Co-Precipitation Synthesis and Optical Properties of Mn4+-Doped Hexafluoroaluminate w-LED Phosphors

Mn4+-activated hexafluoroaluminates are promising red-emitting phosphors for white light emitting diodes (w-LEDs). Here, we report the synthesis of Na3AlF6:Mn4+, K3AlF6:Mn4+ and K2NaAlF6:Mn4+ phosphors through a simple two-step co-precipitation method. Highly monodisperse large (~20 μm) smoothed-octahedron shaped crystallites are obtained for K2NaAlF6:Mn4+. The large size, regular shape and small size distribution are favorable for application in w-LEDs. All Mn4+-doped hexafluoroaluminates show bright red Mn4+ luminescence under blue light excitation. We compare the optical properties of Na3AlF6:Mn4+, K3AlF6:Mn4+ and K2NaAlF6:Mn4+ at room temperature and 4 K. The luminescence measurements reveal that multiple Mn4+ sites exist in M3AlF6:Mn4+ (M = Na, K), which is explained by the charge compensation that is required for Mn4+ on Al3+ sites. Thermal cycling experiments show that the site distribution changes after annealing. Finally, we investigate thermal quenching and show that the luminescence quenching temperature is high, around 460–490 K, which makes these Mn4+-doped hexafluoroaluminates interesting red phosphors for w-LEDs. The new insights reported on the synthesis and optical properties of Mn4+ in the chemically and thermally stable hexafluoroaluminates can contribute to the optimization of red-emitting Mn4+ phosphors for w-LEDs.


Introduction
White light emitting diodes (w-LEDs) are nowadays widely applied in general lighting and consumer electronics because of their high energy efficiency and long operation lifetime [1][2][3][4][5]. Commercial w-LEDs generate white light by combining blue-emitting (In,Ga)N LED chips with inorganic phosphors that convert part of the blue LED emission to green, yellow and/or red light [5,6]. Currently, the typical red phosphors in w-LEDs are Eu 2+ -doped nitrides. These phosphors can have quantum yields (QYs) exceeding 90%, but a drawback is that the Eu 2+ emission band extends into the deep red and near-IR regions where the sensitivity of the human eye is low [6][7][8]. As a result, there are significant efficacy losses (reduced lumen/W output) at high color rendering indices (CRIs) when using Eu 2+ -doped nitrides as red phosphors in w-LEDs [9]. To overcome this issue, efficient narrow band red-emitting phosphors with λ max~6 20 nm have to be developed.
Mn 4+ -doped fluorides are a promising class of materials to improve the color rendering of w-LEDs while maintaining a high luminous efficacy [8][9][10][11]. Upon blue photoexcitation Mn 4+ -doped fluorides show narrow red line emission in the 600-640 nm spectral region. Furthermore, they can have a QY > 90% and are prepared through simple wet-chemical synthesis at room temperature [8,12]. These characteristics make Mn 4+ -doped fluorides interesting narrow band red phosphors for w-LEDs. In recent years, a large number of Mn 4+ -activated fluorides with the general chemical formulas A 2 MF 6 :Mn 4+ (A = Na, K, Rb, Cs and NH 4 ; M = Si, Ge, Ti, Zr and Sn) and BaMF 6 :Mn 4+ (M = Si, Ge, Ti and Sn) have been reported [11][12][13]. In these compounds Mn 4+ substitutes for the M 4+ cation that is octahedrally coordinated by six F − ions. Most work

Materials and Methods
The M 3 AlF 6 :Mn 4+ (M = Na, K) phosphors were synthesized through a two-step chemical co-precipitation method. In the first step the Mn 4+ -precursor K 2 MnF 6 was synthesized and in the second step the M 3 AlF 6 :Mn 4+ phosphor was prepared. Since K 2 MnF 6 and M 3 AlF 6 :Mn 4+ are synthesized in corrosive HF solutions, all reactions described were carried out in plastic or Teflon beakers.
2.2. Synthesis of K 2 MnF 6 K 2 MnF 6 was prepared according to the method of Bode [27,28]. KMnO 4 (4 g) and KF (59.5 g) were dissolved in 250 mL of a 40% HF solution. The black-purple solution obtained was stirred for 30 min and then cooled with an ice bath. Under constant cooling and stirring, a 30 wt % H 2 O 2 solution was added dropwise which resulted in the gradual precipitation of yellow K 2 MnF 6 powder. The dropwise addition of H 2 O 2 was stopped when the reaction solution turned from purple to red-brown, indicating the formation of Mn 4+ . The K 2 MnF 6 product was isolated by decanting the red-brown solution, washing the precipitate twice with 100 mL of acetone and finally drying the precipitate at 60 • C for 5 h.

Synthesis of M 3 AlF 6 :Mn 4+
Al(OH) 3 (10 mmol, 0.78 g) was dissolved in 15 mL 40% HF by stirring and heating at 60 • C. After cooling down to room temperature, 0.1 mmol K 2 MnF 6 (1 mol % doping concentration) was added. Simultaneously, a solution of M + ions (M = Na, K) in 40% HF was prepared by gradually adding M 2 CO 3 or MF to 40% HF (aq) while stirring. Table 1 lists the amounts of Na 2 CO 3 , K 2 CO 3 , KF and 40% HF used for preparing the M + /HF solutions. The M + /HF solution was added to the Al 3+ /Mn 4+ /HF solution, which resulted in the precipitation of Na 3 AlF 6 :Mn 4+ and K 2 NaAlF 6 :Mn 4+ but not K 3 AlF 6 :Mn 4+ phosphor. K 3 AlF 6 :Mn 4+ was precipitated by adding 50 mL of ethanol to the mixed M + /Al 3+ /Mn 4+ /HF solution (ethanol acts as anti-solvent). The synthesized phosphors were isolated by decanting the solution, washing the precipitate twice with ethanol and subsequently drying at 75 • C for 3 h. Different Mn 4+ doping concentrations could be obtained by changing the amount of K 2 MnF 6 that was used in the synthesis.

Characterization
Powder X-ray diffraction (XRD) patterns were measured on a Philips PW1729 X-ray diffractometer using Cu K α radiation (λ = 1.5418 Å). Scanning electron microscopy (SEM) images of the phosphors were obtained using a Philips XL30S FEG microscope operating at 20 keV. The manganese concentrations in the phosphors were determined with inductively coupled plasma optical emission spectroscopy (ICP-OES) performed on a Perkin-Elmer Optima 8300 spectrometer. For the ICP-OES measurements the M 3 AlF 6 :Mn 4+ phosphors were dissolved in aqua regia.
Photoluminescence (PL) measurements were performed on an Edinburgh Instruments FLS900 fluorescence spectrometer equipped with a double 0.22 m excitation monochromator. For recording emission and excitation spectra, we used a 450 W Xe lamp as excitation source and a Hamamatsu R928 photomultiplier tube (PMT) to detect the emission. For PL measurements down to 4 K, the phosphors were cooled in an Oxford Instruments liquid helium flow cryostat. PL measurements between 300 K and 600 K were performed by heating the phosphors with a Linkam THMS600 temperature controlled stage. PL quantum yields were determined with a calibrated home-built setup which consisted of a 65 W Xe lamp, excitation monochromator, integrating sphere (Labsphere) and CCD camera (Avantes AvaSpec-2048).
The XRD measurements show that incorporation of Mn 4+ on the Al 3+ sites does not significantly change the crystal structure of M 3 AlF 6 , which is expected as the ionic radii of Mn 4+ and Al 3+ are similar (0.53 versus 0.54 Å) [21]. The space groups of Na 3 AlF 6 , K 2 NaAlF 6 and K 3 AlF 6 are listed in Table 2. K 2 NaAlF 6 has a highly symmetric cubic crystal structure (space group is Fm3m), while Na 3 AlF 6 and K 3 AlF 6 have a crystal structure with lower symmetry (space groups are P2 1 /n and I4 1 /a, respectively) [29][30][31]. In the M 3 AlF 6 crystal structure, the Al 3+ ions are octahedrally coordinated by six F − ions. Depending on the M 3 AlF 6 lattice, the AlF 6 octahedron can be highly symmetric (O h in K 2 NaAlF 6 ) or distorted (C i in Na 3 AlF 6 and C 1 in K 3 AlF 6 ). The average Al-F distances in the AlF 6 octahedra are around 1.8 Å. By using ICP-OES, we measured the manganese concentrations in the synthesized M 3 AlF 6 :Mn 4+ phosphors (see Materials and Methods section). The XRD patterns displayed in Figure 1 were measured for M 3 AlF 6 :Mn 4+ phosphors containing 0.4 mol % (Na 3 AlF 6 :Mn 4+ ), 1.2 mol % (K 3 AlF 6 :Mn 4+ ) and 0.9 mol % Mn (K 2 NaAlF 6 :Mn 4+ ). The results presented in the rest of this work were obtained for M 3 AlF 6 :Mn 4+ phosphors having these doping concentrations. For K 3 AlF 6 :Mn 4+ and K 2 NaAlF 6 :Mn 4+ the measured Mn concentration is very close to the 1 mol % Mn added during the synthesis, which demonstrates that our synthesis method provides good control over the Mn 4+ doping concentration. Also a substantially higher fraction of Mn 4+ is incorporated into K 3 AlF 6 compared to previously reported cation exchange methods for preparing K 3 AlF 6 :Mn 4+ [18]. Figure 2 displays SEM images of Na 3 AlF 6 :Mn 4+ (0.4%), K 3 AlF 6 :Mn 4+ (1.2%) and K 2 NaAlF 6 :Mn 4+ (0.9%) phosphor particles. The Na 3 AlF 6 :Mn 4+ (Figure 2a) and K 3 AlF 6 :Mn 4+ (Figure 2b) phosphors consist of irregularly shaped particles and clusters of particles with sizes ranging from~100 nm to >10 µm. For K 3 AlF 6 :Mn 4+ , we attribute the large variety in shape and size to the rapid and forced crystallization following addition of the anti-solvent ethanol. In contrast, the synthesis of K 2 NaAlF 6 :Mn 4+ (Figure 2c,d) yields highly monodisperse phosphor particles with a large average diameter of 22.5 ± 6.1 µm. The K 2 NaAlF 6 :Mn 4+ particles have a smoothed octahedral shape, as expected from the cubic elpasolite structure of K 2 NaAlF 6 [10,19,29]. The K 2 NaAlF 6 :Mn 4+ particles prepared with our co-precipitation method have a more uniform shape and size than the K 2 NaAlF 6 :Mn 4+ particles prepared by Zhu et al. via cation exchange [19]. Moreover, the K 2 NaAlF 6 :Mn 4+ (0.9%) phosphor shown in Figure 2c,d exhibits particle size and shape distributions that are superior to the size and shape distributions of other reported Mn 4+ -doped fluoride phosphors [10,24,[32][33][34][35][36][37]. The high monodispersity of the K 2 NaAlF 6 :Mn 4+ crystallites reported here may originate from the synthesis method used. In order to achieve a narrow size distribution, it is necessary to temporally separate the particle nucleation and growth stages [38]. As described in the Materials and Methods section, we dissolve all the starting materials in HF solutions prior to the formation of the phosphor particles. Mixing of the precursor solutions results in oversaturation and the rapid formation of crystal nuclei. Once the nuclei have been formed, the precursor concentrations drop and no new nuclei are formed. The particles can grow to monodisperse and large crystallites during the growth stage. This differs from syntheses where Mn 4+ -doped particles are prepared via cation exchange or chemical etching [14]. With these methods, new precursor ions are constantly supplied to the reaction solution preventing temporal separation of nucleation and growth.
The superior size distribution gives K 2 NaAlF 6 :Mn 4+ potential advantages in LED applications. Monodisperse and large crystallites are favorable for uniform and reproducible packing of phosphors, which is very important in the production of w-LEDs. In addition, phosphors with large and highly crystalline particles often display higher quantum yields because of reduced concentrations of defects which act as quenching sites. Finally, a large particle size aids the long-term stability of a phosphor. Figure 3 shows the room-temperature emission and excitation spectra of M 3 AlF 6 :Mn 4+ (M = Na, K). Upon blue photoexcitation the Mn 4+ -doped hexafluoroaluminates show narrow red emission lines around 620 nm (see Figure 3b). The emission lines are in a spectral range where the eye sensitivity is still relatively high, which is beneficial for applications. The red emission lines are assigned to spin-and parity-forbidden Mn 4+ 2 E → 4 A 2 transitions. Figure 3a shows that the red luminescence of M 3 AlF 6 :Mn 4+ has two broad excitation bands in the ultraviolet (UV) to blue spectral region. These bands correspond to the 4 A 2 → 4 T 1 and 4 A 2 → 4 T 2 transitions of Mn 4+ . In all three lattices the 4 A 2 → 4 T 2 excitation band is positioned at 460 nm, which indicates that the crystal field splitting is approximately equal for the investigated M 3 AlF 6 hosts.  4+ (1.2%) (red, λ em = 628 nm), Na 3 AlF 6 :Mn 4+ (0.4%) (green, λ em = 628 nm) and K 2 NaAlF 6 :Mn 4+ (0.9%) (blue, λ em = 631 nm). The broad excitation bands correspond to the Mn 4+ 4 A 2 → 4 T 1 and 4 A 2 → 4 T 2 transitions; (b) Emission spectra (λ exc = 460 nm) of K 3 AlF 6 :Mn 4+ (1.2%) (red), Na 3 AlF 6 :Mn 4+ (0.4%) (green) and K 2 NaAlF 6 :Mn 4+ (0.9%) (blue). The Mn 4+ emission spectra consist of zero-phonon (ZPL) and (anti-)Stokes vibronic (ν i ) 2 E → 4 A 2 emission lines.

Mn 4+ Luminescence
The emission spectra of M 3 AlF 6 :Mn 4+ in Figure 3b resemble the emission spectra observed for other Mn 4+ -doped fluoride phosphors [33,39,40]. By analogy, the 2 E → 4 A 2 emission spectra of M 3 AlF 6 :Mn 4+ consist of a zero-phonon line (ZPL) at~620 nm and anti-Stokes and Stokes vibronic (ν 3 , ν 4 and ν 6 ) lines on the high and low energy side of the ZPL, respectively [39]. The Mn 4+ emission spectra are dominated by vibronic lines because the parity selection rule is relaxed by coupling of the 2 E → 4 A 2 electronic transition with odd-parity vibrations (ν 3 , ν 4 and ν 6 vibrational modes) [39,41]. It is noted that the 2 E → 4 A 2 ZPL intensity of M 3 AlF 6 :Mn 4+ is relatively strong when compared to other Mn 4+ -doped fluorides. For example, in K 2 SiF 6 :Mn 4+ the ZPL intensity is less than 5% of the Stokes ν 6 intensity, while in K 3 AlF 6 :Mn 4+ the ZPL intensity is almost half of the Stokes ν 6 intensity [40]. The intense ZPL is an interesting property for (w-LED) applications, as it improves the color quality of the red Mn 4+ phosphor. Furthermore, the observation of relatively strong zero-phonon lines is a sign that the MnF 6 centers lack inversion symmetry. The presence of odd-parity crystal field components relaxes the parity selection rule by inducing mixing with opposite parity states. As a result, the radiative lifetime of the 2 E → 4 A 2 emission is shorter (which is beneficial to reduce saturation effects) and the 4 A 2 → 4 T 2 absorption is stronger (and thus less material or a lower Mn 4+ concentration is needed to absorb the desired fraction of blue LED light).
The strong ZPL intensity in K 3 AlF 6 :Mn 4+ can be attributed to the low symmetry for Mn 4+ on the Al 3+ sites, i.e., the AlF 6 octahedron lacks an inversion center (C 1 symmetry, see Table 2). As discussed, without inversion symmetry, the 2 E → 4 A 2 ZPL intensity increases due to relaxation of the parity selection rule by odd-parity crystal field components that mix odd-parity states into the 3d wavefunctions [42]. In Na 3 AlF 6 and K 2 NaAlF 6 the AlF 6 octahedra have inversion centers (C i and O h symmetry, respectively) and the 2 E → 4 A 2 ZPL is expected to be weak, since there are no odd-parity crystal field components to relax the parity selection rule. The emission spectra in Figure 3b however show that the ZPLs of Na 3 AlF 6 :Mn 4+ and K 2 NaAlF 6 :Mn 4+ are significant, which indicates that at least for a part of the Mn 4+ ions the site symmetry is lower than C i (no inversion symmetry). This we explain by the charge compensation required for the Mn • Al sites (the Kröger-Vink notation is used to identify defects). The proximity of charge compensating defects (probably V K,Na vacancies or F i interstitials) gives rise to local deformation of the MnF 6 octahedra and lifts the inversion symmetry, causing the 2 E → 4 A 2 ZPL intensity of Na 3 AlF 6 :Mn 4+ and K 2 NaAlF 6 :Mn 4+ to increase.
Besides influencing the 2 E → 4 A 2 ZPL intensity, the charge compensating defects have a large influence on the integrated luminescence intensity and emission lifetime of M 3 AlF 6 :Mn 4+ . Typical luminescence quantum yield (QY) values measured for the M 3 AlF 6 :Mn 4+ phosphors are 39% for Na 3 AlF 6 :Mn 4+ , 53% for K 2 NaAlF 6 :Mn 4+ and 55% for K 3 AlF 6 :Mn 4+ . These QY values are below unity, which is attributed to non-radiative transfer of excitation energy from Mn 4+ to crystal defects. At the defects the excitation energy is lost non-radiatively as heat, i.e., the defects act as quenching sites. The defect concentration will increase with the Mn 4+ concentration, and it is therefore expected that the luminescence quenching becomes stronger at higher Mn 4+ concentrations. Previously, it has been measured that the luminescence intensity and emission lifetime of M 3 AlF 6 :Mn 4+ significantly decrease with increasing Mn 4+ concentration already at doping levels of 4% Mn 4+ [17][18][19]. In these works, the decrease in intensity and lifetime with the Mn 4+ concentration was explained by concentration quenching, i.e., energy migration between Mn 4+ ions to quenchers (defects, impurities). Energy migration is however not expected at doping concentrations of 4%, as most Mn 4+ ions will not have Mn 4+ neighbors in this concentration range (see also [43]). The results presented here indicate that quenching becomes stronger due to an increase in the defect concentration connected to the need for charge compensation and not because of enhanced energy migration among the Mn 4+ ions.
We performed low-temperature (T = 4 K) spectral measurements to get more insight into the Mn 4+ sites in M 3 AlF 6 :Mn 4+ . In addition, the measurements at 4 K allow us to accurately compare the energy of the emitting 2 E level in the different M 3 AlF 6 hosts. Figure 4 displays emission spectra (λ exc = 460 nm) at T = 4 K of M 3 AlF 6 :Mn 4+ . In line with the luminescence spectra at room temperature, the 4 K emission spectra of M 3 AlF 6 :Mn 4+ consist of zero-phonon and vibronic 2 E → 4 A 2 emission lines (labeled ZPL, ν 3 , ν 4 and ν 6 ). Multiple lines are observed in the ZPL region. The peaks marked with a star can be due to 2 E → 4 A 2 electronic transitions that couple with low energy rotatory or translatory lattice vibrational modes [40,44]. Vibronic lines due to these modes are usually found at 50-150 cm −1 relative to the ZPL in emission spectra of Mn 4+ . Alternatively, the peaks marked with a star can be ZPLs of Mn 4+ ions located on different sites than the Mn 4+ ions yielding the most intense zero-phonon emission line (labeled ZPL in Figure 4). Mn 4+ emission lines caused by lattice modes are typically very weak, and therefore it is more probable that the peaks marked with stars are ZPLs of Mn 4+ ions with other geometric environments [33,44]. In addition, multiple Mn 4+ sites can be expected, based on the need for charge compensation. Below, further evidence is given which indicates that the various sharp emission lines around 620 nm arise from MnF 6 groups with different local geometries related to charge compensation. In Figure 5 we investigate the presence of multiple Mn 4+ sites by measuring 4 K luminescence spectra of K 2 NaAlF 6 :Mn 4+ for various excitation and emission wavelengths. The excitation spectra in Figure 5a show that the structure in the 4 A 2 → 4 T 2 excitation band of K 2 NaAlF 6 :Mn 4+ changes significantly with emission wavelength. If only one Mn 4+ site would be present in K 2 NaAlF 6 :Mn 4+ , the excitation spectrum will have the same shape and structure for all emission wavelengths. However, here, the excitation spectrum changes significantly with emission wavelength, which indicates that more than one Mn 4+ site is present in K 2 NaAlF 6 :Mn 4+ . Furthermore, the spectra in Figure 5b show that the shape of the 2 E → 4 A 2 spectrum is different for various excitation wavelengths within the 4 A 2 → 4 T 2 band. This confirms that multiple Mn 4+ sites exist in K 2 NaAlF 6 :Mn 4+ , and likely also in K 3 AlF 6 :Mn 4+ and Na 3 AlF 6 :Mn 4+ . The presence of more than one Mn 4+ site in M 3 AlF 6 :Mn 4+ was not observed in previous reports on Mn 4+ -doped hexafluoraluminates [17,18]. Instead, from time-resolved measurements it was concluded that only one Mn 4+ emission site was present in M 3 AlF 6 :Mn 4+ . The formation of geometrically different Mn 4+ sites in M 3 AlF 6 :Mn 4+ is expected as charge compensation is required for the Mn • Al center. The charge compensating defect can be local or distant, i.e., in the first shell of cations around the Mn 4+ ion or further away in the lattice. A distant defect will not influence the local geometry around the Mn 4+ ion, whereas a local defect can cause a deformation of the fluorine octahedron around the Mn 4+ ion. This will give rise to multiple differently charge compensated Mn 4+ sites within the lattice, depending on the type and local geometry of charge compensation.
To study the influence of the M 3 AlF 6 (M = Na, K) host on the energy of the Mn 4+2 E level, we compare the positions of the highest-energy 2 E → 4 A 2 ZPLs in M 3 AlF 6 :Mn 4+ . The energies of these ZPLs (labeled ZPL in Figure 4) are 16,200 cm −1 (K 3 AlF 6 :Mn 4+ ), 16,167 cm −1 (Na 3 AlF 6 :Mn 4+ ) and 16,082 cm −1 (K 2 NaAlF 6 :Mn 4+ ). The 2 E level energies are also listed in Table 2. The energy of the 2 E level for Mn 4+ in M 3 AlF 6 is in good agreement with the 2 E level energy observed for Mn 4+ in other fluoride hosts [45,46]. Furthermore, it is observed that the energy of the Mn 4+ 2 E level increases from K 2 NaAlF 6 to Na 3 AlF 6 to K 3 AlF 6 (see dashed line in Figure 4). This indicates that the local structure and type of M + cation in the second coordination sphere around the Mn 4+ ion has an influence on the 2 E level energy. Previous studies on M 2 SiF 6 :Mn 4+ (M = Na, K, Rb or Cs) also show an influence of the M + cation on the 2 E level energy [33,40,44]. In these compounds the energy E of the 2 E level follows the trend E(Na + ) > E(K + ) > E(Rb + ) > E(Cs + ), which suggests that the 2 E level energy decreases with the radius or electron affinity of the M + ion [21]. This is however not confirmed by our results for the M 3 AlF 6 :Mn 4+ phosphors, where the highest 2 E energy is found for K 3 AlF 6 :Mn 4+ . Instead, the results in Table 2 indicate that the energy of the 2 E level increases when the Mn-F (Al-F) distance becomes longer or when the symmetry of the Mn 4+ site (Al 3+ site) is reduced. It is however not possible to draw definite conclusions from the data in Table 2 as the symmetry and distances in (part of) the MnF 6 octahedra will change due to deformations caused by nearby charge compensating defects.

Thermal Quenching in M 3 AlF 6 :Mn 4+
For high-power w-LED applications, the thermal quenching behavior of a phosphor is very important, as the temperature of the on-chip phosphor layer in a high-power w-LED reaches temperatures as high as 450 K. The thermal quenching behavior of K 3 AlF 6 :Mn 4+ and Na 3 AlF 6 :Mn 4+ has been investigated by Song et al. [17,18]. They reported that thermal quenching sets in around 423 K (150 • C) for K 3 AlF 6 :Mn 4+ and Na 3 AlF 6 :Mn 4+ . More interestingly, they found that the integrated photoluminescence (PL) intensity of these phosphors doubles between room temperature and 423 K. This is a very useful property for high temperature applications, but is also very unexpected. For most Mn 4+ -doped fluorides, the PL intensity is relatively constant between room temperature and the temperature at which thermal quenching sets in [8][9][10].
To investigate the thermal quenching of the Mn 4+ emission in M 3 AlF 6 :Mn 4+ (M = Na, K), we measure the integrated PL intensity of M 3 AlF 6 :Mn 4+ as a function of temperature between 298 K and 600 K. Figure 6a shows emission spectra of K 3 AlF 6 :Mn 4+ recorded in this temperature range. Emission spectra of Na 3 AlF 6 :Mn 4+ and K 2 NaAlF 6 :Mn 4+ between 298 K and 600 K can be found in Figure S1. The results show that the PL intensity of M 3 AlF 6 :Mn 4+ slowly decreases up to 423 K (150 • C). Above this temperature, there is rapid quenching, with no emission intensity remaining at 573 K. After heating to 573 K, around half of the initial room-temperature PL intensity is retained. Part of the PL intensity is lost upon heating due to e.g., degradation of the phosphor, reduction/oxidation of the Mn 4+ ions and formation of Mn-oxide impurities.
From the emission spectra recorded between 298 K and 600 K we obtain the temperature dependence of the integrated PL intensity (I PL ), which is displayed in Figure 6b. The intensity is given relative to the integrated PL intensity at room temperature (I RT ). The PL intensity gradually decreases between 300 K and 450 K, but then at higher temperatures rapidly drops due to an increased probability for non-radiative transitions from the 2 E excited state (luminescence quenching). The luminescence quenching temperature T1 ⁄2 , the temperature at which the PL intensity is half of its initial value, is around 460 K for K 3 AlF 6 :Mn 4+ and Na 3 AlF 6 :Mn 4+ and 485 K for K 2 NaAlF 6 :Mn 4+ . The T1 ⁄2 values fall in the range of quenching temperatures reported for Mn 4+ -doped fluoride phosphors [39,47]. The quenching temperatures of M 3 AlF 6 :Mn 4+ are above the phosphor operating temperatures of high-power w-LEDs. Figure 6. (a) Emission spectra of K 3 AlF 6 :Mn 4+ (1.2%) at various temperatures between 298 K and 573 K (λ exc = 450 nm); (b) Integrated PL intensity of K 3 AlF 6 :Mn 4+ (1.2%) (blue), Na 3 AlF 6 :Mn 4+ (0.4%) (green) and K 2 NaAlF 6 :Mn 4+ (0.9%) (red) as a function of temperature between 300 and 600 K. The integrated PL intensity I PL is scaled to the integrated PL intensity at room temperature I RT .
The temperature dependences we obtain for K 3 AlF 6 :Mn 4+ and Na 3 AlF 6 :Mn 4+ (Figure 6b) are significantly different from the work by Song et al. [17,18]. We observe a small decrease in the P intensity between 298 K and 423 K, while they reported a doubling of the PL intensity between these temperatures. The decrease in PL intensity between 298 K and 423 K for M 3 AlF 6 :Mn 4+ we ascribe to an increase of the energy transfer from Mn 4+ ions to quenching sites (defects and impurities) with temperature [48]. The rapid quenching of the Mn 4+ luminescence above 430 K is attributed to thermally activated crossing of the Mn 4+ 4 T 2 excited state and 4 A 2 ground state, as is explained in [49].
Finally, we observe some interesting changes in the emission spectrum of K 2 NaAlF 6 :Mn 4+ upon heating to 573 K. This is illustrated in Figure 7, which displays emission spectra recorded at T = 4 K and 298 K of K 2 NaAlF 6 :Mn 4+ (0.9%) for as-synthesized K 2 NaAlF 6 :Mn 4+ (blue spectra) and K 2 NaAlF 6 :Mn 4+ phosphor that was heated to 573 K (red spectra). The room-temperature spectra in Figure 7a show that the structure of the 2 E → 4 A 2 ZPL emission in K 2 NaAlF 6 :Mn 4+ changes upon heating to 573 K. This effect is even more pronounced in the spectra measured at 4 K (Figure 7b). Four ZPLs of similar intensity are observed for the as-synthesized phosphor, while two ZPLs dominate the spectrum after heating at 573 K. In addition, the results in Figure 7b show that heating to 573 K changes the structure and intensity of the vibronic 2 E → 4 A 2 emission lines. The changes in the emission spectra can be caused by a phase transition in the K 2 NaAlF 6 crystal structure. However, the XRD patterns of as-synthesized K 2 NaAlF 6 :Mn 4+ and K 2 NaAlF 6 :Mn 4+ phosphor heated to 573 K both match the reference pattern of elpasolite K 2 NaAlF 6 , which indicates that no phase transition occurs (see Figure S2). We therefore explain the changes in the emission spectra upon high temperature annealing by a rearrangement of the Mn 4+ sites in K 2 NaAlF 6 :Mn 4+ upon heating to 573 K. Furthermore, the fact that two ZPLs dominate the emission spectrum of K 2 NaAlF 6 :Mn 4+ after heating indicates that there is a redistribution in the abundance of different charge compensated Mn 4+ sites. The results presented in Figure 7 show that post-synthesis heating can have a large effect on the luminescence properties of M 3 AlF 6 :Mn 4+ and other Mn 4+ -doped fluoride phosphors. (a) Room-temperature emission spectra (λ exc = 450 nm) of K 2 NaAlF 6 :Mn 4+ (0.9%) for as-synthesized K 2 NaAlF 6 :Mn 4+ phosphor (blue) and K 2 NaAlF 6 :Mn 4+ phosphor that has been heated to 573 K (red); (b) Emission spectra of K 2 NaAlF 6 :Mn 4+ (0.9%) at T = 4 K (λ exc = 460 nm) for as-synthesized K 2 NaAlF 6 :Mn 4+ phosphor (blue) and K 2 NaAlF 6 :Mn 4+ phosphor that has been heated to 573 K (red).

Conclusions
Mn 4+ -doped fluorides have recently attracted considerable attention due to their potential for application in w-LEDs. For application in w-LEDs, it is important to understand and control the synthesis and luminescence properties of Mn 4+ -doped fluoride phosphors. Here, we report the synthesis of different M 3 AlF 6 :Mn 4+ (M = Na, K) phosphors via a simple two-step co-precipitation method. Our synthesis method provides good control over Mn 4+ doping and yields highly monodisperse~20 µm smoothed octahedron shaped crystallites for K 2 NaAlF 6 :Mn 4+ , while irregularly shaped particles with a broad size distribution are obtained for K 3 AlF 6 :Mn 4+ and Na 3 AlF 6 :Mn 4+ . All synthesized M 3 AlF 6 :Mn 4+ phosphors show narrow red Mn 4+ 2 E → 4 A 2 luminescence that can be excited in the UV and blue spectral region. Luminescence spectra recorded at T = 4 K reveal that multiple Mn 4+ sites are present in M 3 AlF 6 :Mn 4+ , which was not observed in previous reports. The presence of multiple Mn 4+ sites is attributed to charge compensation that is required for Mn 4+ on Al 3+ sites. The results show that charge compensating defects have a large influence on the luminescence properties (e.g., spectra, QY, luminescence lifetime) of Mn 4+ -doped fluorides. Lowering of the QY by defect quenching is a problem for application of this class of Mn 4+ phosphors. Finally, we investigated the thermal quenching behavior for M 3 AlF 6 :Mn 4+ . The luminescence quenching temperature of M 3 AlF 6 :Mn 4+ is between 460 K and 490 K, which is above the phosphor operating temperature in high-power w-LEDs. If the QY can be improved by suppressing defect quenching, Mn 4+ -doped hexafluoroaluminates are a promising class of materials as their chemical and thermal stability is superior to the presently used commercial Mn 4+ phosphors.