Structure, site occupancies, and luminescence properties of Ca10M(PO4)7: Ce3+ (M = Li, Na, K) phosphors

Accepted Manuscript Structure, site occupancies, and luminescence properties of Ca10M(PO4)7: Ce = Li, Na, K) phosphors

3+

(M

Mubiao Xie PII:

S0925-8388(18)33836-2

DOI:

10.1016/j.jallcom.2018.10.162

Reference:

JALCOM 47980

To appear in:

Journal of Alloys and Compounds

Received Date: 13 September 2018 Revised Date:

11 October 2018

Accepted Date: 13 October 2018

Please cite this article as: M. Xie, Structure, site occupancies, and luminescence properties of 3+ Ca10M(PO4)7: Ce (M = Li, Na, K) phosphors, Journal of Alloys and Compounds (2018), doi: https:// doi.org/10.1016/j.jallcom.2018.10.162. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Structure, Site Occupancies, and Luminescence Properties of Ca10M(PO4)7: Ce3+ (M = Li, Na, K) Phosphors Mubiao Xie*

* To whom correspondence should be addressed

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Tel: 86-759-3183245

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Zhanjiang 524048, China

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School of Chemistry and Chemical Engineering, Lingnan Normal University,

Fax: 86-759-3183510

Abstract

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E-mail: [email protected] (Mubiao Xie)

In this work, we reported a detailed studies on photoluminescence properties of

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Ce3+-doped Ca10M(PO4)7: Ce3+ (M = Li, Na, K) phosphors. VUV-UV-vis

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photoluminescence (PL) were employed to discuss the site occupancies of Ce3+ions in the host. Two kinds of Ce3+centers were observed in Ca10M(PO4)7 (M = Li, Na, K). The Ce(I)3+ emission at ∼355 nm is contributed to Ce3+ ions in Ca(1), Ca(2) or Ca(3) sites, while the Ce(II)3+ emission at ∼430 nm are assigned to Ce3+ entering Ca(5) site. The effects of temperature on Ce3+ luminescence properties show Ce(2)3+ emissions have a faster thermal quenching in Ca10M(PO4)7.

Keywords Luminescence; Phosphors; Ca10M(PO4)7; Ce3+ ions

ACCEPTED MANUSCRIPT 1. Introduction Nowadays, one of the popular researches of the inorganic luminescent materials is the material doped with the Ce3+ ions because of their various applications [1-2]. Because

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the parity allowed 4f→5d transitions of Ce3+ ions can be greatly influenced by coordination environments on the outer 5d state energies, the Ce3+-doped luminescent materials can be utilized to be designed as tunable emission luminescence materials

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for lighting and displays [3-10]. Generally, tunable emission can be achieved in Ce3+

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or Eu2+ single doped or co-doped phosphors by the methods as below: 1) Tune the emission by adjusting the host structure through cation or anion substituting[11]; 2) Tunable emissions can be realized through energy transfer between sensitizer Ce3+ and activators Eu2+, Tb3+, Mn2+, and so on [12]; 3) singly doping Ce3+ in a multi-site

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host compound which can result in multi-color emissions [13-14]. The multi-color emissions based on a single activator in a certain host would be a more convenient approach to obtain tunable emission.

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β-Ca3(PO4)2 is a whitlockite-type crystal with space group R3c, which has three P

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sites (P1-P3) and six metal sites (M1-M6) [15]. Because the heterovalent (Ca2+) ions can be substituted by quadrivalent (R4+), monovalent (M+), and trivalent (R3+) cations, the initial structure β-Ca3(PO4)2 can generate many types of compounds, such as Ca9Ln(PO4)7 (Ln = La, Gd, Y, Lu, Sc, Al, Bi, et al) [16-17], Ca8MgR(PO4)7:Eu3+ (R = La3+, Gd3+, Y3+) [18], Ca10M(PO4)7 (M = Li, Na, K) [19-21], and so on. Such β-Ca3(PO4)2-type structure compounds are considered as good hosts for rare earth ions doped phosphors. To the best of our knowledge, the luminescence properties of

ACCEPTED MANUSCRIPT Ce3+–, Eu2+– and their co-oped Ca10M(PO4)7 (M = Li, Na, K) for w-LEDs applications have been reported [19-28], but the site occupancy luminescence properties have not been investigated in detail so far. In this paper, the luminescence

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properties of Ce3+-doped Ca10Li(PO4)7 (CLP), Ca10Na(PO4)7 (CNP), Ca10K(PO4)7 (CKP) phosphors under vacuum ultraviolet-ultraviolet (VUV-UV) excitation are reported. The assignment of site occupancies, thermal quenching of Ce3+ at two sites

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have been systematically studied.

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2. Experimental

The phosphors Ca10M(PO4)7:0.10Ce3+ (M = Li, Na, K) were prepared by a high temperature solid state reaction method in a H2–N2 reducing ambient. The starting materials were CaCO3 (analytical reagent, A.R.), Li2CO3 (A.R.), Na2CO3 (A.R.),

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K2CO3 (A.R.), NH4H2PO4 (A.R.) and CeO2 (99.99%). Firstly, these raw materials were weighed stoichiometrically and mixed thoroughly in an agate mortar, and then the mixture heated at 1323 K for 6 h. The final products were cooled to room

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temperature (RT) by switching off the muffle furnace.

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The phase purity and structure of the final products were checked by X-ray powder diffraction (XRD) using a D8 ADVANCE diffractometer with Cu Kα radiation (λ = 0.15418 nm) at room temperature (RT). The structural diagrams were drawn with Diamond software using the CIF file. The luminescence spectra in the UV− vis range at different temperatures were recorded on an Edinburgh FLS 920 spectrometer. A 450 W xenon lamp was used as the excitation source for the steady-state UV − vis spectra. The vacuum

ACCEPTED MANUSCRIPT ultraviolet-ultraviolet excitation measurements were measured using a water-cooled 150 W deuterium lamp as an excitation source in combination with a vacuum monochromator_model ARC VM 502.

3.1 XRD patterns and crystal structure

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3. Results and Discussion

The XRD patterns of phosphors CLP:0.10Ce3+ (a), CNP:0.10Ce3+ (b), CKP:0.10Ce3+

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(c) are shown in Figure 1. The diffraction peaks are in good agreement with the

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corresponding Joint Committee for Powder Diffraction Standard (JCPDS) files of Ca10Li(PO4)7, Ca10Na(PO4)7 and Ca10K(PO4)7, respectively. It indicates that all the samples are of a pure phase.

Ca10M(PO4)7: (M = Li, Na, K) is firstly reported by Morozov et al, which is

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isostructural to β-Ca3(PO4)2 [29]. As seen in Figure 2(a-c), The Ca10M(PO4)7 structure consists of two columns with M, CaOn and PO4 groups in chains. Figure 2(d) shows that Ca2+ ions have four different coordination environments: Ca(2) and Ca(3) are

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eight-coordinated, Ca(1) is seven-coordinated, and Ca(5) is six-coordinated. The

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alkali metal M+ ions in Ca10M(PO4)7 occupy the M(4) site with three-coordinated. 3.2 Luminescence of Ce3+ in Ca10Li(PO4)7 The emission spectra of the sample CLP:0.10Ce3+ under different wavelength excitations recorded on deuterium lamp at 10 K are shown in Figure 3. Under short-wavelength excitations (for example 202 nm ), two emission bands at ∼352 nm and ∼435 nm can be observed clearly. With an increase in excitation wavelength, the emission band at the long-wavelength side disappear gradually, and the emission band

ACCEPTED MANUSCRIPT at 352 nm become dominant. The energy difference of the emission bands between 352 nm and 435 nm is 5420 cm-1, which is much larger than the energy difference (2000 cm-1) coming from the lowest 5d excited state to the 2F5/2 and 2F7/2 spine orbit

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split 4f ground states for the Ce3+ ions in a definite lattice site. Therefore, it can be concluded that two kinds of Ce3+ luminescence centers surely exist, and different excitation light could excite certain Ce3+ centers more effectively, which result in the

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varying intensity ratio of the two emission bands. Accordingly, we denote the Ce3+

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ions with shorter emission wavelength as Ce(I)3+ centers and that with longer emission as Ce(II)3+ centers. It is known that the emission of Ce3+ ions in a definite lattice site often shows two bands with the energy difference 2000 cm-1 due to the transitions from the lowest 5d excited state to the 2F5/2 and 2F7/2 ground states. In order

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to further find out such positions of the Ce3+ emissions. The emission spectrum under 225 nm excitation was fitted by a sum of four Gaussian functions, as presented in Figure 4. It reveals that the emission spectrum can be well fitted into four peaks with

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maximum peaks at ∼337, ∼361, ∼430 and ∼465 nm. The energy difference of peak 1

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(∼ 330 nm) and peak 2 (∼ 355 nm) is 1973 cm−1, whereas that of peak 3 (∼ 430 nm) and peak 4 (∼ 460 nm) is ∼ 1751 cm−1. The energy difference is nearly equal to the usual energy difference (~ 2000 cm−1) between the Ce3+ 2F5/2 and 2F7/2 states, which further implies that two types of Ce3+ luminescence centers may exist. This estimation and the occupation of two Ce3+ centers are further analyzed below through the excitation spectra. To further study the luminescence of Ce3+ in different sites, the VUV excitation

ACCEPTED MANUSCRIPT spectra monitoring by different emissions of the sample CLP:0.10Ce3+ were recorded at 10K, as shown in Figure 5(a-d). The asymmetric band with a maximum at ∼165 nm appearing in all excitation spectra is attributed to the host absorption, which has been

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found in the reported phosphors Ca10K(PO4)7:RE3+ (RE = Ce, Tb, Dy, Tm and Sm) and Ca9Y(PO4)7:Ce [28,30]. As seen in the Figure 5(a,b), by monitoring emissions 360 and 380 nm, which are mainly corresponding to Ce(I)3+ centers, the excitation

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curves show nearly the same shape. That is, three intensive excitation bands are

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observed above 250 nm. The dominant absorption locates at ∼293 nm. When the emission wavelength is shifted to 415 and 430 nm, which dominantly belong to Ce(II)3+ centers, only one evident excitation band centered at ∼313 nm is observed above 250 nm in Figure 5(c,d). It is found that the intensity of the band peaking at

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∼313 nm and ∼200 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(II)3+, while the band peaking at ∼293 nm is attributed to the lowest 5d excitation

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band of Ce(I)3+. Additionally, the band peaking at ∼200 nm may originate from

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Ce(II)3+ centers. The normalized emission spectra under excitations at 293 and 313 nm are shown in Figure 5(f). When the excitation wavelength is shifted from 293 nm to 313 nm, the emission spectrum band has an evident shifting tendency to longer wavelength side. It further confirms the above assignment about 5d1 states of Ce3+ centers. The emission spectra of the sample CLP:0.10Ce3+ under 220 nm excitation at 10 K and 277 K are presented in Figure 5(g). When the sample is excited at 10K, both Ce(I)3+ and Ce(II)3+ emissions are observed. However, the Ce(II)3+ emission

ACCEPTED MANUSCRIPT disappears at 277K, resulting into an asymmetric emission band. It further certifies the facts that two types of Ce3+ emissions exist in CLP:0.10Ce3+, and a more serious thermal quenching of Ce(II) emission is due to its larger Stokes shift.

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3.3 Luminescence of Ce3+ in Ca10Na(PO4)7 and Ca10K(PO4)7 As CLP:0.10Ce3+ phosphor discussed above, the VUV excitation spectra of CNP:0.10Ce3+ and CKP:0.10Ce3+ are measured, as shown in Figure 6. As presented in

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Figure 6(a-d), the excitation spectra monitored by short-wavelength emission (curve a,

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c: 360 nm) and longer-wavelength emission (curve b, d: 430 nm) exhibit great difference, which are similar with that in Figure 5(a-d). Accordingly, it is concluded that two types of Ce3+ centers also exist in CNP:0.10Ce3+ and CKP:0.10Ce3+. The emission spectra of CNP:0.10Ce3+ and CKP:0.10Ce3+ excited by different wavelength

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based on the excitation bands in Figure 6(a-d) are plotted in Figure 6(f-g). For CNP:0.10Ce3+ sample in Figure 6(f), with excitation wavelength increasing from 200 to 265, the Ce(I) emissions increase, while the Ce(II) emissions decrease gradually.

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For CKP:0.10Ce3+ sample in Figure 6(g), with an increase in excitation wavelength,

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the corresponding emission spectra show different shapes and the Ce(I) emissions gradually become dominant. It further implies that two types of Ce3+ luminescence surely exist.

Figure 7 shows the emission spectra of the samples CNP:0.10Ce3+ and CKP:0.10Ce3+ under 245 nm at the temperature range of 10−300 K. As we expected, Ce(1) and Ce(2) emissions in CNP:0.10Ce3+ and CKP:0.10Ce3+ are observed together in Figure 7. With the temperature increasing, the emission intensities from Ce(I) and Ce(II)

ACCEPTED MANUSCRIPT simultaneously decrease. It also can be observed that more serious decreasing occurs on Ce(II) emissions. The inset of Figure 7 shows that emission intensity of Ce(I) decreases gradually with temperature increasing, while that of Ce(I) declines sharply,

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especially from 10 to 200 K. More prone to thermal quenching of Ce(II) is mainly due to its larger Stokes shift. It also should be point out that, comparing with CLP:0.10Ce3+ and CNP:0.10Ce3+ samples, Ce(II) emission in CKP:0.10Ce3+ at low

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temperature is more significant, and its emission intensity has a greater decrease with

evidence to reveal. 3.4 Assignment of site occupation

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temperature increasing. The reason for this phenomenon needs more to supporting

The effective ionic radii of Ca2+ [coordination number (CN) = 8], Ca2+ (CN = 7), Ca2+

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(CN = 6), Ce3+ (CN = 6), Ce3+ (CN = 7) and Ce3+ (CN = 8) ions are 1.12, 1.06, 1.00, 1.14, 1.07 and 1.08 Å, respectively. Based on the effective ionic radii of cations with different coordination numbers and electric charge balance, we propose that Ce3+ ions

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are expected to occupy the Ca2+ ion sites (Ca(1), Ca(2), Ca (3) and Ca(5)) rather than

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alkali metal site (M(4)) in the crystal structure. In the crystal structure of Ca10K(PO4)7, there isn't a large difference between the bond distances of Ca(3)-O (2.36-2.76 Å), Ca(2)-O (2.39-2.71 Å) and Ca(1)-O (2.33-2.52 Å), but they are shorter than that of Ca(5)-O (2.24-2.27 Å). It is known that the crystal field is stronger in a structure with shorter Ce-O distance, and results in lowering of the 5d band of Ce3+. Thus, it is reasonable to assign that the short-wavelength Ce(I) emission should correspond to the Ca(1), Ca(2) and Ca(3) sites, while the long-wavelength Ce(II) emission is

ACCEPTED MANUSCRIPT originated from the Ca(5) site in CLP, CNP and CKP. To further analyze the occupancy of Ce ions and build the relationship between the coordination environment and emission peaks, the following experiential equation (1)

−n × Ea × r   V E = Q 1-( )1/ V 10  80  4

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reported by Van Uitert is employed to estimated the emission peak position [31]. (1)

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where E is emission peak position (cm−1) for Ce3+; Q represents the position in energy for the lower d-band edge for the free ion (Q = 50000 cm−1 for Ce3+); V is the valence

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of the activator (V = 3 for Ce3+); n is the coordination number of the activator Ce3+, and r is the radius of the host cations (Ca2+) replaced by the Ce3+ ions. Ea is the electron affinity of the atoms that form anions, which is different when the activator is introduced into different anion complexes, and here Ea was determined as

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approximately 3.12 eV [31]. Accordingly, when Ce3+ ions enter into the Ca(1)(n = 7, rCa = 1.06), Ca(2)(n = 8, rCa = 1.12), Ca(3)(n = 8, rCa = 1.12), Ca(5)(n = 6, rCa = 1.00),

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the E values can be calculated by equation (1). The detail calculated results are presented in Table 1.

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The calculated E value 426 nm for Ca(5) is consistent with the measure emission peaks of Ce(II)3+ (430 nm) in CLP, CNP and CKP, while the Ce(1)3+ emission (355 nm) is found between the E values for Ca(2,3) (337) and Ca(1)(375 nm). Therefore, it is concluded that the Ce(II)3+ ions with long-wavelength emissions correspond to the six-coordinated Ca(5) site, and the Ce(I)3+ ions with short-wavelength emissions are believed to randomly occupy the seven-coordinated and eight-coordinated Ca2+ ion sites (Ca(1), Ca(2) and Ca (3)). Based on the assignment of Ce3+ occupation and the

ACCEPTED MANUSCRIPT spectra analysis, the energy levels diagram of Ce3+ in Ca10M(PO4)7 are presented in Figure 8. 4. Conclusions

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In summary, we reported a systematic study on the crystal structure and luminescence properties of CLP:0. 10Ce3+, CNP:0.10Ce3+ and CKP: 0.10Ce3+ phosphors. The XRD patterns indicate that the prepared samples are pure phase. Two kinds of Ce3+ centers

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are found in CLP, CNP and CKP. The Ce(I)3+ ions with shorter wavelength emissions

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are thought to enter Ca(1), Ca(2) or Ca(3),sites, while the Ce(II)3+ ions with longer wavelength emissions are assigned to enter six-coordinated Ca(5) site. The temperature-dependent luminescence properties of CLP:0. 10Ce3+, CNP:0.10Ce3+ and CKP: 0.10Ce3+ phosphors indicate that the emission from Ce(II)3+ is more prone to

Acknowledgments

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thermal quenching in comparison to Ce(I)3+.

The work was supported by the Research Group of Rare Earth Resource Exploiting

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and Luminescent Materials [grant number 2017KCXTD022], National Natural

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Science Foundation of China [grant number 21401165], Natural Science Foundation of Guangdong Province [grant number 2014A030307040], Training Program for Excellent Youth Teachers in Universities of Guangdong Province [grant number YQ2015110]. References (1) L.T. Lin, R. Shi, R.F. Zhou, Q. Peng, C.M. Liu, Y. Tao, Y. Huang, P. Dorenbos, H.B. Liang, The Effect of Sr2+ on Luminescence of Ce3+-Doped (Ca,Sr)2Al2SiO7,

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ACCEPTED MANUSCRIPT Table 1 Calculated E values and emissions in CLP, CNP and CKP. Emissions CLP

in Emissions CNP

Ca(2),Ca(3) n=8, rCa = 1.12

29674 cm-1 (337 nm)

355

355

Ca(1) n=7, rCa = 1.06

26667 cm-1 375 nm

Ca(5) n=6, rCa = 1.00

23474 cm-1 426 nm

430

430

in Emissions CKP 355

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in

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E

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Ca2+ sites

430

ACCEPTED MANUSCRIPT Figure

1

XRD

patterns

of

the

phosphors

Ca10Li(PO4)7:0.10Ce3+

(a),

Ca10Na(PO4)7:0.10Ce3+(b), Ca10K(PO4)7:0.10Ce3+(c) and the standard data. Figure 2 Unit cell representation of Ca10Li(PO4)7 (a), Ca10Na(PO4)7 (b), Ca10K(PO4)7

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(c) and the Ca coordination polyhedra in Ca10Na(PO4)7 (d). Figure 3 emission spectra of the sample CLP:0.10Ce3+ at different wavelength excitations at 10K.

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Figure 4 Gaussian functions for emission spectrum under 225 nm excitation.

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Figure 5 VUV excitation and emission spectra of the sample CLP:0.10Ce3+ at 10K. Figure 6 VUV excitation spectra and emission spectra of the samples CNP:0.10Ce3+ and CKP:0.10Ce3+ at 10K.

Figure 7 Emission spectra (λex = 245 nm) of sample CNP:0.10Ce3+ and

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CKP:0.10Ce3+ from 10 to 300 K. The inset shows temperature-dependent luminescence intensities of Ce(I)3+ and Ce(II)3+, respectively.

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Figure 8 Energy level diagram of Ce3+ in Ca10M(PO4)7:Ce3+ systems (M = Li, Na, K).

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Figure 8

ACCEPTED MANUSCRIPT 1. Ce3+-doped Ca10M(PO4)7: Ce3+ (M = Li, Na, K) phosphors are were prepared by a high temperature solid state reaction method. 2. VUV-UV-vis photoluminescence (PL) were employed to discuss the site occupancies of Ce3+ions in the host.

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3. Two kinds of Ce3+centers were determined in Ca10M(PO4)7 (M = Li, Na, K). 4. The effects of temperature on Ce3+ luminescence properties have been

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systematically investigated.