Comparative investigation on luminescence properties of Mn4+ doped Ba6Y2W3O18 and Ba6Gd2W3O18 phosphors

Journal Pre-proof Comparative Investigation on luminescence properties of Mn4+ doped Ba6 Y2 W3 O18 and Ba6 Gd2 W3 O18 phosphors Wenbo Wang, Siguo Xiao

PII:

S0025-5408(19)32695-9

DOI:

https://doi.org/10.1016/j.materresbull.2019.110709

Reference:

MRB 110709

To appear in:

Materials Research Bulletin

Received Date:

17 October 2019

Revised Date:

25 November 2019

Accepted Date:

28 November 2019

Please cite this article as: Wang W, Xiao S, Comparative Investigation on luminescence properties of Mn4+ doped Ba6 Y2 W3 O18 and Ba6 Gd2 W3 O18 phosphors, Materials Research Bulletin (2019), doi: https://doi.org/10.1016/j.materresbull.2019.110709

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Comparative Investigation on luminescence properties of Mn4+ doped Ba6Y2W3O18 and Ba6Gd2W3O18 phosphors Wenbo Wang, Siguo Xiao*

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School of Physics and Optoelectronics, Xiangtan University, Hunan 411105, China *Author to whom correspondence should be addressed: Electronic mail: [email protected] Fax number: +8673158292468 Phone number: +8673158292468

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Graphical abstract

Mn4+ doped Ba6Y2W3O18 and Ba6Gd2W3O18 phosphors have been synthesized via

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solid-state reaction method at high-temperature. Excitation spectra show the phosphors have broad excitation band in region of 300-600 nm with four Gaussian fitting bands

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peaking at around 331 nm, 366 nm, 440 nm and 518 nm for Ba6Y2W3O18:Mn4+ and peaking at around 347 nm, 374 nm, 436 nm and 511 nm for Ba6Gd2W3O18:Mn4+. Far-red emission band locates at 693 nm for Ba6Y2W3O18:Mn4+ and at 676 nm for

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Ba6Gd2W3O18:Mn4+, respectively.

Highlights:

 ‧ Far-red emission of Mn4+ in Ba6Gd2W3O18 is observed for the first time.  ‧ The difference of Mn4+ doped Ba6Y2W3O18 and Ba6Gd2W3O18 phosphors is discussed.  ‧ The maximum quantum efficiency of Mn4+ emission in present

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work reaches to 59.82%.

Abstract

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Mn4+ doped Ba6Y2W3O18 and Ba6Gd2W3O18 phosphors of double perovskite structure have been synthesized via solid-state reaction method at high-temperature.

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Excitation spectra show the phosphors have broad excitation band in region of 300-600 nm with four Gaussian fitting bands peaking at around 331 nm, 366 nm, 440 nm and

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518 nm for Ba6Y2W3O18:Mn4+ and peaking at around 347 nm, 374 nm, 436 nm and 511 nm for Ba6Gd2W3O18:Mn4+. Far-red emission band locates at 693 nm for Ba6Y2W3O18:Mn4+ and at 676 nm for Ba6Gd2W3O18:Mn4+, respectively. The Maximum

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IQC is as high as 59.82% for Ba6Y2W3O18:Mn4+ and 58.68% for Ba6Gd2W3O18:Mn4+. The difference of the crystal field strength and nephelauxetic effect of Mn4+ ion in

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Ba6Y2W3O18 and Ba6Gd2W3O18 matrix are also discussed. The substitution between Y3+ and Gd3+ induced variation of luminescence properties might give meaningful reference

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for designing new Mn4+-doped phosphor or regulating its luminescence performance. KEYWORDS: A. optical materials, B. optical properties, C. X-ray diffraction, D. phosphors

1. Introduction

Luminescent materials have wide application in many areas because of their excellent optical properties, such as anti-counterfeiting, displays, biological detections, laser, light sensor, solar cell, solid-state lighting, etc[1-10]. The combination of phosphors and LED gives people to have lighting technology with high reliability, long working lifetime, environment friendliness and energy saving[11]. Thus in the foreseeable future, phosphors will have a more and more important role in field of solid-state light source. Among the various phosphors, red phosphor is always indispensable. The red phosphors with light emission within 600-680 nm region can be

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applied to improve the color rendering index and reduce the color temperature of

w-LED[12], while that emitting in the 650-750 nm region has great application value in

the field of indoor cultivation, since the phytochromes Pr and Pfr, which are closely

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related to plant growth, have strong absorption of red light at about 665 nm and 730 nm, respectively[13, 14]. However, presently the red phosphors used in solid state light

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source have many shortcomings in chemical stability, synthesis process and luminous intensity, etc[15-17]. Thus the shortcomings of the existing red phosphor and its

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important role prompted us to explore and research red phosphors with better performance.

To achieve red luminescence of phosphors, transition metal Mn4+ and rare-earth

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element Eu3+ (Eu2+) are top picks, and Eu-doped red phosphors are currently the mainstream of commerce[10, 12, 18]. Yet the transition metal Mn4+ ion with the

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electronic configuration 3d3 has its unique luminescence advantages. The emission wavelength of typical 2E → 4A2 transition of Mn4+ ion is determined by the covalent

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interaction between Mn4+ ions and its surrounding environment, meaning that the luminescence emission of Mn4+ has favourable tenability. So far the luminescence emission at various wavelengths ranging from 600 nm to 800 nm for Mn4+-activated phosphors has been widely reported[11, 19]. This means that it is possible to achieve arbitrary wavelength in red or far red region by doping Mn4+ ions into suitable host. Additionally, Mn4+-based phosphor is also much cheaper than rare-earth ions activated

phosphors. In these reason, the red phosphor activated by Mn4+ ion has attracted more and more attention[12]. It is well known that the luminescence of Mn4+ ions usually requires the formation of octahedral coordination in the matrixes[12, 20], and the luminescence properties of Mn4+ in fluoride/oxyfluoride or oxide hosts have been widely investigated[10, 21]. For Mn4+ activated fluoride/oxyfluoride phosphor, there is a distinct red emission line peak at ～630 nm, and the IQE can exceed 80%[22]. Regretfully, HF reagent is necessary during the synthesis process, which is very harmful to the environment. Additionally,

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the chemical stability of fluoride/oxyfluoride is not ideal. In contrast, Mn4+ activated

oxides exhibit much high chemical stability with an eco-friendly preparation procedure, showing promising capabilities as good red phosphor for warm W-LEDs or plant

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cultivation. Especially, tungstate oxides are believed to be the excellent matrix materials for Mn4+ activated phosphors because they have many merits including low price, easy

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synthesis, good stability and outstanding self-activation properties[23, 24]. Thus recently tungsten-based double perovskite phosphors doped with Mn4+, such as Sr2ZnWO6:Mn4+[23],

LiLaMgWO6:Mn4+[25],

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Ca3La2W2O12:Mn4+[11],

NaLaMgWO6:Mn4+[26], SrMg2La2W2O12:Mn4+[27] have been discovered and reported. In this work, double perovskite Ba6Y2W3O18:Mn4+ and Ba6Gd2W3O18:Mn4+ far-red

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phosphor have been prepared. A comparative study was conducted on the excitation and emission spectra, fluorescence lifetime, thermal stability and quantum efficiency of

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Mn4+ in the Ba6Y2W3O18 and Ba6Gd2W3O18. The difference of the crystal field strength and nephelauxetic effect of Mn4+ ion in Ba6Y2W3O18 and Ba6Gd2W3O18 matrix are also

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analyzed on the basis of experimental results. 2. Experimental 2.1. Synthesis

A series of Ba6Y2W3-xO18:xMn4+ (abbreviated as: BYW:xMn, x=0.001, 0.003, 0.006, 0.009, 0.012, 0.015, 0.018) and Ba6Gd2W3-xO18:xMn4+ (abbreviated as: BGW:xMn, x=0.001, 0.003, 0.006, 0.009, 0.012, 0.015, 0.018) samples were prepared

with high temperature solid state reaction method. The used reagents are BaCO3 (99%), Y2O3 (99.99%), Gd2O3 (99.99%), MnCO3 (99%) and WO3 (A.R.). First, all raw materials were weighed in terms of stoichiometric ratio, then uniformly mixed and thoroughly ground in a porcelain mortar. Lastly, the mixture was placed in a corundum crucible, heated to 1400 ℃ in air atmosphere for 7 h, and naturally cooled down to room temperature for obtaining the final sample. 2.2. Characterization The phase purity of the samples was ascertained by X-ray diffraction (XRD)

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patterns recorded on a Bruker D8 Focus Diffractometer (Instrument model: PIGAKV Ultima IV) with Cu Ka radiation (λ=1.5406 Å). The 2θ range was varied between 15°

and 90° with scanning step of 0.02°, and scanning rate of 10°/min. The acquisition of

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photoluminescence excitation (PLE) and emission (PL) spectra used a monochrometer

(Zolix Instrument, Omni-λ320i) equipped with photomultiplier tube (PMTH-S1-CR928)

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and Data Acquisition System, in which another monochrometer (Zolix Instrument, Omni-λ300) equipped with xenon lamp (LSP-X150) was used to generate

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monochromatic excitation light. In addition, the temperature-dependent PL spectra and the luminescence decay curves of Mn4+ in the samples were measured using the variable temperature steady-state/transient fluorescence spectrometer (Edinburgh Instruments,

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FLS980). The internal quantum efficiency (IQE) was given directly by the Edinburgh FLS920 spectrofluorometer equipped with an integrating sphere. All measurements

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were performed at room temperature except the temperature-dependent PL spectra. 3. Results and discussion

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3.1. Crystal structural properties For a compound with A2B’B”O6 stoichiometry, the Goldschmidt tolerance factor

can be used to estimate the possibility for formation of double perovskite structure[28]. Goldschmidt tolerance factor is written as: 𝑡 = (𝑟𝐴 + 𝑟𝑂 )⁄√2(𝑟𝐵 + 𝑟𝑂 )

(1)

where 𝑟𝐴 is ionic radius of A, 𝑟𝐵 is average radius of B' and B" ions, and 𝑟𝑂

represents the ionic radius of oxygen. According to the report of Vasala S[28], the tolerance factor of a compound 𝑡 in the range of 0.85～1.05 implies a relatively ideal double perovskite structure. For Ba6RE2W3O18 (RE=Y, Gd) compounds, A=Ba, B’=RE (Y, Gd), B”=W. The Goldschmidt tolerance factor 𝑡 is calculated to be 1.004 for BYW and 0.997 for BGW and thus double perovskite structure for the BREW (RE=Y, Gd) compound is expected. The X-ray diffraction (XRD) patterns of BYW:xMn (x=0.001, 0.003, 0.006, 0.009, 0.012) and BGW:xMn (x=0.001, 0.003, 0.006, 0.009, 0.012) are shown in Fig. 1. All

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diffraction peaks can be indexed with the Joint Committee on Powder Diffraction

Standards (JCPDS) card NO.38-0219. The double perovskite structure (Space group

Fm-3m (225)) for the prepared Ba6RE2W3O18 (RE=Y, Gd) compounds is consistent with

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the theoretical prediction. The peak position of BGW:xMn shifts to the left as a whole comparing with that of BYW:xMn. The reason lies in the fact that the ionic radius of

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Gd3+ (𝑟=0.938 Å) is larger than that of Y3+ (𝑟=0.90 Å), leading in the expansion of BGW lattice and the increase of interplanar distance. According to the Bragg diffraction

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formula (2𝑑 𝑠𝑖𝑛 𝜃 = 𝜆), the increase of interplanar distance (𝑑) will make the crystal

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XRD diffraction peak leftward shift.

Fig. 1 Powder X-ray diffraction patterns for (a) BYW:xMn (x=0.001, 0.003, 0.006, 0.009, 0.012) and (b) BGW:xMn (x=0.001, 0.003, 0.006, 0.009, 0.012).

In order to ascertain the possibility that Mn4+ ions can substitute Y3+, Gd3+ or W6+ ions in the host cells, the radius percentage difference between the doped Mn4+ and Y3+, Gd3+ or W6+ ions in BYW and BGW host are calculated using following equation[27]:

𝐷𝑟 = [(𝑅𝑠 − 𝑅𝑑 )/𝑅𝑠 ] × 100%

(2)

where 𝐷𝑟 represents the radius percentage difference, and 𝑅𝑠 and 𝑅𝑑 represent the host cation radius and the doped ion radius, respectively. The values of 𝐷𝑟 (Y3+), 𝐷𝑟 (Gd3+) and 𝐷𝑟 (W6+) were calculated to be 41.1%, 43.5% and 11.7% respectively. Obviously, 𝐷𝑟 (W6+)< 30% < 𝐷𝑟 (Y3+)< 𝐷𝑟 (Gd3+), thus Mn4+ ions are more likely to occupy the lattice site of W6+ ions. From the XRD patterns in Fig. 1, it is seen that the substitution of W6+ sites by Mn4+ ions has little influence on the structure of the prepared phosphors. The crystal structure of BYW and BGW are shown as Fig. 2, in

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which Ba2+ ions are coordinated by twelve oxygen atoms and Y3+ (Gd3+) and W6+ ions

are surrounded by six oxygen atoms to form [Y(Gd)O6]9- and [WO6]6- octahedras. These octahedras appear alternately in unit cell and are connected to each other by sharing

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oxygen atoms at the vertices[23, 29].

Fig. 2 Schematic diagram of BY(Gd)W crystal structure.

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3.2. Optical properties

Fig. 3 PLE and PL spectra of (a) BYW:0.003Mn and (b) BGW:0.003Mn phosphors and Gaussian fitting of the PLE spectra.

Fig. 3 exhibits the PLE and PL spectra of BYW:0.003Mn and BGW:0.003Mn phosphors, respectively. Both the phosphors show broad excitation band covering from 300 to 600 nm. Four different excitation bands can be obtained by Gaussian fitting， peaking at around 331 nm, 366 nm, 440 nm and 518 nm for BYW:00.03Mn and peaking at around 347 nm, 374 nm, 436 nm and 511 nm for BGW:0.003Mn. The fitted four bands from UV to green light correspond to the Mn-O charge transfer band (CTB),

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and the 4A2g → 4T1g, 4A2g → 2T2g, 4A2g → 4T2g transitions of Mn4+ ions, respectively. Significantly, the maximum excitation peaks of both phosphors locate at 366 nm,

matching well with the emission of near-UV chips. It indicates that as-prepared phosphors might be used as phosphors with the efficient excitation of commercial LED

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chips. The PL spectra of BYW:xMn and BGW:xMn phosphors excited at 366 nm consist of a far-red luminescence band in the range of 640-750 nm, peaking at around

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693 nm for BYW:xMn and 676 nm for BGW:xMn, respectively. The far red emissions both originate in the 2Eg→4A2g radiative transition of Mn4+. Obviously, the multiple

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vibrational modes of Mn4+ in [WO6]6- octahedral environment cause some vibrational levels near the 2Eg level, and they participate in luminescence process such that the PL

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spectra have a relative broad full width at half maximum (35 nm) and exhibits

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asymmetric characteristics[30, 31].

Fig. 4 PL spectra of (a) BYW:xMn (x=0.001, 0.003, 0.006, 0.009, 0.012) and (b) BGW:xMn (x= 0.001, 0.003, 0.006, 0.009, 0.012) phosphors excited at 366 nm.

Fig. 4 presents the far-red light PL spectra of BYW:xMn and BGW:xMn at different Mn4+ doping concentrations. It can be seen that the position and shape of PL

spectra has not changed with the increase of Mn4+ contentration for both BYW:xMn and BGW:xMn phosphors. Fig. 5 further shows the variation of integral intensity of the BYW:xMn and BGW:xMn phosphors with Mn4+ concentration. Their emission intensities both increase with Mn4+ content increasing, reaching the maximum at x=0.006. After that, the emission intensities of the both phosphors decrease gradually with the increase of Mn4+ content due to the concentration quenching effect. In addition, it is also seen from Fig. 5 that the integral intensity of the emission of BYW:xMn is slightly higher than that of BGW:xMn at the same Mn4+-doping concentration. At the

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optimum doping concentration, the integral intensity of BYW:0.006Mn is 1.06 times

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that of BGW:0.006Mn.

Fig. 5 Integral luminescence intensity as function of Mn4+ concentration.

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The concentration quenching is due to the increase of nonradiative decay channels, which can be promoted via the interaction with quenching centers in the process of

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energy transfer among Mn4+ ions. The energy transfer among Mn4+ ions might arise from radiation re-absorption, exchange interaction or multi-pole interaction[32, 33].

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According to Fig. 3, we can know that there is no overlap between the PLE and PL spectra of the samples, which means that the concentration quenching mechanism of BYW:xMn and BGW:xMn phosphors cannot be the radiation reabsorption among Mn4+ ions. The other energy transfers induced by the exchange interaction and multi-polar interaction rely on the distance between Mn4+ ions. The critical distance 𝐷𝑐 between Mn4+ ions can be approximately calculated via following equation obtained by Blasse et al[34]:

𝐷𝑐 ≈ 2 × (3𝑉 ⁄4𝜋𝑁𝑥𝑐 )1⁄3

(3)

where 𝑥𝑐 is the critical concentration, 𝑁 is the number of available site for the dopant in a unit cell and 𝑉 is the volume of unit cell. In BYW:xMn phosphor, 𝑉=584.28 Å3, 𝑁 =4, 𝑥𝑐 =0.006, thereupon 𝐷𝑐 is calculated to be about 35.97 Å. Similarly, In BGW:xMn phosphor, 𝑉=596.95 Å3, 𝑁=4, 𝑥𝑐 =0.006, thus 𝐷𝑐 ≈36.22 Å. Obviously, exchange interaction is also invalid according to Blasse's theory since the 𝐷𝑐 value in these phosphors is much higher than 5 Å. Therefore, the dominant approach for the

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concentration quenching should be the electric multi-pole interaction.

Fig. 6 Relationship between the 𝑙𝑜𝑔(𝐼/𝑥) and 𝑙𝑜𝑔(𝑥) of BYW:xMn and BGW:xMn.

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According to reports by Van Uitert[35], the specific type of electric multi-pole interaction can be determined by the following equation: −1

(4)

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𝐼 ⁄𝑥 = 𝐶 ∙ (1 + 𝛽 ∙ 𝑥 𝜃⁄3 )

where 𝐼 refers to the intensity and 𝑥 is Mn4+ concentration not lower than the critical quenching concentration. 𝐶 and 𝛽 are constants under the same excitation conditions

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for the given host material. 𝜃 is the hallmark of electrical multi-pole interaction, and

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the interaction mechanisms corresponding of 𝜃=6, 8 and 10 are electrical dipole-dipole (d-d), electrical dipole-quadrupole (d-q) and electrical quadrupole-quadrupole (q-q) interactions, respectively[27, 36]. The relationship between 𝑙𝑜𝑔(𝐼/𝑥) and 𝑙𝑜𝑔(𝑥) of BYW:xMn and BGW:xMn phosphors is shown Fig. 6, which are fitted to linear correlations with the slope (−𝜃/3) equaling to -1.43 for BYW:xMn and -1.56 for BGW:xMn, respectively. The 𝜃 for both BYW:xMn and BGW:xMn is obviously close to 6, indicating the quenching mechanism in both kinds of phosphors is due to the

dipole-dipole (d-d) interaction[25, 36]. The decay curves for 2Eg→4A2g emission of Mn4+ in BYW:xMn and BGW:xMn phosphors are measured and plotted in Fig. 7. All these curves can be fitted into following single-exponential equation [11]: 𝐼𝑡 = 𝐴 + 𝐼0 ∙ 𝑒𝑥𝑝(−𝑥⁄𝜏)

(5)

where 𝐼𝑡 represent luminescence intensity at time 𝑡, 𝐼0 is the initial (and maximum) luminescence intensity, 𝐴 is constant, and 𝜏 is the radiative decay lifetime. The final results are displayed in Table 1. As expected, all the decay lifetimes are within a few

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hundred microseconds, indicating that the Mn4+ emission cames from the spin-forbidden transition within the 3d-shell[37]. At the same Mn4+ concentration, the

radiative decay lifetime of BYW:xMn is about 200 μs longer than that of BGW:xMn.

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This suggests that the radiative transition probability of Mn4+ in BGW matrix should be larger than that in the BYW one. On the other hand, it is also noted from Table 1 that the

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decay lifetime for both BYW:xMn and BGW:xMn phosphors decreases monotonously with the increase of the Mn4+ concentration, which can be explained as follows. As the

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Mn4+ concentration increases, the distance between the Mn4+ ions is shortened. Subsequently, the excitation energy exchange among Mn4+ ions increases and the

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energy loss rate increases, which thus shortens the decay lifetime[25].

Fig. 7 Normalized decay curves of Mn4+: 2Eg→4A2g in (a) BYW:xMn and (b) BGW:xMn under 366 nm excitation. Table 1 Radiative decay lifetime (τ) of Mn4+: 2Eg→4A2g in BYW:xMn and BGW:xMn phosphors.

Mn4+ concetration

BYW:xMn (693 nm)

BGW:xMn (676 nm)

x

τ (μs)

τ (μs)

0.001 0.003 0.006 0.009 0.012 0.015 0.018

472.46 467.30 459.96 435.55 423.27 411.81 405.42

274.16 266.63 259.77 248.25 241.38 234.84 226.44

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3.3. Crystal field analysis

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Fig. 8 (a) Tanabe-Sigano energy level diagram of Mn4+ in the BYW/BGW host and (b) simple energy level diagram of Mn4+.

Fig. 8(a) is the Tanabe-Sugano energy level diagram of d3 electron configuration,

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which can illustrate the electron transition between different energy levels of Mn4+ in octahedral crystal field[38]. In the BREW:xMn (RE=Y, Gd) phosphor, the 4F ground

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level of Mn4+ will split into three sub-configurations: 4A2g, 4T1g and 4T2g, and the 2G level will split into four sub-configurations: 2A1g, 2T2g, 2T1g and 2Eg. When excited by

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near-UV/green light, the electrons will transit from the ground state 4A2g to the excited state (4T1g(4F), 4T2g(4F), 2T2g(2G)), forming two spin-allowed transitions 4

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A2g→4T1g,

A2g→4T2g and a spin-forbidden transition of 4A2g→2T2g. Then, the excited state

electrons relax to the lowest excited level of 2Eg(2G) by means of non-radiation transition, and eventually return to the ground state 4A2g through radiative transition (2Eg→4A2g), meanwhile emitting red light (as shown in Fig. 8(b)). Based on the energy gap of the 4A2g→4T2g transition (19303 cm-1), the crystal-field

strength (𝐷𝑞 ) of Mn4+ suffered in BYW matrix can be roughly estimated by the following equation [30]: 𝐷𝑞 = 𝐸( 4𝑇2𝑔 → 4𝐴2𝑔 )/10

(6).

Moreover, the Racah parameter 𝐵 and 𝐶 can be acquired according to the following equations[39]: (7)

𝑥 = 𝐸( 4𝐴2𝑔 → 4𝑇1𝑔 ) −𝐸( 4𝐴2𝑔 → 4𝑇2𝑔 )⁄𝐷𝑞

(8)

𝐸( 2𝐸𝑔 → 4𝐴2𝑔 )⁄𝐵 = 3.05𝐶 ⁄𝐵 + 7.9 − 1.8𝐵⁄𝐷𝑞

(9).

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𝐷𝑞 ⁄𝐵 = 15(𝑥 − 8)⁄(𝑥 2 − 10𝑥)

On the basis of PLE and PL spectra in Fig. 3, the crystal field parameters 𝐷𝑞 , 𝐵 and 𝐶

are calculated to be 1930 cm-1, 813 cm-1 and 2827 cm-1 for the BYW:xMn and 1958

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cm-1, 701 cm-1 and 3184 cm-1 for the BGW:xMn, respectively. The Tanabe-Sugano

energy level diagram shown in Fig. 8(a) clearly illustrates the dependence between the

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orbital energy level for Mn4+ in the host and the crystal field splitting energy. When 𝐷𝑞 /𝐵 ≥ 2.1, the crystal field will be considered as a strong one. The calculated value

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of the 𝐷𝑞 /𝐵 is about 2.4 in BYW and 2.8 in BGW. Thus it is evident that the crystal field in BYW and BGW are both strong ones. This further confirms that the PL spectra of Mn4+ is performed by the spin-forbidden transition 2Eg→4A2g, and the PLE spectra

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are consisted of three broad-bands attributed to the 4A2g→4T1g, 4A2g→4T2g spin-allowed transitions and the 4A2g→2T2g spin-forbidden transition. Table 2

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𝐷𝑞 is the crystal field splitting energy, 𝐵 and 𝐶 are the Racah parameters, nephelauxetic ratio (𝛽1 ), E(2Eg) is energy of the Mn4+-2Eg level. Dq/cm-1

B/cm-1

C/cm-1

β1

E(2Eg)/cm-1

Ref.

NaLaMgTeO6

2008

790

2949

0.966

14676

[40]

Y2Sn2O7

2100

700

3515

1.016

15563

[41]

CaAl12O19

2132

807

3088

0.999

15244

[42]

K2Ge4O9

2163

785

3146

0.996

15281

[43]

SrGe4O9

2362

832

3024

1.004

15267

[44]

SrMgAl10O17

2237

791

3084

0.989

15152

[45]

Ba2LaNbO6

1780

670

3290

0.958

14679

[46]

LiLaMgWO6

2101

724

2870

0.913

14025

[25]

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Crystal

SrMg2La2W2O12

2088

746

2856

0.924

14124

[27]

LiAl5O8

2014

725

2900

0.92

13977

[47]

YAlO3

2100

720

3025

0.938

14450

[48]

CaZrO3

1850

754

3173

0.983

15054

[39]

SrTiO3

1818

719

2839

0.905

13827

[49]

Li2MgTiO4

2101

724

3122

0.957

14793

[50]

Mg2TiO4

2096

700

3348

0.985

15267

[51]

BaTiO3

1780

738

2820

0.913

13862

[52]

Ba6Y2W3O18

1930

813

2827

0.960

14430

This work

Ba6Gd2W3O18

1958

701

3184

0.955

14793

This work

From the Fig. 8(a), it can be seen that the emission energy of 2Eg→4A2g is barely

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dependent on the crystal field strength. In fact, Brik et al have found through extensive research that the emission energy of Mn4+ is singularly dependent on the covalence of

the Mn4+-ligands bonding (nephelauxetic effect). To describe this relationship, they

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introduced a dimensionless parameter 𝛽1[39]: 𝛽1 = √(𝐵⁄𝐵0 )2 + (𝐶 ⁄𝐶0 )2

(10)

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where 𝐵0 and 𝐶0 , representing the Racah parameters of Mn4+ free ions are 1160 cm-1 and 4303 cm-1, respectively. Herein, the 𝛽1 of BYW:xMn and BGW:xMn is calculated

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to be 0.960 and 0.955, respectively. Table 2 lists the value of 𝛽1 and spectroscopic parameters of Mn4+ ions in various oxide crystals. Based on these, the linear fitted plot

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of 2Eg energy level versus 𝛽1 is shown in Fig. 9. The linear equation 𝐸( 2𝐸𝑔 ) = −312.98 + 15640.81𝛽1 may predict the emission energy of Mn4+ in oxides on the

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basis of 𝛽1 parameter.

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Fig. 9 Dependence of the Mn4+-2Eg level on the nephelauxetic ratio β1.

However, the data points of BYW:xMn and BGW:xMn do not well fall on the fitted line, just as shown in Fig. 9. One reasonable explanation is that very often the

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Mn4+ 4A2g→4T1g, 4A2g→4T2g and 2Eg→4A2g transitions are accompanied by the vibronic

progressions, and thus it is difficult to ascertain the position of the zero-phonon line

2

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unambiguously[53, 54]. For instance, the position of the zero-phonon line of the Eg→4A2g emission for the BREW:xMn (RE=Y, Gd) phosphor cannot be ascertained

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even at low temperature. Fig. 10 exhibits PL spectra of the BYW:0.006Mn and BGW:0.006Mn sample at 77K and 300K. All PL spectra manifest three distinguishable

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peaks (A, B and C) ascribed to 2Eg→4A2g Stokes/Anti-stokes phonon sidebands of different vibrational modes. Unlike the case of low (77K) temperature, more confined phonon vibrations participate in the luminescence process at room (300K) temperature.

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The emission band is thermally broadened and the anti-Stokes sideband (A-peak) is significantly enhanced. Meanwhile, the existence of thermal quenching effect obviously

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reduces the intensity of B-peak and C-peak at 300K. Although a more structured spectral profile at 77K than that at 300K, the zero-phonon line of Mn4+ still cannot be distinguished even at 77 K, because of the parity- and spin-forbidden nature of the 2

Eg→4A2g transition. Therefore, we can only use the peaks at the center of the excitation

and emission band to give a rough calculation of 𝐷𝑞 , 𝐵 and 𝐶, and thus the error is naturally inevitable.

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Fig. 10 PL spectra of BREW:0.006Mn (RE=Y, Gd) excited by 366 nm at 77K and 300K temperature. (a) and (b) are relative intensity, (c) and (d) are normalized intensity.

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3.4. Thermal stability and IQE

Fig. 11 Temperature-dependent PL spectra of (a) BYW:0.006Mn and (b) BYW:0.006Mn.

The luminescence thermal stability is an important parameter for the phosphors

used in solid-state light source[16], especially in high-power ones because the temperature might seriously influence the luminescence performance of phosphors, with changing light output consequently. Therefore, the temperature-dependent PL spectra of

BYW:0.006Mn and BYW:0.006Mn excited by 366 nm near-UV light were measured with increasing temperature from 300K to 480K, as displayed in Fig. 11. The dependence of PL integral intensity for 2Eg→4A2g from 640 nm to 750 nm of the phosphors on the temperature is portrayed in Fig. 12(a). The PL integral intensities of the BYW:0.006Mn and BYW:0.006Mn phosphors both decrease with the increase of temperature from 300K to 480K. It is found that the values of integral luminescence intensity of the BYW:0.006Mn and BYW:0.006Mn phosphors at 390 K decrease to 62.0% and 50.1% of their initial value at 300K and those at 420K further decrease to

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39.1% and 25.6% respectively, due to the temperature quenching. There is no doubt that the thermal stability of BYW:0.006Mn is better than that of BGW:0.006Mn.

The thermal quenching originates from non-radiative relaxation process the

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intersection between the excited and ground states, and the specific process can be

illustrated by the Fig. 12(b). When the Mn4+ ions are excited, the electrons will

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transition from the ground state 4A2g to the excited state (4T1g, 4T2g, 2T2g). Then the electrons relax to the lowest excited state 2Eg through the non-radiative transition, and

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finally return to the ground state 4A2g in the way of a radiative transition. With the temperature increases, part of the electrons in the excited state 2Eg may transfer to other energy levels through the intersection, and then return to the ground state 4A2g, as

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described in path a→b→c in Fig. 12(b). This will inevitably lead to the decrease of the radiation transition probability of 2Eg→4A2g, consequently the PL intensity of Mn4+ ion

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weakens with the temperature rising.

Fig. 12 (a) PL integral intensity and (b) configuration diagram of Mn4+ in the octahedron.

Further, the PL intensity as a function of temperature can be described with Arrhenius equation[26]: 𝐼𝑇 = 𝐼0 ∙ [1 + 𝐶 ∙ 𝑒𝑥𝑝(−𝐸𝑎 ⁄𝑘𝑇)]−1

(11)

where 𝐼0 and 𝐼𝑇 mean the initial intensity at room (300K) temperature and the intensity at temperature 𝑇, respectively. 𝐶 is a constant that does not affect the calculation, 𝑘 is the Boltzmann constant, and 𝐸𝑎 is the activation energy for thermal quenching. According to Eq (11), the plot of 𝑙𝑛[(𝐼0 /𝐼𝑇 ) − 1] versus 1/𝑘𝑇 for

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BYW:0.006Mn and BGW:0.006 can be linearly fitted into a straight lines, respectively, as shown in Fig. 13. The slopes of the lines are -0.437 and -0.499, suggesting that the of

activation energy (𝐸𝑎 ) are 0.437 eV and 0.499 eV for BYW:0.006Mn and BGW:0.006 phosphors, respectively.

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Notably, the increase in temperature causes a red-shift in the position of the PL peak in addition to the decrease in the PL intensity, as shown in Fig. 10 and Fig. 11. It is

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well known that the thermal shifts of spectra come from two contributions, one is the

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implicit or static contribution due to the changes in the geometry of the transition metal or rare-earth ion clusters in crystals induced by lattice thermal expansion and the other is the explicit or vibrational contribution due to the electron–phonon interaction[55].

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Usually, the former contributes to the blue-shift of the spectrum, while the latter contributes to the red-shift of the spectrum[56-58]. The electron-phonon interaction

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might dominate in the present phosphors since their emission spectra show red-shift with the temperature increasing in the experiment. When the temperature rises from

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300K to 420K, the CIE chromaticity coordinate of BYW:0.006Mn shifts from (0.7316,0.2684) to (0.7302,0.2698) and that of BGW:0.006Mn shifts from (0.7316,0.2684) to (0.7305,0.2695), fully meeting the requirement that the color shift of the applicable luminescence materials should be less than 0.0150 (value of the Δx and Δy) at 150℃ defined by Enterprise Europe Network[59].

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Fig. 13 Plot of 𝑙𝑛[(𝐼0 /𝐼𝑇 ) − 1] versus 1/𝑘𝑇 for BYW:0.006Mn and BGW:0.006Mn phosphors (λex=366 nm).

IQE is a crucial factor to evaluate the applicability as phosphor used in solid-state lighting source. The IQE of the present phosphors were measured and calculated by the instrument based on the following equation[49]: (12)

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𝜂 = (∫ 𝐿𝑆 − ∫ 𝐿𝑅 )⁄(∫ 𝐸𝑅 − ∫ 𝐸𝑆 )

where 𝜂 is the IQE, 𝐿𝑆 is the PL spectra of the studied samples, 𝐿𝑅 is bias light. 𝐸𝑅

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and 𝐸𝑆 present the PLE spectra of the excitation light without and with synthesized

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phosphors, respectively. The IQE value of BYW:0.006Mn4+ and BGW:0.006Mn4+ phosphors under 366 nm excitation are found to be 59.82% and 58.68%, respectively, which are much higher than many red or far-red phosphors doped with Mn4+ reported

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recently like Ca3La2W2O12:Mn4+ (47.9%, λex=360 nm)[11], Li2MgTiO4:Mn4+ (32%, λex=330 nm)[50], Gd2ZnTiO6:Mn4+ (39.7%, λex=365 nm)[30], Li2Mg3SnO6:Mn4+ (36.3%,

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λex=330 nm)[60].

4. Conclusions

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In summary, Mn4+ doped perovskite phosphors Ba6RE2W3-xO18:x (RE=Y, Gd) phosphors were successful synthesized with high-temperature solid-state reaction method. The PLE and PL spectra show that BYW:xMn and BGW:xMn phosphors both have broad excitation band in the range of 300-600 nm with strongest peak at around 366 nm, and can give far-red luminescence centered at 693 and 676 nm, respectively. The optimum doping concentration for Mn4+ 0.6 mol% for both phosphors, beyond which energy transfer between adjacent Mn4+ ions based on d-d interaction results in

concentration quenching. The phosphors can both give strong far red emission and the IQE is as high as 59.82% for BYW:0.006Mn and 58.68% for BGW:0.006Mn under near-UV excitation, respectively. However, the temperature-dependent emission spectra reveals that the BYW:0.006Mn has better thermal stability than that of BGW:0.006Mn. The tenability of luminescence properties caused by the substitution between Y3+ and Gd3+ in the double perovskite tungstate oxide might give important reference for designing new Mn4+ doped phosphor or regulating its luminescence performance.

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Author Statement Siguo Xiao (Corresponding Author):

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Conceptualization, Resources, Writing - Review & Editing, Supervision, Project administration, Funding acquisition.

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Wenbo Wang (First Author):

Conflict of Interest

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Methodology, Validation, Formal analysis, Investigation, Data Curation, Writing Original Draft, Visualization.

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No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication.

Acknowledgments

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This work was supported by the National Science Foundation of China (No.11674272). References

[1] M.A. Noginov, G.B. Loutts, Spectroscopic studies of Mn4+ ions in yttrium orthoaluminate, JOSA B, 16 (1999) 3-11. [2] G.B. Loutts, M. Warren, L. Taylor, R.R. Rakhimov, H.R. Ries, G. Miller III, M.A. Noginov, M. Curley, N. Noginova, N. Kukhtarev, H.J. Caulfield, P. Venkateswarlu, Manganese-doped yttrium orthoaluminate: a potential material for holographic recording and data storage, Phys. Rev. B, 57

Jo

ur

na

lP

re

-p

ro of

(1998) 3706. [3] A. Pan, Y.A. Li, Y.S. Wu, K. Yan, M.J. Jurow, Y. Liu, L. He, Stable luminous nanocomposites of CsPbX3 perovskite nanocrystals anchored on silica for multicolor anti-counterfeit ink and white-LEDs, Mater. Chem. Front., 3 (2019) 414-419. [4] X.Y. Huang, Dual-model upconversion luminescence from NaGdF4: Nd/Yb/Tm@NaGdF4:Eu/Tb core–shell nanoparticles, J. Alloys Compd., 628 (2015) 240-244. [5] R.J. Yu, M.X. Li, N.N. Xie, T. Wang, N. Xue, Structure and luminescence characteristics of Eu3+‐activated trigonal layered perovskite Ba2La2ZnW2O12, J. Am. Ceram. Soc., 98 (2015) 3849-3855. [6] X.Y. Huang, Broadband dye-sensitized upconversion: A promising new platform for future solar upconverter design, J. Alloys Compd., 690 (2017) 356-359. [7] A.A. Lagatsky, O.L. Antipov, W. Sibbett, Broadly tunable femtosecond Tm:Lu2O3 ceramic laser operating around 2070 nm, Opt. Express, 20 (2012) 19349-19354. [8] A. Fafin, J. Cardin, C. Dufour, F. Gourbilleau, Theoretical investigation of the more suitable rare earth to achieve high gain in waveguide based on silica containing silicon nanograins doped with either Nd3+ or Er3+ ions, Opt. Express, 22 (2014) 12296. [9] X.G. Zhang, Z.C. Wu, F.W. Mo, N. Li, Z.Y. Guo, Z.P. Zhu, Insight into temperature-dependent photoluminescence of LaOBr:Ce3+, Tb3+ phosphor as a ratiometric and colorimetric luminescent thermometer, Dyes Pigm., 145 (2017) 476-485. [10] E.H. Song, Y.Y. Zhou, X.B. Yang, Z.F. Liao, W.R. Zhao, T.T. Deng, L.Y. Wang, Y.Y. Ma, S. Ye, Q.Y. Zhang, Highly efficient and stable narrow-band red phosphor Cs2SiF6:Mn4+ for high-power warm white LED applications, ACS Photonics, 4. (2017) 2556-2565. [11] X.Y. Huang, H. Guo, Finding a novel highly efficient Mn4+-activated Ca3La2W2O12 far-red emitting phosphor with excellent responsiveness to phytochrome PFR: Towards indoor plant cultivation application, Dyes Pigm., 152 (2018) 36-42. [12] S. Adachi, Photoluminescence properties of Mn4+-activated oxide phosphors for use in white-LED applications: a review, J. Lumin., 202 (2018) 263-281. [13] N. Yeh, J.P. Chung, High-brightness LEDs—Energy efficient lighting sources and their potential in indoor plant cultivation, Renewable Sustainable Energy Rev., 13 (2009) 2175-2180. [14] H. Smith, Phytochromes and light signal perception by plants—an emerging synthesis, Nature, 407 (2000) 585. [15] J.X. Hu, T.H. Huang, Y.P. Zhang, B. Lu, H.Q. Ye, B.J. Chen, H.P. Xia, C.Y. Ji, Enhanced deep-red emission from Mn4+/Mg2+ co-doped CaGdAlO4 phosphors for plant cultivation, Dalton Trans., 48 (2019) 2455-2466. [16] J. Zhao, C.F. Guo, T. Li, X.Y. Su, N.M. Zhang, J.Y. Chen, Synthesis, electronic structure and photoluminescence properties of Ba2BiV3O11:Eu3+ red phosphor, Dyes Pigm., 132 (2016) 159-166. [17] J.S. Zhong, S. Zhou, D.Q. Chen, J.J. Li, Y.W. Zhu, X.Y. Li, L.F. Chen, Z.G. Ji, Enhanced luminescence of a Ba2GdSbO6:Mn4+ red phosphor via cation doping for warm white light-emitting diodes, Dalton Trans., 47 (2018) 8248-8256. [18] J.S. Zhong, D.Q. Chen, X. Chen, K.Y. Wang, X.Y. Li, Y.W. Zhu, Z.G. Ji, Efficient rare-earth free red-emitting Ca2YSbO6:Mn4+, M(M= Li+, Na+, K+, Mg2+) phosphors for white light-emitting diodes, Dalton Trans., 47 (2018) 6528-6537. [19] P.F. Li, M.Y. Peng, X.W. Yin, Z.J. Ma, G.P. Dong, Q.Y. Zhang, J.R. Qiu, Temperature dependent

Jo

ur

na

lP

re

-p

ro of

red luminescence from a distorted Mn4+ site in CaAl4O7:Mn4+, Opt. Express, 21 (2013) 18943-18948. [20] M.H. Du, Chemical trends of Mn4+ emission in solids, J. Mater. Chem. C, 2 (2014) 2475-2481. [21] Z.L. Wang, Z.Y. Yang, Z.F. Yang, Q.W. Wei, Q. Zhou, L. Ma, X.J. Wang, Red phosphor Rb2NbOF5:Mn4+ for warm white light-emitting diodes with a high color-rendering index, Inorg. Chem., 58 (2018) 456-461. [22] Y. Liu, H. Li, S. Tang, Q. Zhou, K.M. Wang, H.J. Tang, Z.L. Wang, A red-emitting phosphor K2[MoO2F4]•H2O:Mn4+ for warm white light-emitting diodes with a high color rendering index, Mater. Res. Bull., (2019) 110675. [23] R.P. Cao, X.F. Ceng, J.J. Huang, X.W. Xia, S.L. Guo, J.W. Fu, A double-perovskite Sr2ZnWO6:Mn4+ deep red phosphor: synthesis and luminescence properties, Ceram. Int., 42 (2016) 16817-16821. [24] P. Du, L.K. Bharat, J.S. Yu, Strong red emission in Eu3+/Bi3+ ions codoped CaWO4 phosphors for white light-emitting diode and field-emission display applications, J. Alloys Compd., 633 (2015) 37-41. [25] J. Liang, L.L. Sun, B. Devakumar, S.Y. Wang, Q. Sun, H. Guo, B. Li, X.Y. Huang, Novel Mn4+-activated LiLaMgWO6 far-red emitting phosphors: high photoluminescence efficiency, good thermal stability, and potential applications in plant cultivation LEDs, RSC Adv., 8 (2018) 27144-27151. [26] X.Y. Huang, J. Liang, B. Li, L.L. Sun, J. Lin, High-efficiency and thermally stable far-red-emitting NaLaMgWO6:Mn4+ phosphorsfor indoor plant growth light-emitting diodes, Opt. Lett., 43 (2018) 3305-3308. [27] S.Y. Wang, Q. Sun, B. Devakumar, L.L. Sun, J. Liang, X.Y. Huang, Novel SrMg2La2W2O12:Mn4+ far-red phosphors with high quantum efficiency and thermal stability towards applications in indoor plant cultivation LEDs, RSC Adv., 8 (2018) 30191-30200. [28] S. Vasala, M. Karppinen, A2B′B″O6 perovskites: a review, Prog. Solid State Chem., 43 (2015) 1-36. [29] P.F. Niu, X.H. Liu, Z.J. Xie, W.R. Zhao, Photoluminescence properties of a novel red-emitting nanowire phosphor Ba6Gd2W3O18:Eu3+, J. Lumin., 198 (2018) 34-39. [30] H. Chen, H. Lin, Q.M. Huang, F. Huang, J. Xu, B. Wang, Z.B. Lin, J.C. Zhou, Y.S. Wang, A novel double-perovskite Gd2ZnTiO6:Mn4+ red phosphor for UV-based w-LEDs: structure and luminescence properties, J. Mater. Chem. C, 4 (2016) 2374-2381. [31] M.J. Reisfeld, N.A. Matwiyoff, L.B. Asprey, The electronic spectrum of cesium hexafluoromanganese (IV), J. Mol. Spectrosc., 39 (1971) 8-20. [32] D.L. Dexter, J.H. Schulman, Theory of concentration quenching in inorganic phosphors, J. Chem. Phys., 22 (1954) 1063-1070. [33] T. Förster, Intermolecular energy transfer and fluorescence, Ann. Phys. Leipzig., 2 (1948) 55-75. [34] G. Blasse, Energy transfer in oxidic phosphors, Phys. Lett. A, 28 (1968) 444-445. [35] L.G. Van Uitert, Characterization of energy transfer interactions between rare earth ions, J. Electrochem. Soc., 114 (1967) 1048-1053. [36] B. Wang, H. Lin, F. Huang, J. Xu, H. Chen, Z.B. Lin, Y.S. Wang, Non-Rare-Earth BaMgAl10–2xO17:xMn4+, xMg2+: A Narrow-Band Red Phosphor for Use as a High-Power Warm

Jo

ur

na

lP

re

-p

ro of

w-LED, Chem. Mater., 28 (2016) 3515-3524. [37] X. Ding, G. Zhu, W.Y. Geng, Q. Wang, Y.H. Wang, Rare-earth-free high-efficiency narrow-band red-emitting Mg3Ga2GeO8:Mn4+ phosphor excited by near-UV light for white-light-emitting diodes, Inorg. Chem., 55 (2015) 154-162. [38] Y. Tanabe, S. Sugano, On the absorption spectra of complex ions II, J. Phys. Soc. Jpn., 9 (1954) 766-779. [39] M.G. Brik, A.M. Srivastava, Electronic energy levels of the Mn4+ ion in the perovskite, CaZrO3, ECS J. Solid State Sci. Technol., 2 (2013) R148-R152. [40] A.M. Srivastava, M.G. Brik, S.J. Camardello, H.A. Comanzo, F. Garcia-Santamaria, Optical spectroscopy and crystal field studies of the Mn4+ ion (3d3) in the double perovskite NaLaMgTeO6, Z. Naturforsch., B: Chem. Sci., 69 (2014) 141-149. [41] M.G. Brik, A.M. Srivastava, N.M. Avram, Comparative analysis of crystal field effects and optical spectroscopy of six-coordinated Mn4+ ion in the Y2Ti2O7 and Y2Sn2O7 pyrochlores, Opt. Mater., 33 (2011) 1671-1676. [42] T. Murata, T. Tanoue, M. Iwasaki, K. Morinaga, T.J.J.o.l. Hase, Fluorescence properties of Mn4+ in CaAl12O19 compounds as red-emitting phosphor for white LED, J. Lumin., 114 (2005) 207-212. [43] F. Baur, T. Jüstel, Dependence of the optical properties of Mn4+ activated A2Ge4O9 (A= K, Rb) on temperature and chemical environment, J. Lumin., 177 (2016) 354-360. [44] T.M. Chen, J.T. Luo, Highly saturated red-emitting Mn (IV) activated phosphors and method of fabricating the same, U.S. Patent No. 7846350, (2010). [45] L.L. Meng, L.F. Liang, Y.X. Wen, Deep red phosphors SrMgAl10O17:Mn4+, M (M= Li+, Na+, K+, Cl−) for warm white light emitting diodes, J. Mater. Sci.: Mater. Electron., 25 (2014) 2676-2681. [46] A.M. Srivastava, M.G. Brik, Ab initio and crystal field studies of the Mn4+-doped Ba2LaNbO6 double-perovskite, J. Lumin., 132 (2012) 579-584. [47] WM Yen, S. Shionoya, H. Yamamoto, Handbook, Phosphor, CRC Press, Boca Raton, 2007. [48] M.G. Brik, I. Sildos, M. Berkowski, A. Suchocki, Spectroscopic and crystal field studies of YAlO3 single crystals doped with Mn ions, J. Phys.: Condens. Matter, 21 (2008) 025404. [49] Z. Bryknar, Application of spectroscopic probes in study of ferroelectrics, Ferroelectrics, 298 (2004) 43-48. [50] Y.H. Jin, Y.H. Hu, H.Y. Wu, H. Duan, L. Chen, Y.R. Fu, G.F. Ju, Z.F. Mu, M. He, A deep red phosphor Li2MgTiO4:Mn4+ exhibiting abnormal emission: potential application as color converter for warm w-LEDs, Chem. Eng. J., 288 (2016) 596-607. [51] J. Stade, D. Hahn, R. Dittmann, New aspects of the luminescence of magnesiumtitanate part II: Activation with manganese, J. Lumin., 8 (1974) 318-325. [52] X.X. Wu, W. Fang, W.L. Feng, W.C. Zheng, Electron paramagnetic resonance parameters of Mn4+ ion in h-BaTiO3 crystal from a two-mechanism model, Pramana, 72 (2009) 569-575. [53] A.G. Paulusz, Efficient Mn (IV) emission in fluorine coordination, J. Electrochem. Soc., 120 (1973) 942-947. [54] M.G. Brik, S.J. Camardello, A.M. Srivastava, Influence of covalency on the Mn4+ 2Eg→4A2g emission energy in crystals, ECS J. Solid State Sci. Technol., 4 (2015) R39-R43. [55] Z. Wen-Chen, S. Ping, L. Hong-Gang, F. Guo-Ying, Relative importance of static contribution to the thermal shifts of spectral lines in Nd3+-doped Y3Al5O12 laser crystals, J. Phys. D: Appl. Phys., 45 (2012) 345305.

Jo

ur

na

lP

re

-p

ro of

[56] T. Kushida, M. Kikuchi, R, R'and B absorption linewidths and phonon-induced relaxations in ruby, J. Phys. Soc. Jpn., 23 (1967) 1333-1348. [57] T. Kushida, Linewidths and thermal shifts of spectral lines in neodymium-doped yttrium aluminum garnet and calcium fluorophosphate, Phys. Rev., 185 (1969) 500. [58] G.H. Dieke, Spectra and energy levels of rare earth ions in crystals, Am. J. Phys., 38 (1970) 399-400. [59] N.E.M.A.J.A.N.E.M.A.A. C, American national standard for electric lamps—specifications for the chromaticity of solid state lighting (SSL) products, Arlington(VA): National Electrical Manufacturers Association. ANSI C, 78 (2015) 377-2015. [60] R.P. Cao, J.L. Zhang, W.D. Wang, Z.F. Hu, T. Chen, Y.X. Ye, X.G. Yu, Preparation and photoluminescence characteristics of Li2Mg3SnO6:Mn4+ deep red phosphor, Mater. Res. Bull., 87 (2017) 109-113.