K3WOF7:Mn4+ ‒ A red oxyfluoride phosphor

Journal Pre-proof K3 WOF7 :Mn4+ null A Red Oxyfluoride Phosphor Christiane Stoll, Gunter Heymann, Markus Seibald, Dominik Baumann, Hubert Huppertz

PII:

S0022-1139(19)30246-5

DOI:

https://doi.org/10.1016/j.jfluchem.2019.109356

Article Number:

109356

Reference:

FLUOR 109356

To appear in:

FLUOR

Received Date:

12 July 2019

Revised Date:

1 August 2019

Accepted Date:

6 August 2019

Please cite this article as: Stoll C, Heymann G, Seibald M, Baumann D, Huppertz H, K3 WOF7 :Mn4+ x2012; A Red Oxyfluoride Phosphor, Journal of Fluorine Chemistry (2019), doi: https://doi.org/10.1016/j.jfluchem.2019.109356

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K3WOF7:Mn4+ ‒ A Red Oxyfluoride Phosphor Christiane Stoll a, Gunter Heymann a, Markus Seibald b, Dominik Baumann b, and Hubert Huppertz a,*

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Institut für Allgemeine, Anorganische und Theoretische Chemie, Universität Innsbruck, Innrain 80-82, 6020 Innsbruck, Austria OSRAM Opto Semiconductors GmbH, Mittelstetter Weg 2, 86830 Schwabmünchen, Germany

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Graphical Abstract

K3WOF7:Mn4+ was synthesized via a solid-state synthesis route. Main feature of the structure

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is a quasi-isolated [WOF5]– unit. When excited by blue light K3WOF7:Mn4+ exhibits a red luminescence.

Highlights    

Solid-state synthesis of oxyfluoride phosphor K3WOF7:Mn4+ in two facile steps. Single-crystal structure determination reveals novel structure type of K3WOF7 with [WOF5]‒ as main units. Luminescence analysis shows line emission spectrum with maximum wavelength of 627 nm. Temperature dependent luminescence spectra show a good thermal stability up to 75 °C. 1

ABSTRACT Synthesis of K3WOF7:Mn4+ was accomplished via a facile two-step solid-state method. Characterization of the material was carried out by means of single-crystal and powder X-ray diffraction, energy dispersive X-ray spectrometry, as well as luminescence spectroscopy. The substance crystallizes in the monoclinic crystal system in space group P21/c (no. 14) with the lattice parameters a = 884.2(1), b = 1379.9(1), c = 679.7(1) pm, and β = 93.04(1)°. The crystal structure features a quasi-isolated [WOF5]‒ building block as its main motif. The material exhibits a red luminescence under excitation with blue light. The maximum-intensity emission line is located at λmax ≈ 627 nm and the CIE1931 coordinates of the phosphor are 0.689 and

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0.310 for x and y, respectively. Keywords: Crystal-Structure Analysis, Luminescence, Solid-state Reactions * Corresponding author. Fax: +43 512 50757099

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E-mail address: [email protected] (H. Huppertz)

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1. Introduction Red luminescence materials have been investigated intensively in recent years in part for their use in phosphor-converted light-emitting diodes. Currently, there are three known kinds of red phosphors: quantum dots, rare earth substituted compounds, and transition metal substituted materials. Representatives of these substance classes are, e.g. CdSe [1], Sr[LiAl3N4]:Eu2+ [2] as well as the recently discovered SrAl2Li2O2N2:Eu2+ [3], and K2SiF6:Mn4+ [4]. Although the emission of quantum dots is tunable, they tend to suffer from low chemical stability [1]. Rare earth substituted materials, e.g. Eu2+ substituted phosphors are quite difficult to prepare and are often expensive. In comparison, transition metal substituted materials like Mn4+ substituted

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phosphors, which show a line emitting nature, are relatively abundant. The emission of Mn4+ substituted materials is therefore being investigated intensely [5-6]. Interestingly, various host materials such as oxides or fluorides can be used for Mn4+ substitution [5] (see also recent reviews on the oxides [7] and the fluorides [8]). Oxide materials often show an emission

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spectrum in the deep red to infrared region and are more suitable for applications in greenhouse lighting than in general lighting. These materials can be excited by blue light [7]. Additionally, oxides exhibit great thermal and chemical stability. The most investigated representatives of

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manganese substituted oxides are germanates like Mg4GeO6:Mn4+ [9] and aluminates like CaAl12O19:Mn4+ [10]. In comparison, the emission of fluoride materials is located well within

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the red spectral region (λmax ≈ 630 nm) and can also be excited by blue light [8]. Therefore, fluorides can be considered suitable for application in general lighting. K2SiF6:Mn4+ was the first manganese doped fluoride phosphor, discovered in 2008, and found adoption in industry

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[4, 11]. This led to an increase of research in this field and lots of new manganese substituted fluoride phosphors have been discovered until today. Most prominent is the substance class of

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the hexafluoridometallates with the general composition A2XF6 (A = monovalent cation and X = tetravalent cation) [4, 12-25], where most of the compounds have already been successfully

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substituted with manganese. Additionally, a few substances with the general compositions A3XF7 (A = monovalent cation and X = tetravalent cation) [26-28], BaXF6 (X = tetravalent cation) [29-31], and Ba2HfF8 [32] have been employed as host materials for manganese. In 2017, Seo and coworkers started to investigate oxyfluoride materials as possible hosts for substituting with manganese [33]. Soon afterwards, a few more manganese substituted oxyfluoride phosphors had been discovered, with A2NbOF5:Mn4+ (A = Rb, Cs) [34-36], BaNbOF5:Mn4+ [37], A’2WO2F4:Mn4+ (A’ = Na, Cs) [33, 38-40], BaTiOF4:Mn4+ [41], Rb5Nb3OF18:Mn4+ [42], K3TaO2F4 [43], K3HF2WO2F4 [44], and K3MoOF7:Mn4+ [45], being 3

known examples. These compounds exhibit narrow line emissions, as is usually observed for Mn4+ fluoride hosts, with a maximum wavelength (λmax) of approx. 630 nm. This work presents a novel member, K3WOF7:Mn4+, of the still young substance class of oxyfluoride phosphors and its luminescence properties.

2. Results and discussion 2.1. Structural properties The compound K3WOF7 crystallizes in the monoclinic crystal system with space group P21/c

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(no. 14). The lattice parameters are a = 884.2(1), b = 1379.9(1), c = 679.7(1) pm, β = 93.04(1)° and the volume of the unit cell amounts to V = 0.828(1) nm3. The asymmetric unit contains 12 atoms, all located at the general Wyckoff positions 4e. Four formula units make up the unit cell which therefore contains 48 atoms. Parameters of the crystal structure solution and refinement

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can be found in Table 1. The atomic positions, anisotropic displacement parameters, important distances, as well as angles are reported in Table S1-4.

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The main motif of the structure is a quasi-isolated [WOF5]‒ unit, with tungsten, coordinated octahedrally by five fluorine atoms and one oxygen atom (Figure 1). Two of the positions

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(O1/F1 and O2/F2) exhibit mixed occupancy (50:50) by fluorine and oxygen. Regarding these two positions, shorter W‒O/F bond-lengths (177.8(3) and 178.2(2) pm) can be noticed compared to the other four W‒F distances (189.2(2) to 199.5(2) pm). Ag(WOF5)2, a substance

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showing an ordered [WOF5]‒ anion as well as an unordered [WOF5]‒ anion (two mixed O/F positions are present), shows similar bond-lengths for W‒O and W‒F. In Ag(WOF5)2, the bondlengths for the ordered [WOF5]‒ are dW‒O = 172.9 pm and dW‒F = 182.6 to 207.5 pm. For the

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unordered [WOF5]‒ unit, where tungsten gets deflected in the direction of the oxygen positions, the W‒O bond-lengths are 177.2 and 172.2 pm, whereas the W‒F bond-lengths vary between

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184.8 and 203.2 pm [46]. Therefore, the [WOF5]‒ anion in K3WOF7 is consistent with the data given for [WOF5]‒ in Ag(WOF5)2 (Figure S1) [46]. The shorter bond-lengths are most likely due to the more covalent character of the W‒O bond in comparison to the W‒F bond. Hence, tungsten is slightly deflected in the direction of the mixed occupied sites. For further verification of the mixed occupancy at these positions, the sites were refined as fluorine atoms only and the constraint of the occupancy was released. Due to this release, the occupancy decreased for both fluorine positions, in contrast to the other fluoride positions. If refined as oxygen atoms, the occupancy at those positions increased. The refinement with mixed 4

occupancy led to better R-values and feasible displacement parameters in contrast to the refinement with pure oxygen or fluorine occupation at these positions. This is consistent with the theory of a mixed occupancy at these two positions. Additionally, two crystallographically independent fluorine atoms F7 and F8 are present in the structure. These fluoride anions

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coordinate solely to potassium.

Figure 1. Octahedral coordination sphere of tungsten in the structure of K3WOF7 (ellipsoids are drawn

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at 50 % probability).

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The [WOF5]‒ units are isolated by potassium cations. Within the structure, there are three crystallographically independent potassium positions. Figure 2 shows the coordination spheres of the potassium cations of which two are coordinated eightfold (K1, K2) and one nine fold

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(K3). The K‒F bond-lengths vary between 257.8(2) and 315.9(3) pm, and the K‒O/F bond-

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lengths between 283.1(3) and 288.7(3) pm.

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Figure 2. Coordination spheres of the potassium cations in K3WOF7 (ellipsoids are drawn at 50 % probability).

The structure shows a layer-like setup of wave-like arranged, quasi isolated [WOF5]‒ units

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alternating with additional non-bonded fluoride anions, when viewed along [1;¯00] (Figure 3, left). Viewed along [001], the [WOF5]‒ units are arranged in straight lines and build channels

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for the quasi-independent fluoride anions (Figure 3, right). The cation ratio K:W was determined by energy dispersive X-ray spectrometry (EDX) to be 3.2(2):1 and could therefore

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be verified. A scanning electron microscope (SEM) image is shown in Figure S2. The crystals can be stored in closed vessels without decomposition for several weeks, but dissolve readily by contact with water.

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Figure 3. Structure of K3WOF7 viewed approximately along [1;¯00] (left) and along [001] (right).

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The quite unique arrangement of the simple [WOF5]‒ units and the independent fluoride atoms can, to the best of our knowledge, not be described by a known structure type. Therefore,

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K3WOF7 represents its own: P21/c, Pearson code mP48, and Wyckoff sequence e12.

Figure 4. Rietveld plot (after ball milling) of K3WOF7:Mn4+. The experimental data are plotted as black crosses. The simulation is shown in red, the difference curve in blue, and the reflections of K3WOF7:Mn4+ are marked in green. An enlarged section of the powder diffractogram, in which the unknown side phase is well visible (black asterisk), is plotted in the top right corner.

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2.2. Photoluminescence properties Phase analysis via the Rietveld refinement technique determined the main component to be K3WOF7 with an unknown side phase (Figure 4). After the substitution experiment, the sample showed a red luminescence when excited with blue light. As the substance was yielded with sufficient purity for further characterization, an emission spectrum of K3WOF7:Mn4+ (after ballmilling) was measured (Figure 5). It shows seven emission lines with the highest intensity at λmax ≈ 627 nm. In this case, Mn4+ most likely replaces a W6+ cation as the ionic radii are similar (Mn4+ = 67 pm, W6+ = 74 pm). As described for other oxyfluoride phosphors [33, 39], it is postulated that the substitution of a tungsten atom by a manganese atom changes the

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coordination polyhedron from a mixed [WOF5]‒ to a pure [MnF6]2‒ octahedron, where the coordination sphere consists solely of fluorine atoms. Charge compensation is likely to be realized by the creation of defects, like oxygen or fluorine vacancies, or by the exchange of oxygen with fluorine, or a combination of both. However, it could also be realized by a reduction of the oxidation state of tungsten in neighboring octahedra. Therefore, a local

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coordination sphere built exclusively of fluorine atoms is assumed for the Mn4+ in

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K3WOF7:Mn4+.

Figure 5. Excitation spectrum (monitored at λem = 627 nm) and the emission spectrum (excited at λexc = 460 nm) of K3WOF7:Mn4+.

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The excitation spectrum of K3WOF7:Mn4+ (Figure 5) exhibits three broad bands at ≈ 250, ≈ 360, and ≈ 470 nm, which can be attributed to the charge transfer transition of the [WOF5]‒ group and the spin-allowed 4A2g → 4T1g and 4A2g → 4T2g transitions, respectively. The emission spectrum reveals seven emission lines at ≈ 596, ≈ 606, ≈ 611, ≈ 619, ≈ 627, ≈ 632, and ≈ 644 nm. These can be attributed to the transitions of the vibronic modes ν3(t1u), ν4(t1u), ν6(t2u), zerophonon-line (ZPL), ν6(t2u), ν4(t1u), and ν3(t1u) of the spin-forbidden 2Eg  4A2g transition, respectively. In comparison to the state-of-the-art phosphor K2SiF6:Mn4+, K3WOF7:Mn4+ presents a visible ZPL and thus exhibits seven instead of six emission lines at room temperature [4]. This is likely based on the distorted coordination sphere around manganese in K3WOF7:Mn4+. In comparison, manganese is coordinated perfectly octahedral by fluorine

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atoms in cubic K2SiF6:Mn4+. This inhibits a zero-phonon line, as the transition is prohibited by the Laporte parity selection rule. A distortion of the highly symmetric coordination sphere weakens this prohibition. This is already described for oxyfluoride phosphors like K3MoOF7:Mn4+ and Na2WO2F4:Mn4+, showing an intense ZPL, and fluoride phosphors like

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K2MF7:Mn4+ (M = Nb, Ta) [38-40, 47-48]. With λmax located at 627 nm, the emission spectrum is blue shifted in comparison to the phosphor K2SiF6:Mn4+ (λmax = 631 nm). This blue shift leads

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to an increase of the luminous efficacy of radiation (LER) from 204 lmWopt−1 for K2SiF6:Mn4+ to 225 lmWopt−1 for K3WOF7:Mn4+ (Table 2). K3WOF7:Mn4+ exhibits a dominant wavelength

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(λdom) of 619 nm with CIE1931 coordinates of 0.689(1) and 0.311(1) for x and y, respectively. The quantum efficiency of the samples is still below 10%, which is likely due to small particle size and defects caused by the doping method by ball-milling. It might be improved by a post

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treatment of the phosphor or by modification of the synthesis strategy. Additionally, the temperature dependent emission profiles were recorded (Figure 6, top). The

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position of λmax is shifted a few nm in the direction of longer wavelengths by the heating process. Furthermore, the dominant wavelength λdom is shifted from 616 to 603 nm by an increase of the temperature to 423.15 K (Figure S3). This leads to a small change of the CIE1931 coordinates

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of ∆ = 0.01 for x and y for a temperature raise from 298.15 K to 373.15 K and a further change of ∆ = 0.03 by heating up to 423.15 K (Figure S4). The integrated intensities of the temperature dependent emission spectra (normalized to 298.15 K) are shown in Figure 6, bottom. K3WOF7:Mn4+ exhibits a small decrease (12.7%) in intensity by heating to 348.15 K. This is a comparable behavior to K3MoOF7:Mn4+ (7.2%) [45]. If heated further, the intensity decreases more rapidly with a further loss of 53.2% within the next 50 K. At 423.15 K, a residual intensity of 16.1% could be detected. Additionally, the temperature dependent behavior was analyzed using the following Arrhenius expression [7-8]: 9

𝐼em (𝑇) =

𝐼0 1 + 𝑎 exp (‒

𝐸𝑎 ) 𝑘B 𝑇

Here, T is the absolute temperature, 𝐼0 is the emission intensity at 0 K, 𝑎 is a constant, 𝑘B is the Boltzmann constant, and 𝐸𝑎 is the activation energy for the thermal-quenching process. Figure 7 shows the normalized emission intensity plotted against 1/T. The solid line marks the resulting function, when fitted with the Arrhenius expression (stated above). The variables could be

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determined to 𝐼0 = 0.9972, 𝑎 = 1.37452×108 and 𝐸𝑎 = 0.62 eV.

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Figure 6. (top) Temperature dependent emission spectra of K3WOF7:Mn4+ and (bottom) temperature

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dependent integrated intensity data normalized to that at 298.15 K.

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Figure 7. Fit according to the Arrhenius expression stated above (solid line) of the normalized intensity

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data against 1/T. Experimental values are given as black squares.

3. Conclusions

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A novel oxyfluoride phosphor K3WOF7:Mn4+ was successfully synthesized via a solid-state route. It shows a unique structure, with quasi-isolated [WOF5]‒ units as its main feature. The phosphor can be excited by ultraviolet to blue light, and it shows a narrow line emission with

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the most prominent emission line at λmax ≈ 627 nm. Surprisingly, its emission spectrum is blueshifted in comparison to K2SiF6:Mn4+, which results in an increased luminous efficacy of

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radiation of about 10%. Its CIE1931 coordinates are 0.689(1) and 0.311(1) for x and y, respectively and it exhibits a dominant wavelength of λdom ≈ 619 nm. The thermal quenching

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behavior is similar to that of K3MoOF7:Mn4+ [45], and compared to Na2WO2F4:Mn4+ (approx. 50% intensity loss by heating up to 343.15 K) [38] it shows a better thermal stability. An activation energy for the thermal-quenching process of 𝐸𝑎 = 0.58(1) eV was calculated. The phosphors properties could enable applications in the low-power/ low-temperature regime. The quantum efficiency of the sample is quite low, however may be improved by the addition of an annealing step to the synthesis strategy. Additionally, the study of various oxyfluoride phosphors will be invaluable to better understand and optimize their optical properties. Therefore, K3WOF7:Mn4+ is another representative of the still young substance class of manganese-substituted oxyfluorides and offers the possibility to study their properties further. 11

4. Experimental section 4.1. Synthesis K3WOF7:Mn4+ was synthesized via a two-step synthesis approach. This synthesis route was first described for K3MoOF7:Mn4+ by our group in 2019 [45]. The first step results in the synthesis of the unsubstituted substance K3WOF7 and is carried out in closed copper ampules. For this purpose, 75 mg KHF2 (Alfa Aesar, Haverhill, USA, 99+%) and 27.83 mg WO3 were weighed in and ground in an agate mortar in an argon filled glove box (MBraun Inertgas-System

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GmbH, Germany). It is recommended by the authors to conduct the grinding step under argon atmosphere, because of the hygroscopic nature of KHF2. Subsequently, the mixture was transferred into a copper ampule, and welded shut (miniaturized arc-welding according to [49]). After positioning the ampule into a silica ampule, which was subsequently filled with 400 mbar argon gas, the silica ampule was inserted into a tube furnace. The temperature was raised to

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673.15 K with a heating rate of 3.0 Kmin-1 and kept at that temperature for 48 h. Subsequently, the temperature was lowered to 623.15 K with a cooling rate of 0.1 Kmin-1, followed by the

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quenching in the oven to room temperature. The recovered product revealed a colorless appearance and contained small plate-like crystals.

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The second step was performed by ball-milling. For this purpose, the above prepared K3WOF7 and Cs2MnF6 (substituent) were weighed in with a ratio of 1:0.025 under inert gas atmosphere.

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The mixture, together with 1 g of milling balls (ZrO2, Ø = 3 mm), was transferred into a zirconia vessel and placed into a planetary ball mill (Pulverisette 7, FRITSCH, Idar-Oberstein, Germany). The sample was subjected to a milling program (6 × 10 min - 300 rpm; 15 min

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breaks). A slightly yellow powder was recovered.

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4.2. Single-crystal X-ray Structure Determination Single-crystal structure analysis was carried out on a colorless plate-like crystal, which was selected and isolated under a polarization microscope. The crystal seems to be sensitive to ambient conditions, therefore the measurement was conducted at 186(2) K. A Bruker D8 Quest diffractometer (BRUKER, Billerica, USA) equipped with a molybdenum radiation source (MoKα radiation, λ = 71.07 pm), an Incoatec microfocus X-ray tube (Incoatec, Geesthacht, Germany), and a Photon 100 detector was used for collection of the intensity data. The program 12

SADABS 2014/5 [50] was used to perform a multi-scan absorption correction of the intensity data. Space group P21/c (no. 14) was identified on the basis of the extinction conditions and used for the structure solution (SHELXTL-XT-2014/4) and refinement. The program suite WinGX-2013.3 [51] employing SHELXL-2013 [52-53] was used for the parameter refinement (full-matrix least-squares against F2). Values of 0.0328 and 0.0461 for R1 and wR2 (all data), respectively, were obtained by the anisotropic refinement. Further information on the crystal structure investigation can be obtained from the joint CCDC/FIZ Karlsruhe deposition service on quoting the deposition number CCDC: 1939087 for K3WOF7.

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4.3. Powder X-ray Diffraction Powder samples of K3WOF7 were analyzed using a Stoe Stadi P diffractometer (Stoe, Darmstadt, Germany). The set-up used a focusing Ge(111) primary beam monochromator and operated in transmission geometry. A molybdenum radiation source (Mo-Kα1-radiation;

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λ = 70.93 pm) was used, which enabled the detection of the diffraction data via a Mythen 2 DCS4 detector. The sample was measured in Debye Scherrer mode in the 2θ range of

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2.0 − 40.4° with a step size of 0.015° in a soda-lime glass capillary to prevent the deterioration of the sample under ambient conditions. Profile fitting for the Rietveld analysis was

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accomplished using the Pseudo-Voigt approach employing the program suite TOPAS 4.2 [54] for hardware-parameter refinement and fitting of the reflection shape.

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4.4. Luminescence spectroscopy

A Fluoromax 4 spectrophotometer (HORIBA, Japan) was used to record temperature dependent

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luminescence data, emission spectra and the excitation spectrum of the powder sample. The emission spectrum was taken within the wavelength region of 480 to 800 nm (step size 0.5 nm;

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excitation wavelength 460 nm), whereas the excitation spectrum was measured in the range of 332 to 600 nm (step size 1 nm; monitored at 628 nm).

4.5. EDX spectroscopy Analysis of the chemical composition was carried out with the help of energy dispersive X-ray spectroscopy (EDX) using a SUPRATM35 scanning electron microscope (SEM, CARL ZEISS,

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Oberkochen, Germany, field emission). This set-up was equipped with a Si/Li EDX detector (OXFORD INSTRUMENTS, Abingdon, United Kingdom, model 7426).

Acknowledgment We thank Ass.-Prof. Dr. K. Wurst for the support with the crystal-structure refinement/determination. We want to express our gratitude to Prof. Dr. F. Kraus for the donation of Cs2MnF6 and want to acknowledge the time and effort spent by Christian Koch for

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the SEM and EDX measurements.

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Table 1: Crystal data and structure refinement of K3WOF7. K3WOF7 450.15 monoclinic P21/c

ro of

Stoe Stadi P Mo-Kα1 (λ = 70.93 pm) 293(2) 886.7(2) 1393.0(3) 679.9(6) 92.90(1) 0.839(1)

-p

Bruker D8 Quest Photon 100 Mo-Kα (λ = 71.07 pm) 884.2(1) 1379.9(1) 679.7(3) 93.04(1) 0.828(1) 4 3.61 186(2) 15.53 808 4.6-70.0 ±14, ±22, ±10 63868 3643 / 0.0462 3040 3643 / 110 0.987 multi-scan 0.0247 / 0.0440 0.0328 / 0.0461 3.81 / –3.00

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empirical formula molar mass, g mol-1 crystal system space group powder data powder diffractometer radiation temperature, K a, pm b, pm c, pm β, deg V, nm3 single-crystal data single-crystal diffractometer radiation a, pm b, pm c, pm β, deg V, nm3 formula units per cell, Z calculated density, g cm-3 temperature, K absorption coefficient, mm-1 F(000), e 2 range, deg range in hkl total no. of reflections independent reflections / Rint reflections with I > 2 σ(I) data / ref. parameters goodness-of fit on Fi2 absorption correction final R1/wR2 (I ≥ 2σ(I)) final R1/wR2 (all data) largest diff. peak/hole, e Å−3

18

Table 2. Dominant and maximum emission wavelength, x and y coordinates (rounded to significant numerical values regarding the measurement accuracy) in the CIE1931-Diagram, and LER value for K3WOF7:Mn4+.

K3WOF7:Mn4+

λmax / nm

xCIE

yCIE

LER / lm Wopt−1

619

627

0.689(1)

0.311(1)

225

621

631

0.693(1)

0.307(1)

204

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ro of

K2SiF6:Mn

4+

λdom / nm

19