Room temperature broadband upconversion luminescence in Yb3+ and Mn2+ codoped Sr5(PO4)3Cl phosphors

Room temperature broadband upconversion luminescence in Yb3+ and Mn2+ codoped Sr5(PO4)3Cl phosphors

Journal Pre-proof 3+ 2+ Room temperature broadband upconversion luminescence in Yb and Mn codoped Sr5(PO4)3Cl phosphors Fen Xiao, Shikun Xie, Rongxi Y...

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Journal Pre-proof 3+ 2+ Room temperature broadband upconversion luminescence in Yb and Mn codoped Sr5(PO4)3Cl phosphors Fen Xiao, Shikun Xie, Rongxi Yi, Shu Peng, Chengning Xie PII:

S0022-2313(19)31773-9

DOI:

https://doi.org/10.1016/j.jlumin.2019.116943

Reference:

LUMIN 116943

To appear in:

Journal of Luminescence

Received Date: 9 September 2019 Revised Date:

30 November 2019

Accepted Date: 2 December 2019

Please cite this article as: F. Xiao, S. Xie, R. Yi, S. Peng, C. Xie, Room temperature broadband 3+ 2+ upconversion luminescence in Yb and Mn codoped Sr5(PO4)3Cl phosphors, Journal of Luminescence (2020), doi: https://doi.org/10.1016/j.jlumin.2019.116943. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.

Room temperature broadband upconversion luminescence in Yb3+ and Mn2+ codoped Sr5(PO4)3Cl phosphors Fen Xiao, Shikun Xie, Rongxi Yi, Shu Peng, Chengning Xie∗ College of Mechanical and Electrical Engineering, Jinggangshan University, Ji’an, 343009, China Abstract Broadband yellowish emission from Sr5(PO4)3Cl:Yb3+,Mn2+ phosphors at room temperature is demonstrated upon 976 nm excitation. Two kinds of Yb3+-Mn2+ dimers with different structural geometry are proposed according to the substitution of Mn2+ for different coordinated Sr2+ ion. The upconversion luminescence at 506 nm and 585 nm are originated from the |2F5/2, 2F5/2> -> |2F7/2, 2F7/2> transition of Yb3+-Yb3+ dimers and the |2F7/2, 4T1(4G)> -> |2F7/2, 6A1(6S)> transition of Yb3+-Mn2+ dimers, respectively. From the analysis of crystal structure and measured spectra, the UC mechanism of Mn2+ ions in Sr5(PO4)3Cl:Yb3+,Mn2+ phosphors is deduced to be ground state absorption (GSA) / excited state absorption (ESA) process based on the dimer model. The abnormal photon number deviated from the double photons process can be explained by the large ESA rate of intermediate state and additional depletion of emitting state within the Yb3+-Mn2+ dimers. The obtained results provide a better understanding of the intrinsical UC luminescence of Mn2+ ions at room temperature. Keywords: Phosphate; Upconversion; Manganese; Super-exchange interaction.



To whom correspondence should be addressed, electronic mail: [email protected]

1. Introduction Upconversion (UC) luminescence of Lanthanide (Ln) activators shows great attractive to scientists for its excellent luminescence performances such as plenty emitting color and high chromatic purity, which may have huge potential applications in solar harvesting [1,2], biological label [3,4] and lighting displays [5,6], etc. However, the intrinsic luminescence of Ln3+ ions is originated from the forbidden intra-4f transition and shielded by the outer 5s2 and 5p6 orbits. The shielding effect keeps Ln3+ ions out of interactions with surroundings, so the obtained spectrum is composed of sharp lines and hard to be tuned by crystal field. Although the co-doping of Ln3+ ions may somehow realize the color-tuning of UC luminescence, the complex cross-relaxation processes between them would cause additional energy loss and the final performance strictly depends on the experimental procedures [7,8]. Fortunately, the transition metal (TM) ions can show a broadband emission due to the d-d transitions which are more sensitive to the local environment. Especially for Mn2+ ions, the emission color can be easily tuned from green to deep red due to the crystal field effect on outermost unfulfilled d orbitals [8,9], which makes Mn2+ ions a new candidate for color-tunable broadband UC luminescence. The broadband UC luminescence of Mn2+ was first realized in manganese halide sensitized by Yb3+ ions [10–13]. Owing to the characteristic parity-forbidden and spin-forbidden 4T1(4G) -> 6A1(6S) transition of Mn2+, heavy doping strategy is usually adopted to enhance the emission intensity. Unfortunately, the high concentration of Mn2+ ions would trigger a strong exchange interaction between the activators making the UC

luminescence easily quenched [14]. Moreover, the nonradiative transition rate would rise with the temperature increasing because of the serious multi-phonon relaxation, and thus the UC luminescence from Mn2+ is only available at cryogenic temperature [15]. Until recently, the Mn2+ UC luminescence at room temperature (RT) has been achieved in some specific hosts such as GdMgB5O10[16], KZnF3[17], LaMgAl11O19[18] and MgGa2O4[19]. Within the above hosts, the effect of the octahedral or tetrahedral Mn2+ configuration on UC luminescence properties has been carefully investigated, while the effect of other polyhedral Mn2+ configurations has not yet been identified. Therefore, it’s still a challenge for seeking new proper hosts with efficient RT UC luminescence by accommodating Mn2+ and Yb3+ ions simultaneously. As an important branch of luminescent materials, phosphates are benefited from the rigid three dimensional matrix based on the phosphorus oxygen tetrahedron, which is considered as an ideal charge stabilization structure [20,21]. Herein, the Sr5(PO4)3Cl phosphate is selected as host to realize RT UC luminescence of Mn2+ ions. With Yb3+/Mn2+ co-doping, an obvious visible (VIS) broadband UC emission is obtained at RT under 976 nm excitation. The Mn2+-concentration-dependent Stokes and UC luminescence properties are investigated and discussed in detail. Combined with structural analysis, the emissions from Mn2+ ions with different ligand configurations and the UC mechanism are illustrated. A comprehensive understanding on the UC luminescence of Sr5(PO4)3Cl:Yb3+,Mn2+ phosphor is important for the next design of new broadband UC materials. 2.Experimental procedure

2.1 Sample preparation A series of Sr5(PO4)3Cl:0.05Yb3+,xMn2+ (x = 0, 0.25, 0.5, 0.75, 1, 1.25) phosphors were synthesized by conventional solid-state reaction. The starting materials SrCO3 (AR), SrCl2.6H20 (AR), NH4H2PO4 (AR), MnCO3 (AR) and Yb2O3 (99.998%) were weighed in stoichiometric proportions and grounded in an agate mortar for thoroughly mixing. After calcined at 500 oC for 4h, the foamy product was sufficiently grinded to powder again. One more calcination at 950 oC for 5h within activated carbon reducing atmosphere was performed to prepare the final Sr5(PO4)3Cl:Yb3+,Mn2+ samples with an additional grinding at room temperature. 2.2 Characterizations The crystallization information of the as-prepared phosphors was examined by X-ray diffractometer (Philips Model PW 1830 diffractometer, Cu Kα). The Stokes Photoluminescence (PL) and photoluminescence excitation (PLE) spectra as well as the decay curve of the samples were measured using a FLS920 fluorimeter (Edinburgh Instruments, Livingston, U.K.) equipped with a 450 W Xenon lamp as the excitation source. The UC emission spectra were recorded on a TRIAX320 Fluorescence spectrofluorometer (Jobin-Yvon Co., France) equipped with a R928 photomultiplier tube (PMT) with 976 nm laser diode (LD, Coherent Corp., USA) continuous wave excitation. 2.3 Theoretical calculations Based on the crystallographic information of Sr5(PO4)3Cl, density functional theory (DFT) implemented in the Vienna ab initio simulation package (VASP) was adopted to calculate the total energy at different substitution situations. The cutoff energy Ecut and

K-point mesh were set as 400 eV with a 2×2×2 Monk horst-Pack grid. The convergence criterion for the electronic energy is 10−5eV and the structures were relaxed when the Hellmann-Feynman forces were less than 0.02 eV/Å. 3.Results and discussions 3.1 Structural characterization Fig. 1 shows the XRD patterns of Sr5(PO4)3Cl, Sr5(PO4)3Cl:0.05Yb3+ and Sr5(PO4)3Cl:0.05Yb3+,0.5Mn2+ samples. Compared with the standard pattern (JCPDS 83-0974), all the diffraction peaks are well matched suggesting that the doping Yb3+ and Mn2+ ions are successfully incorporated into Sr5(PO4)3Cl host and haven’t caused any impurity phase. In addition, the diffraction peak has shifted to high angles slightly with the increased concentration of doping ions, as shown on the right of Fig. 1. According to the Bragg’s equation 2dsinθ = nλ [22], the atomic lattice interplanar distance d is inversely proportional to sinθ, for n is an integer and λ is the wavelength of incident X-ray. A smaller d is obtained due to the shrinkage of the host lattice caused by continuous ion substitutions with non-equal radius, which leads to the shifting of the specific diffraction to higher angle θ [21]. The compound Sr5(PO4)3Cl is crystallized in a hexagonal structure (space group P63/m (No.176)) with the unit cell parameters of a = 9.8777 Å, c = 7.1892 Å, V = 607.47 Å3 and Z = 2 [23]. Fig. 2 presents the crystal structure of Sr5(PO4)3Cl unit cell as well as the coordination geometry of Sr2+ ions. Considering the effective ionic radius of different cations, the tetrahedral P5+ ions are too small to be occupied by the doping ions, while the Sr2+ ions are suitable for substitution, as the ionic radius of (Sr2+, Yb3+, Mn2+) are (1.18, 0.863, 0.83 Å CN = 6) and (1.21, 0.925, 0.90 Å CN =

7), respectively. Obviously, the six-coordinated and seven-coordinated Sr2+ ions both can be replaced by the doping Yb3+ and Mn2+ ions, which are abbreviated as Sr1 and Sr2, respectively. To evaluate the site occupancy tendency of Mn and Yb in Sr5(PO4)3Cl, the comparison of formation energy Ef at different situations are carried out based on the formula [24]: Ef = Ed - Ep - ∑niui

(1)

here Ed and Ep are the total energy of doped and perfect crystal, ui and ni are chemical potential and variation number (add or subtract) of i-type atoms, respectively. With the calculation assumption that only one Sr atom is substituted by single doping atom, the items of Ep and ∑niui are both constant for the same type doping atom. So the value of Ef can be evaluated by magnitude of Ed. The calculated total energy Ed for different site occupancies of Mn and Yb are listed in Table 1. It can be seen that the substitution at Sr1 site shows a smaller Ed, which implies that Mn and Yb are preferentially occupy Sr1 site for owning a lower formation energy. 3.2 Stokes luminescence properties Fig. 3 shows the PLE and PL spectra of Sr5(PO4)3Cl:0.05Yb3+,0.5Mn2+ phosphor. Under the excitation of 406 nm, an asymmetric yellowish emission band around 585 nm is observed, originating from the spin-forbidden 4T1(4G) -> 6A1(6S) transition of Mn2+ ions. This asymmetric emission band is comprised of two Gaussian peaks centered at 573 and 617 nm, actually resulting from Mn2+ ions located at two different Sr sites in Sr5(PO4)3Cl. Based on structural geometry of the Sr5(PO4)3Cl host, the average distance

between Mn2+ ions and ligand at Sr1 site is shorter than that at Sr2 site, which suggests that the crystal field strength around Sr1 site is stronger than Sr2 site [25]. Therefore, it can be concluded that the emission bands peaked at 573 and 617 nm are ascribed to the Mn2+ ions located at Sr2 and Sr1 sites, respectively. Moreover, the integrated intensity of emission band centered at 617 nm is stronger than that at 573 nm from Fig. 3, which means more Sr1 sites have been occupied by Mn2+ ions, agreeing well with the substitution tendency of Mn in Sr5(PO4)3Cl derived from DFT analysis. The excitation spectrum monitored at 585 nm shows a series of sub-bands located at 340, 355, 406 and 470 nm, which are originated from the transitions from ground state 6A1(6S) to the 4E(4D), 4T2(4D), [4A1(4G), 4E(4D)]

and 4T2(4G) excited states of Mn2+ ions, respectively.

The PL spectra of Sr5(PO4)3Cl:0.05Yb3+,xMn2+ (x = 0.25, 0.5, 0.75, 1, 1.25) phosphors under the excitation of 406 nm are presented in Fig. 4(a). All the samples show similar peak profile ascribing to the 4T1(4G) -> 6A1(6S) transition of Mn2+ ions. However, with Mn2+ ions concentration increasing, the emission intensity firstly rises to the maximum at x = 0.5 and then falls due to the concentration quenching effect [26]. Meanwhile, the peak wavelength of emission bands shift from 578 to 592 nm as shown in the inset of Fig. 4(a). As the emission band of Mn2+ ions is the combination of two sub-bands originating from Mn2+ ions located at Sr1 and Sr2 sites, their integrated intensity ratio (I617/I573) varied with x is also plotted in the inset. The ratio is monotonically increased indicating that more and more Mn2+ ions preferred to occupy Sr1 sites with the Mn2+ concentration increasing, which demonstrates the substituting tendency of Mn2+ ions again. Simultaneously, the increment of Mn2+ ions at Sr1 sites will shorten Mn2+-ligand

average distance and thus enhance the average crystal filed strength, which causes the overall red-shift phenomenon [7]. In order to evaluate the luminescence dynamic of Mn2+ ions, the semi-log decay curves monitored at 585 nm are recorded and depicted in Fig. 4(b). All the curves are well fitted by a double exponential function and the effective average lifetimes τ* are calculated by the formula bellows [27]: I(t) = I0 + A1 * exp(-t/τ1) + A2 * exp(-t/τ2)

(2)

τ* = (A1τ12 + A2τ22) / (A1τ1 + A2τ2)

(3)

here I(t) and I0 are the emission intensity at time t and initial time 0, Ai (i = 1,2) is a constant and τi (i = 1,2) is the decay lifetime corresponding to different sites. The obtained effective lifetime τ* decreases from 11.4 ms to 8.21 ms with the elevated Mn2+ concentration. The gradually shorten decay lifetime is ascribed to the nonradiative transition caused by the shrinking distance between Mn2+ ions with the rising concentration [28]. 3.3 UC luminescence properties Fig. 5 shows the UC spectra of Sr5(PO4)3Cl:0.05Yb3+,xMn2+ upon 976 nm excitation. A broad yellowish emission band from 530 to 710 nm is obtained for all the samples, which is similar with the profile of emission band under 406 nm excitation. Since Yb3+ ions show no emission in such region, this band belongs to the transition 4T1(4G) -> 6A1(6S)

of Mn2+ ions, which can also be well fitted by two Gaussian peaks. Simultaneously,

a relatively weak peak around 506 nm is observed in the UC spectra, which is undetected in the Stokes spectrum and inferred to UC luminescence of Yb3+ ions [7,29,30].

Considering the low Yb3+ concentration in present work (1 mol%), the cooperative luminescence of Yb3+ ions is impractical because no UC emission of Yb3+ ions is observed for the Yb3+ single-doped sample [31], so the emission band centered at 506 nm is originated from the Yb3+-Yb3+ dimers [16,32]. It should be noted that the emission wavelength is longer than half-wavelength of Yb3+ excitation, which means the energy of the emission photon is lower than twice the energy of excitation photon. This phenomenon is generally attributed to the energy levels splitting of Yb3+-Yb3+ dimers under the effect of super-exchange coupling [14]. After absorbing two excitation photons, the higher splitting component of emitting states of Yb3+-Yb3+ dimers is populated and then decay to the lowest splitting component of the same state, where an emission photon with lower energy is obtained. With Mn2+ ions content increasing, the intensity of emission band around 506 nm is varied little within acceptable error, implying the formation of Yb3+-Yb3+ dimer in Sr5(PO4)3Cl [16]. In addition, the emission intensity of Mn2+ ions around 585 nm reaches its maximum at x = 0.5, as shown in the inset of Fig. 5. Comparing with the Stokes luminescence (Fig. 4(a)), the UC intensity variation curve shows a similar tendency, but the UC luminescence is quenched more quickly once the content x exceeds the optimum, which reveals that the UC emission of Mn2+ is more sensitive to the Mn2+ concentration and then results in a larger non-radiative transfer probability in UC materials [7,17,19]. The UC spectra of the Sr5(PO4)3Cl:0.05Yb3+,0.5Mn2+ under different 976 nm LD excitation power are displayed in the Fig. 6. All the emission bands show similar shapes and identical peak positions, and the integrated emission intensity originating from Mn2+

ions is gradually enhanced with the increasing pump power. However, the UC emission of Yb3+ ions at 506 nm is weak and nearly unchanged at low pump power, which suggests that the UC luminescence from Yb3+-Yb3+ dimers is less efficient than that of Mn2+ ions in Sr5(PO4)3Cl. Under the low pumping power, the relation expression I ∝ Pn is generally applied for estimating the photon number participated in UC process by exploring the relationship between the integrated emission intensity I at 585 nm and LD excitation power P [13]. The inset of Fig. 6 shows the linear fitting of the integrated intensity of Mn2+ varied with excitation power in log-log coordinates. The obtained slope n is around 0.9, which is less than 1 and indicating the photon number to be 1. Such anomalous situation deviated from two-photons process [28,31] will be discussed subsequently. In general, two possible UC mechanisms are proposed to explain the UC behavior of Mn2+ and Yb3+ ions codoped materials [33,34]. One is cooperative sensitization mechanism, in which two Yb3+ ions simultaneously transfer their excitation energy to Mn2+ ions in close proximity. In Sr5(PO4)3Cl, the cooperative optical phenomena (including absorption and emission [35]) of Yb3+ ions are unavailable in present work due to the low Yb3+ concentration [31] and the formation of Yb3+-Yb3+ dimers, which greatly bring down the possibility of the cooperative sensitization process. So the cooperative sensitization mechanism can be ruled out for the observed UC luminescence here. Another potential UC mechanism is ground state absorption / excited state absorption (GSA/ESA) based on a super-exchange interaction model of Yb3+-Mn2+ dimer

[10,36]. In order to clarify the existence of Yb3+-Mn2+ dimer, the near-infrared (NIR) PLE and PL spectra of Sr5(PO4)3Cl:0.05Yb3+ and Sr5(PO4)3Cl:0.05Yb3+,0.5Mn2+ are measured and shown in Fig. 7. For both samples, the emission bands (curves b and d) with the maximum peak at 976 nm are corresponding to the characteristic 2F5/2 -> 2F7/2 transition of Yb3+ ions. But the profiles are dissimilar suggesting that the activator in codoped sample is distinguished from the single-doped Yb3+ ions, which can also be verified by comparison of the excitation spectra patterns monitored at 976 nm. For Sr5(PO4)3Cl:0.05Yb3+ sample, only the charge transition band (CTB) of Yb3+-O2- around 260 nm is observed (curve c), while the codoped sample Sr5(PO4)3Cl:0.05Yb3+,0.5Mn2+ shows a series of excitation bands beyond 300 nm besides the CTB (curve a). Compared with the excitation spectrum in Fig. 3, these excitation bands from 300 nm to 525 nm in curve a are attributed to the characteristic absorptions of Mn2+ ions, which proves the existence of the energy transfer process from Mn2+ to Yb3+ ions. It should be noted that such energy transfer process is non-resonant because there is no spectral overlap between the excitation band of Yb3+ ions and emission band of Mn2+ ions [17]. Therefore, super-exchange interaction between the Mn2+ and Yb3+ ions is responsible for the energy transfer. Besides, super-exchange coupled dimer requires involved ions non-directly connected with a distance less than 5 Å and bridged by certain anions [37], which is satisfied with the distribution of Yb3+ and Mn2+ ions in Sr5(PO4)3Cl. The existence of Mn2+-Yb3+ dimer is confirmed, which is considered as a single-ion activator accounting for the yellowish UC emission from its new formed emitting state upon 976 nm excitation. Thus the UC luminescence of Mn2+ ions in Sr5(PO4)3Cl:Yb3+,Mn2+ can be well

interpreted by the GSA/ESA mechanism. Fig.

8

shows

the

schematic

representation

of

the

UC

process

in

Sr5(PO4)3Cl:Yb3+,Mn2+ as well as the geometric structure of Yb3+-Mn2+ dimers. Depending on the substitution of doping ions and the formation conditions of dimer, the face-sharing Yb3+(Sr1)-Mn2+(Sr1) and corner-sharing Yb3+(Sr1)-Mn2+(Sr2) dimers have been easily formed. Through the super-exchange interaction, the Yb3+-Mn2+ dimer has constructed the new ground, intermediate and emitting state as |2F7/2, 6A1(6S)>, |2F5/2, 6A1(6S)>

and |2F7/2, 4T1(4G)>, respectively. In the GSA step, the dimer is promoted from

ground state |2F7/2, 6A1(6S)> to intermediate state |2F5/2, 6A1(6S)>, which is mainly localized on the Yb3+ ions. And then the ESA step populates the emitting state |2F7/2, 4T

4 1( G)>

localized at Mn2+, from which the dimer return to its ground state and a visible

photon is released. The same GSA/ESA mechanism is also applicable to the UC process of Yb3+-Yb3+ dimer. All the detail UC steps are described as follows: Mn2+-Yb3+ dimer: |2F7/2, 6A1(6S)> + hv(976 nm) => |2F5/2, 6A1(6S)> (GSA) |2F5/2, 6A1(6S)> + hv(976 nm) => |2F7/2, 4T1(4G)> (ESA) |2F7/2, 4T1(4G)> => |2F7/2, 6A1(6S)> + hv(~585 nm) Yb3+-Yb3+ dimer: |2F7/2, 2F7/2> + hv(976 nm) => |2F5/2, 2F7/2> (GSA) |2F5/2, 2F7/2>+ hv(976 nm) => |2F5/2, 2F5/2> (ESA) |2F5/2, 2F5/2> => |2F7/2, 2F7/2> + hv(~506 nm) With the simple three energy level model shown in the UC schematic diagram, the abnormal photon number obtained in the inset of Fig. 6 can be elucidated. Assuming that the population density N0 at ground state without bleaching is constant, the rate

equations are listed below: dN1/dt = ρPσ0N0 - ρPσ1N1 - D1N1 dN2/dt = ρPσ1N1 - D2N2

(4) (5)

where ρP is the pump constant proportional to the exciting power P [32], σi, Ni and Di are the absorption cross section, population density and linear decay constant of level i (as indicated in Fig. 8), respectively. Under steady condition, i.e. dN1/dt = dN2/dt = 0, the equations can be deduced as follow:

ρPσ0N0 = ρPσ1N1 + D1N1 ρPσ1N1 = D2N2

(6) (7)

If the item ρPσ1N1 is much smaller than D1N1, it can be omit in eqn (6), which means the linear decay is dominated in the depopulation process of N1, so N1 ∝ ρP ∝ P is obtained. Further, N2 ∝ ρPN1 ∝ P2 can be derived from eqn (7), corresponding to the two photons UC process. But if ESA rate is large enough to suppress the linear decay process (ρPσ1N1 >> D1N1), N1 can be viewed as a constant, so N2 is only proportional to ρP, that is N2 ∝ P1. In our case, the photon number is estimated to be 1 meaning that a large ESA rate is acquired under the test conditions. What is worth mentioning is that other depopulation processes of N2 level are not be considered in the proposed rate equations, such as the energy transfer process from Mn2+ to Yb3+ ions in the Fig. 7. This additional depletion item would make N1 become a variable depended on P again, which leads to the situation of N2 ∝ Pn (n < 1) with the large ESA rate [11,16]. 4. Conclusion In summary, a yellowish broadband RT UC luminescence has been obtained in

Sr5(PO4)3Cl:Yb3+,Mn2+ phosphate under 976 nm excitation. The six-coordinated and seven-coordinated Sr Sites can be both substituted by Mn2+ ions to form different luminescence centers for the Stokes and UC luminescence. The UC spectrum is composed of the green (506 nm) and yellow (585 nm) emission bands corresponding to the super-exchange coupled Yb3+-Yb3+ and Yb3+-Mn2+ dimers, respectively. The UC mechanism of both emission bands are deduced to be GSA/ESA according to the analysis of the crystal structure and concentration-dependent UC spectra. The obtained photon number of the UC luminescence of Yb3+-Mn2+ dimers deviated far from two-photons process is ascribed to the large ESA rate of intermediate state |2F5/2, 6A1(6S)> and the additional depletion of emitting state |2F7/2, 4T1(4G)>. The broadband UC emission of Sr5(PO4)3Cl:Yb3+,Mn2+ phosphors at RT has strengthened the comprehension on UC process of Mn2+ ions and provides an insight into the design of new broadband UC materials. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (grant number 51562011), Natural Science Foundation of Jiangxi Province of China (grant number 20181BAB206028), Science and technology research project of Jiangxi Educational Committee of China (grant number GJJ160732) and Natural Science Foundation of Jinggangshan University (grant number JZ1904).

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Tables and table captions: Table 1. The total energy for different site occupancies of Mn and Yb in Sr5(PO4)3Cl host.

Position

[email protected]

[email protected]

[email protected]

[email protected]

Ed(eV)

-293.16103

-292.66312

-294.24441

-294.00814

Figures and figure captions: Fig.

1

The

XRD

patterns

of

Sr5(PO4)3Cl,

Sr5(PO4)3Cl:0.05Yb3+

and

Sr5(PO4)3Cl:0.05Yb3+,0.5Mn2+ samples as well as the standard pattern (JCPDS 83-0974). The diffraction peaks around 30.5 degree are shown on the right for comparison.

Fig. 2 The crystal structure of Sr5(PO4)3Cl unit cell and the coordination geometry of Sr2+ ion.

Fig. 3 The PLE and PL spectra of Sr5(PO4)3Cl:0.05Yb3+,0.5Mn2+ phosphor.

Fig. 4 (a) The PL spectra of Sr5(PO4)3Cl:0.05Yb3+,xMn2+ (x = 0.25, 0.5, 0.75, 1, 1.25). The peak wavelength and intensity ratio of the two sub-bands (I617/I573) varied with x are shown in the inset. (b) The semi-log luminescence decay curves of Sr5(PO4)3Cl:0.05Yb3+,xMn2+ samples under excitation at 406 nm.

Fig. 5 The UC spectra of Sr5(PO4)3Cl:0.05Yb3+,xMn2+ (x = 0.25, 0.5, 0.75, 1, 1.25) upon 976 nm excitation at RT. The inset shows the dependence of integrated intensity on Mn2+ content x.

Fig. 6 Power-dependent UC spectra and the corresponding log(I585)-log(P) plots (inset) of Sr5(PO4)3Cl:0.05Yb3+,0.5Mn2+ upon 976 nm excitation.

Fig. 7 The PLE and PL spectra (in NIR region) of Sr5(PO4)3Cl:0.05Yb3+ and Sr5(PO4)3Cl:0.05Yb3+,0.5Mn2+.

Fig. 8 Schematic representation of the UC process in Sr5(PO4)3Cl:Yb3+,Mn2+ as well as the geometric structure of Mn2+-Yb3+ dimers.

Fig. 1

Sr5(PO4)3Cl:0.05Yb3+,0.5Mn2+

Intendity (a.u.)

Sr5(PO4)3Cl:0.05Yb3+

Sr5(PO4)3Cl

JCPDS 83-0974 20

30

40

50

2Theta(degree)

Fig. 2

60

70

30.2 30.4 30.6

Fig. 3

λex = 406 nm

Intensity (a.u.)

λem = 585 nm

Sr1 Sr2

300

350

400

450

500

550

600

Wavelength (nm)

650

700

750

Fig. 4

(a) Intensity(a.u.)

x Mn2+ 0.25 0.50 0.75 1.00 1.25

590

4

585

3

580

2

575 1 570 0.00 0.25 0.50 0.75 1.00 1.25

Mn2+ content x

λex = 406 nm

450

500

550

600

650

700

750

Wavelength(nm)

1

log[Intensity(a.u)]

x Mn2+ τ* 0.25 11.4 ms 0.50 10.2 ms 0.75 9.74 ms 1.00 8.38 ms 1.25 8.21 ms

0.1

(b) 0

10

20

Time(ms)

30

800

I617/I573

Peak Wavelength(nm)

5

Fig. 5

x Mn2+ 0.25 0.50 0.75 1.00 1.25

Intensity(a.u.)

λem = 585 nm

Intensity(a.u.)

λex = 976 nm

0.2

0.4

0.6

0.8

1.0

1.2

Mn2+ Content x

450

500

550

600

650

700

750

800

Wavelength (nm)

Fig. 6

λex = 976 nm

log[I585(a.u.)]

6.7

Intensity(a.u.)

500 mW 450 mW 400 mW 350 mW 300 mW 250 mW 200 mW 150 mW

450

500

λem= 585 nm

6.6 6.5 6.4 6.3

Slope = 0.9

6.2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8

log[P(mW)]

550

600

650

Wavelength(nm)

700

750

800

Fig. 7

λem = 976 nm

λex = 406 nm

Sr5(PO4)3Cl:0.05Yb3+:

λem = 976 nm

λex = 260 nm

Intensity(a.u)

Sr5(PO4)3Cl:0.05Yb3+,0.5Mn2+:

a c 250

300

350

400

450

500

b d 900

Wavelength(nm)

Fig. 8

950 1000 1050 1100

Highlights (1) A broadband upconversion luminescence of Yb3+/Mn2+ codoped Sr5(PO4)3Cl were obtained at room temperature. (2) The upconversion mechanisms of emission bands peak at 506 nm and 585 nm were discussed. (3) The prepared phosphor provide a new insight for upconversion luminescent materials in lighting and displays applications .

Author Statement: Fen Xiao: Conceptualization, Methodology, Writing - Original Draft. Shikun Xie: Investigation. Rongxi Yi: Data Curation. Shu Peng: Resources. Chengning Xie: Supervision, Writing - Review & Editing.

Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.