Mn2+ codoped barium octaborate

Mn2+ codoped barium octaborate

Journal of Alloys and Compounds 587 (2014) 177–182 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.e...

1MB Sizes 1 Downloads 30 Views

Journal of Alloys and Compounds 587 (2014) 177–182

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom

Abnormal broadband photoluminescence from Yb3+/Mn2+ codoped barium octaborate F. Xiao a,b, E.H. Song a, S. Ye a, Q.Y. Zhang a,⇑ a b

State Key Laboratory of Luminescent Materials and Devices, and Institute of Optical Communication Materials, South China University of Technology, Guangzhou 510641, PR China School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, PR China

a r t i c l e

i n f o

Article history: Received 16 September 2013 Received in revised form 21 October 2013 Accepted 22 October 2013 Available online 30 October 2013 Keywords: Upconversion luminescence Exchange interaction Cooperative sensitization Phosphor

a b s t r a c t Concentration-dependent anomalous broadband down- and upconversion luminescence of Yb3+/Mn2+ codoped barium octaborate have been demonstrated. Upon ultraviolet (UV) excitation, dual Gaussian emission bands depend on the Mn2+ concentration located at 550 and 656 nm were respectively ascribed to the Mn2+ pairs and single Mn2+ ion. Moreover, abnormal broadband upconversion luminescence at room temperature in BaB8O13:Yb3+, Mn2+ under near-infrared (NIR) excitation of Yb3+ at 976 nm was demonstrated by the cooperative sensitization luminescence process. The experimental results could provide new insights into developing lighting and display systems, and biomedical imaging. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Recently, as the increasing demand for tunable light emitting and high efficiency sunlight-powered devices, the investigation on efficient upconversion (UC) luminescence has attracted more and more interests. Most of the reported UC processes were realized by the mixed lanthanide ion pairs with fixed and narrow emission band, such as Tm3+, Er3+, Nd3+ and Ho3+ with Yb3+ ions as the excellent UC sensitizer [1–4]. In order to obtain the broadband UC luminescence, the d–d transitions of Mn2+ ions and Yb3+ pairs has led to some new and unexpected results in the search for novel UC materials and process, such as Yb3+ doped CsMnCl3, Rb2MnCl4, CsMnBr3, and so on [5–7]. However, this kind of Yb3+ doped manganese chlorides and bromides only exhibits UC luminescence at low temperature, while the UC luminescence is quenched at room temperature (RT) due to the high non-radiative rate of Mn2+ ions with the increase of temperature. In the last few years, there are some reports on RT UC luminescence in Mn2+/Yb3+ codoped aluminates and borates [8–10]. In all these Mn2+–Yb3+ systems, the excitation band at around 1 lm is resonant with 2 F7/2 ? 2F5/2 transition of Yb3+ ions which results in the visible UC emission from Mn2+ ions. Since the excited states of Mn2+ ions are more sensitive to chemical and structural changes of the ligands than those of rare earth (RE) ions, both of which can give rise to tunable emission properties from Mn2+ ions. For example,

⇑ Corresponding author. Tel.: +86 20 87113681. E-mail address: [email protected] (Q.Y. Zhang). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.10.177

changing the crystal field strength on Mn2+ can tune the emission from green (strong crystal field) to orange/red (weak crystal field). Therefore, the UC luminescence properties of Mn2+–Yb3+ systems can be easily modified by changing their chemical composition. However, the previous references mainly focused on the low temperature UC luminescence or the single emission band of Mn2+. In the present study, barium octaborate BaB8O13 was selected as the host material, which is well known for the self-reduction properties and can obtain strong green and red emission by doping Mn2+ ions [11]. BaB8O13 was reported to be crystallized in an orthorhombic crystal system, which consists of two separate interlocking three-dimensional infinite networks as alternating triborate and pentaborate groups. Triborate groups are linked only to pentaborate groups [12]. Furthermore, we also observed the UC luminescence of Yb3+ doped BaB8O13 phosphors. As twice the energy of 2F5/2 ? 2F7/2 of Yb3+ ions around 500 nm originated from the cooperative luminescence of a pair excited Yb3+ ions, which is higher than the emission energy of the Mn2+. Therefore, the UC emission may be highly expected in the Yb3+ and Mn2+ codoped BaB8O13 at room temperature. Herein, we demonstrated an unusual Mn2+ ions concentration dependent down-conversion luminescence based on steady-state and time-resolved spectroscopy techniques. It was demonstrated that the two different emission bands originated from Mn2+ pairs and single Mn2+ ion respectively. An anomalous broadband UC luminescence ranged from 460 to 750 nm was observed in Yb3+/ Mn2+ codoped BaB8O13 phosphors at room temperature. We proposed that this broadband UC luminescence mechanism was dominated by the cooperation sensitization process.

178

F. Xiao et al. / Journal of Alloys and Compounds 587 (2014) 177–182

2. Experimental section 2+

3+

2+

Series BaB8O13:x%Mn and BaB8O13:5%Yb , x%Mn phosphors were synthesized by a solid-state reaction method. According to the desired stoichiometric ratios of each sample, the starting materials of BaCO3, H3BO3, Mn(CH3COO)2, Yb2O3(99.99%) and K2CO3 (charge compensator) were thoroughly grounded including a 5% excess of H3BO3. The mixtures were put into alumina crucible and preheated at 500 °C for 2 h. After that, the samples were regrounded and heated at 700 °C for 5 h. Finally, the as-synthesized white powder samples were obtained after cooling to room temperature. Crystalline phases were checked by X-ray powder diffraction (XRD) analysis with a Bruker D8 Endeavor with Cu Ka radiation (k = 1.5405 Å). Diffraction data were collected by step scanning from 10° to 65° in 2h with a step size of 0.02° and a counting time of 0.2 s per step. Rietveld refinement was conducted using the TOPAS Academic program. Photoluminescence (PL), photoluminescence excitation (PLE) and time-resolved PL spectra of the samples were measured using a FLS920 fluorimeter (Edinburgh Instruments, Livingston, U.K.) with a 450 W Xenon lamp as the excitation source. The NIR emission was detected by a R5509-72 PMT in liquid nitrogen. UC luminescence was obtained by a Jobin–Yvon TRIAX320 spectrofluorimeter equipped with a photomultiplier tubes Hamamatsu R928 and a 976 nm continuous wave LD.

3. Results and discussion The powder XRD for the samples were firstly measured to verify the phase purity and to check the crystal structure. The XRD patterns of BaB8O13, BaB8O13:25%Mn2+, and BaB8O13:5%Yb3+, 10%Mn2+ phosphor are presented in Fig. 1. All the diffraction peaks match well with the JCPDS card no. 20-0097, indicating that the obtained samples are pure phase. Fig. 2(a) shows the XRD pattern and Rietveld refinement profiles of BaB8O13:5%Mn2+, and the structure refinements data are summarized in Table 1. The red line and blue circles in Fig. 2(a) represent the experiment and calculated patterns, respectively. The as-obtained fit parameter v2 = 1.73 and Rwp = 6.19% ensure the purity of the sample phase, and the crystal structure of BaB8O13 has been proved to be included in orthorhombic crystal system with space group P2221 and lattice constants of a = 8.55 Å, b = 17.35 Å, c = 13.21 Å, V = 1959.98 Å3 and Z = 8 [12]. Fig. 2(b) presents the crystal structure of BaB8O13 viewed from b-axis direction. This crystal structure is built up by two separate interlocking three-dimensional (B8O13)1 infinite networks as connected by six BO3 and two BO4 groups [13]. Notably, the six-coordinated Ba2+ has one crystal site surrounded by BO3 and BO4, all of the O2 is shared by BO3 and BO4. Since there are two crystallographically independent cation sites in BaB8O13 host, one for Ba2+ and one for B3+. Considering the effective ionic radius of cations with different coordination (CN) numbers, Mn2+ (CN = 6,0.83 Å) prefer to occupy the Ba2+

(CN = 6, 1.35 Å) sites due to the smaller ionic radii difference compared with B3+ (CN = 4, 0.11 Å) [14]. Moreover, the electric charge of Mn2+ is the same as Ba2+, which is beneficial for the occupation. Therefore, it can be deduced that the Mn2+ doping ions will substitute Ba sites obviously. Moreover, The smaller refinement cell volume in Table 1 was found to be V = 1952.409 Å3 for BaB8O13:5%Mn2+, which further proves the substitution of smaller Mn2+ ions for the larger Ba2+ ions. Moreover, the luminescence of Mn2+ occupy Ba site was previously reported [15–17]. Since the radius of Yb3+ (0.868 Å, CN = 6) is more close to Ba2+ ions, it will also occupied Ba sites and their charge difference has been eliminated by the addition of charge compensator K+ ions. Fig. 3(a) presents PL spectra of BaB8O13:x%Mn2+ (x = 0.5, 5, 10, 15, 20, 25) phosphors under the excitation of 407 nm. For the low Mn2+ doping concentration (x = 0.5), the emission spectrum exhibits a Gaussian profile centered at 600 nm. It should be noted here that with the increasing Mn2+ doping concentration from x = 5 to 25, there exhibits an asymmetric emission band and the emission intensities increase continuously without any concentration quenching. The normalized PLE and PL spectra of BaB8O13:10%Mn2+ phosphor are shown in Fig. 3(b). Under the excitation of 407 nm, the PL spectrum consists of a broad band ranged from 500 to 800 nm with maximum wavelength at 656 nm. The broad asymmetry emission band could be deconvoluted into two Gaussian peaks at 550 and 656 nm, and both of the PLE spectra exhibit the same series absorption bands. The excitation spectrum monitored at 656 nm consists of several bands located at 316, 342, 369, 407, 441 and 485 nm ascribing to the transition from the ground state 6 A1(6S) to the excited states of 4T1(4P), 4E(4D), 4T2(4D), [4A1(4G), 4 4 E( G)], 4T2(4G) and 4T1(4G) energy levels, respectively [18]. In order to offer interpretation on the two emission bands of Mn2+ in BaB8O13 host, the luminescence decay measurements upon 407 nm excitation are provided. To eliminate the minor disturbance of the two different emission band (except for the sample of BaB8O13:0.5%Mn2+, measured at 600 nm), the lifetime measurements were detected at 530 (I) and 670 nm (II), which deviated from the corresponding peak wavelengths in the emission spectra. Fig. 4(a) and (b) show the fluorescence decay curves of BaB8O13:x%Mn2+ phosphors, which can be well fitted by a bi-exponential equation as follow:

I ¼ A1 expðt=s1 Þ þ A2 expðt=s2 Þ þ I0

ð1Þ

where I and I0 are the luminescence intensities at time t and 0, A1 and A2 are fitting constants, s1 and s2 are the fast and slow luminescence lifetimes, respectively. The effective average lifetime seff can be estimated by the following equation [19]:

seff ¼ ðA1 s21 þ A2 s22 Þ=ðA1 s1 þ A2 s2 Þ

ð2Þ

Intensity (a.u.)

(d)

(c)

(b)

(a) 10

15

20

25

30

35

40

45

50

2θ (Degree) Fig. 1. XRD patterns of JCPDS card no. 20-0097 (a), BaB8O13 (b), BaB8O13:25%Mn2+ (c) and BaB8O13:5%Yb3+, 10%Mn2+ (d).

Thus seff for x = 0.5 was obtained with the value of 6.70 ms. The average decay lifetimes of peak I and II at various concentrations were calculated and summarized in Table 2. Peak II exhibited an increasing average lifetime from 10.6 ms to 16.1 ms which are typical characterization for the parity and spin-forbidden 4T1 ? 6A1 transition of Mn2+ ions [20,21]. Therefore, it can be concluded that peak II originated from Mn2+ occupied octahedral Ba site in BaB8O13 host. With the increasing Mn2+ concentration, there is a red-shift from 600 nm (x = 0.5) to 656 nm (x = 25), which could be ascribe to the enhancement of crystal field strength caused by the substitution of smaller ionic radius of Mn2+ (0.83 Å) for Ba2+ (1.35 Å) ions. The increasing average lifetime may be ascribed to the energy transfer from peak I to peak II since there are spectral overlap between the excitation band of peak II and emission band of peak I. The obtained lifetimes for peak I showed a fast decay ranged from 1.4 ms to 718 ls, which was different from the isolated

179

F. Xiao et al. / Journal of Alloys and Compounds 587 (2014) 177–182

Fig. 2. (a) Experimental (blue circle), calculated (red solid line) and difference (gray solid line) XRD patterns of the Rietveld refinement of BaB8O13:5%Mn2+. Blue vertical lines represent the position of Bragg reflection in the bottom. (b) Crystal structure of BaB8O13 viewed along b axis. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 1 Crystallographic data and Refinement Parameters of BaB8O13:5%Mn2+.

v

Mn2+ ions. Since there was only one kind of site (Ba) for the occupation of Mn2+ in BaB8O13 and the lattice contraction will only result in red shift of emission band, the origination of peak I may come from some other mechanism. It is well known that Mn2+ pairs or clusters emission generally shows a fast decay rate in oxide, nitride and sulfide compounds [22–24]. For the transition metal Mn2+ (3d5) ions, the ground state and first excited state are 6A1 (total spin S = 5/2) and 4T1 (S = 3/2), respectively. Moreover, as the total spin S for the exchange coupled pairs is determined by the linear combination of two spin states SI and SII, which can be expressed as: (SI + SII),. . .,(SI  SII). Therefore, this will result in the total spin state S = 5, 4, 3, 2, 1 and 0 when both Mn2+ ions are located in ground state. When one of the Mn2+ located in ground state (S = 5/2) and the other ion were in the first excited state (S = 3/2), the values of S will be 4, 3, 2, and 1. For the antiferromagnetic exchange-coupled Mn2+ pair, the ground state and excited state can be denoted by 6A16A1 and 6 A14T1 respectively, and the emission originate from the lowest excited state to different spin component of ground state which will result in the shift of emission band [25]. When DS = 0, the spin selection rule will be changed from spin-forbidden to spin-allowed optical transition and the corresponding decay lifetime shortening. Moreover, it has been reported that the exchange interaction will also result in blue shift of the emission band from single ions to Mn2+ pairs [26]. According to the crystal structure of BaB8O13, the octahedral Ba2+ ions are structurally isolated from each other, consequently, the most probably pairs formation will be Mn2+–O–O0 –Mn2+ after Mn2+ occupying the Ba2+ site, which exhibit a super-exchange way with a shortest distance of 6.596 Å [9]. Therefore, it can be deduced that peak I is probably originated from Mn2+ pairs with increasing Mn2+ concentration.

x = 0.5 x=5 x = 10

Relative Intensity (a.u.)

2

(a)

BaB8O13 Orthorhombic P2221 1952.409 (77) 8.5458 (2) 17.3359 (3) 13.1786 (3) 6.19 4.72 1.73

x = 15 x = 20 x = 25

500

550

600

650

700

750

800

Wavelength (nm)

1.0

(b)

4

4

A1( G)

λem = 550 nm λem = 656 nm

Relative Intensity (a.u.)

Compounds Crystal system Space group Cell volume (Å3) a (Å) b (Å) c (Å) Rwp (%) Rp (%)

λex = 407 nm

0.8

FittedCurves 1 FittedCurves 2 0.6

4

4

E( D)

0.4

4

4

T2( G)

0.2

4 4

4

T1( P)

4

T2( D)

4

4

T1( G)

0.0 300

350

400

450

500

550

600

650

700

750

800

Wavelength (nm) Fig. 3. PL spectra of BaB8O13:x%Mn2+ phosphors (kex = 407 nm) (a), normalized PLE and PL spectra of BaB8O13:10%Mn2+ (b).

F. Xiao et al. / Journal of Alloys and Compounds 587 (2014) 177–182

Normalized Intensity (a.u.)

180 1

(a)

x = 5% x = 10% x = 15% x = 20% x = 25%

0.1 0.01 1E-3 1E-4

Normalized Intensity (a.u.)

0.0 1

0.5

1.0

1.5

(b)

x = 0.5% x = 5% x = 10% x = 15% x = 20% x = 25%

0.1 0.01 1E-3 1E-4 0

5

10

15

20

Time (ms) Fig. 4. Decay curves of the peak I (a, kem = 530 nm) and II (b, kem = 670 nm) in BaB8O13:x%Mn2+ phosphors (kex = 407 nm). BaB8O13:0.5%Mn2+ in (b) measured at emission wavelength of 600 nm.



x = 10

x = 15

x = 20

x = 25

1.47 ms 10.6 ms

804 ls 11.1 ms

778 ls 11.4 ms

791 ls 12.2 ms

718 ls 16.1 ms

Fig. 5 presents the UC spectrum of BaB8O13:5%Yb3+. Upon the 976 nm excitation, the PL spectrum exhibit green emission centered at around 500 nm which is nearly twice the NIR emission energy of Yb3+ ion (1.0 lm). It is well known that the Yb3+ ion only has one excited state (2F5/2) located at about 104 cm1 above the ground state (2F7/2) and usually makes the 1.0 lm NIR emission from the 2F5/2 ? 2F7/2 transition. The green emission should originate from the cooperative luminescence of a pair excited Yb3+ ions, as the single Yb3+ ions has no stationary states. This process can be expressed by the following equation [27,28]:

Intensity (a.u.)

Log[Intensity (a.u.)]

500nm slope = 1.51

3.07 3.12 3.17 3.22 3.27 3.32

Log[exitation power (mw)]

450 460

480

500

520

540

560

580

Wavelength (nm) Fig. 5. The UC spectra of BaB8O13:5%Yb3+. The inset shows the dependence of integral intensity of Yb3+ luminescence on excitation power.

ð3Þ

Generally, for an unsaturated UC process, the emission intensity (Iuc) is proportional to the power of the NIR excitation intensity n (INIR) according to Iuc ¼ kINIR , where n indicates the number of photons absorbed to produce the visible emission. The dependence of green UC luminescence intensity upon the pump power Iuc is plotted in the inset of Fig. 5, which was fitted to a straight line with the slope of 1.51 illustrating that a two-photon processes were involved to generate this green emission. The UC luminescence of series BaB8O13:5%Yb3+, x%Mn2+ phosphor under 976 nm excitation at room temperature are detected and shown in Fig. 6. All the UC spectra exhibit a broad emission band ranging from 460 to 750 nm with two peak wavelengths at 550 nm and 656 nm, which were consistent with the emission bands as shown in Fig. 3(a). The emission intensity of the two x = 5% x = 10% x = 15% x = 20% x = 25%

550 nm slope = 1.12 656 nm slope = 1.04

Log[Intensity (a.u.)]

x=5

Relative Intensity (a.u.)

s530 s670



Yb ð2 F5=2 Þ þ Yb ð2 F5=2 Þ ! Ybð2 F7=2 Þ þ Ybð2 F7=2 Þ þ hc

Table 2 Average decay lifetime in BaB8O13:x%Mn2+ phosphors (kex = 407 nm).

3.10

3.15

3.20

3.25

3.30

Log[exitation power (mw)]

500

550

600

650

700

750

Wavelength (nm) Fig. 6. UC spectra for BaB8O13:5%Yb3+, x%Mn2+ phosphors (kex = 976 nm). The inset shows the dependence of integral intensity of green and red emission of Mn2+ on excitation power. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

181

F. Xiao et al. / Journal of Alloys and Compounds 587 (2014) 177–182

b

Intensity (a.u.)

a

PL

PLE

d

c 200

300

400

500 900

1000

1100

Wavelength (nm) Fig. 7. PLE (a, kem = 976 nm) and PL (b, kex = 407 nm) spectra of BaB8O13:5%Yb3+, 10%Mn2+, PLE (c, kem = 976 nm) and PL (d, kex = 264 nm) spectra of BaB8O13:5%Yb3+.

emission bands exhibit the same changing trend with a maximum at x = 10, and then decreases sharply with the increasing doping concentration due to the concentration quenching effect. Moreover, the relative emission intensity of green band (550 nm) is stronger than the red one (656 nm), which is just opposite to that spectrum upon 407 nm excitation. Here we would like to ascribe this phenomenon to the different luminescence mechanisms. The inset of Fig. 6 shows the double logarithmic plots of the green and red UC luminescence intensity as a function of excitation power for the BaB8O13:5%Yb3+, 10%Mn2+ sample. The emission intensities at 550 and 656 nm under different power intensity could be well fitted with slopes of 1.12 and 1.04, respectively. In general, a fitted slope of k is indicative of the UC process involved at least n photons, where n is the smallest integer greater than k (or equal to k if k is an integer). In addition, the value of the UC luminescence intensity depending on absorbed pumping power P has been proved to range from the limit Pn at low pump power and the limit P1 at high pump power [29]. Therefore, a two-photon process is expected for the two different visible UC luminescence

peaks of Mn2+ in the present work. Here the slopes are slightly larger than 1 may be caused by two reasons, one is the not very strong UC emission intensity which is easily get to saturation under the high pump power excitation, the other one is the additional down conversion channels such as the energy transfer from Mn2+ to Yb3+ ions that will be discussed subsequently [30]. To figure out the UC mechanism, the NIR PL and PLE spectra of BaB8O13:5%Yb3+, 10%Mn2+ are shown in Fig. 7 to provide more detailed information. The NIR emission centered at 1.0 lm, due to the transition of 2F5/2 ? 2F7/2 from Yb3+ ions, was observed under 407 nm excitation (Line b). The PLE spectrum monitored at 976 nm not only exhibits the series excitation bands of Mn2+ ions, but also a new broad band from 200 to 300 nm (Line a). To clarify its origination, we simultaneously provided the PLE and PL spectra of BaB8O13:5%Yb3+ phosphor (Line c and d). The broad excitation band centered at 264 nm originated from the charge transfer of O2  Yb3+, which coincided with the broad band in BaB8O13:5%Yb3+, 10%Mn2+. JØrgensen has deduced an empirical formula to describe the relationship between the charge transfer band and the optical electro-negativity of the new introduced ions from a large number of experiments [31]:

h

i

r ¼ vopt ðXÞ  vuncorr ðMÞ 30; 000 cm1

ð4Þ

where vopt(X) is the optical electro-negativity of the ligand ion, which is an approximation of the Pauling’s electro-negativity. vuncorr(M) is the optical electro-negativity of central cation which has close relationship with the composition and structure of host. With the value of vopt(O) = 3.2 and the energy of O2 ? Yb3+ transition observed in the present experiment, the vuncorr(Yb) is estimated to be 1.9, which is higher than the value of 1.6 provided by JØrgensen. Here we propose that the red shift of the charge transfer absorption band was attributed to the relative big ionic radius of the cation site (Ba) compared with the rare earth ions (Y, Gd, La) with the same coordination number [32,33]. Since the UC process was reported from 1966, most UC processes of rare earth ion pairs can be classified into ground-state-absorption (GSA)/excited state absorption (ESA), GSA/energy-transfer-upconversion (ETU) and cooperative sensitization process [34,35]. For the GSA/ESA mechanism, a single ion would accept a second photon by ESA after reaching a metastable

Fig. 8. Schematic representation of the proposed UC mechanism in BaB8O13:Yb3+, Mn2+.

182

F. Xiao et al. / Journal of Alloys and Compounds 587 (2014) 177–182

intermediate state by GSA, while it would transfer nonradiative energy to the ion in close proximity for the case of GSA/ETU mechanism. The cooperative sensitization process needs two ions getting close and being excited to the same excited state, then combines and transfers excitation energy to an acceptor ion to emit one photon. Based on the UC spectra of BaB8O13:Yb3+, Mn2+ phosphor, after absorbing the pumping energy that caused 2F7/2 ? 2F5/2 transition, only the emission from the excited state 4T1 of Mn2+ can be observed. Because Mn2+ ions have no intermediate excitation states resonant with Yb3+ ions, a conclusion can be obtained that the GSA/ESA and GSA/ETU cannot occur simultaneously in single Yb3+ or Mn2+, and both Yb3+ and Mn2+ ions should be involved to explain the UC mechanism. As we have obviously observed the UC luminescence of Yb3+ ions, it can be deduced that the most possible mechanism for UC luminescence is cooperative sensitization process [36]. Fig. 8 schematically presents the potential UC mechanisms of cooperative sensitization process of one Mn2+ ions by two nearby Yb3+ excitations. When two Yb3+ ions occupying different Ba2+ sites locate in close position, they combine their excitation energy by a cooperative process to transfer it to the nearby Mn2+ (or Mn2+ pairs) simultaneously, and then gives the red (or green) emission. The same UC mechanism for transition metal ions has been previously proposed [8,37]. 4. Conclusions In summary, Mn2+ ions concentration depended luminescence under UV excitation was investigated in BaB8O13:Mn2+ phosphors. The two broad emission bands centered at 550 and 656 nm were demonstrated to originate from the Mn2+ exchange pairs and single Mn2+ ion, respectively. Furthermore, an efficient broadband UC luminescence upon 976 nm laser excitation on BaB8O13:Yb3+, Mn2+ phosphors was investigated systematically at room temperature. The UC mechanism was proposed to be a cooperative sensitization process. These results indicate that BaB8O13:Yb3+, Mn2+ phosphors may be considered as a potential candidate for lighting and displays. Acknowledgements This work is financially supported by NSFC (Grant Nos. 51125005, 21101065 and U0934001) and China Postdoctoral Science Foundation funded project (2012M511802).

References [1] I. Etchart, M. Bérard, M. Laroche, A. Huignard, I. Hernández, W.P. Gillin, R.J. Curryd, A.K. Cheetham, Chem. Commun. 47 (2011) 6263. [2] Q. Zhang, G.R. Chen, G. Zhang, J.R. Qiu, D.P. Chen, J. Appl. Phys. 106 (2009) 113102. [3] X.F. Wang, Y.Y. Bu, S.G. Xiao, J.K. Yang, J.W. Ding, Mater. Lett. 62 (2008) 3865. [4] T. Li, C.F. Guo, P.J. Zhao, L. Li, J.H. Jeong, J. Am. Ceram. Soc. 96 (2013) 1193. [5] R. Valiente, O. Wenger, H.U. Güdel, Chem. Phys. Lett. 320 (2000) 639. [6] C. Reinhard, P. Gerner, R. Valiente, O.S. Wenger, H.U. Güdel, J. Lumin. 94–95 (2001) 331. [7] P. Gerner, O.S. Wenger, R. Valiente, H.U. Güdel, Inorg. Chem. 40 (2001) 4534. [8] R. Martín-Rodríguez, R. Valiente, M. Bettinelli, Appl. Phys. Lett. 95 (2009) 091913. [9] R. Martín-Rodríguez, R. Valiente, F. Rodríguez, F. Piccinelli, A. Speghini, M. Bettinelli, Phys. Rev. B 82 (2010) 075117. [10] S. Ye, Y.J. Li, D.C. Yu, G.P. Dong, Q.Y. Zhang, J. Mater. Chem. 21 (2011) 3735. [11] R. Cheng, J. Huang, Y. Xu, J. Fudan Univ. Nat. Sci. 28 (1989) 304. [12] J. Krogh-Moe, M. Ihara, Acta Crystallogr. B 25 (1969) 2153. [13] K.-I. Machida, D. Ueda, S. Inoue, G.-Y. Adachi, Electrochem. Solid State Lett. 2 (1999) 597. [14] R.D. Shannon, Acta Crystallogr. 32 (1976) 751. [15] Y.F. Wang, X. Xu, L.J. Yin, L.Y. Hao, Electrochem. Solid State Lett. 13 (2010) J119. [16] K.-S. Sohn, E.S. Park, C.H. Kim, H.D. Park, J. Electrochem. Soc. 147 (2000) 4368. [17] S. Ye, Z.S. Liu, X.T. Wang, J.G. Wang, L.X. Wang, X.P. Jing, J. Lumin. 129 (2009) 50. [18] L. Shi, Y.L. Huang, H.J. Seo, J. Phys. Chem. A 114 (2010) 6927. [19] Z.G. Xia, X.M. Wang, Y.X. Wang, L.B. Liao, X.P. Jing, Inorg. Chem. 50 (2011) 10134. [20] S. Okamoto, H. Yamamoto, J. Electrochem. Soc. 158 (2011) J363. [21] F. Li, Y.H. Wang, Electrochem. Solid State Lett. 9 (2006) J24. [22] C. Kulshreshtha, J.H. Kwak, Y.J. Park, K.S. Sohn, Opt. Lett. 34 (2009) 794. [23] C. Barthou, J. Benoit, P. Benalloul, A. Morell, J. Electrochem. Soc. 141 (1994) 524. [24] O. Goede, W. Heimbrodt, D.D. Thong, Phys. Status Solidi B 126 (1984) K159. [25] A.P. Vink, M.A. de Bruin, S. Roke, P.S. Peijzel, A. Meijerink, J. Electrochem. Soc. 148 (2001) E313. [26] C.R. Ronda, T. Amrein, J. Lumin. 69 (1996) 245. [27] E. Nakazawa, J. Lumin. 12 (1976) 675. [28] E. Nakazawa, Phys. Rev. Lett. 25 (1970) 1710. [29] M. Pollnau, D.R. Gamelin, S.R. Lüthi, H.U. Güdel, Phys. Rev. B: Condens. Matter 61 (2000) 3337. [30] G.J. Gao, L. Wondraczek, J. Mater. Chem. C 1 (2013) 1952. [31] C.K. JØrgensen, Prog. Inorg. Chem. 12 (1970) 101. [32] L. van Pieterson, M. Heeroma, E. de Heer, A. Meijerink, J. Lumin. 91 (2000) 177. [33] C.K. JØrgensen, Mol. Phys. 5 (1962) 271. [34] F. Auzel, C. R. Acad. Sci. 262 (1966) 1016. [35] F. Auzel, J. Lumin. 45 (1990) 341. [36] S. García-Revilla, P. Gerner, H.U. Güdel, R. Valiente, Phys. Rev. B 72 (2005) 125111. [37] S. Heer, M. Wermuth, K. Krämer, H.U. Güdel, Phys. Rev. B 65 (2002) 125112.