthesis. A series of polycrystalline samples LiY9−xCex(SiO4)6O2 (x = 0−3) was prepared by a traditional hightemperature solid-state reaction method using Li2CO3 (analytical reagent, AR), SiO2 (AR), Y2O3 (99.99%) and CeO2 (99.99%) as raw Received: July 11, 2016

A

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are in Wyckoff 6h sites and are denoted as Y(1)3+, while the other three Y3+ ions are in Wyckoff 4f sites and are marked as Y(2)3+. Accordingly, the chemical formula of this compound can be written as [Y(1)6]6h[Li,Y(2)3]4f(SiO4)6O(1)2. As shown in Figure S1 in the Supporting Information, all polyhedra around Y3+ ions share their edges or corners with the [SiO4]4− tetrahedra, forming a structural framework.17 Table 1 gives the

materials. The powder reactants were weighed stoichiometrically and ground thoroughly in an agate mortar for 30 min. Then the mixtures were preheated at 973 K for 3 h in an air atmosphere. After the samples were cooled to room temperature, they were reground and sintered at 1523 K for 6 h under a thermal carbon reducing atmosphere to give the final products. 2,2. Characterization Method. The phase purity of all samples was characterized by powder X-ray diffraction (PXRD) using a Bruker D8 Advance powder diffractometer with Cu Kα radiation (λ = 1.5418 Å) at 40 kV and 26 mA. High-quality XRD data for Rietveld refinements were collected over a 2θ range from 10 to 110° with an interval of 0.02°, and the refinements were performed using TOPAS Academic software.21 The structural diagrams were drawn with VESTA software.22 The luminescence spectra in the UV−vis range and luminescence decay curves at different temperatures were recorded on an Edinburgh FLS 920 combined fluorescence lifetime and steady-state spectrometer. A 450 W xenon lamp was used as the excitation source for the steady-state UV−vis spectra, while an nF900 ns flash lamp (150 W, with a pulse width of 1 ns and pulse repetition rate of 40−100 kHz) was used as the excitation source for the luminescence decay curves. The sample temperature was varied by a temperature controller (Oxford, CRYTEMP), which was provided with a closed cycle liquid helium apparatus to cool the sample that was built in-house. The VUV−UV excitation spectra and corresponding emission spectra were recorded on the beamline 4B8 of the Beijing Synchrotron Radiation Facility (BSRF) by a FASCO Radmin remote access desktop system. The synchrotron radiation VUV−UV excitation light was dispersed through a 1 m Seya monochromator (1200 grooves/mm, 120−350 nm, 1 nm bandwidth), and the change in the incident flux was corrected using sodium salicylate (oC6H4OHCOONa) as a standard. The emitted light was spectrally filtered by an Acton SP-308 monochromator (600 grooves/mm, 330− 900 nm). The signal was collected with a Hamamatsu H8259-01 photon counting unit. The vacuum degree of the sample chamber was about 1 × 10−5 mbar. More details about the optical layout and the measurements can be found elsewhere.23,24

Table 1. Structure Data of two Y3+ Sites in LiY9(SiO4)6O2 site occupying ions polyhedral shape point symmetry coordination no. av Y−O bond length (Å) a

6h 100% Y(1)3+ distorted pentagonal bipyramid Cs 7a 2.396(4)

4f 75% Y(2)3+ + 25% Li(1)+ highly distorted tricapped trigonal prism C3 9 2.481(4)

There is a free oxygen ion O(1).

detailed structure data of two Y3+ sites. It is interesting to mention that in the Y(1)O7 polyhedron of the 6h site there is the so-called free oxygen ion O(1)2− which fails to participate in the formation of an [SiO4]4− tetrahedron. The bond length (2.193 Å) between this free oxygen ion O(1)2− and Y(1)3+ is relatively shorter than those of other Y(1)−O bonds, which indicates more covalency of this Y(1)−O(1)(free) bond.25,26 This nature may have a significant influence on the luminescence of Ce3+ at the Y(1)3+ sites, as discussed later. The phase purities of Ce3+-doped samples have been confirmed by XRD measurements, as displayed in Figure 2a,

3. RESULTS AND DISCUSSION 3.1. Crystal Structure Refinements and Site Occupancies of Ce3+ Ions. Figure 1 presents the Rietveld refinement of

Figure 2. (a) XRD patterns of samples LiY9−xCex(SiO4)6O2 (x = 0.005−3.00) at room temperature. (b) Enlargements of XRD patterns over a range of 31.45−33.45°. Figure 1. Rietveld refinement of laboratory XRD data of compound LiY9(SiO4)6O2 at room temperature. The calculated line, Bragg positions, and difference between experimental data and the calculated curve are shown.

in which the refined pattern of the compound LiY9(SiO4)6O2 is presented at the bottom as a reference. All XRD patterns are similar, and no impurity diffraction peaks are found, which indicates that all samples are of a single LiY9(SiO4)6O2 phase. Of more interest is that with an increase in Ce3+ ions the diffraction peaks gradually shift toward the low-angle side as shown in Figure 2b. This phenomenon implies that larger Ce3+ ions are effectively incorporated into the Y3+ sites and the unit cell parameters of samples become larger with increasing doping contents, which can be further confirmed by the Rietveld refinements of the corresponding powder XRD data, as mentioned below. The occupancies of Ce3+ ions in different crystallographic sites have crucial influences on the lumines-

laboratory XRD data of the synthesized compound LiY 9 (SiO4 ) 6 O 2 at room temperature using the P6 3/m (hexagonal) structure as an initial model.20 The obtained reliability factors Rwp, Rp, and Rb demonstrate a good fitting quality. All observed peaks satisfy the reflection conditions, confirming the formation of a single phase without impurities. The refined structure parameters of compound LiY9(SiO4)6O2 are given in Table S1 in the Supporting Information. There are two kinds of Y3+ sites in LiY9(SiO4)6O2; six out of nine Y3+ ions B

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Table 2. Chemical Formulas of LiY9−xCex(SiO4)6O2 (x = 1−3) and Percentages of Ce3+ Ions in 6h and 4f Sites to the Total Ce3+ Doping Concentration x

chem formula

6h (%)

4f (%)

0 1 2 3

[Y(1)6]6h[Li,Y(2)3]4f(SiO4)6O(1)2 [Y(1)5.225Ce(1)0.775]6h[Li,Y(2)2.775Ce(2)0.225]4f(SiO4)6O2 [Y(1)4.397Ce(1)1.603]6h[Li,Y(2)2.603Ce(2)0.397]4f(SiO4)6O2 [Y(1)3.521Ce(1)2.479]6h[Li,Y(2)2.479Ce(2)0.521]4f(SiO4)6O2

77.5 80.1 82.6

22.5 19.9 17.4

Figure 3. (a) Height-normalized 3D emission spectra of the sample LiY8.995Ce0.005(SiO4)6O2 at different wavelength excitations at 4.5 K and their contour projection. (b) Height-normalized emission (λex = 280, 300, 340 nm) spectra. (c) Height-normalized emission (λex = 340 nm) spectra and fitting results via a sum of two Gaussian components. (d) Height-normalized emission (λex = 280 nm) spectra and fitting results via a sum of four Gaussian components.

cence properties of phosphors.27,28 According to previous reports, for apatite-type hosts the Wyckoff 6h sites with free oxygen ions are favorable for highly charged cations.29,30 Therefore, it is expected that Ce3+ ions preferentially occupy 6h sites with an increase in doping concentration. However, preferential occupancies of Eu3+ ions in 4f sites are also reported in the Sr2Y6(SiO4)6O2 host due to the occurrences of larger Sr2+ ions in 4f sites.31 To examine the occupancy tendency of Ce3+ in two possible (4f and 6h) sites in our case, the Rietveld refinements of three high-doping samples LiY9−xCex(SiO4)6O2 (x = 1−3) are conducted. The final refined results are given in Table S2 and Figure S2 in the Supporting Information. They reveal that Ce3+ ions can be incorporated into both Wyckoff 6h and 4f sites, and the occupancies of Ce3+ ions in both sites increase with an increase in Ce3+ concentration. According to the occupancies of Ce3+ ions in 4f and 6h sites, the chemical formulas of LiY9−xCex(SiO4)6O2 (x = 1−3) are given in Table 2 to clearly show the amounts of two different Ce3+ ions. The percentages of Ce3+ ions in 6h and 4f sites to the total Ce3+ doping concentration are also estimated to investigate the preferential occupancy properties of Ce3+ ions. These results directly demonstrate that the percentage of Ce3+ ions in 6h sites increases but that in 4f sites decreases with an increase in doping concentration, implying preferential occupancies of Ce3+ ions in 6h sites at high-doping levels. Since Y(1)3+ ions occupy 6h sites and Li+/Y(2)3+ ions occupy 4f sites, the Ce3+

ions in 6h and 4f sites are correspondingly denoted as Ce(1)3+ and Ce(2)3+ in Table 2 and the following sections, respectively. For samples at low-doping levels, the weak influence of Ce3+ occupancies at different sites on laboratory XRD data is hard to detect; thus, the Ce3+ occupancy information cannot be evaluated via Rietveld refinement. In light of the data in Table 2, it is expected that the occupancies of Ce3+ ions also obey this tendency at low-doping levels. 3.2. Luminescence of Ce3+ in LiY9(SiO4)6O2. To understand the influence of coordination environments on the luminescence of Ce3+ in two distinct sites, the emission spectra at different wavelength excitations and the excitation spectra at different wavelength emissions of the sample LiY8.995Ce0.005(SiO4)6O2 were recorded at 4.5 and 16 K, respectively. Figure 3a shows the height-normalized 3D emission spectra under different wavelength excitations at 4.5 K and their contour projection. With an increase in excitation wavelength, the corresponding emission spectra show different shapes and their maximum positions gradually shift toward longer wavelengths. The typical emission spectra under 280, 300, and 340 nm excitations are presented in Figure 3b, implying that two types of Ce3+ luminescence surely exist. Accordingly, we denote the Ce3+ ions with longer emission wavelength as Ce(L)3+ centers and the higher energy ions as Ce(H)3+ centers. A sum of two Gaussian functions was used to fit the Ce(L)3+ emission spectrum (λex 340 nm), and the corresponding results are displayed in Figure 3c. The fitting C

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all excitation spectra originates from the host exciton creation absorption. With an excess of 8% for exciton binding energy, the bottom of the conduction band is estimated to be ∼7.40 eV higher than the top of the valence band at 16 K.32,33 This estimated band gap energy is in good agreement with that of other silicates.34−36 With reference to the Gaussian fitting results in Figure 3d, by monitoring 366 nm emission dominantly arising from Ce(H)3+ centers, two evident excitation bands were observed above 250 nm in Figure 4b. We attribute the band with a maximum at ∼286 nm to the transition from the ground 4f to lowest excited 5d (5d1) state of Ce(H)3+ centers. The weaker excitation band at ∼321 nm belongs to the 4f → 5d1 excitation band of Ce(L)3+, as discussed below, and its appearance indicates that the emission from Ce(L)3+ may slightly extend to 366 nm. By monitoring of a longer wavelength emission at 390 nm (see Figure 4c), the excitation band at ∼321 nm increases. When the emission wavelength is shifted to 480 nm, the excitation band at ∼321 nm is dominant, as shown in Figure 4d. The intensity of the band peaking at ∼321 nm gradually increases with an increase in monitoring emission; thus, it is reasonable that this band should be the lowest 5d excitation band of Ce(L)3+. Since the free oxygen ion O(1)2− occurs in 6h sites, the Ce− O bond in this site is expected to be short and show a high covalency. This plays an important role in lowering the 5d1 excitation energy of Ce3+ via enhancing the centroid shift and crystal field splitting.37 Therefore, Ce(L)3+ is assigned to Ce(1)3+ in 6h sites while Ce(H)3+ is assigned to Ce(2)3+ in 4f sites, as given in Table 3.30,38 According to the peak of the

curve shows good consistence with the experimental data, and the energies of the two band maxima are at ∼2.65 eV (∼468 nm) and ∼2.92 eV (∼425 nm), which can be assigned to the doublet emission bands from the relaxed lowest 5d state to the 2 F5/2 and 2F7/2 4f levels, respectively. Their energy difference is ∼2.16 × 103 cm−1, which is near the typical value (∼2000 cm−1) due to a spin−orbital coupling of 4f ground states. The emission spectrum under 280 nm excitation was fitted via a sum of four Gaussian functions, as presented in Figure 3d. It reveals that the energies of peaks 1 and 2 attributed to Ce(L)3+ are well in line with that in Figure 3c and the doublet emission from Ce(H)3+ is aligned to peak 3 (∼3.16 eV, ∼392 nm) and peak 4 (∼3.39 eV, ∼366 nm) with an energy difference of ∼1.81 × 103 cm−1. These estimations for the emission band positions of two Ce3+ centers are indispensable to further analyze their excitation spectra in the following sections. Figure 4a shows the intensity map of height-normalized synchrotron radiation VUV−UV excitation spectra of the

Table 3. Luminescent Properties of Two Different Ce3+ Centers in LiY9(SiO4)6O2 5d1 excitation (nm) emission (nm) Stokes shift (103 cm−1) lifetime (ns)

Ce(1)3+ at 6h

Ce(2)3+ at 4f

∼321 ∼425, ∼468 ∼7.62 ∼44.7

∼286 ∼366, ∼392 ∼7.64 ∼30.6

lowest f−d excitation band and that of the 5d−2F5/2 emission band of two different Ce3+ ions, their corresponding Stokes shifts are also estimated as presented in Table 3. It appears that these two values are quite similar. Finally, the luminescence decay curves of two distinct Ce3+ ions on monitoring of their corresponding excitation and emission wavelengths at 79 K are shown in Figure S3 in the Supporting Information. Both ions possess a single-exponential decay, and Ce(1)3+ ions have a longer lifetime of ∼44.7 ns due to a long-wavelength emission, in comparison to the Ce(2)3+ ions with a decay constant of ∼30.6 ns. 3.3. Influence of Temperature on Emissions of Ce3+ at Two Sites. Figure 5 presents the height-normalized emission spectra of the sample LiY8.995Ce0.005(SiO4)6O2 under 305 nm (under this wavelength, Ce3+ ions at two sites can be excited simultaneously; see Figure 4) excitation in the temperature range of 79−500 K, in which the emission wavelengths corresponding to Ce(1)3+ and Ce(2)3+ as given in Table 3 are marked for further analysis. The full width at half-maximum (fwhm) of the total emission profile becomes narrower from ∼6.66 × 103 to ∼5.10 × 103 cm−1 and the peak position shifts about 40 nm toward the shorter wavelengths when the temperature rises from 79 to 500 K. These phenomena are mainly due to the different thermal-quenching characteristics of

Figure 4. (a) Intensity map of height-normalized VUV−UV excitation spectra of the sample LiY8.995Ce0.005(SiO4)6O2 by monitoring different wavelength emissions at 16 K. (b−d) Synchrotron radiation VUV−UV excitation (λem = 366, 390, 480 nm) spectra of the sample LiY8.995Ce0.005(SiO4)6O2 recorded at 16 K, respectively.

sample LiY8.995Ce0.005(SiO4)6O2 by monitoring different wavelength emissions at 16 K. There are three intense areas, i.e. red parts A−C, corresponding to excitation wavelengths of 184, 286, and 321 nm, respectively. The intensities of bands A and B decrease with an increase in emission wavelength, and the band C becomes dominant when the monitoring emission wavelength is above 425 nm, which indicates that Ce3+ ions in two sites have been successively probed. The VUV−UV excitation spectra upon three representative wavelength emissions (366, 390, and 480 nm) are displayed in Figure 4b−d, respectively. The asymmetric band with a maximum at 181 nm appearing in D

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Figure 5. Height-normalized emission spectra (λex = 305 nm) of sample LiY8.995Ce0.005(SiO4)6O2 from 79 to 500 K. The inset shows temperature-dependent luminescence intensities of Ce(1)3+ at 468 nm and Ce(2)3+ at 366 nm, respectively.

Figure 6. (a) Temperature dependence of average fluorescent lifetimes of Ce(1)3+ and Ce(2)3+ centers. (b) Ftting results using eq 3.

emissions from Ce(1)3+ and Ce(2)3+ centers, although we cannot exclude the contributions of temperature-dependent shifts of both emissions.39 In reference to the discussion in section 3.2, the luminescence intensities at 468 and 366 nm are suitable to serve as indicators of the emission intensities of Ce(1)3+ and Ce(2)3+ ions to discuss the thermal stabilities of their emissions, respectively. The inset shows that the emission of Ce(1)3+ centers at the longer wavelength side is more prone to quenching with an increase in temperature than the emission of Ce(2)3+ centers. Therefore, the contribution from longwavelength emission of Ce(1)3+ gradually decreases while that from short-wavelength emission of Ce(2)3+ relatively increases to the overall normalized spectra with increasing temperature, which leads to the short-wavelength shifting of the emission peak and the narrowing of the whole emission profile. To further study the different thermal-quenching properties of Ce3+ ions at two sites, the luminescence decay curves of Ce(1)3+ and Ce(2)3+ at different temperatures were collected, respectively. The representative results are shown in Figure S4 in the Supporting Information. The figure shows that with rising temperature the lifetimes of two types of Ce3+ centers tend to be shortened. Especially for Ce(1)3+ ions, a severe thermal quenching occurs, which is consistent with the temperature-dependent intensity results in Figure 5. At each temperature, the average fluorescent lifetime ⟨τ⟩ of each Ce3+ center is calculated via eq 140

the host,43 τ0 is the radiative decay time of the Ce3+ excited state, A is a pre-exponential factor relating to the rate constant for the thermally activated escape,4 and k is the Boltzmann constant ([8.6173324(78)] × 10−5 eV K−1). For convenience, eq 2 can be transformed to eq 3 ⎡ τ ⎤ E 1 ln⎢ 0 − 1⎥ = − a · + ln A ⎣ τ (T ) ⎦ k T

(3) 3+

The lifetime data for two types of Ce at increasing temperature were fitted via eq 3. The results are shown in Figure 6b. According to the slopes of the fitted lines, the thermal activation energies are estimated to be ∼0.0493 eV for Ce(1)3+ and ∼0.106 eV for Ce(2)3+, respectively. To clearly illustrate the physical processes responsible for these two activation energies, a schematic diagram involving energy levels of two types of Ce3+ ions and the host band structure is built as presented in Figure 7. With reference to

∞

⟨τ ⟩ =

∫0 tI(t ) dt ∞

∫0 I(t ) dt

(1)

where I(t) is the emission intensity at time t. The estimated results are given in Table S3 in the Supporting Information. Figure 6a shows the temperature dependence of the lifetimes of Ce(1)3+ and Ce(2)3+, respectively. The thermal quenching of Ce3+ f−d emission is mainly due to the thermal ionization of 5d excited state electrons into the conduction band in most cases.41 Consequently, the temperature-dependent fluorescent lifetime data of Ce(1)3+ and Ce(2)3+ in Figure 6a have been analyzed via the Mott formula (2) of a thermal ionization framework42 −1 ⎡ ⎛ Ea ⎞⎤ ⎜ ⎟ τ(T ) = τ(0)⎢1 + A exp − ⎥ ⎝ kT ⎠⎦ ⎣

Figure 7. Schematic energy level diagram of Ce(1)3+ and Ce(2)3+ in the LiY9(SiO4)6O2 host.

section 3.2, the bottom of the conduction band (CB) of the host compound is estimated to be ∼7.40 eV higher than the top of the valence band (VB). For the Ce(2)3+ ion, its lowest 5d excited state (5d1) is ∼0.106 eV lower than the bottom of CB when we assume that the activation energy Ea for thermal quenching of Ce(2)3+ corresponds to the energy gap between 5d1 and the bottom of CB, indicating that the energy of Ce(2)3+ 5d1 is about ∼7.29 eV higher than the top of VB.44 The lowest f−d excitation band of Ce(2)3+ is at a maximum at about 286 nm (∼4.33 eV); therefore, the 4f ground state 2F5/2 is estimated to be about 2.96 eV higher than the top of VB. Herein, the energy of the 4f ground state of Ce(1)3+ is expected to be nearly the same as that of Ce(2)3+ due to a complete

(2)

where Ea is the activation energy for thermal quenching required for the ionization from the 5d1 state to a high-lying quenching level, usually the bottom of the conduction band of E

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Inorganic Chemistry shielding of 4f electrons by outer 5s and 5p electronic configurations. Consequently, the 4f ground state 2F5/2 of Ce(1)3+ is also regarded to be ∼2.96 eV above the top of VB. The first f−d excitation band of Ce(1)3+ is at a maximum at about 321 nm, meaning that 5d1 of Ce(1)3+ is energetically higher, about 6.82 eV, than the top of VB. Accordingly, the energy gap of 5d1 and the bottom of CB is calculated to be ∼0.58 eV for Ce(1)3+. This value is about 12 times that obtained from eq 3, provided that the thermal quenching of Ce(1)3+ is also due to thermal ionization of 5d excited state electrons into the conduction band. This large discrepancy reminds us that not the bottom of the CB but another highlying state serves as a thermal quenching level for luminescence of Ce(1)3+. Considering that there is a higher covalency at 6h sites due to the shorter Ce−O bond length and the existence of free oxygen, the anharmonic charge-transfer vibronic exciton (CTVE) might play an important part in strongly quenching the emission of Ce(1)3+.45−47 The CTVE is an effective framework to understanding the intra-3d transitions of transition-metal ions (TMs) due to high covalency of the ion-ligand.46,48 Similarly to the spectra of the intra-3d transitions of TMs, the vibronic coupling of 5d excitation states of Ce(1)3+ and ligand is also strong, since free oxygen O(1)2− occurs around this site.47 Consequently, the CTVE state of Ce(1)3+ is assumed to serve as a nonradiative channel to quench photoluminescence in the emitting state through vibronic coupling. That is, instead of ionizing to the bottom of the conduction band, the electrons in the 5d1 excited state of Ce(1)3+ prefer jumping into the quenching CTVE state. As presented in Figure 7, the CTVE state is just slightly higher than the 5d1 excited state of Ce(1)3+, resulting in a smaller activation energy required to quench the luminescence of Ce(1)3+. 3.4. Influence of Doping Concentration on Emissions of Ce3+ at Two Sites. Figure 8a shows the height-normalized

Meanwhile, the relative intensities of the 5d1 excitation bands of Ce(1)3+ at ∼321 nm gradually increase. This is due to an increasing emission of Ce(1)3+ at ∼360 nm, which is mainly caused by the preferential occupancies of Ce3+ in 6h sites as indicated by Rietveld refinement results (Table 2) and a possible broadening of emission bands of Ce(1)3+ with increasing concentration. The height-normalized excitation spectra of samples LiY9−xCex(SiO4)6O2 (x = 0.005, 0.05, 0.15, 0.50, 1, 2, 3) on monitoring the 480 nm emission of Ce(1)3+ at room temperature are presented in Figure 8b. The 5d1 excitation band of Ce(1)3+ at ∼321 nm dominates. When the doping concentration increases, an obvious excitation band broadening to the longer wavelength side is observed, especially for highly concentrated samples (x = 1−3). This may be related to the inhomogeneous broadening induced by the high-level doping of impurity Ce3+ ions into the host.51 Moreover, the relative absorption at shorter wavelength range rises due to an increasing energy transfer rate from Ce(2)3+ to Ce(1)3+ with increasing concentration. The height-normalized emission spectra of LiY9−xCex(SiO4)6O2 samples (x = 0.005, 0.05, 0.015, 1, 2, 3) under 305 nm excitation at room temperature are presented in Figure 9a. The full width at half-maximum (fwhm) of the

Figure 9. (a) Height-normalized emission spectra (λex = 305 nm) of LiY9−xCex(SiO4)6O2 samples (x = 0.005, 0.05, 0.15, 0.50, 1, 2, 3) at room temperature. (b) Corresponding CIE chromaticity coordinates of the samples. (c) Concentration-dependent emission intensities of Ce(1)3+ at 468 nm and Ce(2)3+ at 366 nm together with integrated intensities of total spectra.

emission band has been broadened from ∼5.13 × 103 to ∼7.96 × 103 cm−1 and the peak position has been shifted from ∼392 to ∼432 nm when the x value increases from 0.005 to 3. The CIE chromaticity coordinates of representative spectra are calculated and given in Table S4 in the Supporting Information. The corresponding chromaticity points marked in the chromaticity diagram show a tunable luminescence from blue to cyan with an increase in doping concentration, as depicted in Figure 9b. In fact, the total emission profile is determined by the cooperative contributions of Ce3+ in two sites. Factors such as site occupancy, possible energy transfer, and emission band broadening may show their influences on the shape of total emission spectra. First, the data in Table 2 have demonstrated that the amounts of Ce(1)3+ and Ce(2)3+ ions simultaneously increase with increasing Ce3+ content, and the Ce(1)3+/Ce(2)3+ ratio also increases. This effect will broaden the total emission profile and shift the emission peak to longer wavelengths before

Figure 8. Height-normalized excitation spectra (a, λem = 360 nm; b, λem = 480 nm) of LiY9−xCex(SiO4)6O2 samples at room temperature.

excitation spectra of samples LiY9−xCex(SiO4)6O2 (x = 0.005, 0.05, 0.15, 0.50) of 360 nm emission at room temperature. The band at ∼286 nm is the lowest 5d (5d1) excitation band of Ce(2)3+ while the shoulder band at ∼321 nm the 5d1 excitation band of Ce(1)3+ as presented in Figure 4b. The peak of the 5d1 excitation band of Ce(2)3+ remains stable at ∼286 nm at different doping concentrations, which implies the almost invariable collaborative effects of crystal field and covalency on Ce(2)3+ luminescence centers at different doping levels.49,50 F

DOI: 10.1021/acs.inorgchem.6b01656 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry 3/ S ⎤ ⎡ ⎛ ⎞ ⎛ ⎞ C ⎛ I (t ) t 3 ⎞⎛ t ⎞ = exp⎢ −⎜ ⎟ − ⎜ A ⎟Γ⎜1 − ⎟⎜ ⎟ ⎥ ⎢⎣ ⎝ τ0 ⎠ ⎝ C0 ⎠ ⎝ I(0) S ⎠⎝ τ0 ⎠ ⎥⎦

the occurrence of concentration quenching. Second, the possible emission band broadening of Ce3+ at a high-doping level will also have an effect on the total emission spectrum shape.52 Third, the possible energy transfer from higher energy Ce(2)3+ to Ce(1)3+, which will be discussed below, results in a relatively more intense Ce(1)3+ emission and will push the total emission toward longer wavelengths. Figure 9c shows the concentration-dependent emission intensities of Ce(1)3+ at 468 nm and Ce(2)3+ at 366 nm together with integrated intensities of total spectra. The emission intensity comes to the top at x = 0.50 for Ce(1)3+, at x = 0.15 for Ce(2)3+, and at x = 0.50 for the total integrated emission, respectively. This shows that the sample LiY8.50Ce0.50(SiO4)6O2 has the strongest total emission intensity. Because the f−d emission bands of Ce(2)3+ partially overlap with the lowest 5d excitation band of Ce(1)3+ (see Figures 3 and 4), this implies possible energy transfer from higher energy Ce(2)3+ to Ce(1)3+. The site-selected concentration-dependent luminescence decay curves of Ce(2)3+ centers were collected at room temperature to investigate the energy transfer process. Figure 10 shows the luminescence decay curves of Ce(2)3+ in

(4) 3+

where I(t) is the luminescence intensity from donor Ce(2) at time t and I(0) denotes an initial emission intensity. τ0 is the intrinsic lifetime of Ce(2)3+. CA is the acceptor Ce(1)3+ concentration, C0 is the critical concentration, and Γ(1 − (3/ S)) is the γ function. The S value depends on the mechanism of energy transfer and is 6, 8, and 10 for the electric dipole−dipole (EDD), electric dipole−quadrupole (EDQ), and electric quadrupole−quadrupole (EQQ) interaction, respectively. For S = 6, the best fitting results for the experimental data are achieved, which suggests that the mechanism responsible for energy transfer from Ce(2)3+ to Ce(1)3+ is mainly the electric dipole−dipole (EDD) interaction. As a matter of fact, when donor to acceptor transitions are electric dipole allowed, a longer critical interaction distance is expected due to stronger oscillator strength. A dipole−dipole mechanism appears to be therefore the most probable.54 The fitted curves are displayed in Figure S5 in the Supporting Information, and the obtained parameters are given in Table 4. One can see that the fitted Table 4. Fitting Parameters via Inokuti−Hirayama Energy Transfer Model Ce3+

Radj2

τ0 (ns)

CA/C0

0.01 0.05 0.15

0.9960 0.9967 0.9947

30.2(4) 30.8(1) 31.2(5)

0.380(5) 0.492(5) 0.775(8)

intrinsic lifetimes of donor Ce(2)3+ are approximately equal to the value (∼30.6 ns) as estimated in Figure S3 in the Supporting Information. In addition, with an increase in Ce3+ concentration the CA/C0 ratio increases, which indicates an increase in acceptor Ce(1)3+ ions. When x continues to increase, both the concentration quenching (CQ) effect and energy transfer (ET) process contribute to the faster decay of Ce(2)3+ ions. The concentration-dependent decay curves of Ce(1)3+ at 79 K and room temperature are also shown in Figure S6, and a brief discussion is given in the Supporting Information. 3.5. Thermometric Properties of Sample LiY8.50Ce0.50(SiO4)6O2. Due to the obvious thermal quenching of emission of Ce(1)3+ in comparison with that of Ce(2)3+, the total emission spectra shift toward shorter wavelengths and undergo a temperature-dependent color variation from blue to violet when the temperature rises, as shown in Figure 5. This feature of tunable Ce3+ emission at different temperatures endows the materials with possible thermometric applications. According to section 3.4, the sample LiY8.50Ce0.50(SiO4)6O2, which has the strongest total emission under 305 nm excitation, is selected to investigate the possible thermometric applications. The height-normalized emission spectra (λex 305 nm) of the sample LiY8.50Ce0.50(SiO4)6O2 at increasing temperatures from 100 to 475 K are shown in Figure S7 in the Supporting Information. The raw data of emission spectra are given in Figure S8. In comparison to the blue shift (about 40 nm) of the sample LiY 8.995 Ce 0.005 (SiO 4 ) 6 O 2 in Figure 5, the CIE chromaticity coordinates of representative spectra are calculated and given in Table S5 in the Supporting Information. The corresponding chromaticity points marked in the chromaticity

Figure 10. Luminescence decay curves (λex = 286 nm, λem = 366 nm) of LiY9−xCex(SiO4)6O2 samples (x = 0.005, 0.05, 0.15, 0.50, 1, 2, 3) at room temperature and 79 K.

samples LiY9−xCex(SiO4)6O2 (x = 0.005−3, λex 286 nm, λem 366 nm) at 79 K (black line) and room temperature (other lines). It appears that the decay of Ce(2)3+ at room temperature (violet curve) slightly deviates from the single exponential in comparison with that at 79 K (black curve) for the low-dosing sample (x = 0.005), implying that the influence of temperature on the decay properties of Ce(2)3+ emission is limited, as shown in Figure 6. With an increase in x values, the decay curves at room temperature further gradually deviate from the exponential and the lifetimes of the Ce(2)3+ 5d1 excited state become shorter. The Ce(2)3+ → Ce(1)3+ energy transfer and possible concentration quenching of Ce(2)3+ are thought to accelerate the decay of Ce(2)3+ emission, and their respective contributions are different at different doping levels. As shown in Figure 9c, the emission intensities at ∼366 nm mainly from Ce(2)3+ first go up with x increasing and then come to the top when x = 0.15, which implies that the concentration quenching effect is trivial in the concentration range (0.005−0.15). Therefore, in this range the energy transfer from Ce(2)3+ to Ce(1)3+ is the dominating reason for the gradually faster decays of Ce(2)3+ in Figure 10. According to the Inokuti−Hirayama (I-H) energy transfer model,40,53 the nonexponential decay of donor Ce(2)3+ luminescence can be described as eq 4 G

DOI: 10.1021/acs.inorgchem.6b01656 Inorg. Chem. XXXX, XXX, XXX−XXX

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where B, C, D, E, and ΔE are parameters related to the I(0), A, and E constants of Ce(1)3+ and Ce(2)3+. As displayed in Figure 11b, the measured plot of FIR versus temperature can be fitted well by eq 6. Consequently, the obtained function can be employed to estimate the practical temperature through the FIR values, which indicates that the phosphor LiY8.50Ce0.50(SiO4)6O2 has a possible thermometric property. The temperature sensitivity (S), i.e. the absolute value of the first derivative of FIR with respect to temperature, is also presented in Figure S9 in the Supporting Information. To further evaluate the repeatability of the temperature-sensing property of this phosphor, the temperature-recycle measurements were carried out twice. All of the obtained temperaturedependent emission spectra (λex 305 nm) in the range of 100− 475 K are shown in Figure S10 in the Supporting Information, from which the values of FIR(I468/I366) for twice cycling are also calculated and presented in Figure 11b. It appears that the values of FIR(I468/I366) for certain temperature points are approximately constant, which implies a good repeatability of thermometric properties of the sample LiY8.50Ce0.50(SiO4)6O2.

diagram along with three photographs at 100, 350, and 475 K both show a color variation from cyan to deep blue with an increase in temperature, as depicted in Figure 11a. More

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CONCLUSIONS In summary, we have presented a systematic study on the crystal structure and luminescence properties of LiY9(SiO4)6O2:Ce3+ phosphors. Through the Rietveld refinements, the Ce3+ ions are found to prefer occupying the 6h sites with an increase in doping concentration. Two kinds of Ce3+ luminescence properties have been studied in detail. The Ce(1)3+ ions with longer wavelength emissions are thought to enter the 6h sites due to the higher covalency induced by the free oxygen ion in their coordination environment, while the Ce(2)3+ ions with shorter wavelength emissions are assigned to 4f sites. With an increase in Ce3+ concentration, the collaborative influences of site occupancies, energy transfer, and emission band broadening push the total emission of Ce3+ toward the longer wavelength side, which results in the concentration-dependent emission properties of phosphors. Moreover, the phosphors also possess tunable emission from cyan to blue with an increase in temperature. Through temperature-dependent luminescence decay measurements for these two Ce3+ ions, the emission from Ce(1)3+ is found to be more prone to thermal quenching in comparison to Ce(2)3+. We deem that the charge-transfer vibronic exciton (CTVE) state induced by the existence of free oxygen ion is responsible for the poorer thermal stability of Ce(1)3+ at 6h sites. This tunable Ce3+ emission at different temperatures endows the materials with possible thermometric properties. The relationship between the fluorescence intensity ratio FIR(I468/I366) and temperature has been established using the sample LiY8.50Ce0.50(SiO4)6O2 to demonstrate the potential thermometric applications. The temperature recycle measurements further reveals the good repeatability of the temperaturesensing properties.

Figure 11. (a) CIE chromaticity coordinates at seven temperature points and luminescence photos (λex = 305 nm) of the sample LiY8.50Ce0.50(SiO4)6O2 at 100, 350, and 475 K. (b) Temperature dependence of FIR(I468/I366) with error bars, fitting function, and temperature-cycling results.

quantitatively, the temperature-dependent fluorescence intensity ratio of emission intensity at 468 nm to that at 366 nm (FIR(I468/I366)) is employed to judge the thermometric properties of the phosphor LiY8.50Ce0.50(SiO4)6O2.55,56 As discussed in section 3.3, the temperature-dependent luminescence intensities at 468 and 366 nm emissions, which are related to the emissions of Ce(1)3+ and Ce(2)3+, respectively, are extracted from Figure S8 in the Supporting Information. Then the obtained FIR(I468/I366) values with error bars are shown in Figure 11b, showing that these values decrease with an increase in temperature. Because the Ce3+ emissions quench thermally with an increase in temperature, the temperaturedependent luminescence intensity of Ce3+ can be expressed as eq 5 ICe

−1 ⎡ ⎛ Ea ⎞⎤ ⎜ ⎟ = I(0)⎢1 + A exp − ⎥ ⎝ kT ⎠⎦ ⎣

(5)

where ICe and I(0) represent the luminescence intensities of Ce3+ at certain temperatures and 0 K, respectively. The other physical parameters A, Ea, k, and T have the same meanings as those in eq 2. Consequently, the temperature dependence of FIR(I468/I366) can be described as in eq 655 I468 1 + A 2 exp( −E2 /kT ) =B I366 1 + A1 exp( −E1/kT ) C ≈ +E 1 + D exp( −ΔE /kT )

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ASSOCIATED CONTENT

* Supporting Information S

he Supporting Information is available free of charge on the ACS Publications Web site at . (PDF). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01656. Refined structure parameters of host compound and Ce3+-doped samples at room temperature, calculated

FIR =

(6) H

DOI: 10.1021/acs.inorgchem.6b01656 Inorg. Chem. XXXX, XXX, XXX−XXX

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lifetimes of two Ce 3+ centers, CIE chromaticity coordinates of samples with different doping concentrations and at increasing temperature, crystal structure diagram of the host compound, Rietveld refinement results of high-doping samples, luminescence decay curves of two Ce3+ centers at increasing temperature, fitting results via Inokuti−Hirayama model, luminescence decay curves of Ce(1)3+ centers with increase of contents along with a brief discussion, temperature-dependent emission spectra (λex 305 nm) of LiY8.50Ce0.50(SiO4)6O2 with increasing temperature, temperature sensitivity of the phosphor LiY8.50Ce0.50(SiO4)6O2, and temperature cycling measurements (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for H.L.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was financially supported by the National Natural Science Foundation of China (21171176, U1232108, U1432249, and 21671201) and the Natural Science Foundation of Guangdong Province (S2013030012842).

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