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Synthesis and luminescence properties of double perovskite Sr2ZnMoO6:Mn4+ deep red phosphor
Optical Materials 62 (2016) 706e710
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Synthesis and luminescence properties of double perovskite Sr2ZnMoO6:Mn4þ deep red phosphor Renping Cao*, Xiangfeng Ceng, Jijun Huang, Hui Ao, Guotai Zheng, Xiaoguang Yu, Xinqin Zhang College of Mathematics and Physics, Jinggangshan University, Ji'an 343009, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 August 2016 Received in revised form 23 October 2016 Accepted 24 October 2016 Available online 29 October 2016
A double perovskite Sr2ZnMoO6:Mn4þ (SZM:Mn4þ) phosphor is synthesized by high-temperature solidstate reaction method in air. Emission band peaking at ~705 nm of SZM:Mn4þ phosphor in the range of 650e790 nm is attributed to the 2E / 4A2 transition of Mn4þ ion with activated different lattice vibration modes. Three excitation bands in the range of 210e610 nm are assigned to the O2 / Mn4þ charge transfer and the 4A2 / (4T1, 2T2, and 4T2) transition of Mn4þ ion. The optimal Mn4þ ion concentration is ~0.8 mol% in SZM:Mn4þ phosphor. Fluorescence lifetime of SZM:Mn4þ phosphor decreases from ~132 ms to 108 ms with increasing Mn4þ ion concentration in the range of 0.2e1.0 mol%. Time-resolved spectra and fluorescence lifetime data indicate that luminescent center is caused by Mn4þ ion. The luminous mechanism of SZM:Mn4þ phosphor is explained by Tanabe-Sugano energy level diagram of Mn4þ ion. The results are useful to understand the influences of the neighboring coordination environment around Mn4þ and host crystal structure to the luminescence properties of Mn4þ ion and develop other novel Mn4þ-doped materials. © 2016 Elsevier B.V. All rights reserved.
Keywords: Double perovskite Phosphors Mn4þ ions Optical properties Deep red-emitting
1. Introduction Double perovskite oxides with A2BMO6 formula (A: alkalineearth or rare-earth element, B: electropositive ion with a lower oxidation state than M, and M: d0 transition metal e.g., Ti4þ, Nb5þ, Ta5þ, Mo6þ or W6þ) are a group of materials with a variety of photocatalytic and optoelectronic applications and have attracted much interest because of their physical properties [1e3]. The B and M cations are octahedrally coordinated by oxygen and formed the BO6 and MO6 octahedra [4]. In recent years, the luminescence materials based on double perovskite molybdates/tungstates A2BMO6 (M ¼ Mo or W) have been reported widely due to their higher excitation efficiency of the M - O charge transfer band (CTB) in the MO6 group, such as Sr2ZnWO6:RE3þ (RE ¼ Eu, Dy, Sm and Pr), (Ba,Sr)2CaMoO6:Eu3þ,Yb3þ, Sr2MgMoxW1-xO6:Eu3þ, Sr2ZnMoO6:Tb3þ, KLa(MoO4)2:Sm3þ, Sr2ZnMoO6:Dy3þ, CaLa2(MoO4)4:Eu3þ [5e11]. In orde to reduce cost of materials, nonrare-earth ions as activator in luminescence materials are becoming a new research object.
Mn4þ with outer 3d3 electron configuration as a non-rare-earth activator has been extensively researched [12e14]. Mn4þ-doped materials usually show red or deep red emission in the range of 620e760 nm due to the 2Eg - 4A2g transition under blue/UV light irradiation [13]. In recent years, Mn4þ-doped materials have two type hosts: one is hexafluorides (e.g., K2SiF6, KNaSiF6, K3ZrF7, BaSiF6, BaTiF6, (NH4)2TiF6:Mn4þ, ZnSnF6$6H2O:Mn4þ, and Cs2SnF6:Mn4þ) [14e21] and the other is oxides (e.g., CaAl2O4, CaAl12O19, Mg2TiO4, Li3Mg2NbO6, NaLaMgTeO6, Li2AlGe2O6, La2LiTaO6, Gd2MgTiO6, Ba2GeO4, and Ca14Al10Zn6O35) [22e31]. However, to the best of our knowledge, there has few reports about the luminescence properties of Sr2ZnMo6:Mn4þ (SZM:Mn4þ). In the present work, double perovskite SZM:Mn4þ phosphor is synthesized by high-temperature solid-state reaction method in air. The crystal structures, luminescence properties, and fluorescence lifetimes are investigated. The influences of Mn4þ ion concentration on luminescence properties and lifetimes are analyzed. The luminous mechanism of SZM:Mn4þ phosphor is explained by Tanabe-Sugano energy level diagram of Mn4þ ion. 2. Experimental procedures
* Corresponding author. E-mail address: [email protected] (R. Cao). http://dx.doi.org/10.1016/j.optmat.2016.10.047 0925-3467/© 2016 Elsevier B.V. All rights reserved.
Here, raw materials such as SrCO3 (A.R. 99.5%), ZnO (99.9%),
R. Cao et al. / Optical Materials 62 (2016) 706e710
MoO3 (99.9%), and MnCO3 (A.R. 99.5%) are purchased from the Aladdin Chemical Reagent Company in Shanghai, China. We synthesize many novel SZM:xMn4þ (x ¼ 0, 0.2, 0.4, 0.6, 0.8, and 1.0 mol %) phosphors by high-temperature solid-state reaction method in air. The stoichiometric amounts of raw materials are well grounded in an agate mortar without performing any purification, sintered at 650 C for 5 h, grounded again, and finally sintered at 1150 C for 6 h in air. Repeated grindings are performed between two sintering processes to improve the homogeneity. 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 2q range of 10e90 . Luminescence properties, timeresolved spectra, and fluorescence lifetimes of phosphors are investigated by using a steady-state FLS980 spectrofluorimeter (Edinburgh Instruments, UK, Edinburgh) with a high spectral resolution (signal to noise ratio >12000:1) at room temperature. A 450 W ozone free xenon lamp is used for steady-state measurements. A microsecond pulsed xenon flash lamp mF900 with an average power of 60 W is available to record the emission decay curves for lifetimes. 3. Results and discussion The unit cell of SZM drawn on the basis of the Inorganic Crystal Structure Database (ICSD) #28602 is shown in Fig. 1. SZM belongs to one of the double perovskite oxides and is described as the cubic crystal system with space-group Fm - 3 m (225) and the lattice parameters of a ¼ 7.954 Å, V ¼ 503.22(38) Å3, and z ¼ 4 [32]. The SZM crystal structure contains [MoO6] and [ZnO6] octahedra surrounding the Sr2þ ions. Sr2þ ions are in the vacancies in the normal tetrahedra. Mn4þ ion can occupy the cation site in the octahedral symmetry. According to ionic radius similar principle (Zn2þ: ~0.74 Å, Mo6þ: ~0.62 Å, and Mn4þ: ~0.54 Å) [33], it can be infered that Mn4þ ions will replace mainly the Mo6þ ion sites in host SZM lattice. It is well known that valence states between Mo6þ and Mn4þ ions are different. So, the charge balance in host SZM will be difficult to be kept after Mn4þ ions will replace the Mo6þ ions sites. In order to keep the charge balance, some Mn4þ ions will occupy Zn2þ ions. Thus, the new charge balance equation (2 Mn4þ ¼ Mo6þ þ Zn2þ) may be expected. According to XRD patterns of Joint Committee on Powder Diffraction Standards (JCPDS) card (no. 74e2474) (SZM), blank SZM, and SZM:xMn4þ (x ¼ 0.2, 0.6, and 1.0 mol%) phosphor in Fig. 2, it can be understood that the XRD patterns of the samples are all matched well with the standard data of JCPDS card (no. 74e2474). The XRD patterns of other SZM:xMn4þ (0 x 1.0 mol%) phosphors are not displayed in Fig. 2, but those patterns are also in line with those of JCPDS card (no. 74e2474). No other crystalline phases
Fig. 1. The unit cell of SZM drawn on the basis of ICSD #28602.
707
Fig. 2. XRD patterns of JCPDS card no. 74e2474 (SZM), blank SZM, and SZM:xMn4þ (x ¼ 0.2, 0.6, and 1.0 mol%) phosphors.
are observed by Mn4þ ion doping. The results indicate that the doping of Mn4þ ion does not cause significant influence on the SZM crystal structure. So, it is said that all samples are pure phase SZM. Photoluminescence excitation (PLE) and photoluminescence (PL) spectra of SZM:0.8%Mn4þ phosphor at room temperature are shown in Fig. 3. Manganese belongs to a transition metal and has many valence states (e.g., þ2, þ3, þ4, þ5, þ6, and þ7). Mn2þ ion has usually different PL bands in the range of 450e610 nm, Mn3þ, Mn5þ, and Mn6þ ions show mainly near-infrared emission, and Mn7þ ion does not show emission [34]. So, the emission of SZM:Mn4þ phosphor is derived from Mn4þ ion. With excitation 430 and 520 nm, SZM:0.8%Mn4þ phosphor shows deep red emission and the Commission Internationale de L'Eclairage (CIE) chromaticity coordinate is about (0.732, 0.268). PL band peaking at ~705 nm of SZM:0.8%Mn4þ phosphor in the range of 650e790 nm is attributed to the 2E / 4A2 electron transition of Mn4þ ion activated with different lattice vibration modes of the octahedra [35,36]. Monitored at 705 nm, PLE spectrum of SZM:0.8%Mn4þ phosphor in the range of 210e610 nm contains three PLE bands, which are assigned to the O2 / Mn4þ charge transfer (CT) and the 4 A2 / 4T1 electron transition of Mn4þ ion (210e375 nm), the 4A2 / 2 T2 electron transition of Mn4þ ion (375e470 nm), and the 4A2 / 4 T2 electron transition of Mn4þ ion (470e600 nm), respectively [37,38]. Comparison of the PL and PLE spectra in Fig. 3 with those of Sr2ZnWO6 phosphor reported in Ref. [30] shows that the cations in the octahedral symmetry have some influences on luminescence properties of Mn4þ ion. PL band peaking at ~702 nm of Sr2ZnWO6:Mn4þ phosphor does not split and the strongest PLE
Fig. 3. PL and PLE spectra of SZM:0.8%Mn4þ phosphor at room temperature (lex ¼ 430 and 520 nm; lem ¼ 705 nm).
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band lies in the range of 210e450 nm [39]. However, PL band peaking at ~705 nm of SZM:Mn4þ phosphor shows split and the strongest PLE band lies in the range of 470e600 nm. It is posible reason that the lattice vibration modes of the octahedra are affected by the host crystal field. Now, we consider the luminous mechanism of SZM:Mn4þ phosphor. Mn4þ ion as a non-rare-earth activator has a 3d3 electronic configuration. In octahedral environment, Mn4þ ion can be stabilized by substituting the cation (e.g., Al3þ, Ti4þ, Sn4þ, Si4þ, or Ge4þ etc) in host lattice. Its dependence of energy levels on crystal field strength can be well explained by Tanabe-Sugano energy level diagram in Fig. 4. 4F ground level and 2H excited level in Mn4þ ion are the most important free ion states [40]. The 3d state of Mn4þ ion can split into two- and three-fold degenerate T2g and Eg states by the crystal field strength. The 4F ground level can split into the 4A2 ground state, the 4T2 and 4T1 excited states by the crystal field strength. Because of the admixture of the 2E level state to the 4T2 level state through spin-orbital interaction, the spin-forbidden 2E / 4A2 transition may appear [41]. The absorption bands of Mn4þ ion are usually assigned to the O2 / Mn4þ CT and the spinallowed 4A2 / (4T2, 2T2, and 4T1) transitions. Emission of Mn4þ ion is always dominated by the 2E / 4A2 electron transition aided by lattice vibrations, whose energy is significantly influenced by the Mn4þ - ligand hybridization in host lattice. PL spectra of SZM:xMn4þ (0.2 x 1.0 mol%) phosphors with excitation at 430 nm at room temperature are shown in Fig. 5. It can be seen that PL band peaks are the same except PL intensity with changing Mn4þ ion concentration in the range of 0.2e1.0 mol%. Their CIE chromaticity coordinates are about (0.732, 0.268) PL intensity increases with increasing Mn4þ ion concentration from 0.2 to 0.8 mol% then decreases with further increasing Mn4þ ion concentration. The distance between Mn4þ ions decreases with increasing Mn4þ ion concentration from 0.2 to 0.8 mol%. The smaller distance is helpful for energy transfer and superposition between Mn4þ ions, so emission will be enhanced. If the distance between Mn4þ ions further decreases the concentration quenching of Mn4þ ions will be formed, which can decrease the emission. The result indicates that the optimal Mn4þ ion concentration is ~0.8 mol % in SZM:Mn4þ phosphor. The critical transfer distance (Rc) between Mn4þ ions can be calculated using the formula suggested by Blasse et al. [34]:
Rc z2½3V=ð4pXc NÞ1=3
(1)
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. By taking the experimental and analytic values of V, Xc, and N (503.22, 0.008, and 4), the critical transfer distance of Mn4þ
Fig. 4. Tanabe-Sugano energy level diagram of Mn4þ in the octahedron.
Fig. 5. PL spectra of SZM:xMn4þ (0.2 x 1.0 mol%) phosphors at room temperature (lex ¼ 430 nm) and the CIE chromaticity coordinate diagram.
ions in SZM:Mn4þ phosphor is ~31 Å. There are usually exchange interaction, radiation reabsorption, and electric multipolar interactions for nonradiate energy transfer. If exchange interaction is occurred, the Rc should be less than 5 Å. The radiation reabsorption interaction may be formed if the absorption and fluorescence spectra are widely overlapping. As a result, we infer that the concentration quenching mainly occurs via electric multipolar interactions between Mn4þ ions [42]. Decay curves of SZM:xMn4þ (0.2 x 1.0 mol%) phosphors at room temperature are shown in Fig. 6. The monitoring wavelength is 705 nm with excitation at 430 nm. The luminescence decay curves are well fitted by a first-order exponential function [43].
IðtÞ ¼ Ið0Þ expðt=tÞ þ A
(2)
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 t is the decay time for the exponential components. All decay curves of SZM:xMn4þ (0.2 x 1.0 mol%) phosphors can be fitted by firstorder exponential function and the fluorescence lifetimes are ~ 132, 124, 116, 112, and 108 ms, respectively. The result shows that lifetime decreases from ~132ms to 108 ms with increasing Mn4þ ion concentration in the range of 0.2e1.0 mol%. Time-resolved spectrum may provide more information on the same time scale as the fluorescence decay than steady-state fluorescence measurements and is used to analyze the transient process and gain insight into the chemical surroundings of the fluorophore [34]. According to time-resolved spectra in Fig. 7, time-
Fig. 6. Decay curves of SZM:xMn4þ (0.2 x 1.0 mol%) phosphors at room temperature (lex ¼ 430 nm and lem ¼ 705 nm).
R. Cao et al. / Optical Materials 62 (2016) 706e710
Fig. 7. Time-resolved spectra of SZM:0.8%Mn4þ phosphor with excitation at 430 nm at room temperature.
resolved spectral shape and peak position are the same except PL intensity with largely changing decay time. However, because the different lattice vibration modes of Mn4þ ion may be affected by changing decay time there are some slight variations between time-resolved spectral shape and steady-state PL shape, such as the ratio of peak intensity at ~705 and 708 nm. Combining with the fluorescence lifetime data in Fig. 6, it can be indicated that there is a luminescent center Mn4þ ion in SZM:Mn4þ phosphor. 4. Conclusion In summary, double perovskite SZM:Mn4þ phosphor is synthesized by high-temperature solid-state reaction method in air. XRD patterns of samples indicate that all samples have only a pure phase SZM. With excitation at 430 and 520 nm, PL band peaking at ~705 nm of SZM:Mn4þ phosphor in the range of 650e790 nm is attributed to the 2E / 4A2 electron transition of Mn4þ ion. Three PLE bands in the range of 210e610 nm are assigned to the O2 / Mn4þ CT and the (4A2 / 4T1, 2T2, and 4T2) electron transition of Mn4þ ion. The optimal Mn4þ ion concentration is ~0.8 mol% in SZM:Mn4þ phosphor. Fluorescence lifetime of SZM:Mn4þ phosphor decreases from ~132 ms to 108 ms with increasing Mn4þ ion concentration in the range of 0.2e1.0 mol%. The luminous mechanism of SZM:Mn4þ phosphor is analyzed by Tanabe-Sugano energy level diagram of Mn4þ ion. The experimental results are useful to develop and research new Mn4þ-doped materials. 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 llez, D.M. Buitrago, R. Cardona, C.E.W. Barrera, J. Roa-Rojas, Crystalline [1] D.A.L. Te structure, magnetic response and electronic properties of RE2MgTiO6 (RE ¼ Dy, Gd) double perovskites, J. Mol. Struct. 1067 (2014) 205e209. [2] E.A. Filonova, A.S. Dmitriev, Crystal structure and thermal properties of Sr2ZnMoO6, Inorg. Mater 49 (6) (2013) 602e605. [3] C. Guo, H.K. Yang, J.H. Jeong, Preparation and luminescent properties of phosphor MGd2(MoO4)4:Eu3þ (M ¼ Sr and Ba), J. Lumin 130 (2010) 1390e1393. [4] C. Tablero, Optical absorption analysis of quaternary molybdate- and tungstateordered double perovskites, J. Alloys Compd. 639 (2015) 203e209. [5] K.V. Dabre, K. Park, S.J. Dhoble, Synthesis and photoluminescence properties of microcrystalline Sr2ZnWO6:RE3þ (RE ¼ Eu, Dy, Sm and Pr) phosphors, J. Alloys Compd. 617 (2014) 129e134. [6] S. Ye, Y. Li, D. Yu, Z. Yang, Q. Zhang, Structural effects on Stokes and antiStokes luminescence of double-perovskite (Ba,Sr)2CaMoO6: Yb3þ,Eu3þ, J. Appl. Phys. 110 (2011) 013517.
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Synthesis and luminescence properties of double perovskite Sr2ZnMoO6:Mn4þ deep red phosphor Renping Cao*, Xiangfeng Ceng, Jijun Huang, Hui Ao, Guotai Zheng, Xiaoguang Yu, Xinqin Zhang College of Mathematics and Physics, Jinggangshan University, Ji'an 343009, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 August 2016 Received in revised form 23 October 2016 Accepted 24 October 2016 Available online 29 October 2016
A double perovskite Sr2ZnMoO6:Mn4þ (SZM:Mn4þ) phosphor is synthesized by high-temperature solidstate reaction method in air. Emission band peaking at ~705 nm of SZM:Mn4þ phosphor in the range of 650e790 nm is attributed to the 2E / 4A2 transition of Mn4þ ion with activated different lattice vibration modes. Three excitation bands in the range of 210e610 nm are assigned to the O2 / Mn4þ charge transfer and the 4A2 / (4T1, 2T2, and 4T2) transition of Mn4þ ion. The optimal Mn4þ ion concentration is ~0.8 mol% in SZM:Mn4þ phosphor. Fluorescence lifetime of SZM:Mn4þ phosphor decreases from ~132 ms to 108 ms with increasing Mn4þ ion concentration in the range of 0.2e1.0 mol%. Time-resolved spectra and fluorescence lifetime data indicate that luminescent center is caused by Mn4þ ion. The luminous mechanism of SZM:Mn4þ phosphor is explained by Tanabe-Sugano energy level diagram of Mn4þ ion. The results are useful to understand the influences of the neighboring coordination environment around Mn4þ and host crystal structure to the luminescence properties of Mn4þ ion and develop other novel Mn4þ-doped materials. © 2016 Elsevier B.V. All rights reserved.
Keywords: Double perovskite Phosphors Mn4þ ions Optical properties Deep red-emitting
1. Introduction Double perovskite oxides with A2BMO6 formula (A: alkalineearth or rare-earth element, B: electropositive ion with a lower oxidation state than M, and M: d0 transition metal e.g., Ti4þ, Nb5þ, Ta5þ, Mo6þ or W6þ) are a group of materials with a variety of photocatalytic and optoelectronic applications and have attracted much interest because of their physical properties [1e3]. The B and M cations are octahedrally coordinated by oxygen and formed the BO6 and MO6 octahedra [4]. In recent years, the luminescence materials based on double perovskite molybdates/tungstates A2BMO6 (M ¼ Mo or W) have been reported widely due to their higher excitation efficiency of the M - O charge transfer band (CTB) in the MO6 group, such as Sr2ZnWO6:RE3þ (RE ¼ Eu, Dy, Sm and Pr), (Ba,Sr)2CaMoO6:Eu3þ,Yb3þ, Sr2MgMoxW1-xO6:Eu3þ, Sr2ZnMoO6:Tb3þ, KLa(MoO4)2:Sm3þ, Sr2ZnMoO6:Dy3þ, CaLa2(MoO4)4:Eu3þ [5e11]. In orde to reduce cost of materials, nonrare-earth ions as activator in luminescence materials are becoming a new research object.
Mn4þ with outer 3d3 electron configuration as a non-rare-earth activator has been extensively researched [12e14]. Mn4þ-doped materials usually show red or deep red emission in the range of 620e760 nm due to the 2Eg - 4A2g transition under blue/UV light irradiation [13]. In recent years, Mn4þ-doped materials have two type hosts: one is hexafluorides (e.g., K2SiF6, KNaSiF6, K3ZrF7, BaSiF6, BaTiF6, (NH4)2TiF6:Mn4þ, ZnSnF6$6H2O:Mn4þ, and Cs2SnF6:Mn4þ) [14e21] and the other is oxides (e.g., CaAl2O4, CaAl12O19, Mg2TiO4, Li3Mg2NbO6, NaLaMgTeO6, Li2AlGe2O6, La2LiTaO6, Gd2MgTiO6, Ba2GeO4, and Ca14Al10Zn6O35) [22e31]. However, to the best of our knowledge, there has few reports about the luminescence properties of Sr2ZnMo6:Mn4þ (SZM:Mn4þ). In the present work, double perovskite SZM:Mn4þ phosphor is synthesized by high-temperature solid-state reaction method in air. The crystal structures, luminescence properties, and fluorescence lifetimes are investigated. The influences of Mn4þ ion concentration on luminescence properties and lifetimes are analyzed. The luminous mechanism of SZM:Mn4þ phosphor is explained by Tanabe-Sugano energy level diagram of Mn4þ ion. 2. Experimental procedures
* Corresponding author. E-mail address: [email protected] (R. Cao). http://dx.doi.org/10.1016/j.optmat.2016.10.047 0925-3467/© 2016 Elsevier B.V. All rights reserved.
Here, raw materials such as SrCO3 (A.R. 99.5%), ZnO (99.9%),
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MoO3 (99.9%), and MnCO3 (A.R. 99.5%) are purchased from the Aladdin Chemical Reagent Company in Shanghai, China. We synthesize many novel SZM:xMn4þ (x ¼ 0, 0.2, 0.4, 0.6, 0.8, and 1.0 mol %) phosphors by high-temperature solid-state reaction method in air. The stoichiometric amounts of raw materials are well grounded in an agate mortar without performing any purification, sintered at 650 C for 5 h, grounded again, and finally sintered at 1150 C for 6 h in air. Repeated grindings are performed between two sintering processes to improve the homogeneity. 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 2q range of 10e90 . Luminescence properties, timeresolved spectra, and fluorescence lifetimes of phosphors are investigated by using a steady-state FLS980 spectrofluorimeter (Edinburgh Instruments, UK, Edinburgh) with a high spectral resolution (signal to noise ratio >12000:1) at room temperature. A 450 W ozone free xenon lamp is used for steady-state measurements. A microsecond pulsed xenon flash lamp mF900 with an average power of 60 W is available to record the emission decay curves for lifetimes. 3. Results and discussion The unit cell of SZM drawn on the basis of the Inorganic Crystal Structure Database (ICSD) #28602 is shown in Fig. 1. SZM belongs to one of the double perovskite oxides and is described as the cubic crystal system with space-group Fm - 3 m (225) and the lattice parameters of a ¼ 7.954 Å, V ¼ 503.22(38) Å3, and z ¼ 4 [32]. The SZM crystal structure contains [MoO6] and [ZnO6] octahedra surrounding the Sr2þ ions. Sr2þ ions are in the vacancies in the normal tetrahedra. Mn4þ ion can occupy the cation site in the octahedral symmetry. According to ionic radius similar principle (Zn2þ: ~0.74 Å, Mo6þ: ~0.62 Å, and Mn4þ: ~0.54 Å) [33], it can be infered that Mn4þ ions will replace mainly the Mo6þ ion sites in host SZM lattice. It is well known that valence states between Mo6þ and Mn4þ ions are different. So, the charge balance in host SZM will be difficult to be kept after Mn4þ ions will replace the Mo6þ ions sites. In order to keep the charge balance, some Mn4þ ions will occupy Zn2þ ions. Thus, the new charge balance equation (2 Mn4þ ¼ Mo6þ þ Zn2þ) may be expected. According to XRD patterns of Joint Committee on Powder Diffraction Standards (JCPDS) card (no. 74e2474) (SZM), blank SZM, and SZM:xMn4þ (x ¼ 0.2, 0.6, and 1.0 mol%) phosphor in Fig. 2, it can be understood that the XRD patterns of the samples are all matched well with the standard data of JCPDS card (no. 74e2474). The XRD patterns of other SZM:xMn4þ (0 x 1.0 mol%) phosphors are not displayed in Fig. 2, but those patterns are also in line with those of JCPDS card (no. 74e2474). No other crystalline phases
Fig. 1. The unit cell of SZM drawn on the basis of ICSD #28602.
707
Fig. 2. XRD patterns of JCPDS card no. 74e2474 (SZM), blank SZM, and SZM:xMn4þ (x ¼ 0.2, 0.6, and 1.0 mol%) phosphors.
are observed by Mn4þ ion doping. The results indicate that the doping of Mn4þ ion does not cause significant influence on the SZM crystal structure. So, it is said that all samples are pure phase SZM. Photoluminescence excitation (PLE) and photoluminescence (PL) spectra of SZM:0.8%Mn4þ phosphor at room temperature are shown in Fig. 3. Manganese belongs to a transition metal and has many valence states (e.g., þ2, þ3, þ4, þ5, þ6, and þ7). Mn2þ ion has usually different PL bands in the range of 450e610 nm, Mn3þ, Mn5þ, and Mn6þ ions show mainly near-infrared emission, and Mn7þ ion does not show emission [34]. So, the emission of SZM:Mn4þ phosphor is derived from Mn4þ ion. With excitation 430 and 520 nm, SZM:0.8%Mn4þ phosphor shows deep red emission and the Commission Internationale de L'Eclairage (CIE) chromaticity coordinate is about (0.732, 0.268). PL band peaking at ~705 nm of SZM:0.8%Mn4þ phosphor in the range of 650e790 nm is attributed to the 2E / 4A2 electron transition of Mn4þ ion activated with different lattice vibration modes of the octahedra [35,36]. Monitored at 705 nm, PLE spectrum of SZM:0.8%Mn4þ phosphor in the range of 210e610 nm contains three PLE bands, which are assigned to the O2 / Mn4þ charge transfer (CT) and the 4 A2 / 4T1 electron transition of Mn4þ ion (210e375 nm), the 4A2 / 2 T2 electron transition of Mn4þ ion (375e470 nm), and the 4A2 / 4 T2 electron transition of Mn4þ ion (470e600 nm), respectively [37,38]. Comparison of the PL and PLE spectra in Fig. 3 with those of Sr2ZnWO6 phosphor reported in Ref. [30] shows that the cations in the octahedral symmetry have some influences on luminescence properties of Mn4þ ion. PL band peaking at ~702 nm of Sr2ZnWO6:Mn4þ phosphor does not split and the strongest PLE
Fig. 3. PL and PLE spectra of SZM:0.8%Mn4þ phosphor at room temperature (lex ¼ 430 and 520 nm; lem ¼ 705 nm).
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band lies in the range of 210e450 nm [39]. However, PL band peaking at ~705 nm of SZM:Mn4þ phosphor shows split and the strongest PLE band lies in the range of 470e600 nm. It is posible reason that the lattice vibration modes of the octahedra are affected by the host crystal field. Now, we consider the luminous mechanism of SZM:Mn4þ phosphor. Mn4þ ion as a non-rare-earth activator has a 3d3 electronic configuration. In octahedral environment, Mn4þ ion can be stabilized by substituting the cation (e.g., Al3þ, Ti4þ, Sn4þ, Si4þ, or Ge4þ etc) in host lattice. Its dependence of energy levels on crystal field strength can be well explained by Tanabe-Sugano energy level diagram in Fig. 4. 4F ground level and 2H excited level in Mn4þ ion are the most important free ion states [40]. The 3d state of Mn4þ ion can split into two- and three-fold degenerate T2g and Eg states by the crystal field strength. The 4F ground level can split into the 4A2 ground state, the 4T2 and 4T1 excited states by the crystal field strength. Because of the admixture of the 2E level state to the 4T2 level state through spin-orbital interaction, the spin-forbidden 2E / 4A2 transition may appear [41]. The absorption bands of Mn4þ ion are usually assigned to the O2 / Mn4þ CT and the spinallowed 4A2 / (4T2, 2T2, and 4T1) transitions. Emission of Mn4þ ion is always dominated by the 2E / 4A2 electron transition aided by lattice vibrations, whose energy is significantly influenced by the Mn4þ - ligand hybridization in host lattice. PL spectra of SZM:xMn4þ (0.2 x 1.0 mol%) phosphors with excitation at 430 nm at room temperature are shown in Fig. 5. It can be seen that PL band peaks are the same except PL intensity with changing Mn4þ ion concentration in the range of 0.2e1.0 mol%. Their CIE chromaticity coordinates are about (0.732, 0.268) PL intensity increases with increasing Mn4þ ion concentration from 0.2 to 0.8 mol% then decreases with further increasing Mn4þ ion concentration. The distance between Mn4þ ions decreases with increasing Mn4þ ion concentration from 0.2 to 0.8 mol%. The smaller distance is helpful for energy transfer and superposition between Mn4þ ions, so emission will be enhanced. If the distance between Mn4þ ions further decreases the concentration quenching of Mn4þ ions will be formed, which can decrease the emission. The result indicates that the optimal Mn4þ ion concentration is ~0.8 mol % in SZM:Mn4þ phosphor. The critical transfer distance (Rc) between Mn4þ ions can be calculated using the formula suggested by Blasse et al. [34]:
Rc z2½3V=ð4pXc NÞ1=3
(1)
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. By taking the experimental and analytic values of V, Xc, and N (503.22, 0.008, and 4), the critical transfer distance of Mn4þ
Fig. 4. Tanabe-Sugano energy level diagram of Mn4þ in the octahedron.
Fig. 5. PL spectra of SZM:xMn4þ (0.2 x 1.0 mol%) phosphors at room temperature (lex ¼ 430 nm) and the CIE chromaticity coordinate diagram.
ions in SZM:Mn4þ phosphor is ~31 Å. There are usually exchange interaction, radiation reabsorption, and electric multipolar interactions for nonradiate energy transfer. If exchange interaction is occurred, the Rc should be less than 5 Å. The radiation reabsorption interaction may be formed if the absorption and fluorescence spectra are widely overlapping. As a result, we infer that the concentration quenching mainly occurs via electric multipolar interactions between Mn4þ ions [42]. Decay curves of SZM:xMn4þ (0.2 x 1.0 mol%) phosphors at room temperature are shown in Fig. 6. The monitoring wavelength is 705 nm with excitation at 430 nm. The luminescence decay curves are well fitted by a first-order exponential function [43].
IðtÞ ¼ Ið0Þ expðt=tÞ þ A
(2)
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 t is the decay time for the exponential components. All decay curves of SZM:xMn4þ (0.2 x 1.0 mol%) phosphors can be fitted by firstorder exponential function and the fluorescence lifetimes are ~ 132, 124, 116, 112, and 108 ms, respectively. The result shows that lifetime decreases from ~132ms to 108 ms with increasing Mn4þ ion concentration in the range of 0.2e1.0 mol%. Time-resolved spectrum may provide more information on the same time scale as the fluorescence decay than steady-state fluorescence measurements and is used to analyze the transient process and gain insight into the chemical surroundings of the fluorophore [34]. According to time-resolved spectra in Fig. 7, time-
Fig. 6. Decay curves of SZM:xMn4þ (0.2 x 1.0 mol%) phosphors at room temperature (lex ¼ 430 nm and lem ¼ 705 nm).
R. Cao et al. / Optical Materials 62 (2016) 706e710
Fig. 7. Time-resolved spectra of SZM:0.8%Mn4þ phosphor with excitation at 430 nm at room temperature.
resolved spectral shape and peak position are the same except PL intensity with largely changing decay time. However, because the different lattice vibration modes of Mn4þ ion may be affected by changing decay time there are some slight variations between time-resolved spectral shape and steady-state PL shape, such as the ratio of peak intensity at ~705 and 708 nm. Combining with the fluorescence lifetime data in Fig. 6, it can be indicated that there is a luminescent center Mn4þ ion in SZM:Mn4þ phosphor. 4. Conclusion In summary, double perovskite SZM:Mn4þ phosphor is synthesized by high-temperature solid-state reaction method in air. XRD patterns of samples indicate that all samples have only a pure phase SZM. With excitation at 430 and 520 nm, PL band peaking at ~705 nm of SZM:Mn4þ phosphor in the range of 650e790 nm is attributed to the 2E / 4A2 electron transition of Mn4þ ion. Three PLE bands in the range of 210e610 nm are assigned to the O2 / Mn4þ CT and the (4A2 / 4T1, 2T2, and 4T2) electron transition of Mn4þ ion. The optimal Mn4þ ion concentration is ~0.8 mol% in SZM:Mn4þ phosphor. Fluorescence lifetime of SZM:Mn4þ phosphor decreases from ~132 ms to 108 ms with increasing Mn4þ ion concentration in the range of 0.2e1.0 mol%. The luminous mechanism of SZM:Mn4þ phosphor is analyzed by Tanabe-Sugano energy level diagram of Mn4þ ion. The experimental results are useful to develop and research new Mn4þ-doped materials. 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 llez, D.M. Buitrago, R. Cardona, C.E.W. Barrera, J. Roa-Rojas, Crystalline [1] D.A.L. Te structure, magnetic response and electronic properties of RE2MgTiO6 (RE ¼ Dy, Gd) double perovskites, J. Mol. Struct. 1067 (2014) 205e209. [2] E.A. Filonova, A.S. Dmitriev, Crystal structure and thermal properties of Sr2ZnMoO6, Inorg. Mater 49 (6) (2013) 602e605. [3] C. Guo, H.K. Yang, J.H. Jeong, Preparation and luminescent properties of phosphor MGd2(MoO4)4:Eu3þ (M ¼ Sr and Ba), J. Lumin 130 (2010) 1390e1393. [4] C. Tablero, Optical absorption analysis of quaternary molybdate- and tungstateordered double perovskites, J. Alloys Compd. 639 (2015) 203e209. [5] K.V. Dabre, K. Park, S.J. Dhoble, Synthesis and photoluminescence properties of microcrystalline Sr2ZnWO6:RE3þ (RE ¼ Eu, Dy, Sm and Pr) phosphors, J. Alloys Compd. 617 (2014) 129e134. [6] S. Ye, Y. Li, D. Yu, Z. Yang, Q. Zhang, Structural effects on Stokes and antiStokes luminescence of double-perovskite (Ba,Sr)2CaMoO6: Yb3þ,Eu3þ, J. Appl. Phys. 110 (2011) 013517.
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