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Luminescence property and emission enhancement of YbAlO3:Mn4+ red phosphor by Mg2+ or Li+ ions
Optical Materials 53 (2016) 169–173
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Luminescence property and emission enhancement of YbAlO3:Mn4+ red phosphor by Mg2+ or Li+ ions Renping Cao a,⇑, Wenjie Luo a, Haidong Xu a, Zhiyang Luo b, Qianglin Hu a, Ting Fu a, Dedong Peng a a b
College of Mathematics and Physics, Jinggangshan University, Ji’an 343009, China College of Mechanical Manufacture and Automation, Jinggangshan University, Ji’an 343009, China
a r t i c l e
i n f o
Article history: Received 9 December 2015 Received in revised form 10 January 2016 Accepted 18 January 2016
Keywords: Mn4+ ion Red-emitting Phosphors Charge compensation
a b s t r a c t YbAlO3:Mn4+, YbAlO3:Mn4+, Li+, and YbAlO3:Mn4+, Mg2+ phosphors are synthesized by high temperature solid-state reaction method in air. Their crystal structures and luminescence properties are investigated. Photoluminescence excitation (PLE) spectrum monitored at 677 nm contains broad PLE band with three PLE peaks located at 318, 395, and 470 nm within the range 220–600 nm. Emission spectra with excitation 318 and 470 nm exhibit three emission band peaks located at 645, 677, and 700 nm in the range of 610–800 nm and their corresponding chromaticity coordinates are about (x = 0.6942, y = 0.3057). The possible luminous mechanism of Mn4+ ion is analyzed by the simple energy level diagram of Mn4+ ion. The optimum Mn4+-doped concentration in YbAlO3:Mn4+ phosphor is about 0.4 mol% and the luminescence lifetime of YbAlO3:0.4%Mn4+ phosphor is 0.59 ms. Emission intensity of YbAlO3:0.4%Mn4+ phosphor can be enhanced 6 times after Mg2+ ion is co-doped and it is 2 times when Li+ ion is co-doped. The content in the paper is useful to research new Mn4+-doped luminescence materials and improve luminescence property of other Mn4+-doped phosphors. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction It is well known that Mn4+ ion belongs to transition metal ion and has an incompletely filled d-shell with outer 3d3 electron configuration [1]. Generally, Mn4+ ion can be generated and stabilized in oxygen-coordinated with six nearest neighbors and octahedral host systems and gives rather complicated optical spectra [2,3]. The broad absorption spectrum of Mn4+-doped luminescence materials covers the region from 200 to 550 nm. Red-emitting of Mn4+-doped luminescence materials can be observed within the range 600–780 nm with excitation ultraviolet (UV) and blue light [4,5]. The luminescence properties of Mn4+-doped materials have be reported widely because Mn4+-doped materials are interest for potential applications in the fields of lighting, white light-emitting diodes (LEDs), graphic recording, and optical data storages [6,7]. In 1947, Williams reported the emission of Mn4+ -doped magnesium germanate red phosphor [8]. Later, Kemeny et al. studied red emitting 3.5MgO0.5MgF2GeO2:Mn4+ phosphor in 1960, which has been used as commercial red phosphor in fluorescent lamp for providing light [9]. In recent years, Mn4+-doped other luminescence materials have also been reported widely, such
⇑ Corresponding author. E-mail address: [email protected] (R. Cao). http://dx.doi.org/10.1016/j.optmat.2016.01.034 0925-3467/Ó 2016 Elsevier B.V. All rights reserved.
as K2GeF6:Mn4+, K2SiF6:Mn4+, BaSiF6:Mn4+, CaAl2O4:Mn4+, Ba2GeO4:Mn4+, Sr2MgAl22O36:Mn4+, Sr4Al14O25:Mn4+, Ca14Zn6M10O35: Mn4+ (M = Al3+ and Ga3+), LiRGe2O6:Mn4+ (R = Al or Ga), Li2MgGeO4:Mn4+, Ba2LaNbO6:Mn4+, and Y2Ti2O7:Mn4+ [10–21]. However, it is necessary to further study the luminescence properties of Mn4+-doped novel luminescence materials in order to meet the requirements for practical application in different fields. Host doped is one of important influence factors to the luminescence properties of optical materials. Ytterbium aluminum perovskite (e.g., YbAlO3) attracted specific attention for its high melting point and satisfactory stability in combustion environments [22]. Due to high optical quality, YbAlO3 has been considered as good candidate of effective potential application in many fields, such as optical data storage, radiation detectors, and fast scintillators [23,24]. YbAlO3 has the distorted orthorhombic GdFeO3 type of crystal structure and contains AlO6 octahedron with six O atoms and YbO9 polyhedron [25]. So, Mn4+ ion may occupy the Ai3+ ion site in AlO6 octahedron. Up to now, luminescence property of YbAlO3:Mn4+ phosphor has few been reported. It is well known that charge compensation is one of effective methods to enhance the luminescence properties of optical materials [26]. In order to improve luminescence properties of YbAlO3:Mn4+ phosphor, Mg2+ and Li+ ions may usually be used as charge compensators and added into YbAlO3:Mn4+ phosphor in the paper.
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In the paper, YbAlO3:Mn4+, YbAlO3:Mn4+, Li+, and YbAlO3:Mn4+, Mg2+ phosphors are synthesized by high temperature solid-state reaction method in air. Their crystal structures, luminescence properties, and fluorescence lifetimes are investigated, respectively. The relation between Mn4+ doping concentration and luminescence properties of YbAlO3:Mn4+ phosphor is discussed. The influences of Mg2+ and Li+ ions as charge compensators to luminescence properties of YbAlO3:Mn4+ phosphors are investigated and analyzed. The possible luminous mechanism of Mn4+ ion is analyzed and explained by the simple energy level diagram of Mn4+ ion. 2. Experimental All the chemicals are purchased from the Aladdin Chemical Reagent Company in Shanghai China, such as Al2O3 (A.R. 99.5%), MgO (A.R. 99.5%), Li2CO3 (A.R. 99.5%), Yb2O3 (99.99%), and MnCO3 (A.R. 99.5%). A series of YbAlO3:xMn4+ (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0 mol%), YbAlO3:0.4%Mn4+, 2% Li+, and YbAlO3:0.4%Mn4+, yMg2+ (y = 2 and 4 mol%) phosphors are synthesized by high temperature solid-state reaction method in air. The molar% is defined against one mole of a phosphor formula. The stoichiometric amount of raw materials are well grounded in an agate mortar without further purification, then sintered at 600 °C for 5 h, and subsequently 1450 °C for 4 h in air. Repeated grindings are performed between two sintering processes to improve the homogeneity. All products are obtained after natural cooling to room temperature. The crystal structures of phosphors are checked by X-ray Powder Diffraction (XRD) (Philips Model PW1830) with Cu-Ka radiation at 40 kV and 40 mA at room temperature. The data are collected in the 2h range of 10°–70°. Luminescence properties and fluorescence lifetimes of these phosphors are investigated by using a steady-state FLS980 spectrofluorimeter (Edinburgh Instruments, UK, Edinburgh) with a high spectral resolution (signal to noise ratio >12,000:1) at room temperature. A 450 W ozone free xenon lamp is used for steady-state measurements. A microsecond pulsed xenon flash lamp lF900 with an average power of 60 W is available to record the emission decay curves for fluorescence lifetimes. 3. Results and discussion XRD patterns of (a) Joint Committee on Powder Diffraction Standards (JCPDS) card no. 20-1392 (YbAlO3), (b) blank YbAlO3, (c) YbAlO3:0.4%Mn4+, (d) YbAlO3:0.8%Mn4+, (e) YbAlO3:0.4%Mn4+, 2%Li+, (f) YbAlO3:0.4%Mn4+, 2%Mg2+, and (g) YbAlO3:0.4%Mn4+, 4%
Fig. 1. (i) XRD patterns of (a) JCPDS card no. 20-1392 (YbAlO3), (b) blank YbAlO3, (c) YbAlO3:0.4%Mn4+, (d) YbAlO3:0.8%Mn4+, (e) YbAlO3:0.4%Mn4+, 2%Li+, (f) YbAlO3:0.4%Mn4+, 2%Mg2+, and (g) YbAlO3:0.4%Mn4+, 4%Mg2+ phosphors at room temperature; (j) the enlarged figure in the 2h range of 35–36.5°.
Mg2+ phosphors at room temperature are shown in Fig. 1i. The XRD patterns of these samples all match well with the standard data of JCPDS card (no. 20-1392). The XRD patterns of other YbAlO3:xMn4+ (0 6 x 6 1.0%) phosphors are not displayed in Fig. 1i, but those patterns are also in line with those of JCPDS card (no. 201392). No other crystalline phase is formed after Mn4+, Li+, and Mg2+ ions are added. This is said that all samples are pure phase YbAlO3. After Mn4+, Li+, and Mg2+ ions are doped, the doped samples show regular changes accordingly with increasing Mn4+ ion doping concentration, which can be illustrated more clearly in Fig. 1j. The XRD diffraction peaks shift toward lower angles due to the different ionic radii (Al3+: 0.5 Å, Mg2+: 0.65 Å, Mn4+: 0.54 Å, and Li+: 0.6 Å) [27]. This indicates that a contraction of the lattice cell according to Bragg equation (2dsin h = k, where k and h are the wavelength of the X-ray and diffraction angle, respectively.) owing to the substitution of Mn4+, Li+, and Mg2+ ions for Al4+ ions in host lattice. YbAlO3 is described in the orthorhombic crystal system with space-group Pbnm (no. 62) and the lattice parameters a = 5.13 Å, b = 5.33 Å, c = 7.32 Å, and z = 4 [28,29]. YbAlO3 crystallize possesses GdFeO3-type perovskite structure, and contains AlO6 octahedron with six O atoms and YbO9 polyhedron [25]. Mn4+ ion is capable of substituting in the pure octahedral and distorted octahedral symmetry, so, Mn4+, Mg2+, and Li+ ions occupy the Al3+ ions site and replace Al3+ ions in the host YbAlO3 lattice owing to their similar ionic radii. Photoluminescence (PL) and Photoluminescence excitation (PLE) spectra of YbAlO3:0.4%Mn4+ phosphor at room temperature, the simple energy level diagram of Mn4+ ion, and Commission Internationale Ed I’eclairage (CIE) chromaticity diagram and chromaticity coordinates are shown in Fig. 2. PLE spectrum monitored at 677 nm contains broad PLE band with three PLE peaks located at 318, 395, and 470 nm within the range 220–600 nm. PLE band peaking at 318 nm within the range 200–380 nm is assigned to the 4A2 ? 4T1 transition and the O2–Mn4+ change transfer band (CTB), other two PLE bands peaking at 395 and 470 nm are attributed to 4A2 ? 2T1, and 4A2 ? 4T2 transitions of Mn4+ ion, respectively [30,31]. These PLE bands indicate that the kind of phosphor can be effectively excited by UV ( 318 nm), near UV (395 nm), and blue light (470 nm) LED chips. PL spectra with excitation 318 and 470 nm exhibit three PL peaks located at 645, 677, and 700 nm in the range of 610–800 nm due to antistokes vibronic sidebands associated with the excited state 2E of Mn4+ ion, the 2E ? 4A2 transition of Mn4+ ion, and the vibronics transition of Mn4+ ion with zero-phonon line, respectively [32]. The corresponding CIE chromaticity coordinates are about (x = 0.6942, y = 0.3057) (see Fig. 2c). Now the possible luminous mechanism of Mn4+ ion is analyzed by the simple energy level diagram shown in Fig. 2b. Mn4+ ion belongs to transition metal ion with a 3d3 electronic configuration as well as Cr3+ ion, and its energetic structure can be well described by standard crystal-field theory [2]. Mn4+ ion is capable of substituting in the pure octahedral, distorted octahedral symmetry, and invariably oxygen-coordinated with six nearest neighbors crystal system [5]. The most important free ion states contain 2H excited level and 4F ground level. The 4F level may split into the 4T2 and 4T1 excited states and the 4 A2 ground state in an octahedral field. The spin allowed electron transitions could be used to populate the excited states directly correspond to 4A2 ? 4T2 and 4A2 ? 4T1. The emission is attributed to the 2E ? 4A2 electron transition. As shown in Fig. 2b, electrons absorb energy and are raised from the 4A2 ground state to the 4T2 and 4T1 excited states, the excited electrons then relax to the lower level by nonradiative process and populate the excited 2 E state, finally the possible 2E ? 4A2 electron transition may occur, thus the phosphor emits red light.
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Fig. 2. (a) PLE and PL spectra of YbAlO3:0.4% Mn4+ phosphor at room temperature (kex = 318 and 470 nm; kem = 677 nm); (b) the simple energy level diagram of Mn4+ ion; (c) the CIE chromaticity diagram and chromaticity coordinates.
PL spectra of YbAlO3:xMn4+ (0.2 mol% 6 x 6 1.0 mol%) phosphors with excitation 318 nm at room temperature and the relation between PL intensity and Mn4+ ion doping concentration are shown in Fig. 3. PL spectra shape and peak positions of YbAlO3: xMn4+ phosphors (0.2% 6 x 6 1.0%) are the same except their PL intensity with changing Mn4+ doping concentration. PL intensity increases with increasing Mn4+ doping concentration in the range of 0.2–0.4%, and decreases with further increasing Mn4+ doping concentration. The former observation could be attributed to the distance between Mn4+ ions, and the intensity is proportional to the content of Mn4+ ion. The critical transfer distance (Rc) can be calculated via using the formula suggested by Blass [2]: Rc 2 [3V/(4pXcN)]1/3, where V is the unit cell volume of host lattice, Xc is the critical concentration, N is the number of sites available for the dopant in the unit cell. The latter observation is presumably due to the concentration quenching of Mn4+ ions. Therefore, the optimal Mn4+ doping concentration in YbAlO3:Mn4+ phosphor is about 0.4 mol%. The decay curve of YbAlO3:0.4%Mn4+ phosphor at room temperature is shown in Fig. 4. The monitoring wavelength is at 677 nm with excitation 318 nm. The red curve is a fit of the experimental data to a first order exponential decay equation. The luminescence lifetime of YbAlO3:0.4%Mn4+ phosphor is 0.593 ms. The luminescence decay curve is well fitted by a first-order exponential function [2].
Fig. 4. Decay curve of YbAlO3:0.4%Mn4+ phosphor monitored at 677 nm with excitation 318 nm at room temperature. The inset: The relation between lifetime and Mn4+ ion doping concentration. The red curve is a fit of the experimental data to a first order exponential decay equation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
where I(t) is the luminescence intensity at time t, I(0) is the initial luminescence intensity, A is the value for different fitting, and s is
the decay time for the exponential components. All decay curves of YbAlO3:xMn4+ phosphors (0.2% 6 x 6 1.0%) can be fitted by first-order exponential function and their luminescence lifetimes are about 0.601, 0.593, 0.584, 0.581, and 0.576 ms, respectively, and decreases with increasing Mn4+ ion doping concentration in the range of 0.2–1.0%. Time resolved emission spectra of YbAlO3:0.4%Mn4+ phosphor within delay time range 3–120 ls at room temperature are shown in Fig. 5. The excitation wavelength is 318 nm, and the PL scanned
Fig. 3. PL spectra of YbAlO3:xMn4+ (0.2 mol% 6 x 6 1.0 mol%) phosphors at room temperature (kex = 318 nm). The inset: The relation between PL intensity and Mn4+ ion doping concentration.
Fig. 5. Time resolved emission spectra of YbAlO3:0.4%Mn4+ phosphor within delay time range 3–120 ls at room temperature (kex = 318 nm).
IðtÞ ¼ Ið0Þ expðt=sÞ þ A
ð1Þ
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area is from 620 to 760 nm. PL peak shapes and positions are almost the same with increasing delay time. Combining with lifetime data in Fig. 4, these results indicate that it is only a single type of Mn4+ ion luminescent center in YbAlO3:Mn4+ phosphor. PL spectra of (a) YbAlO3:0.4%Mn4+, (b) YbAlO3:0.4%Mn4+, 2% Mg2+, (c) YbAlO3:0.4%Mn4+, 4%Mg2+, and (b) YbAlO3:0.4%Mn4+, 2% Li+ phosphors with excitation 318 nm at room temperature are shown in Fig. 6. Their PL spectra shape and peak positions are the same except PL intensity. Mg2+ and Li+ ions co-doped can improve obviously the luminescence properties of YbAlO3:0.4% Mn4+ phosphor. PL intensity of YbAlO3:0.4%Mn4+ phosphor may be enhanced about 6 times after Mg2+ ion is co-doped and it is about 2 times when Li+ ion is co-doped. The possible reason can be assigned to the charge compensation of Mg2+ and Li+ ions. In YbAlO3:Mn4+ phosphor, it is difficult to keep charge balance in the host lattice due to the different valence state between Mn4+ ion and Al3+ ion. Mn4+ ions cannot be fully introduced into the Al3+ ions site in the host YbAlO3 lattice. In order to keep their charge balance, Mn4+ ions have to form pairs by trapping an interstitial O2 ion and partial occupy Al3+ ions sites [33]. When Mg2+ and Li+ ions as charge compensators are co-doped into YbAlO3: Mn4+ phosphor, they will tend to occupy the Al3+ ions sites in the host YbAlO3 lattice due to their similar ionic radii. Thus, the charge compensation is formed without involving additional O2 ions. As a result, the electron radiative transition of Mn4+ ion in the excited state will be increased, thus, the emission of Mn4+ ion can be enhanced. In addition, the trend of forming the Al3+–Mn4+–Mg2+– Mn4+–Al3+ will enhance more Mn4+ doping concentration than that of the Al3+–Mn4+–Li+ forming, therefore Mg2+ ions as charge compensators are more beneficial to improve the PL intensity of YbAlO3:0.4%Mn4+ phosphor than Li+ ions. Decay curves of (a) YbAlO3:0.4%Mn4+, (b) YbAlO3:0.4%Mn4+, 2% Mg2+, (c) YbAlO3:0.4%Mn4+, 4%Mg2+, and (b) YbAlO3:0.4%Mn4+, 2% Li+ phosphors at room temperature are shown in Fig. 7. The monitoring wavelength is 677 nm with excitation 318 nm. These decay curves can be fitted by first-order exponential function (1) and their luminescence lifetimes are about (a) 0.595 ms, (b) 0.642 ms, (c) 0.675 ms, and (d) 0.614 ms. The experimental observed decay time of the luminescence is given by s = 1/(kr + ki), where kr is the probability of radiative decay and ki is the probability of nonradiative decay processes from the same state [2]. After Mg2+ and Li+ ions as charge compensators are co-doped intoYbAlO3:Mn4+ phosphor, kr and ki will be possible to decrease. Therefore, these luminescence lifetimes are increase.
Fig. 7. Decay curves of (a) YbAlO3:0.4%Mn4+, (b) YbAlO3:0.4%Mn4+, 2%Mg2+, (c) YbAlO3:0.4%Mn4+, 4%Mg2+, and (b) YbAlO3:0.4%Mn4+, 2%Li+ phosphors monitored at 677 nm with excitation 318 nm at room temperature.
4. Conclusions In summary, YbAlO3:xMn4+ (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0 mol %), YbAlO3:0.4%Mn4+, 2% Li+, and YbAlO3:0.4%Mn4+, yMg2+ (y = 2 and 4 mol%) phosphors are synthesized by high temperature solid-state reaction method in air. Their crystal structures, fluorescence lifetime, and luminescence properties are investigated, respectively. The XRD patterns indicate that all samples are pure phase YbAlO3. PLE spectrum monitored at 677 nm contains broad band with three peaks located at 318, 395, and 470 nm within the range 220–600 nm, which are assigned to the O2–Mn4+ CTB, 4 A2 ? 4T1, 4A2 ? 2T1, and 4A2 ? 4T2 transitions of Mn4+ ion, respectively. PL spectra with excitation 318 and 470 nm exhibit three peaks located at 645, 677, and 700 nm in the range of 610– 800 nm and their corresponding CIE chromaticity coordinates are about (x = 0.6942, y = 0.3057). The possible luminous mechanism of Mn4+ ion is analyzed by its simple energy level diagram. The optimal Mn4+ doping concentration in YbAlO3:Mn4+ phosphor is about 0.4 mol% and the luminescence lifetime of YbAlO3:0.4% Mn4+ phosphor is 0.59 ms. Mg2+ and Li+ ions as charge compensators can improve obviously the luminescence properties of YbAlO3:0.4%Mn4+ phosphor. PL intensity of YbAlO3:0.4%Mn4+ phosphor may be enhanced about 6 times after Mg2+ ion is co-doped and it is about 2 times when Li+ ion is co-doped. The content in the paper is help to research new Mn4+-doped luminescence materials and improve luminescence properties of other Mn4+-doped phosphors. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 11464021) and Natural Science Foundation of Jiangxi Province of China (No. 20151BAB202008). References
Fig. 6. PL spectra of (a) YbAlO3:0.4%Mn4+, (b) YbAlO3:0.4%Mn4+, 2%Mg2+, (c) YbAlO3:0.4%Mn4+, 4%Mg2+, and (b) YbAlO3:0.4%Mn4+, 2%Li+ phosphors at room temperature (kex = 318 nm).
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Luminescence property and emission enhancement of YbAlO3:Mn4+ red phosphor by Mg2+ or Li+ ions Renping Cao a,⇑, Wenjie Luo a, Haidong Xu a, Zhiyang Luo b, Qianglin Hu a, Ting Fu a, Dedong Peng a a b
College of Mathematics and Physics, Jinggangshan University, Ji’an 343009, China College of Mechanical Manufacture and Automation, Jinggangshan University, Ji’an 343009, China
a r t i c l e
i n f o
Article history: Received 9 December 2015 Received in revised form 10 January 2016 Accepted 18 January 2016
Keywords: Mn4+ ion Red-emitting Phosphors Charge compensation
a b s t r a c t YbAlO3:Mn4+, YbAlO3:Mn4+, Li+, and YbAlO3:Mn4+, Mg2+ phosphors are synthesized by high temperature solid-state reaction method in air. Their crystal structures and luminescence properties are investigated. Photoluminescence excitation (PLE) spectrum monitored at 677 nm contains broad PLE band with three PLE peaks located at 318, 395, and 470 nm within the range 220–600 nm. Emission spectra with excitation 318 and 470 nm exhibit three emission band peaks located at 645, 677, and 700 nm in the range of 610–800 nm and their corresponding chromaticity coordinates are about (x = 0.6942, y = 0.3057). The possible luminous mechanism of Mn4+ ion is analyzed by the simple energy level diagram of Mn4+ ion. The optimum Mn4+-doped concentration in YbAlO3:Mn4+ phosphor is about 0.4 mol% and the luminescence lifetime of YbAlO3:0.4%Mn4+ phosphor is 0.59 ms. Emission intensity of YbAlO3:0.4%Mn4+ phosphor can be enhanced 6 times after Mg2+ ion is co-doped and it is 2 times when Li+ ion is co-doped. The content in the paper is useful to research new Mn4+-doped luminescence materials and improve luminescence property of other Mn4+-doped phosphors. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction It is well known that Mn4+ ion belongs to transition metal ion and has an incompletely filled d-shell with outer 3d3 electron configuration [1]. Generally, Mn4+ ion can be generated and stabilized in oxygen-coordinated with six nearest neighbors and octahedral host systems and gives rather complicated optical spectra [2,3]. The broad absorption spectrum of Mn4+-doped luminescence materials covers the region from 200 to 550 nm. Red-emitting of Mn4+-doped luminescence materials can be observed within the range 600–780 nm with excitation ultraviolet (UV) and blue light [4,5]. The luminescence properties of Mn4+-doped materials have be reported widely because Mn4+-doped materials are interest for potential applications in the fields of lighting, white light-emitting diodes (LEDs), graphic recording, and optical data storages [6,7]. In 1947, Williams reported the emission of Mn4+ -doped magnesium germanate red phosphor [8]. Later, Kemeny et al. studied red emitting 3.5MgO0.5MgF2GeO2:Mn4+ phosphor in 1960, which has been used as commercial red phosphor in fluorescent lamp for providing light [9]. In recent years, Mn4+-doped other luminescence materials have also been reported widely, such
⇑ Corresponding author. E-mail address: [email protected] (R. Cao). http://dx.doi.org/10.1016/j.optmat.2016.01.034 0925-3467/Ó 2016 Elsevier B.V. All rights reserved.
as K2GeF6:Mn4+, K2SiF6:Mn4+, BaSiF6:Mn4+, CaAl2O4:Mn4+, Ba2GeO4:Mn4+, Sr2MgAl22O36:Mn4+, Sr4Al14O25:Mn4+, Ca14Zn6M10O35: Mn4+ (M = Al3+ and Ga3+), LiRGe2O6:Mn4+ (R = Al or Ga), Li2MgGeO4:Mn4+, Ba2LaNbO6:Mn4+, and Y2Ti2O7:Mn4+ [10–21]. However, it is necessary to further study the luminescence properties of Mn4+-doped novel luminescence materials in order to meet the requirements for practical application in different fields. Host doped is one of important influence factors to the luminescence properties of optical materials. Ytterbium aluminum perovskite (e.g., YbAlO3) attracted specific attention for its high melting point and satisfactory stability in combustion environments [22]. Due to high optical quality, YbAlO3 has been considered as good candidate of effective potential application in many fields, such as optical data storage, radiation detectors, and fast scintillators [23,24]. YbAlO3 has the distorted orthorhombic GdFeO3 type of crystal structure and contains AlO6 octahedron with six O atoms and YbO9 polyhedron [25]. So, Mn4+ ion may occupy the Ai3+ ion site in AlO6 octahedron. Up to now, luminescence property of YbAlO3:Mn4+ phosphor has few been reported. It is well known that charge compensation is one of effective methods to enhance the luminescence properties of optical materials [26]. In order to improve luminescence properties of YbAlO3:Mn4+ phosphor, Mg2+ and Li+ ions may usually be used as charge compensators and added into YbAlO3:Mn4+ phosphor in the paper.
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In the paper, YbAlO3:Mn4+, YbAlO3:Mn4+, Li+, and YbAlO3:Mn4+, Mg2+ phosphors are synthesized by high temperature solid-state reaction method in air. Their crystal structures, luminescence properties, and fluorescence lifetimes are investigated, respectively. The relation between Mn4+ doping concentration and luminescence properties of YbAlO3:Mn4+ phosphor is discussed. The influences of Mg2+ and Li+ ions as charge compensators to luminescence properties of YbAlO3:Mn4+ phosphors are investigated and analyzed. The possible luminous mechanism of Mn4+ ion is analyzed and explained by the simple energy level diagram of Mn4+ ion. 2. Experimental All the chemicals are purchased from the Aladdin Chemical Reagent Company in Shanghai China, such as Al2O3 (A.R. 99.5%), MgO (A.R. 99.5%), Li2CO3 (A.R. 99.5%), Yb2O3 (99.99%), and MnCO3 (A.R. 99.5%). A series of YbAlO3:xMn4+ (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0 mol%), YbAlO3:0.4%Mn4+, 2% Li+, and YbAlO3:0.4%Mn4+, yMg2+ (y = 2 and 4 mol%) phosphors are synthesized by high temperature solid-state reaction method in air. The molar% is defined against one mole of a phosphor formula. The stoichiometric amount of raw materials are well grounded in an agate mortar without further purification, then sintered at 600 °C for 5 h, and subsequently 1450 °C for 4 h in air. Repeated grindings are performed between two sintering processes to improve the homogeneity. All products are obtained after natural cooling to room temperature. The crystal structures of phosphors are checked by X-ray Powder Diffraction (XRD) (Philips Model PW1830) with Cu-Ka radiation at 40 kV and 40 mA at room temperature. The data are collected in the 2h range of 10°–70°. Luminescence properties and fluorescence lifetimes of these phosphors are investigated by using a steady-state FLS980 spectrofluorimeter (Edinburgh Instruments, UK, Edinburgh) with a high spectral resolution (signal to noise ratio >12,000:1) at room temperature. A 450 W ozone free xenon lamp is used for steady-state measurements. A microsecond pulsed xenon flash lamp lF900 with an average power of 60 W is available to record the emission decay curves for fluorescence lifetimes. 3. Results and discussion XRD patterns of (a) Joint Committee on Powder Diffraction Standards (JCPDS) card no. 20-1392 (YbAlO3), (b) blank YbAlO3, (c) YbAlO3:0.4%Mn4+, (d) YbAlO3:0.8%Mn4+, (e) YbAlO3:0.4%Mn4+, 2%Li+, (f) YbAlO3:0.4%Mn4+, 2%Mg2+, and (g) YbAlO3:0.4%Mn4+, 4%
Fig. 1. (i) XRD patterns of (a) JCPDS card no. 20-1392 (YbAlO3), (b) blank YbAlO3, (c) YbAlO3:0.4%Mn4+, (d) YbAlO3:0.8%Mn4+, (e) YbAlO3:0.4%Mn4+, 2%Li+, (f) YbAlO3:0.4%Mn4+, 2%Mg2+, and (g) YbAlO3:0.4%Mn4+, 4%Mg2+ phosphors at room temperature; (j) the enlarged figure in the 2h range of 35–36.5°.
Mg2+ phosphors at room temperature are shown in Fig. 1i. The XRD patterns of these samples all match well with the standard data of JCPDS card (no. 20-1392). The XRD patterns of other YbAlO3:xMn4+ (0 6 x 6 1.0%) phosphors are not displayed in Fig. 1i, but those patterns are also in line with those of JCPDS card (no. 201392). No other crystalline phase is formed after Mn4+, Li+, and Mg2+ ions are added. This is said that all samples are pure phase YbAlO3. After Mn4+, Li+, and Mg2+ ions are doped, the doped samples show regular changes accordingly with increasing Mn4+ ion doping concentration, which can be illustrated more clearly in Fig. 1j. The XRD diffraction peaks shift toward lower angles due to the different ionic radii (Al3+: 0.5 Å, Mg2+: 0.65 Å, Mn4+: 0.54 Å, and Li+: 0.6 Å) [27]. This indicates that a contraction of the lattice cell according to Bragg equation (2dsin h = k, where k and h are the wavelength of the X-ray and diffraction angle, respectively.) owing to the substitution of Mn4+, Li+, and Mg2+ ions for Al4+ ions in host lattice. YbAlO3 is described in the orthorhombic crystal system with space-group Pbnm (no. 62) and the lattice parameters a = 5.13 Å, b = 5.33 Å, c = 7.32 Å, and z = 4 [28,29]. YbAlO3 crystallize possesses GdFeO3-type perovskite structure, and contains AlO6 octahedron with six O atoms and YbO9 polyhedron [25]. Mn4+ ion is capable of substituting in the pure octahedral and distorted octahedral symmetry, so, Mn4+, Mg2+, and Li+ ions occupy the Al3+ ions site and replace Al3+ ions in the host YbAlO3 lattice owing to their similar ionic radii. Photoluminescence (PL) and Photoluminescence excitation (PLE) spectra of YbAlO3:0.4%Mn4+ phosphor at room temperature, the simple energy level diagram of Mn4+ ion, and Commission Internationale Ed I’eclairage (CIE) chromaticity diagram and chromaticity coordinates are shown in Fig. 2. PLE spectrum monitored at 677 nm contains broad PLE band with three PLE peaks located at 318, 395, and 470 nm within the range 220–600 nm. PLE band peaking at 318 nm within the range 200–380 nm is assigned to the 4A2 ? 4T1 transition and the O2–Mn4+ change transfer band (CTB), other two PLE bands peaking at 395 and 470 nm are attributed to 4A2 ? 2T1, and 4A2 ? 4T2 transitions of Mn4+ ion, respectively [30,31]. These PLE bands indicate that the kind of phosphor can be effectively excited by UV ( 318 nm), near UV (395 nm), and blue light (470 nm) LED chips. PL spectra with excitation 318 and 470 nm exhibit three PL peaks located at 645, 677, and 700 nm in the range of 610–800 nm due to antistokes vibronic sidebands associated with the excited state 2E of Mn4+ ion, the 2E ? 4A2 transition of Mn4+ ion, and the vibronics transition of Mn4+ ion with zero-phonon line, respectively [32]. The corresponding CIE chromaticity coordinates are about (x = 0.6942, y = 0.3057) (see Fig. 2c). Now the possible luminous mechanism of Mn4+ ion is analyzed by the simple energy level diagram shown in Fig. 2b. Mn4+ ion belongs to transition metal ion with a 3d3 electronic configuration as well as Cr3+ ion, and its energetic structure can be well described by standard crystal-field theory [2]. Mn4+ ion is capable of substituting in the pure octahedral, distorted octahedral symmetry, and invariably oxygen-coordinated with six nearest neighbors crystal system [5]. The most important free ion states contain 2H excited level and 4F ground level. The 4F level may split into the 4T2 and 4T1 excited states and the 4 A2 ground state in an octahedral field. The spin allowed electron transitions could be used to populate the excited states directly correspond to 4A2 ? 4T2 and 4A2 ? 4T1. The emission is attributed to the 2E ? 4A2 electron transition. As shown in Fig. 2b, electrons absorb energy and are raised from the 4A2 ground state to the 4T2 and 4T1 excited states, the excited electrons then relax to the lower level by nonradiative process and populate the excited 2 E state, finally the possible 2E ? 4A2 electron transition may occur, thus the phosphor emits red light.
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Fig. 2. (a) PLE and PL spectra of YbAlO3:0.4% Mn4+ phosphor at room temperature (kex = 318 and 470 nm; kem = 677 nm); (b) the simple energy level diagram of Mn4+ ion; (c) the CIE chromaticity diagram and chromaticity coordinates.
PL spectra of YbAlO3:xMn4+ (0.2 mol% 6 x 6 1.0 mol%) phosphors with excitation 318 nm at room temperature and the relation between PL intensity and Mn4+ ion doping concentration are shown in Fig. 3. PL spectra shape and peak positions of YbAlO3: xMn4+ phosphors (0.2% 6 x 6 1.0%) are the same except their PL intensity with changing Mn4+ doping concentration. PL intensity increases with increasing Mn4+ doping concentration in the range of 0.2–0.4%, and decreases with further increasing Mn4+ doping concentration. The former observation could be attributed to the distance between Mn4+ ions, and the intensity is proportional to the content of Mn4+ ion. The critical transfer distance (Rc) can be calculated via using the formula suggested by Blass [2]: Rc 2 [3V/(4pXcN)]1/3, where V is the unit cell volume of host lattice, Xc is the critical concentration, N is the number of sites available for the dopant in the unit cell. The latter observation is presumably due to the concentration quenching of Mn4+ ions. Therefore, the optimal Mn4+ doping concentration in YbAlO3:Mn4+ phosphor is about 0.4 mol%. The decay curve of YbAlO3:0.4%Mn4+ phosphor at room temperature is shown in Fig. 4. The monitoring wavelength is at 677 nm with excitation 318 nm. The red curve is a fit of the experimental data to a first order exponential decay equation. The luminescence lifetime of YbAlO3:0.4%Mn4+ phosphor is 0.593 ms. The luminescence decay curve is well fitted by a first-order exponential function [2].
Fig. 4. Decay curve of YbAlO3:0.4%Mn4+ phosphor monitored at 677 nm with excitation 318 nm at room temperature. The inset: The relation between lifetime and Mn4+ ion doping concentration. The red curve is a fit of the experimental data to a first order exponential decay equation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
where I(t) is the luminescence intensity at time t, I(0) is the initial luminescence intensity, A is the value for different fitting, and s is
the decay time for the exponential components. All decay curves of YbAlO3:xMn4+ phosphors (0.2% 6 x 6 1.0%) can be fitted by first-order exponential function and their luminescence lifetimes are about 0.601, 0.593, 0.584, 0.581, and 0.576 ms, respectively, and decreases with increasing Mn4+ ion doping concentration in the range of 0.2–1.0%. Time resolved emission spectra of YbAlO3:0.4%Mn4+ phosphor within delay time range 3–120 ls at room temperature are shown in Fig. 5. The excitation wavelength is 318 nm, and the PL scanned
Fig. 3. PL spectra of YbAlO3:xMn4+ (0.2 mol% 6 x 6 1.0 mol%) phosphors at room temperature (kex = 318 nm). The inset: The relation between PL intensity and Mn4+ ion doping concentration.
Fig. 5. Time resolved emission spectra of YbAlO3:0.4%Mn4+ phosphor within delay time range 3–120 ls at room temperature (kex = 318 nm).
IðtÞ ¼ Ið0Þ expðt=sÞ þ A
ð1Þ
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area is from 620 to 760 nm. PL peak shapes and positions are almost the same with increasing delay time. Combining with lifetime data in Fig. 4, these results indicate that it is only a single type of Mn4+ ion luminescent center in YbAlO3:Mn4+ phosphor. PL spectra of (a) YbAlO3:0.4%Mn4+, (b) YbAlO3:0.4%Mn4+, 2% Mg2+, (c) YbAlO3:0.4%Mn4+, 4%Mg2+, and (b) YbAlO3:0.4%Mn4+, 2% Li+ phosphors with excitation 318 nm at room temperature are shown in Fig. 6. Their PL spectra shape and peak positions are the same except PL intensity. Mg2+ and Li+ ions co-doped can improve obviously the luminescence properties of YbAlO3:0.4% Mn4+ phosphor. PL intensity of YbAlO3:0.4%Mn4+ phosphor may be enhanced about 6 times after Mg2+ ion is co-doped and it is about 2 times when Li+ ion is co-doped. The possible reason can be assigned to the charge compensation of Mg2+ and Li+ ions. In YbAlO3:Mn4+ phosphor, it is difficult to keep charge balance in the host lattice due to the different valence state between Mn4+ ion and Al3+ ion. Mn4+ ions cannot be fully introduced into the Al3+ ions site in the host YbAlO3 lattice. In order to keep their charge balance, Mn4+ ions have to form pairs by trapping an interstitial O2 ion and partial occupy Al3+ ions sites [33]. When Mg2+ and Li+ ions as charge compensators are co-doped into YbAlO3: Mn4+ phosphor, they will tend to occupy the Al3+ ions sites in the host YbAlO3 lattice due to their similar ionic radii. Thus, the charge compensation is formed without involving additional O2 ions. As a result, the electron radiative transition of Mn4+ ion in the excited state will be increased, thus, the emission of Mn4+ ion can be enhanced. In addition, the trend of forming the Al3+–Mn4+–Mg2+– Mn4+–Al3+ will enhance more Mn4+ doping concentration than that of the Al3+–Mn4+–Li+ forming, therefore Mg2+ ions as charge compensators are more beneficial to improve the PL intensity of YbAlO3:0.4%Mn4+ phosphor than Li+ ions. Decay curves of (a) YbAlO3:0.4%Mn4+, (b) YbAlO3:0.4%Mn4+, 2% Mg2+, (c) YbAlO3:0.4%Mn4+, 4%Mg2+, and (b) YbAlO3:0.4%Mn4+, 2% Li+ phosphors at room temperature are shown in Fig. 7. The monitoring wavelength is 677 nm with excitation 318 nm. These decay curves can be fitted by first-order exponential function (1) and their luminescence lifetimes are about (a) 0.595 ms, (b) 0.642 ms, (c) 0.675 ms, and (d) 0.614 ms. The experimental observed decay time of the luminescence is given by s = 1/(kr + ki), where kr is the probability of radiative decay and ki is the probability of nonradiative decay processes from the same state [2]. After Mg2+ and Li+ ions as charge compensators are co-doped intoYbAlO3:Mn4+ phosphor, kr and ki will be possible to decrease. Therefore, these luminescence lifetimes are increase.
Fig. 7. Decay curves of (a) YbAlO3:0.4%Mn4+, (b) YbAlO3:0.4%Mn4+, 2%Mg2+, (c) YbAlO3:0.4%Mn4+, 4%Mg2+, and (b) YbAlO3:0.4%Mn4+, 2%Li+ phosphors monitored at 677 nm with excitation 318 nm at room temperature.
4. Conclusions In summary, YbAlO3:xMn4+ (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0 mol %), YbAlO3:0.4%Mn4+, 2% Li+, and YbAlO3:0.4%Mn4+, yMg2+ (y = 2 and 4 mol%) phosphors are synthesized by high temperature solid-state reaction method in air. Their crystal structures, fluorescence lifetime, and luminescence properties are investigated, respectively. The XRD patterns indicate that all samples are pure phase YbAlO3. PLE spectrum monitored at 677 nm contains broad band with three peaks located at 318, 395, and 470 nm within the range 220–600 nm, which are assigned to the O2–Mn4+ CTB, 4 A2 ? 4T1, 4A2 ? 2T1, and 4A2 ? 4T2 transitions of Mn4+ ion, respectively. PL spectra with excitation 318 and 470 nm exhibit three peaks located at 645, 677, and 700 nm in the range of 610– 800 nm and their corresponding CIE chromaticity coordinates are about (x = 0.6942, y = 0.3057). The possible luminous mechanism of Mn4+ ion is analyzed by its simple energy level diagram. The optimal Mn4+ doping concentration in YbAlO3:Mn4+ phosphor is about 0.4 mol% and the luminescence lifetime of YbAlO3:0.4% Mn4+ phosphor is 0.59 ms. Mg2+ and Li+ ions as charge compensators can improve obviously the luminescence properties of YbAlO3:0.4%Mn4+ phosphor. PL intensity of YbAlO3:0.4%Mn4+ phosphor may be enhanced about 6 times after Mg2+ ion is co-doped and it is about 2 times when Li+ ion is co-doped. The content in the paper is help to research new Mn4+-doped luminescence materials and improve luminescence properties of other Mn4+-doped phosphors. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 11464021) and Natural Science Foundation of Jiangxi Province of China (No. 20151BAB202008). References
Fig. 6. PL spectra of (a) YbAlO3:0.4%Mn4+, (b) YbAlO3:0.4%Mn4+, 2%Mg2+, (c) YbAlO3:0.4%Mn4+, 4%Mg2+, and (b) YbAlO3:0.4%Mn4+, 2%Li+ phosphors at room temperature (kex = 318 nm).
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