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

Abstract

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.

1. 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 Eu²⁺-doped nitrides. These phosphors can have quantum yields (QYs) exceeding 90%, but a drawback is that the Eu²⁺ 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 Eu²⁺-doped nitrides as red phosphors in w-LEDs [9]. To overcome this issue, efficient narrow band red-emitting phosphors with λ_max ~ 620 nm have to be developed.

Mn⁴⁺-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 Mn⁴⁺-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 Mn⁴⁺-doped fluorides interesting narrow band red phosphors for w-LEDs. In recent years, a large number of Mn⁴⁺-activated fluorides with the general chemical formulas A₂MF₆:Mn⁴⁺ (A = Na, K, Rb, Cs and NH₄; M = Si, Ge, Ti, Zr and Sn) and BaMF₆:Mn⁴⁺ (M = Si, Ge, Ti and Sn) have been reported [11,12,13]. In these compounds Mn⁴⁺ substitutes for the M⁴⁺ cation that is octahedrally coordinated by six F⁻ ions. Most work has been done on the phosphor K₂SiF₆:Mn⁴⁺ (KSF) [14,15]. The Mn⁴⁺-doped fluorides have excellent luminescence properties. The deep red color of the 600–640 nm Mn⁴⁺ emission is particularly favorable for extending the color gamut of displays, and KSF is already widely applied in displays. In lighting, large-scale application is still limited, partly hampered by issues related to thermal and chemical stability and saturation at high pump powers [14,16].

Recently, the synthesis and luminescence properties of Mn⁴⁺-doped hexafluoraluminates with the compositions M₃AlF₆:Mn⁴⁺ (M = Li, Na, K) were reported [17,18,19,20]. The ionic radius of Mn⁴⁺ is similar to the ionic radius of Al³⁺ (0.53 versus 0.54 Å), and as a result, Mn⁴⁺ can easily substitute for Al³⁺ in fluoride hosts [17,21]. The M₃AlF₆:Mn⁴⁺ phosphors have potential advantages over K₂SiF₆:Mn⁴⁺ and other Mn⁴⁺-doped fluorides. First, Na₃AlF₆ and K₃AlF₆ have a melting point of ~1000 °C and therefore have a much better thermal stability than K₂SiF₆, which already decomposes at 350–400 °C [10,22,23,24]. Secondly, the M₃AlF₆ compounds have a lower water solubility than K₂SiF₆, making them more chemically stable under high humidity conditions [14,25]. Thirdly, hexafluoroaluminates are already produced worldwide on a large scale, as they are used as solvents for bauxite in the industrial extraction of aluminum [26]. This may facilitate cheap large-scale production of Mn⁴⁺-activated hexafluoroaluminates.

Until now, different methods have been used to synthesize M₃AlF₆:Mn⁴⁺ phosphors. Song et al. prepared Na₃AlF₆:Mn⁴⁺ phosphors via a co-precipitation method [17], while others synthesized K₃AlF₆:Mn⁴⁺ and K₂NaAlF₆:Mn⁴⁺ by cation exchange [18,19]. Furthermore, K₂LiAlF₆:Mn⁴⁺ was synthesized via a hydrothermal method [20]. A single convenient synthesis method for preparing M₃AlF₆:Mn⁴⁺ phosphors is thus so far lacking. The previous works on M₃AlF₆:Mn⁴⁺ have reported luminescence spectra and decay curves for different Mn⁴⁺ doping concentrations in the temperature range of 300 to 500 K [17,18,19,20]. However, they provided no insight into the influence of (charge compensating) defects on the luminescence spectra and quantum yield of M₃AlF₆:Mn⁴⁺. Furthermore, no explanations for the thermal quenching of the Mn⁴⁺ luminescence were given.

In this work we report a new synthesis route for Na₃AlF₆:Mn⁴⁺, K₃AlF₆:Mn⁴⁺ and K₂NaAlF₆:Mn⁴⁺ based on a simple two-step co-precipitation method. We synthesize Mn⁴⁺-doped hexafluoroaluminates by initially preparing the Mn⁴⁺-precursor K₂MnF₆ and then in a second step precipitating M₃AlF₆:Mn⁴⁺ (M = Na, K) from an aqueous HF solution containing Al³⁺, Mn⁴⁺ and Na⁺/K⁺ ions. The presented method gives good control over the composition and homogeneity of the M₃AlF₆:Mn⁴⁺ phosphors. All synthesized M₃AlF₆:Mn⁴⁺ phosphors exhibit bright red Mn⁴⁺ luminescence around 620 nm. For K₂NaAlF₆:Mn⁴⁺ we obtain highly monodisperse and large (~20 µm) phosphor particles that exhibit narrow size and shape distributions that are superior to the size and shape distributions of other reported Mn⁴⁺-doped phosphors. This makes K₂NaAlF₆:Mn⁴⁺ interesting for use in w-LEDs, as monodisperse, uniform and large particles are favorable for reproducible and high packing density of phosphors in w-LEDs.

We perform both room-temperature and low-temperature (T = 4 K) spectral measurements of M₃AlF₆:Mn⁴⁺. The measurements at 4 K reveal that multiple Mn⁴⁺ sites exist in M₃AlF₆:Mn⁴⁺, which was not observed in previous works on Mn⁴⁺-doped hexafluoroaluminates. The formation of multiple Mn⁴⁺ sites can be understood from the need for charge compensation for Mn⁴⁺ ions on an Al³⁺ site. Further evidence for the presence of multiple sites is obtained from thermal cycling experiments, which show a change in site distribution after high temperature annealing. The charge compensation and associated defects have a large influence on the luminescence properties (e.g., quantum yield) of M₃AlF₆:Mn⁴⁺.

Finally, we study the thermal quenching behavior for M₃AlF₆:Mn⁴⁺ by measuring the luminescence intensity as a function of temperature between 300 and 600 K. The luminescence quenching temperature we find for M₃AlF₆:Mn⁴⁺ is 460–490 K, which is above the operating temperature of phosphors in high-power w-LEDs. The thermal quenching is explained by thermally activated crossover from the ⁴T₂ excited state to the ⁴A₂ ground state. Furthermore, there is luminescence quenching due to non-radiative energy transfer from Mn⁴⁺ ions to quenching sites (defects and impurities).

2. Materials and Methods

The M₃AlF₆:Mn⁴⁺ (M = Na, K) phosphors were synthesized through a two-step chemical co-precipitation method. In the first step the Mn⁴⁺-precursor K₂MnF₆ was synthesized and in the second step the M₃AlF₆:Mn⁴⁺ phosphor was prepared. Since K₂MnF₆ and M₃AlF₆:Mn⁴⁺ are synthesized in corrosive HF solutions, all reactions described were carried out in plastic or Teflon beakers.

2.1. Chemicals

The following chemicals were purchased from Sigma-Aldrich: KMnO₄ (≥99.0%), KF (≥99.0%), H₂O₂ (30 wt % solution, ACS reagent), Al(OH)₃ (reagent grade), Na₂CO₃ (≥99.95%) and K₂CO₃ (≥99.0%). Hydrofluoric acid (HF, 40% aqueous solution) was purchased from Riedel de Haën. All chemicals were used without any further purification.

2.2. Synthesis of K₂MnF₆

K₂MnF₆ was prepared according to the method of Bode [27,28]. KMnO₄ (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 % H₂O₂ solution was added dropwise which resulted in the gradual precipitation of yellow K₂MnF₆ powder. The dropwise addition of H₂O₂ was stopped when the reaction solution turned from purple to red-brown, indicating the formation of Mn⁴⁺. The K₂MnF₆ 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.

2.3. Synthesis of M₃AlF₆:Mn⁴⁺

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 K₂MnF₆ (1 mol % doping concentration) was added. Simultaneously, a solution of M⁺ ions (M = Na, K) in 40% HF was prepared by gradually adding M₂CO₃ or MF to 40% HF (aq) while stirring.

Table 1. Amounts of Na₂CO₃, K₂CO₃, KF and 40% HF (aq) used in the synthesis of M₃AlF₆:Mn⁴⁺ (M = Na, K).

Phosphor	Na2CO3	K2CO3	KF	40% HF
Na3AlF6:Mn4+	15 mmol	-	-	15 mL
K2NaAlF6:Mn4+	5 mmol	10 mmol	-	15 mL
K3AlF6:Mn4+	-	-	40 mmol 1	3 mL

¹ A 4:1 ratio of K:Al was used, as this gave K₃AlF₆ without impurity phases. With a 3:1 ratio, the obtained phosphor contained impurities of other crystal phases.

lists the amounts of Na₂CO₃, K₂CO₃, KF and 40% HF used for preparing the M⁺/HF solutions. The M⁺/HF solution was added to the Al³⁺/Mn⁴⁺/HF solution, which resulted in the precipitation of Na₃AlF₆:Mn⁴⁺ and K₂NaAlF₆:Mn⁴⁺ but not K₃AlF₆:Mn⁴⁺ phosphor. K₃AlF₆:Mn⁴⁺ was precipitated by adding 50 mL of ethanol to the mixed M⁺/Al³⁺/Mn⁴⁺/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 Mn⁴⁺ doping concentrations could be obtained by changing the amount of K₂MnF₆ that was used in the synthesis.

2.4. 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 M₃AlF₆:Mn⁴⁺ 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).

3. Results and Discussion

3.1. Structural Properties

To investigate the composition, size and shape of the M₃AlF₆:Mn⁴⁺ (M = Na, K) phosphor particles, we used different characterization techniques such as XRD, SEM and ICP-OES. Figure 1 shows XRD patterns of the M₃AlF₆:Mn⁴⁺ phosphors. The XRD patterns are in perfect agreement with the reference patterns for Na₃AlF₆ (PDF 00-025-0772, red), K₂NaAlF₆ (PDF 01-072-2434, blue) and K₃AlF₆ (PDF 00-057-0227, green). No other crystal phases can be observed, which confirms that the phosphor samples are single phase.

The XRD measurements show that incorporation of Mn⁴⁺ on the Al³⁺ sites does not significantly change the crystal structure of M₃AlF₆, which is expected as the ionic radii of Mn⁴⁺ and Al³⁺ are similar (0.53 versus 0.54 Å) [21]. The space groups of Na₃AlF₆, K₂NaAlF₆ and K₃AlF₆ are listed in

Table 2. Space group, Al³⁺ site symmetry and average Al–F distance for M₃AlF₆ (M = Na, K); zero-phonon line (ZPL) energy of the Mn^{4+ 2}E → ⁴A₂ emission in M₃AlF₆:Mn⁴⁺. Structural data obtained from Refs. [29,30,31].

Lattice	Space Group	Al3+ Symmetry	Al–F Distance (Å)	ZPL Energy (cm−1)
Na3AlF6	P21/n	Ci	1.808	16,167
K2NaAlF6	Fm$\overline{3}$m	Oh	1.778	16,082
K3AlF6	I41/a	C1	1.810	16,200

. K₂NaAlF₆ has a highly symmetric cubic crystal structure (space group is Fm$\overline{3}$m), while Na₃AlF₆ and K₃AlF₆ have a crystal structure with lower symmetry (space groups are P2₁/n and I4₁/a, respectively) [29,30,31]. In the M₃AlF₆ crystal structure, the Al³⁺ ions are octahedrally coordinated by six F⁻ ions. Depending on the M₃AlF₆ lattice, the AlF₆ octahedron can be highly symmetric (O_h in K₂NaAlF₆) or distorted (C_i in Na₃AlF₆ and C₁ in K₃AlF₆). The average Al–F distances in the AlF₆ octahedra are around 1.8 Å.

By using ICP-OES, we measured the manganese concentrations in the synthesized M₃AlF₆:Mn⁴⁺phosphors (see Materials and Methods section). The XRD patterns displayed in Figure 1 were measured for M₃AlF₆:Mn⁴⁺ phosphors containing 0.4 mol % (Na₃AlF₆:Mn⁴⁺), 1.2 mol % (K₃AlF₆:Mn⁴⁺) and 0.9 mol % Mn (K₂NaAlF₆:Mn⁴⁺). The results presented in the rest of this work were obtained for M₃AlF₆:Mn⁴⁺ phosphors having these doping concentrations. For K₃AlF₆:Mn⁴⁺ and K₂NaAlF₆:Mn⁴⁺ 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 Mn⁴⁺ doping concentration. Also a substantially higher fraction of Mn⁴⁺ is incorporated into K₃AlF₆ compared to previously reported cation exchange methods for preparing K₃AlF₆:Mn⁴⁺ [18].

Figure 2 displays SEM images of Na₃AlF₆:Mn⁴⁺ (0.4%), K₃AlF₆:Mn⁴⁺ (1.2%) and K₂NaAlF₆:Mn⁴⁺ (0.9%) phosphor particles. The Na₃AlF₆:Mn⁴⁺ (Figure 2a) and K₃AlF₆:Mn⁴⁺ (Figure 2b) phosphors consist of irregularly shaped particles and clusters of particles with sizes ranging from ~100 nm to >10 µm. For K₃AlF₆:Mn⁴⁺, 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 K₂NaAlF₆:Mn⁴⁺ (Figure 2c,d) yields highly monodisperse phosphor particles with a large average diameter of 22.5 ± 6.1 µm. The K₂NaAlF₆:Mn⁴⁺ particles have a smoothed octahedral shape, as expected from the cubic elpasolite structure of K₂NaAlF₆ [10,19,29]. The K₂NaAlF₆:Mn⁴⁺ particles prepared with our co-precipitation method have a more uniform shape and size than the K₂NaAlF₆:Mn⁴⁺ particles prepared by Zhu et al. via cation exchange [19]. Moreover, the K₂NaAlF₆:Mn⁴⁺ (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 Mn⁴⁺-doped fluoride phosphors [10,24,32,33,34,35,36,37].

The high monodispersity of the K₂NaAlF₆:Mn⁴⁺ 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 Mn⁴⁺-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 K₂NaAlF₆:Mn⁴⁺ 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.

3.2. Mn⁴⁺Luminescence

Figure 3 shows the room-temperature emission and excitation spectra of M₃AlF₆:Mn⁴⁺ (M = Na, K). Upon blue photoexcitation the Mn⁴⁺-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 → ⁴A₂ transitions. Figure 3a shows that the red luminescence of M₃AlF₆:Mn⁴⁺ has two broad excitation bands in the ultraviolet (UV) to blue spectral region. These bands correspond to the ⁴A₂ → ⁴T₁ and ⁴A₂ → ⁴T₂ transitions of Mn⁴⁺. In all three lattices the ⁴A₂ → ⁴T₂ excitation band is positioned at 460 nm, which indicates that the crystal field splitting is approximately equal for the investigated M₃AlF₆ hosts.

The emission spectra of M₃AlF₆:Mn⁴⁺ in Figure 3b resemble the emission spectra observed for other Mn⁴⁺-doped fluoride phosphors [33,39,40]. By analogy, the ²E → ⁴A₂ emission spectra of M₃AlF₆:Mn⁴⁺ consist of a zero-phonon line (ZPL) at ~620 nm and anti-Stokes and Stokes vibronic (ν₃, ν₄ and ν₆) lines on the high and low energy side of the ZPL, respectively [39]. The Mn⁴⁺ emission spectra are dominated by vibronic lines because the parity selection rule is relaxed by coupling of the ²E → ⁴A₂ electronic transition with odd-parity vibrations (ν₃, ν₄ and ν₆ vibrational modes) [39,41]. It is noted that the ²E → ⁴A₂ ZPL intensity of M₃AlF₆:Mn⁴⁺ is relatively strong when compared to other Mn⁴⁺-doped fluorides. For example, in K₂SiF₆:Mn⁴⁺ the ZPL intensity is less than 5% of the Stokes ν₆ intensity, while in K₃AlF₆:Mn⁴⁺ the ZPL intensity is almost half of the Stokes ν₆ intensity [40]. The intense ZPL is an interesting property for (w-LED) applications, as it improves the color quality of the red Mn⁴⁺ phosphor. Furthermore, the observation of relatively strong zero-phonon lines is a sign that the MnF₆ 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 ²E → ⁴A₂ emission is shorter (which is beneficial to reduce saturation effects) and the ⁴A₂ → ⁴T₂ absorption is stronger (and thus less material or a lower Mn⁴⁺ concentration is needed to absorb the desired fraction of blue LED light).

The strong ZPL intensity in K₃AlF₆:Mn⁴⁺ can be attributed to the low symmetry for Mn⁴⁺ on the Al³⁺ sites, i.e., the AlF₆ octahedron lacks an inversion center (C₁ symmetry, see Table 2). As discussed, without inversion symmetry, the ²E → ⁴A₂ 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 Na₃AlF₆ and K₂NaAlF₆ the AlF₆ octahedra have inversion centers (C_i and O_h symmetry, respectively) and the ²E → ⁴A₂ 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 Na₃AlF₆:Mn⁴⁺ and K₂NaAlF₆:Mn⁴⁺ are significant, which indicates that at least for a part of the Mn⁴⁺ ions the site symmetry is lower than C_i (no inversion symmetry). This we explain by the charge compensation required for the ${\mathrm{Mn}}_{\mathrm{Al}}^{•}$ sites (the Kröger-Vink notation is used to identify defects). The proximity of charge compensating defects (probably ${\mathrm{V}}_{\mathrm{K},\mathrm{Na}}^{\prime }$ vacancies or ${\mathrm{F}}_{\mathrm{i}}^{\prime }$ interstitials) gives rise to local deformation of the MnF₆ octahedra and lifts the inversion symmetry, causing the ²E → ⁴A₂ ZPL intensity of Na₃AlF₆:Mn⁴⁺ and K₂NaAlF₆:Mn⁴⁺to increase.

Besides influencing the ²E → ⁴A₂ ZPL intensity, the charge compensating defects have a large influence on the integrated luminescence intensity and emission lifetime of M₃AlF₆:Mn⁴⁺. Typical luminescence quantum yield (QY) values measured for the M₃AlF₆:Mn⁴⁺ phosphors are 39% for Na₃AlF₆:Mn⁴⁺, 53% for K₂NaAlF₆:Mn⁴⁺ and 55% for K₃AlF₆:Mn⁴⁺. These QY values are below unity, which is attributed to non-radiative transfer of excitation energy from Mn⁴⁺ 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 Mn⁴⁺ concentration, and it is therefore expected that the luminescence quenching becomes stronger at higher Mn⁴⁺ concentrations. Previously, it has been measured that the luminescence intensity and emission lifetime of M₃AlF₆:Mn⁴⁺ significantly decrease with increasing Mn⁴⁺ concentration already at doping levels of 4% Mn⁴⁺ [17,18,19]. In these works, the decrease in intensity and lifetime with the Mn⁴⁺ concentration was explained by concentration quenching, i.e., energy migration between Mn⁴⁺ ions to quenchers (defects, impurities). Energy migration is however not expected at doping concentrations of 4%, as most Mn⁴⁺ ions will not have Mn⁴⁺ 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 Mn⁴⁺ ions.

We performed low-temperature (T = 4 K) spectral measurements to get more insight into the Mn⁴⁺ sites in M₃AlF₆:Mn⁴⁺. In addition, the measurements at 4 K allow us to accurately compare the energy of the emitting ²E level in the different M₃AlF₆ hosts. Figure 4 displays emission spectra (λ_exc = 460 nm) at T = 4 K of M₃AlF₆:Mn⁴⁺. In line with the luminescence spectra at room temperature, the 4 K emission spectra of M₃AlF₆:Mn⁴⁺ consist of zero-phonon and vibronic ²E → ⁴A₂ emission lines (labeled ZPL, ν₃, ν₄ and ν₆). Multiple lines are observed in the ZPL region. The peaks marked with a star can be due to ²E → ⁴A₂ 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 Mn⁴⁺. Alternatively, the peaks marked with a star can be ZPLs of Mn⁴⁺ ions located on different sites than the Mn⁴⁺ ions yielding the most intense zero-phonon emission line (labeled ZPL in Figure 4). Mn⁴⁺ 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 Mn⁴⁺ ions with other geometric environments [33,44]. In addition, multiple Mn⁴⁺ 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 MnF₆ groups with different local geometries related to charge compensation.

In Figure 5 we investigate the presence of multiple Mn⁴⁺ sites by measuring 4 K luminescence spectra of K₂NaAlF₆:Mn⁴⁺ for various excitation and emission wavelengths. The excitation spectra in Figure 5a show that the structure in the ⁴A₂ → ⁴T₂ excitation band of K₂NaAlF₆:Mn⁴⁺ changes significantly with emission wavelength. If only one Mn⁴⁺ site would be present in K₂NaAlF₆:Mn⁴⁺, 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 Mn⁴⁺ site is present in K₂NaAlF₆:Mn⁴⁺. Furthermore, the spectra in Figure 5b show that the shape of the ²E → ⁴A₂ spectrum is different for various excitation wavelengths within the ⁴A₂ → ⁴T₂ band. This confirms that multiple Mn⁴⁺ sites exist in K₂NaAlF₆:Mn⁴⁺, and likely also in K₃AlF₆:Mn⁴⁺ and Na₃AlF₆:Mn⁴⁺. The presence of more than one Mn⁴⁺ site in M₃AlF₆:Mn⁴⁺ was not observed in previous reports on Mn⁴⁺-doped hexafluoraluminates [17,18]. Instead, from time-resolved measurements it was concluded that only one Mn⁴⁺ emission site was present in M₃AlF₆:Mn⁴⁺. The formation of geometrically different Mn⁴⁺ sites in M₃AlF₆:Mn⁴⁺ is expected as charge compensation is required for the ${\mathrm{Mn}}_{\mathrm{Al}}^{•}$ center. The charge compensating defect can be local or distant, i.e., in the first shell of cations around the Mn⁴⁺ ion or further away in the lattice. A distant defect will not influence the local geometry around the Mn⁴⁺ ion, whereas a local defect can cause a deformation of the fluorine octahedron around the Mn⁴⁺ ion. This will give rise to multiple differently charge compensated Mn⁴⁺ sites within the lattice, depending on the type and local geometry of charge compensation.

To study the influence of the M₃AlF₆ (M = Na, K) host on the energy of the Mn⁴⁺²E level, we compare the positions of the highest-energy ²E → ⁴A₂ ZPLs in M₃AlF₆:Mn⁴⁺. The energies of these ZPLs (labeled ZPL in Figure 4) are 16,200 cm⁻¹ (K₃AlF₆:Mn⁴⁺), 16,167 cm⁻¹ (Na₃AlF₆:Mn⁴⁺) and 16,082 cm⁻¹ (K₂NaAlF₆:Mn⁴⁺). The ²E level energies are also listed in Table 2. The energy of the ²E level for Mn⁴⁺ in M₃AlF₆ is in good agreement with the ²E level energy observed for Mn⁴⁺ in other fluoride hosts [45,46]. Furthermore, it is observed that the energy of the Mn^{4+ 2}E level increases from K₂NaAlF₆ to Na₃AlF₆ to K₃AlF₆ (see dashed line in Figure 4). This indicates that the local structure and type of M⁺ cation in the second coordination sphere around the Mn⁴⁺ ion has an influence on the ²E level energy. Previous studies on M₂SiF₆:Mn⁴⁺ (M = Na, K, Rb or Cs) also show an influence of the M⁺ cation on the ²E level energy [33,40,44]. In these compounds the energy E of the ²E level follows the trend E(Na⁺) > E(K⁺) > E(Rb⁺) > E(Cs⁺), which suggests that the ²E level energy decreases with the radius or electron affinity of the M⁺ ion [21]. This is however not confirmed by our results for the M₃AlF₆:Mn⁴⁺ phosphors, where the highest ²E energy is found for K₃AlF₆:Mn⁴⁺. Instead, the results in Table 2 indicate that the energy of the ²E level increases when the Mn–F (Al–F) distance becomes longer or when the symmetry of the Mn⁴⁺ site (Al³⁺ 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 MnF₆ octahedra will change due to deformations caused by nearby charge compensating defects.

3.3. Thermal Quenching in M₃AlF₆:Mn⁴⁺

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 K₃AlF₆:Mn⁴⁺ and Na₃AlF₆:Mn⁴⁺ has been investigated by Song et al. [17,18]. They reported that thermal quenching sets in around 423 K (150 °C) for K₃AlF₆:Mn⁴⁺ and Na₃AlF₆:Mn⁴⁺. 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 Mn⁴⁺-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 Mn⁴⁺ emission in M₃AlF₆:Mn⁴⁺ (M = Na, K), we measure the integrated PL intensity of M₃AlF₆:Mn⁴⁺ as a function of temperature between 298 K and 600 K. Figure 6a shows emission spectra of K₃AlF₆:Mn⁴⁺ recorded in this temperature range. Emission spectra of Na₃AlF₆:Mn⁴⁺ and K₂NaAlF₆:Mn⁴⁺ between 298 K and 600 K can be found in Figure S1. The results show that the PL intensity of M₃AlF₆:Mn⁴⁺ 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 Mn⁴⁺ 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 ²E excited state (luminescence quenching). The luminescence quenching temperature T_½, the temperature at which the PL intensity is half of its initial value, is around 460 K for K₃AlF₆:Mn⁴⁺ and Na₃AlF₆:Mn⁴⁺ and 485 K for K₂NaAlF₆:Mn⁴⁺. The T_½ values fall in the range of quenching temperatures reported for Mn⁴⁺-doped fluoride phosphors [39,47]. The quenching temperatures of M₃AlF₆:Mn⁴⁺ are above the phosphor operating temperatures of high-power w-LEDs.

The temperature dependences we obtain for K₃AlF₆:Mn⁴⁺ and Na₃AlF₆:Mn⁴⁺ (Figure 6b) are significantly different from the work by Song et al. [17,18]. We observe a small decrease in the PL 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 M₃AlF₆:Mn⁴⁺ we ascribe to an increase of the energy transfer from Mn⁴⁺ ions to quenching sites (defects and impurities) with temperature [48]. The rapid quenching of the Mn⁴⁺ luminescence above 430 K is attributed to thermally activated crossing of the Mn^{4+ 4}T₂ excited state and ⁴A₂ ground state, as is explained in [49].

Finally, we observe some interesting changes in the emission spectrum of K₂NaAlF₆:Mn⁴⁺ upon heating to 573 K. This is illustrated in Figure 7, which displays emission spectra recorded at T = 4 K and 298 K of K₂NaAlF₆:Mn⁴⁺ (0.9%) for as-synthesized K₂NaAlF₆:Mn⁴⁺ (blue spectra) and K₂NaAlF₆:Mn⁴⁺ phosphor that was heated to 573 K (red spectra). The room-temperature spectra in Figure 7a show that the structure of the ²E → ⁴A₂ ZPL emission in K₂NaAlF₆:Mn⁴⁺ 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 ²E → ⁴A₂ emission lines. The changes in the emission spectra can be caused by a phase transition in the K₂NaAlF₆ crystal structure. However, the XRD patterns of as-synthesized K₂NaAlF₆:Mn⁴⁺ and K₂NaAlF₆:Mn⁴⁺ phosphor heated to 573 K both match the reference pattern of elpasolite K₂NaAlF₆, 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 Mn⁴⁺ sites in K₂NaAlF₆:Mn⁴⁺ upon heating to 573 K. Furthermore, the fact that two ZPLs dominate the emission spectrum of K₂NaAlF₆:Mn⁴⁺ after heating indicates that there is a redistribution in the abundance of different charge compensated Mn⁴⁺ sites. The results presented in Figure 7 show that post-synthesis heating can have a large effect on the luminescence properties of M₃AlF₆:Mn⁴⁺ and other Mn⁴⁺-doped fluoride phosphors.

4. Conclusions

Mn⁴⁺-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 Mn⁴⁺-doped fluoride phosphors. Here, we report the synthesis of different M₃AlF₆:Mn⁴⁺ (M = Na, K) phosphors via a simple two-step co-precipitation method. Our synthesis method provides good control over Mn⁴⁺ doping and yields highly monodisperse ~20 μm smoothed octahedron shaped crystallites for K₂NaAlF₆:Mn⁴⁺, while irregularly shaped particles with a broad size distribution are obtained for K₃AlF₆:Mn⁴⁺ and Na₃AlF₆:Mn⁴⁺. All synthesized M₃AlF₆:Mn⁴⁺ phosphors show narrow red Mn^{4+ 2}E → ⁴A₂ luminescence that can be excited in the UV and blue spectral region. Luminescence spectra recorded at T = 4 K reveal that multiple Mn⁴⁺ sites are present in M₃AlF₆:Mn⁴⁺, which was not observed in previous reports. The presence of multiple Mn⁴⁺ sites is attributed to charge compensation that is required for Mn⁴⁺ on Al³⁺ sites. The results show that charge compensating defects have a large influence on the luminescence properties (e.g., spectra, QY, luminescence lifetime) of Mn⁴⁺-doped fluorides. Lowering of the QY by defect quenching is a problem for application of this class of Mn⁴⁺ phosphors. Finally, we investigated the thermal quenching behavior for M₃AlF₆:Mn⁴⁺. The luminescence quenching temperature of M₃AlF₆:Mn⁴⁺ 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, Mn⁴⁺-doped hexafluoroaluminates are a promising class of materials as their chemical and thermal stability is superior to the presently used commercial Mn⁴⁺ phosphors.