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

Abstract

Eu²⁺, Eu³⁺ and Mn⁴⁺ co-doped Sr₄Al₁₄O₂₅ phosphors were synthesized in air by high temperature solid-state reaction. The coexistence of Eu²⁺, Eu³⁺ and Mn⁴⁺ 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 Eu²⁺ and Mn⁴⁺ were investigated scientifically. Under ultraviolet excitation, a broad emission band with peak at 496 nm was ascribed to a 4f⁶5d–4f⁷ transition of Eu²⁺, and another five narrow emission bands were attributed to 4f–4f transitions of Eu³⁺. The sharp emission band with peak at 659 nm was generated from the spin-forbidden electronic transition ²E_g–⁴T₂ of Mn⁴⁺. The energy transfer between Eu²⁺ and Mn⁴⁺ was demonstrated following the nonradiative electric dipole–dipole interaction, and the energy transfer efficiency reached 58.16%. The emission colour of Sr₄Al₁₄O₂₅:Eu,Mn⁴⁺ can be tuned from blue-green (0.2396, 0.3332) to white (0.3465, 0.2906) with increasing concentrations of Mn⁴⁺.

---

1 Introduction

With the ever-increasing demands of humankind for illumination quality, white light-emitting diodes (WLEDs) are replacing the incandescent and fluorescent lamps gradually and becoming a new generation of solid state lighting sources for buildings and scenery lighting, LCD backlighting, interior lighting, automobiles, agriculture, medicine and aerospace.^1–4 At present, the commercial method to generate white light emission is exciting the yellow-emitting Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺) with an InGaN light-emitting diode (LED) chip which emits blue light with the wavelength between 450 nm and 480 nm typically.^5,6 Because of the emission deficiency in the red region, this WLED shows poor colour rendering and appears a simple cool white. Another feasible method is combining blue, green and red phosphors with a near-ultraviolet (n-UV) chip which emits ultraviolet with the wavelength between 380 nm and 420 nm.^7–9 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.

To the best of our knowledge, aluminate has becoming an important host material because of its excellent thermostability, non-poisonous and competitive price.^10–13 Peng¹⁴ firstly found the coexistence of Eu²⁺ and Eu³⁺ in Sr₄Al₁₄O₂₅ prepared in air. Two broad bands peaking at 400 nm and 490 nm attributed to the 4f–5d transition of Eu²⁺, and line emissions in the region of 550–700 nm belong to the f–f transitions of Eu³⁺. Xu¹⁵ reported the non-RE activated Sr₄Al₁₄O₂₅:Mn⁴⁺ phosphor prepared by solid-state method in air. The Al³⁺ ions in the [AlO₆] octahedral sites of Sr₄Al₁₄O₂₅ were substituted by Mn⁴⁺ 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 ²E–⁴A₂ transition of Mn⁴⁺ 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 Eu²⁺ and Mn⁴⁺ in Sr₄Al₁₄O₂₅. 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 Eu²⁺, Eu³⁺ and Mn⁴⁺ in Sr₄Al₁₄O₂₅.

In our work, a series of Sr₄Al₁₄O₂₅:xEu,Mn⁴⁺ phosphors were synthesized in air through the high temperature solid-state reaction. The coexistence of Eu²⁺, Eu³⁺ and Mn⁴⁺ was revealed by X-ray photoelectron spectroscopy (XPS), diffuse reflection spectra and photoluminescence (PL) spectra. The influences of Mn⁴⁺ on the luminescence properties were discussed in detail, and the fluorescence lifetime, energy transfer between Eu²⁺ and Mn⁴⁺ and applications in WLED were studied respectively.

2 Experimental

Series of Eu³⁺ ions single-doped Sr₄Al₁₄O₂₅:xEu (x = 0.005, 0.01, 0.02, 0.04, 0.06, 0.08) and co-doped Sr₄Al₁₄O₂₅:0.01Eu,yMn⁴⁺ (y = 0.0002, 0.0005, 0.001, 0.002, 0.003, 0.005, 0.01, 0.02) were synthesized by conventional solid-state reaction in air with SrCO₃(A.R.), Al(OH)₃(A.R.), MnCO₃(A.R.), H₃BO₃(A.R.) and Eu₂O₃(4N). The stoichiometric amount of raw materials were well homogenized in an agate mortar, and the H₃BO₃ was added as a flux. All samples were pre-sintered in alumina crucibles in air at 700 °C for 2 h, and further heat-treated at 1400 °C for 5 h with sufficient grindings in the processes.

The phase purity of synthesized phosphors were checked by X-ray diffraction (XRD) with a D8 Advance diffractometer with Cu Kα radiation (λ = 1.5406 Å) by the step of 4° min⁻¹ 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 fluorescence spectrophotometer with a 400 nm cut-off filter. The diffuse reflection 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 spectrofluorometer.

3 Results and discussion

Fig. 1 shows the XRD patterns of Sr₄Al₁₄O₂₅:xEu (x = 0, 0.005, 0.01, 0.02, 0.04, 0.06) and Sr₄Al₁₄O₂₅:0.01Eu,yMn⁴⁺ (y = 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 Sr₄Al₁₄O₂₅ was also listed in Fig. 1 as a reference. It is observed that the samples synthesized in various conditions are all in pure Sr₄Al₁₄O₂₅ phase, and the change of doping of Eu³⁺ and Mn⁴⁺ ions would not impact the host structure.

Fig. 1 XRD patterns of Sr₄Al₁₄O₂₅:xEu (x = 0, 0.005, 0.01, 0.02, 0.04, 0.06) and Sr₄Al₁₄O₂₅:0.01Eu,yMn⁴⁺ (y = 0.0002, 0.0005, 0.001, 0.002, 0.003, 0.005, 0.01) synthesized at different temperatures, and the standard pattern JCPDS 52-1876.

It has been found that the Mn⁴⁺ 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 Mn⁴⁺ substitutes an octahedral site.^16,17 The fragments of Sr₄Al₁₄O₂₅ unit cell are exhibited in Fig. 2. Sr₄Al₁₄O₂₅ 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.¹⁸ Because the radius difference between Eu²⁺ and Sr²⁺ ions is much smaller than that of Eu²⁺ and Al³⁺, and the ionic size of Mn⁴⁺ is much smaller than that of Sr²⁺ but close to Al³⁺, it is reasonable to consider the Eu²⁺ ions are substituted in Sr²⁺ sites, while Mn⁴⁺ ions tend to occupy the octahedral sites of Al³⁺.

Fig. 2 The crystal structures of Sr₄Al₁₄O₂₅.

Fig. 3 shows the observed (black squares), calculated (red lines), and difference (bottom) XRD profiles for the Rietveld refinement of Sr_3.96Al_13.9972O₂₅:0.04Eu²⁺,0.0028Mn²⁺ with λ = 1.5406 Å by TOPAS program. Almost all peaks can be indexed by hexagonal cell with parameters close to Sr₄Al₁₄O₂₅, herein the structural parameters of Sr₄Al₁₄O₂₅ are used as initial parameters in the Rietveld analysis. The final refinement is stable and convergent well with low residual factors R_exp = 6.281%, R_wp = 17.843%, R_p = 13.770% and χ² = 2.841, indicating the single phase with no unidentified diffraction peaks from impurity. The final refined crystallographic data are listed in Table 1. The cell parameters are determined to be a = 24.7177 Å, b = 8.4810 Å, c = 4.8847 Å and V = 1026.22 ų. The crystallographic site coordinates, occupancy factors, and equivalent isotropic displacement parameters are summarized in Table 2.

Fig. 3 Power XRD pattern of Sr_3.96Al_13.9972O₂₅:0.04Eu²⁺,0.0028Mn²⁺ with corresponding Rietveld refinement (red) and residuals (blue).

Table 1 Main parameters of processing and refinement of the Sr_3.96Al_13.9972O₂₅:0.04Eu²⁺,0.0028Mn²⁺ sample

Sp.Gr.	Pmma
a, Å	24.7177 (8)
B, Å	8.4810 (3)
c, Å	4.8847 (2)
V, Å^3	1026.22 (6)
2θ-interval, °	10–80
No. of reflections	313
No. of refined parameters	47
Rwp, %	17.843
Rp, %	13.770
Rexp, %	6.281
χ^2	2.841
RB, %	4.593

---

Table 2 Fractional atomic coordinates and isotropic displacement parameters (Ų) of Sr_3.96Al_13.9972O₂₅:0.04Eu²⁺,0.0028Mn²⁺ sample

Sr3.96Al13.9972O25:0.04Eu^2+,0.0028Mn^2+
	x	y	z	Biso	Occ.
Sr1	0.1387	1/2	0.0355	2.98	0.99
Eu1	0.1387	1/2	0.0355	2.98	0.01
Sr2	0.1204	0	0.1143	1.16	0.99
Eu2	0.1204	0	0.1143	1.16	0.01
Al1	0.1851	0.1956	0.6284	3.68	0.9998
Mn1	0.1851	0.1956	0.6284	3.68	0.0002
Al2	0.0677	0.3178	0.5066	0.35	0.9998
Mn2	0.0677	0.3178	0.5066	0.35	0.0002
Al3	1/4	0.2451	0.4131	1.38	0.9998
Mn3	1/4	0.2451	0.4131	1.38	0.0002
Al4	0	0.1671	0	2.21	0.9998
Mn4	0	0.1671	0	2.21	0.0002
Al5	0	0	1/2	2.98	0.9998
Mn5	0	0	1/2	2.98	0.0002
Al6	0	1/2	0	4.29	0.9998
Mn6	0	1/2	0	4.29	0.0002
O1	0.0462	0.1867	0.3054	1.10	1
O2	0.1359	0.3187	0.5089	1.10	1
O3	0.1904	0.2482	0.9582	1.10	1
O4	1/4	0.2957	0.1281	1.10	1
O5	0.0340	0	0.8224	1.10	1
O6	0.0527	1/2	0.3277	1.10	1
O7	0.1582	0	0.5656	1.10	1
O8	0.0442	0.3234	0.8311	1.10	1
O9	1/4	1/2	0.0580	1.10	1

---

To examine the valence state of europium and manganese in Sr₄Al₁₄O₂₅, the XPS spectra of Sr₄Al₁₄O₂₅:xEu (x = 0.01, 0.02, 0.06) and Sr₄Al₁₄O₂₅:0.01Eu,yMn⁴⁺ (y = 0.005, 0.01, 0.02) is recorded in Fig. 4(a) and (b). It is evident from Fig. 4(a) that the broad bands peaking at 1136.5 eV ascribed to Eu³⁺ 3d_5/2 are observed, and the bands peaking at 1127.5 eV consistent with Eu²⁺ 3d_5/2 emerge gradually with the increasing doping of Eu³⁺ ions.¹⁹ The peak intensities of Mn⁴⁺ 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 Mn²⁺ ions, and no Mn²⁺ peaks are observed as the value of y varies. We can deeply confirm that there is an coexistence of Eu²⁺, Eu³⁺ and Mn⁴⁺ in Sr₄Al₁₄O₂₅ phosphors.

Fig. 4 High-resolution Eu 3d XPS spectra of Sr₄Al₁₄O₂₅:xEu (x = 0.01, 0.02, 0.06) (a) and Mn 2p XPS spectra of Sr₄Al₁₄O₂₅:0.01Eu,yMn (y = 0.005, 0.01, 0.02) (b).

Fig. 5 shows the excitation and emission spectra of Sr₄Al₁₄O₂₅:Eu. We doped the Eu ions originally into the host in the form of Eu₂O₃, and we found that the trivalent and bivalent Eu ions co-existed in Sr₄Al₁₄O₂₅: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 Eu²⁺.²⁰ The 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 Eu³⁺ and O²⁻ 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 Eu³⁺ within its 4f⁶ configuration.²¹ The PL spectra with different excitation factors are demonstrated in Fig. 5(b). The characteristic peaks of Eu²⁺ and Eu³⁺ are both observed in emission spectra with the change of excitation wavelength. The band emission peaking at 469 nm stems from 4f⁶5d–4f⁷ transition of Eu²⁺, and the other five typical line emissions peaking at 596 nm, 620 nm, 652 nm, 688 nm and 707 nm are attributed to 4f–4f transition of ⁵d₀–⁷f_J (J = 1, 2, 3, 4) of Eu³⁺.^22,23 The strongest emission generated from the transition of ⁵d₀–⁷f₂ of Eu³⁺ belongs to the electric dipole transition, which reflects that the electric dipole transition is the dominant factor in the luminescence process of Eu³⁺.²⁴

Fig. 5 PLE spectra of Eu³⁺ (619 nm) and Eu²⁺ (496 nm) of Sr₄Al₁₄O₂₅:0.01Eu (a) and PL spectra with different excitations (b).

The PL spectra of Sr₄Al₁₄O₂₅:xEu (x = 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 Eu²⁺ (496 nm) and Eu³⁺ (594 nm) in the Sr₄Al₁₄O₂₅ 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 Eu²⁺ and Eu³⁺ enhance with the increase of Eu³⁺ ions concentration, and the former reaches the maximum at 1 mol% due to the concentration quenching.²⁵ This observation indicates that the increasing doping of Eu³⁺ ions spawn the V′′_Sr and electrons which can combine with Eu³⁺ ions and promotes the self-reduction reaction in some extent. The self-reduction process can be presented in the following equations:²⁶

3Sr²⁺ + 2Eu³⁺ → V′′_Sr + 2Eu˙_Sr (1)

V′′_Sr → V_Sr^x + 2e (2)

2Eu˙_Sr + 2e → 2Eu_Sr^x (3)

Fig. 6 PL spectra of Sr₄Al₁₄O₂₅:xEu (x = 0.005, 0.01, 0.02, 0.04, 0.06, 0.08) under ultraviolet excitation (325 nm) (a) and dependence of relative emission intensity of Eu²⁺ (496 nm) and Eu³⁺ (619 nm) in Sr₄Al₁₄O₂₅ phosphors as a function of Eu³⁺ doped concentration (b).

As the Mn²⁺ is singly doped in Sr₄Al₁₄O₂₅ host, the PL and PLE spectra of synthesized phosphor (Sr₄Al₁₄O₂₅:0.002Mn⁴⁺) 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 ⁴A₂–⁴T₁ and ⁴A₂–⁴T₂ of Mn⁴⁺.²⁷ The optimal excitation wavelength of Sr₄Al₁₄O₂₅:0.002Mn⁴⁺ is 325 nm, and the half-band 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 ²E_g–⁴T₂ of Mn⁴⁺.²⁸

Fig. 7 The spectral pattern of Sr₄Al₁₄O₂₅:0.002Mn⁴⁺.

As revealed in Fig. 8, series of phosphors with varied Eu³⁺ and Mn⁴⁺ concentrations are synthesized and their PL spectra are recorded upon excitation of 325 nm for the investigation of the energy transfer(ET) between Eu²⁺ and Mn⁴⁺. We can see that the intensities of the Eu²⁺ and Eu³⁺ emissions are found to decrease monotonically with increasing Mn⁴⁺ content due to the conspicuous energy transfer between Eu²⁺ and Mn⁴⁺. The simplified diagram of the energy transfer process from Eu²⁺ to Mn⁴⁺ in Sr₄Al₁₄O₂₅ phosphors is illustrated in Fig. 9. As indicated in this diagram, the relationship of energy transfer between Eu²⁺ and Mn⁴⁺ ions can be attributed to the similar energy level values between the excited 5d state of Eu²⁺ and the ⁴T_1g levels of Mn⁴⁺ ions.²⁹

Fig. 8 PL spectra of Sr₄Al₁₄O₂₅:0.01Eu,yMn⁴⁺ (y = 0, 0.0002, 0.0005, 0.001, 0.002, 0.003, 0.005, 0.01, 0.02) under ultraviolet excitation (325 nm) (a) and dependence of relative emission intensity of Eu²⁺ (496 nm), Eu³⁺ (619 nm) and Mn⁴⁺ (659 nm) in Sr₄Al₁₄O₂₅ phosphors as a function of Mn⁴⁺ doped concentration (b).

Fig. 9 The energy level structure of Eu²⁺ and Mn⁴⁺ and the energy transfer mechanism among them.

Meanwhile, the PL intensity of the Mn⁴⁺ increases with the doping of Mn⁴⁺ ions, and comes to a maximum value at y = 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 Eu²⁺, Eu³⁺ and Mn⁴⁺. To observe the characteristic of change in colour after doping Mn⁴⁺ 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 Sr₄Al₁₄O₂₅:0.01Eu,yMn⁴⁺ (y = 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 Sr₄Al₁₄O₂₅:0.01Eu,yMn⁴⁺ 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 Mn⁴⁺, corresponding to the correlated colour temperature from 11 489 K to 4554 K as listed in Table 3.

Table 3 The CIE chromaticity coordinates (x, y) and correlated colour temperature (CCT) of Sr₄Al₁₄O₂₅:0.01Eu,yMn⁴⁺ (y = 0, 0.0002, 0.0005, 0.001)

Sample no.	Composition (y)	CIE coordinates (x, y)	CCT (K)
1(a)	0	(0.2396, 0.3332)	11 489
2(b)	0.0002	(0.2723, 0.3191)	9380
3(c)	0.0005	(0.3266, 0.3040)	5798
4(d)	0.001	(0.3465, 0.2906)	4554

---

Fig. 10 CIE chromaticity diagram of Sr₄Al₁₄O₂₅:0.01Eu,yMn⁴⁺ (y = 0, 0.0002, 0.0005, 0.001) under 325 nm excitation and digital images in UV box.

To further verify the existence of energy transfer between Eu²⁺ and Mn⁴⁺ in Sr₄Al₁₄O₂₅:0.01Eu,yMn⁴⁺ (y = 0, 0.0005, 0.001, 0.002, 0.003, 0.005, 0.01, 0.02), we investigated the luminescence decay curves of Eu²⁺ 496 nm emission and the corresponding lifetimes of Eu²⁺ with the doping of Mn⁴⁺. Additionally, the energy transfer efficiency is also listed next to the lifetimes in Fig. 11(a). All the decay curves can be well fitted ground on the following non-exponential equation:^30,31

where I(t) stands for the PL intensities at time t. The effective decay lifetimes monitored at 496 nm of Eu²⁺ are calculated to be 1478.69, 1242.81, 888.22, 794.80, 720.49, 652.35, 643.00 and 618.70 ns for y = 0, 0.0005, 0.001, 0.002, 0.003, 0.005, 0.01, 0.02, respectively. The lifetimes of Eu²⁺ emission are monotonically decreasing with the doping of Mn⁴⁺, which indicates that the energy transfer between Eu²⁺ and Mn⁴⁺ ions occurs in the Sr₄Al₁₄O₂₅ host, and the estimate the Eu²⁺–Mn⁴⁺ ET probability (P_Eu–Mn) can be fitted to the following equation:^32,33where τ_y and τ₀ stand for the corresponding lifetimes of Eu²⁺ ions with and without Mn⁴⁺ 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 Mn⁴⁺ content manifests an enhanced ET process between Eu²⁺ and Mn⁴⁺ because of the decrease in the average distance between the ions.³⁴ Furthermore, the energy transfer efficiency η from a donor Eu²⁺ to acceptor Mn⁴⁺ would be estimated according to the expression as follows:³⁵where η stands for the energy transfer efficiency. τ_s and τ_s0 mean the lifetimes of Eu²⁺ with and without the doping of Mn⁴⁺ 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 Mn⁴⁺. The result is ascribed to the shortening distance between the Eu²⁺ ions and Mn⁴⁺ ions.

Fig. 11 Decay curves, lifetime and energy transfer efficiency (η) of Sr₄Al₁₄O₂₅:0.01Eu,yMn⁴⁺ (y = 0, 0.0005, 0.001, 0.002, 0.003, 0.005, 0.01, 0.02) under excitation of 325 nm (a). Energy transfer probability (P_Eu–Mn) as a function of Mn⁴⁺ doped concentration.

Generally, the energy transfer mechanism between the sensitizer (Eu²⁺) to activator (Mn⁴⁺) ions and the relation can be revealed by the Dexter's energy transfer theory:^36–38

in which I_s0 and I_s mean the PL intensities of Eu²⁺ with and without activator Mn⁴⁺ ions, separately. C stands for the doping content of Mn⁴⁺ ions, and the dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions correspond to n = 6, 8, and 10, respectively. The fitting curves of I_s0/I_s versus C^n/-3 are shown in Fig. 12(a–c), which reveal a linearity only when n = 6, demonstrating that the energy transfer mechanism from Eu²⁺ to Mn⁴⁺ in the Sr₄Al₁₄O₂₅ host follows a nonradiative electric dipole–dipole interaction.

Fig. 12 Dependence of I_s0/I_s of Eu²⁺ on C_Mn^6/3, C_Mn^8/3 and C_Mn^10/3.

Fig. 13 presents the diffuse reflection spectra of Sr₄Al₁₄O₂₅, Sr₄Al₁₄O₂₅:0.01Eu and Sr₄Al₁₄O₂₅:0.01Eu,0.003Mn⁴⁺. It is observed that the energy absorption of Sr₄Al₁₄O₂₅ host occurs in ultraviolet region, and the band gap is estimated to be 3.701 eV based on the fitting line of the absorption edge.^39,40 A strong energy absorption band can be seen obviously in the 200–350 nm region from the host material. With the doping of Eu³⁺ ions, an obvious absorption appears around 200–300 nm in the near-UV region as a result of the 4f–5d transition of Eu²⁺, and the relatively weak absorption peaking at 391 nm results from the f–f transition of Eu³⁺, which is in accordance with the excitation spectra of Eu²⁺ and Eu³⁺. When the Mn⁴⁺ doped into the host, it exhibits an obvious absorption from 390 nm to 550 nm assigned to the spin-forbidden electronic transition of the Mn⁴⁺ ions. Above results indicates the potential of Sr₄Al₁₄O₂₅:Eu,Mn⁴⁺ for solid-state lighting.

Fig. 13 Diffuse reflectance spectra of Sr₄Al₁₄O₂₅, Sr₄Al₁₄O₂₅:0.01Eu and Sr₄Al₁₄O₂₅:0.01Eu,0.003Mn⁴⁺.

4 Conclusion

Series of colour-tunable direct-white-emitting phosphors Sr₄Al₁₄O₂₅:Eu,Mn⁴⁺ were synthesized in air by conventional high temperature solid state reactions. The coexistence of Eu²⁺, Eu³⁺ and Mn⁴⁺ was proved, and the self-reduction process of Eu³⁺ was discussed. The luminescence properties were investigated, and three groups of emissions in Sr₄Al₁₄O₂₅ ascribed to Eu²⁺, Eu³⁺ and Mn⁴⁺ were observed under ultraviolet excitation. The energy transfer between Eu²⁺ and Mn⁴⁺ originated from a nonradiative electric dipole–dipole interaction, and the energy transfer efficiency could reach 58.16%. The emission colour of Sr₄Al₁₄O₂₅:0.01Eu,yMn⁴⁺ (y = 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 Mn⁴⁺. These results manifest that the prepared Sr₄Al₁₄O₂₅:Eu,Mn⁴⁺ phosphor can be regarded as a potential approach to obtain colour-tunable direct-white emissions through adjusting the content of Mn⁴⁺.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the National Natural Science Foundation of China (Grant No. 51472223) and the Fundamental Research Funds for Central Universities (Grant No. 2652015090).

Notes and references

1. B. Ma, Q. F. Guo and M. S. Molokeev, et al., Ceram. Int., 2016, 42, 5995–5999.
2. R. Y. Mi, C. L. Zhao and Z. G. Xia, J. Am. Ceram. Soc., 2014, 97, 1802–1808.
3. D. L. Geng, G. G. Li and M. M. Shang, et al., J. Mater. Chem., 2012, 22, 14262–14271.
4. Z. Y. Wang, J. Chen and Y. G. Liu, et al., J. Adv. Ceram., 2017, 6, 81–89.
5. Y. T. Lin, Z. R. Niu and Y. Han, et al., J. Alloys Compd., 2017, 690, 267–273.
6. Y. F. Xia, J. Chen and Y. G. Liu, et al., Mater. Express, 2016, 6, 37–44.
7. M. Lee and W. Jung, Ceram. Int., 2016, 42, 3113–3120.
8. T. Arakawa and N. Nagata, J. Alloys Compd., 2006, 408–412, 864–866.
9. A. D. Deshmukh, S. J. Dhoble and N. S. Dhoble, Adv. Mater. Lett., 2011, 2, 38–42.
10. L. J. Xiao, M. R. He and Y. W. Tian, et al., J. Nanosci. Nanotechnol., 2010, 10, 2131–2134.
11. M. Y. Peng, J. R. Qiu and L. Y. Yang, et al., Opt. Mater., 2004, 27, 591–595.
12. M. Y. Peng and G. Hong, J. Lumin., 2007, 127, 735–740.
13. M. Rezende, M. Valerio and R. Jackson, Mater. Res. Bull., 2014, 61, 348–351.
14. M. Y. Peng, Z. W. Pei and G. Hong, et al., Chem. Phys. Lett., 2003, 371, 1–6.
15. Y. D. Xu, D. Wang and L. Wang, et al., J. Alloys Compd., 2013, 550, 226–230.
16. M. Y. Peng, Z. W. Pei and G. Hong, et al., J. Mater. Chem., 2003, 13, 1202–1205.
17. M. Y. Peng, X. W. Yin and P. Tanner, et al., Chem. Mater., 2015, 27, 2938–2945.
18. Y. H. Lin, Z. L. Tang and Z. T. Zhang, et al., Appl. Phys. Lett., 2002, 81, 996–998.
19. J. Chen, Y. G. Liu and H. K. Liu, et al., Opt. Mater., 2015, 42, 80–86.
20. Z. G. Xia, R. S. Liu and K. W. Huang, et al., J. Mater. Chem., 2012, 22, 15183–15189.
21. R. Y. Mi, J. Chen and Y. G. Liu, et al., RSC Adv., 2016, 6, 28887–28894.
22. H. K. Liu, Q. F. Guo and L. B. Liao, et al., Opt. Commun., 2013, 309, 64–67.
23. Z. G. Xia, J. Zhou and Z. Y. Mao, J. Mater. Chem. C, 2013, 1, 5917–5924.
24. J. Chen, Y. G. Liu and L. F. Mei, et al., J. Mater. Chem. C, 2015, 3, 3451–3455.
25. H. K. Liu, Y. Y. Zhang and L. B. Liao, et al., J. Lumin., 2014, 156, 49–54.
26. Q. H. Zeng, Z. W. Pei and S. B. Wang, et al., J. Alloys Compd., 1998, 275–277, 238–241.
27. Y. Zhydachevskii, D. Galanciak and S. Kobyakov, et al., J. Phys.: Condens. Matter, 2006, 18, 11385.
28. S. Adachi and T. Takahashi, J. Appl. Phys., 2008, 104, 317.
29. S. Adachi and T. Takahashi, J. Appl. Phys., 2009, 106, 339.
30. R. J. Xie, N. Hirosaki and N. Kimura, et al., Appl. Phys. Lett., 2007, 90, 191101.
31. Y. Y. Zhang, Z. G. Xia and H. K. Liu, et al., Chem. Phys. Lett., 2014, 593, 189.
32. M. Xie, T. Ye and Y. Huang, et al., ChemInform, 2011, 42.
33. D. Wen, J. Feng and J. Li, et al., J. Mater. Chem. C, 2015, 3, 2107–2114.
34. J. Chen, C. H. Li and Z. Hui, et al., Inorg. Chem., 2017, 56, 1144–1151.
35. W. J. Yang, L. Y. Luo and T. M. Chen, et al., Chem. Mater., 2005, 17, 3883–3888.
36. D. L. Dexter and J. H. Schulman, J. Chem. Phys., 1954, 22, 1063–1070.
37. S. P. Lee, C. H. Huang and T. M. Chen, J. Mater. Chem. C, 2014, 2, 8925–8931.
38. D. Y. Wang, C. H. Huang and B. M. Cheng, et al., RSC Adv., 2014, 4, 28632–28635.
39. Y. F. Xia, Y. G. Liu and Z. H. Huang, et al., J. Mater. Chem. C, 2016, 4, 4675–4683.
40. C. Zeng, H. W. Huang and Y. M. Hu, et al., RSC Adv., 2015, 5, 68099–68108.

---