Luminescence properties and energy transfer behavior of colour-tunable white-emitting Sr4Al14O25 phosphors with co-doping of Eu2+, Eu3+ and Mn4+

Eu, Eu and Mn co-doped Sr4Al14O25 phosphors were synthesized in air by high temperature solid-state reaction. The coexistence of Eu, Eu and Mn in Sr4Al14O25 was detected and verified from X-ray photoelectron spectra (XPS), photoluminescence (PL)/photoluminescence excitation (PLE) spectra and diffuse reflection spectra. The crystal structure, luminescence properties and energy transfer between Eu and Mn were investigated scientifically. Under ultraviolet excitation, a broad emission band with peak at 496 nm was ascribed to a 4f5d–4f transition of Eu, and another five narrow emission bands were attributed to 4f–4f transitions of Eu. The sharp emission band with peak at 659 nm was generated from the spin-forbidden electronic transition Eg– T2 of Mn . The energy transfer between Eu and Mn was demonstrated following the nonradiative electric dipole–dipole interaction, and the energy transfer efficiency reached 58.16%. The emission colour of Sr4Al14O25:Eu,Mn 4+ can be tuned from blue-green (0.2396, 0.3332) to white (0.3465, 0.2906) with increasing concentrations of Mn.


Introduction
][3][4] At present, the commercial method to generate white light emission is exciting the yellow-emitting Y 3 Al 5 O 12 :Ce 3+ (YAG:Ce 3+ ) with an InGaN light-emitting diode (LED) chip which emits blue light with the wavelength between 450 nm and 480 nm typically. 5,6Because of the emission deciency in the red region, this WLED shows poor colour rendering and appears a simple cool white.8][9] This combination improves the colour rendering performance, but poor chemical stability will be shown as it ages due to the different chemical properties of the three phosphors.Compared with other approaches, studies on the preparation of direct-white-emitting light phosphors have been carried out due to their excellent thermal stability and colour rendering properties.
1][12][13] Peng 14 rstly found the coexistence of Eu 2+ and Eu 3+ in Sr 4 Al 14 O 25 prepared in air.Two broad bands peaking at 400 nm and 490 nm attributed to the 4f-5d transition of Eu 2+ , and line emissions in the region of 550-700 nm belong to the f-f transitions of Eu 3+ .Xu 15 reported the non-RE activated Sr 4 Al 14 O 25 :Mn 4+ phosphor prepared by solid-state method in air.The Al 3+ ions in the [AlO 6 ] octahedral sites of Sr 4 Al 14 O 25 were substituted by Mn 4+ ions in the crystal lattice, and the emission spectrum measurement excited by 365 nm showed that the sharp emission peak at 651 nm was ascribed to the 2 E-4 A 2 transition of Mn 4+ ion, and a broad band at 662 nm was attributed to the phonon sideband transition.Based on the above experimental results, the authors believe that there should be an energy transfer between Eu 2+ and Mn 4+ in Sr 4 Al 14 O 25 .Meanwhile, there has been no one, by far, reporting its luminescence properties and potential applications.The previous studies provide the access to obtain direct white emission for WLED by the coexistence of Eu 2+ , Eu 3+ and Mn 4+ in Sr 4 Al 14 O 25 .
In our work, a series of Sr 4 Al 14 O 25 :xEu,Mn 4+ phosphors were synthesized in air through the high temperature solid-state reaction.The coexistence of Eu 2+ , Eu 3+ and Mn 4+ was revealed by X-ray photoelectron spectroscopy (XPS), diffuse reection spectra and photoluminescence (PL) spectra.The inuences of Mn 4+ on the luminescence properties were discussed in detail, and the uorescence lifetime, energy transfer between Eu 2+ and Mn 4+ and applications in WLED were studied respectively.
The phase purity of synthesized phosphors were checked by X-ray diffraction (XRD) with a D8 Advance diffractometer with Cu Ka radiation (l ¼ 1.5406 Å) by the step of 4 min À1 at room temperature.The X-ray photoelectron spectroscopy (XPS) was obtained on an ESCALAB 250xi (ThermoFisher, England) electron spectrometer.The emission and excitation spectra were analyzed on a Hitachi F-4600 uorescence spectrophotometer with a 400 nm cut-off lter.The diffuse reection spectra were detected by a Shimadzu UV-3600 UV-vis-NIR spectrophotometer attached with an integral sphere.The photoluminescence decay curves were recorded on a Horiba JOBIN YVON FL3-21 spectrouorometer.

Results and discussion
Fig. 1 shows the XRD patterns of Sr 4 Al 14 O 25 :xEu (x ¼ 0, 0.005, 0.01, 0.02, 0.04, 0.06) and Sr 4 Al 14 O 25 :0.01Eu,yMn 4+ (y ¼ 0.0002, 0.0005, 0.001, 0.002, 0.003, 0.005, 0.01) synthesized at 1400 C for 5 h in air.The reference pattern of the standard JCPDS Card no.52-1876 for Sr 4 Al 14 O 25 was also listed in Fig. 1 as a reference.It is observed that the samples synthesized in various conditions are all in pure Sr 4 Al 14 O 25 phase, and the change of doping of Eu 3+ and Mn 4+ ions would not impact the host structure.
It has been found that the Mn 4+ ion is more likely to achieve stability once it is accommodated and stabilized in an octahedral site.Usually, there will be strong absorption and emission spanning from 250 nm to 500 nm and 630 nm to 735 nm respectively when Mn 4+ substitutes an octahedral site. 16,17The fragments of Sr 4 Al 14 O 25 unit cell are exhibited in Fig. 2. Sr 4 Al 14 O 25 is orthorhombic structure with space group Pmma.The structure contains two Sr sites and six Al sites.Sr(1) and Sr( 2) are both coordinated by ten oxygen atoms.Al(1), Al(2) and Al(3) form tetrahedrons with four coordinations, while Al(4), Al( 5) and Al( 6) are six-coordinated that form rigid octahedrons. 18Because the radius difference between Eu 2+ and Sr 2+ ions is much smaller than that of Eu 2+ and Al 3+ , and the ionic size of Mn 4+ is much smaller than that of Sr 2+ but close to Al 3+ , it is reasonable to consider the Eu 2+ ions are substituted in Sr 2+ sites, while Mn 4+ ions tend to occupy the octahedral sites of Al 3+ .Fig. 3 shows the observed (black squares), calculated (red lines), and difference (bottom) XRD proles for the Rietveld renement of Sr 3.96 Al 13.9972 O 25 :0.04Eu 2+ ,0.0028Mn 2+ with l ¼  The nal rened crystallographic data are listed in Table 1.The cell parameters are determined to be a ¼  4(a) that the broad bands peaking at 1136.5 eV ascribed to Eu 3+ 3d 5/2 are observed, and the bands peaking at 1127.5 eV consistent with Eu 2+ 3d 5/2 emerge gradually with the increasing doping of Eu 3+ ions. 19The peak intensities of Mn 4+ 2p 3/2 and 2p 1/2 at 642.5 eV and 654.5 eV, as shown in Fig. 4(b), increase with the increasing content of Mn 2+ ions, and no Mn 2+ peaks are observed as the value of y varies.We can deeply conrm that there is an coexistence of Eu 2+ , Eu 3+ and Mn 4+ in Sr 4 Al 14 O 25 phosphors.Fig. 5 shows the excitation and emission spectra of Sr 4 Al 14 -O 25 :Eu.We doped the Eu ions originally into the host in the form of Eu 2 O 3 , and we found that the trivalent and bivalent Eu ions co-existed in Sr 4 Al 14 O 25 :Eu prepared in air at 1400 C for 5 h.Under 496 nm monitoring, as shown in Fig. 5(a), the broad excitation band from 250 nm to 450 nm with maximum at 362 nm originates from the f-d transition of Eu 2+ . 20The other excitation spectra monitored at 619 nm consists of the broad bands from 200 nm to 350 nm, which is generated from the charge-transfer band (CTB) between Eu 3+ and O 2À with the peak at 261 nm, and the series of narrow bands from 350 nm to 550 nm form from the f-f transition of Eu 3+ within its 4f 6 conguration. 21The PL spectra with different excitation factors are demonstrated in Fig. 5(b).The characteristic peaks of Eu 2+ and Eu 3+ are both observed in emission spectra with the change of excitation wavelength.The band emission peaking at 469 nm stems from 4f 6 5d-4f 7 transition of Eu 2+ , and the other ve typical line emissions peaking at 596 nm, 620 nm, 652 nm, 688 nm and 707 nm are attributed to 4f-4f transition of 5 d 0 -7 f J (J ¼ 1, 2, 3, 4) of Eu 3+ . 22,23The strongest emission generated from the transition of 5 d 0 -7 f 2 of Eu 3+ belongs to the electric dipole transition, which reects that the electric dipole transition is the dominant factor in the luminescence process of Eu 3+ . 24he PL spectra of Sr 4 Al 14 O 25 :xEu (x ¼ 0.005, 0.01, 0.02, 0.04, 0.06, 0.08) prepared at 1400 C for 5 h and the dependence of the relative emission intensities of Eu 2+ (496 nm) and Eu 3+ (594 nm) in the Sr 4 Al 14 O 25 phosphors are labelled in Fig. 6.All investigated phosphors were monitored at an excitation wavelength of 325 nm.It is found that the PL intensities of red and blue emissions change obviously with the concentration variation of the Eu dopant.The emissions of Eu 2+ and Eu 3+ enhance with the increase of Eu 3+ ions concentration, and the former reaches the maximum at 1 mol% due to the concentration  quenching. 25This observation indicates that the increasing doping of Eu 3+ ions spawn the V 00 Sr and electrons which can combine with Eu 3+ ions and promotes the self-reduction reaction in some extent.The self-reduction process can be presented in the following equations: 26 3Sr 2+ + 2Eu 3+ / V 00 Sr + 2Euc Sr (1) As the Mn 2+ is singly doped in Sr 4 Al 14 O 25 host, the PL and PLE spectra of synthesized phosphor (Sr 4 Al 14 O 25 :0.002Mn 4+ ) are shown in the Fig. 7.It contains two excitation bands from 250 nm to 550 nm, which are due to the spin-allowed electronic transition 4 A 2 -4 T 1 and 4 A 2 -4 T 2 of Mn 4+ . 27The optimal excitation wavelength of Sr 4 Al 14 O 25 :0.002Mn 4+ is 325 nm, and the halfband width is approximately 85 nm, much wider than the emission band of UV LED chips.It is indicated that the excitation wavelength at 325 nm can be an alternative for the typical commercial chips in the future market.The sharp emission    band peaking at 659 nm generates from the spin-forbidden electronic transition 2 E g -4 T 2 of Mn 4+ . 28s revealed in Fig. 8, series of phosphors with varied Eu 3+ and Mn 4+ concentrations are synthesized and their PL spectra are recorded upon excitation of 325 nm for the investigation of the energy transfer(ET) between Eu 2+ and Mn 4+ .We can see that the intensities of the Eu 2+ and Eu 3+ emissions are found to decrease monotonically with increasing Mn 4+ content due to the conspicuous energy transfer between Eu 2+ and Mn 4+ .The simplied diagram of the energy transfer process from Eu 2+ to Mn 4+ in Sr 4 Al 14 O 25 phosphors is illustrated in Fig. 9.As indicated in this diagram, the relationship of energy transfer between Eu 2+ and Mn 4+ ions can be attributed to the similar energy level values between the excited 5d state of Eu 2+ and the 4 T 1g levels of Mn 4+ ions. 29anwhile, the PL intensity of the Mn 4+ increases with the doping of Mn 4+ ions, and comes to a maximum value at y ¼ 0.001.Then the concentration quenching leads to the weakening of PL intensity.The synthesized phosphors can be tuned expediently from blue-green to white by the co-existence of Eu 2+ , Eu 3+ and Mn 4+ .To observe the characteristic of change in colour aer doping Mn 4+ more visually, as shown in Table 3 and Fig. 10, the Commission Internationale de l'Eclairage (CIE) chromaticity coordinates, correlated colour temperature (CCT) and CIE chromaticity diagram of Sr 4 Al 14 O 25 :0.01Eu,yMn 4+ (y ¼ 0, 0.0002, 0.0005, 0.001) phosphors are calculated and plotted respectively.As summarized in Table 2, the series of chromaticity coordinates of the Sr 4 Al 14 O 25 :0.01Eu,yMn 4+ are calculated from the corresponding datum of emission spectra.The phosphors can be regulated from blue-green (0.2396, 0.3332) to white (0.3465, 0.2906) with the increasing concentration of Mn 4+ , corresponding to the correlated colour temperature from 11 489 K to 4554 K as listed in Table 3.To further verify the existence of energy transfer between Eu 2+ and Mn 4+ in Sr 4 Al 14 O 25 :0.01Eu,yMn 4+ (y ¼ 0, 0.0005, 0.001, 0.002, 0.003, 0.005, 0.01, 0.02), we investigated the luminescence decay curves of Eu 2+ 496 nm emission and the corresponding lifetimes of Eu 2+ with the doping of Mn 4+ .Additionally, the energy transfer efficiency is also listed next to the lifetimes in Fig. 11(a).All the decay curves can be well tted ground on the following non-exponential equation: where s y and s 0 stand for the corresponding lifetimes of Eu 2+ ions with and without Mn 4+ ions doping for the constant sensitizer concentration.The values of the estimated P Eu-Mn are plotted in Fig. 11(b).The consecutive increasing of the ET probabilities with the various Mn 4+ content manifests an enhanced ET process between Eu 2+ and Mn 4+ because of the decrease in the average distance between the ions. 34Furthermore, the energy transfer efficiency h from a donor Eu 2+ to acceptor Mn 4+ would be estimated according to the expression as follows: where h stands for the energy transfer efficiency.s s and s s 0 mean the lifetimes of Eu 2+ with and without the doping of Mn 4+ ions.As shown in the Fig. 11(a), the energy transfer efficiency decreases constantly from 15.95% to 58.16% with the increasing concentration of Mn 4+ .The result is ascribed to the shortening distance between the Eu 2+ ions and Mn 4+ ions.
Generally, the energy transfer mechanism between the sensitizer (Eu 2+ ) to activator (Mn 4+ ) ions and the relation can be revealed by the Dexter's energy transfer theory: [36][37][38]   and the relatively weak absorption peaking at 391 nm results from the f-f transition of Eu 3+ , which is in accordance with the excitation spectra of Eu 2+ and Eu 3+ .When the Mn 4+ doped into the host, it exhibits an obvious absorption from 390 nm to 550 nm assigned to the spin-forbidden electronic transition of the Mn 4+ ions.Above results indicates the potential of Sr 4 Al 14 -O 25 :Eu,Mn 4+ for solid-state lighting.

Conclusion
Series of colour-tunable direct-white-emitting phosphors Sr 4 -Al 14 O 25 :Eu,Mn 4+ were synthesized in air by conventional high temperature solid state reactions.The coexistence of Eu 2+ , Eu 3+ and Mn 4+ was proved, and the self-reduction process of Eu 3+ was discussed.The luminescence properties were investigated, and three groups of emissions in Sr 4 Al 14 O 25 ascribed to Eu 2+ , Eu 3+ and Mn 4+ were observed under ultraviolet excitation.The energy transfer between Eu 2+ and Mn 4+ originated from a nonradiative electric dipole-dipole interaction, and the energy transfer efficiency could reach 58.16%.The emission colour of Sr 4 Al 14 -O 25 :0.01Eu,yMn 4+ (y ¼ 0, 0.0002, 0.0005, 0.001) could be modulated from blue-green (0.2396, 0.3332) to white (0.3465, 0.2906), corresponding to the colour temperature from 11 489 K to 4554 K with the increasing concentration of Mn 4+ .These results manifest that the prepared Sr 4 Al 14 O 25 :Eu,Mn 4+ phosphor can be regarded as a potential approach to obtain colour-tunable direct-white emissions through adjusting the content of Mn 4+ .
1.5406 Å by TOPAS program.Almost all peaks can be indexed by hexagonal cell with parameters close to Sr 4 Al 14 O 25 , herein the structural parameters of Sr 4 Al 14 O 25 are used as initial parameters in the Rietveld analysis.The nal renement is stable and convergent well with low residual factors R exp ¼ 6.281%, R wp ¼ 17.843%, R p ¼ 13.770% and c 2 ¼ 2.841, indicating the single phase with no unidentied diffraction peaks from impurity.

Fig. 9 Fig. 10
Fig.9The energy level structure of Eu 2+ and Mn 4+ and the energy transfer mechanism among them.