Quenching of the red Mn⁴⁺ luminescence in Mn⁴⁺-doped fluoride LED phosphors

Abstract

Red-emitting Mn⁴⁺-doped fluorides are a promising class of materials to improve the color rendering and luminous efficacy of white light-emitting diodes (w-LEDs). For w-LEDs, the luminescence quenching temperature is very important, but surprisingly no systematic research has been conducted to understand the mechanism for thermal quenching in Mn⁴⁺-doped fluorides. Furthermore, concentration quenching of the Mn⁴⁺ luminescence can be an issue but detailed investigations are lacking. In this work, we study thermal quenching and concentration quenching in Mn⁴⁺-doped fluorides by measuring luminescence spectra and decay curves of K₂TiF₆:Mn⁴⁺ between 4 and 600 K and for Mn⁴⁺ concentrations from 0.01% to 15.7%. Temperature-dependent measurements on K₂TiF₆:Mn⁴⁺ and other Mn⁴⁺-doped phosphors show that quenching occurs through thermally activated crossover between the ⁴T₂ excited state and ⁴A₂ ground state. The quenching temperature can be optimized by designing host lattices in which Mn⁴⁺ has a high ⁴T₂ state energy. Concentration-dependent studies reveal that concentration quenching effects are limited in K₂TiF₆:Mn⁴⁺ up to 5% Mn⁴⁺. This is important, as high Mn⁴⁺ concentrations are required for sufficient absorption of blue LED light in the parity-forbidden Mn⁴⁺ d–d transitions. At even higher Mn⁴⁺ concentrations (>10%), the quantum efficiency decreases, mostly due to direct energy transfer to quenching sites (defects and impurity ions). Optimization of the synthesis to reduce quenchers is crucial for developing more efficient highly absorbing Mn⁴⁺ phosphors. The present systematic study provides detailed insights into temperature and concentration quenching of Mn⁴⁺ emission and can be used to realize superior narrow-band red Mn⁴⁺ phosphors for w-LEDs.

Introduction

White light-emitting diodes (w-LEDs) are the next-generation light sources for display and illumination systems because of their small size, high luminous efficacy, and long operation lifetime^1,2,3,4,5. Conventional w-LEDs are composed of blue-emitting (In,Ga)N LEDs and green/yellow-emitting and orange/red-emitting phosphors that convert part of the blue LED emission^5,6,7. Both phosphors are necessary to generate warm white light with a high color rendering index (CRI > 85). The typical red phosphors in w-LEDs are Eu²⁺-doped nitrides (e.g., CaAlSiN₃:Eu²⁺)^4,8. These phosphors exhibit high photoluminescence (PL) quantum efficiencies (QEs > 90%), but their use also has a serious drawback. The Eu²⁺ emission band is broad and extends into the deep red spectral region (λ > 650 nm) where the eye sensitivity is low. This causes the luminous efficacy of the w-LED to drop (reduced lumen/W output). A worldwide search is therefore aimed at finding efficient narrow-band red-emitting phosphors that can be excited by blue light. In this search, Mn⁴⁺-doped fluoride phosphors, such as K₂SiF₆:Mn⁴⁺ and K₂TiF₆:Mn⁴⁺, have recently attracted considerable attention^{9,10,11,12,13}. Under blue light excitation, Mn⁴⁺-doped fluorides show narrow red line emission (λ_max ~ 630 nm) with high luminescence QEs^13,14,15,16. Furthermore, they are prepared through low-cost, simple wet-chemical synthesis at room temperature^11,17. These aspects make Mn⁴⁺-doped fluorides very promising red-emitting phosphors for developing energy-efficient high color-rendering w-LED systems⁹.

The application of Mn⁴⁺-doped fluoride phosphors in w-LEDs may, however, be hampered by thermal quenching of the Mn⁴⁺ luminescence. Thermal quenching of the phosphor luminescence is a serious issue, as it affects both the efficacy and color stability of the w-LED. In high-power w-LEDs, the temperature of the on-chip phosphor layer easily reaches 450 K. At these elevated temperatures, thermal quenching occurs for Mn⁴⁺-doped fluorides. The luminescence quenching temperature T_½, the temperature at which the emission intensity is reduced to half of its maximum, is typically between 400 and 500 K^15,18,19. Although the temperature dependence of the emission intensity has been measured for many Mn⁴⁺-doped fluorides, the understanding of the thermal quenching behavior is still limited. Most studies do not explain which process quenches the Mn⁴⁺ luminescence^{13,20,21,22,23}. Moreover, the few reports that do propose a quenching mechanism disagree. Paulusz¹⁵ states that the luminescence of Mn⁴⁺-doped fluorides is quenched by thermally activated crossing of the Mn^{4+ 4}T₂ excited state and ⁴A₂ ground state. In contrast, Dorenbos²⁴ finds a relation between the quenching temperature and the energy of the F⁻ → Mn⁴⁺ charge-transfer (CT) state and therefore suggests that quenching involves crossover between the CT state and ⁴A₂ ground state. This CT state crossover mechanism was also used by Blasse and our group to explain thermal quenching in Mn⁴⁺-doped oxides^25,26,27. Finally, other reports claim that the quenching temperature increases if the radius of the cation substituted by Mn⁴⁺ becomes smaller^11,18. A better understanding of the thermal quenching behavior is essential for developing Mn⁴⁺-doped fluoride phosphors with superior quenching temperatures, and thereby improving their potential for application in w-LEDs.

Besides thermal quenching, concentration quenching is an issue for the application of Mn⁴⁺-doped fluorides in w-LEDs. As the Mn⁴⁺ d–d transitions are parity-forbidden, high Mn⁴⁺ doping concentrations (e.g., 5 mol%) are required for sufficient absorption of the blue LED light¹². At high dopant concentrations, energy migration among the Mn⁴⁺ ions can result in concentration quenching^26,28, as is illustrated in Fig. 1. If the distance between the Mn⁴⁺ ions is small, excitation energy may efficiently migrate from one Mn⁴⁺ ion to another until it reaches a quenching site (defect or impurity ion), where the excitation energy is lost non-radiatively (as heat). Studies on concentration quenching in Mn⁴⁺-doped fluorides are limited. Several works have compared the luminescence properties of fluoride phosphors with varying Mn⁴⁺ concentrations, but do not measure the actual Mn⁴⁺ concentration in the phosphors by elemental analysis^{29,30,31,32,33}. Determining the Mn⁴⁺ concentration is crucial, as often only a fraction of the Mn⁴⁺ ions is incorporated during the synthesis^19,34. Reports that do perform elemental analysis study only a small range of Mn⁴⁺ doping concentrations and do not provide insight into the role of concentration quenching in Mn⁴⁺ doped fluorides^13,35,36. An in-depth investigation of concentration quenching in Mn⁴⁺-doped fluorides is thus lacking, despite it being very important for the application of Mn⁴⁺-doped fluorides in w-LEDs.

Fig. 1: Concentration quenching for Mn⁴⁺ in crystals.

At high Mn⁴⁺ doping concentrations the Mn⁴⁺ ions (orange) are in close proximity in the crystal lattice. If the Mn⁴⁺ ions are close together, energy transfer between Mn⁴⁺ ions (dark blue) causes the excitation to migrate through the crystal. Eventually, it may reach a quenching site such as a vacancy or impurity (dashed circle), where the excitation energy is lost as heat. This process competes with radiative emission (red) and reduces the luminescence efficiency

In this work, we systematically investigate concentration quenching and thermal quenching in Mn⁴⁺-doped fluorides. The quenching is studied by measuring luminescence spectra and decay curves in the temperature range of 4 to 600 K for K₂TiF₆:Mn⁴⁺ phosphors with Mn⁴⁺ concentrations ranging from 0.01 to 15.7 mol% (actual Mn⁴⁺ concentration). The temperature-dependent luminescence measurements of K₂TiF₆:Mn⁴⁺ and other Mn⁴⁺-doped phosphors demonstrate that thermal quenching occurs because of thermally activated crossover from the ⁴T₂ excited state to the ⁴A₂ ground state. This insight into the quenching mechanism shows that the Mn⁴⁺ quenching temperature can be raised by finding fluoride hosts that have an increased Mn^{4+ 4}T₂ level energy. Concentration studies show that the luminescence QE of K₂TiF₆:Mn⁴⁺ is high, ~80%, for doping concentrations up to 5 mol% Mn⁴⁺. Concentration quenching is limited for these relatively high Mn⁴⁺ dopant concentrations. At even higher doping concentrations of >10 mol%, the QE of K₂TiF₆:Mn⁴⁺ falls below 60%. Luminescence decay curves indicate that the drop in QE can be attributed to an increased probability for direct energy transfer to quenching sites (e.g., defects, impurity ions, Mn²⁺, and Mn³⁺), the concentration of which increases with the Mn⁴⁺ concentration. The present results provide an improved understanding of thermal quenching and concentration quenching in Mn⁴⁺-doped solids and can be used to develop superior Mn⁴⁺-doped fluoride phosphors for w-LEDs.

Materials and methods

Synthesis and characterization of K₂TiF₆:Mn⁴⁺ phosphors

The K₂TiF₆:Mn⁴⁺ (x%) phosphors were synthesized according to the method of Zhu et al.¹³ For the synthesis of K₂TiF₆:Mn⁴⁺ (0.8%), 0.0488 g of K₂MnF₆ (prepared following refs. ^37,38) was dissolved in 2.5 mL of a 40 wt% HF solution (Fluka, 40 wt% HF in water). Next, the obtained yellow-brown solution was mixed with 4.5730 g of K₂TiF₆ (Sigma-Aldrich, p.a.) and then stirred for 1 h at room temperature to form K₂TiF₆:Mn⁴⁺ crystals. The K₂TiF₆:Mn⁴⁺ phosphor was isolated by decanting the HF solution, washing twice with 15 mL of ethanol and then drying the phosphor for 7 h at 75 °C. The other K₂TiF₆:Mn⁴⁺ (x%) phosphors were prepared following the same procedure but using other amounts of K₂MnF₆ and K₂TiF₆ as to obtain different Mn⁴⁺ doping concentrations.

Powder X-ray diffraction (see Supplementary Figure S1) confirms that the K₂TiF₆:Mn⁴⁺ (x%) phosphors exhibit the hexagonal crystal structure of K₂TiF₆ up to the highest doping concentration of 15.7% Mn⁴⁺. Furthermore, no impurities of K₂MnF₆ or other crystal phases are observed in the diffraction patterns. Scanning electron microscopy (SEM) images show that most K₂TiF₆:Mn⁴⁺ phosphor particles are irregularly shaped and have sizes ranging from 1 to 200 µm (see Supplementary Figure S2a). Some particles have a hexagonal shape, in agreement with the hexagonal crystal structure of K₂TiF₆ (see Supplementary Figure S2b). Energy-dispersive X-ray (EDX) spectra (see Supplementary Figure S2c) confirm that the phosphor particles consist of potassium, titanium, fluorine, and manganese ions. The manganese dopant concentrations in the K₂TiF₆:Mn⁴⁺ phosphors were determined with inductively coupled plasma optical emission spectroscopy (ICP-OES). The ICP-OES measurements were performed on a Perkin-Elmer Optima 8300DV spectrometer (λ_em = 257.61 and 259.37 nm). For the ICP-OES analyses, the K₂TiF₆:Mn⁴⁺ phosphors were dissolved in aqua regia.

Optical spectroscopy

PL measurements were performed on an Edinburgh Instruments FLS920 fluorescence spectrometer, except for the PL decay measurements between 300 and 600 K (see below). For recording excitation and emission spectra, we used a 450 W Xe lamp as excitation source and a Hamamatsu R928 photomultiplier tube (PMT) with a grating blazed at 500 nm for detection of emission. For PL decay measurements, excitation was done with a tunable optical parametric oscillator (OPO) Opotek Opolette HE 355II laser (pulse width 10 ns, repetition rate 10 Hz) and emission was detected with a Hamamatsu H74220–60 PMT. The PL decay curves between 300 and 600 K were recorded on a different setup, which had an Ekspla NT 342B OPO laser (pulse width 5 ns, repetition rate 10 Hz) as excitation source and a 0.55 m Triax 550 monochromator combined with a Hamamatsu H74220–60 PMT for detection of emission. All PL decay curves were obtained by multi-channel scaling (MCS) with a PicoQuant TimeHarp 260 computer card. The K₂TiF₆:Mn⁴⁺ phosphors were cooled down to 4 K with an Oxford Instruments liquid helium flow cryostat. For PL measurements between 300 and 600 K samples were heated in a Linkam THMS600 temperature controlled stage. The PL quantum efficiencies of the phosphors 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).

Results and discussion

Luminescence of K₂TiF₆:Mn⁴⁺

For our quenching studies, we examine the luminescence of K₂TiF₆:Mn⁴⁺ phosphors with a wide range of Mn⁴⁺ doping concentrations. A photographic image of the K₂TiF₆:Mn⁴⁺ (x%) phosphors is displayed in Fig. 2a. The Mn⁴⁺ doping concentrations x (molar percentages with respect to Ti⁴⁺) were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). The body color of K₂TiF₆:Mn⁴⁺ becomes more yellow with increasing Mn⁴⁺ concentration as a result of enhanced absorption in the blue. All of the investigated K₂TiF₆:Mn⁴⁺ phosphors exhibit bright red Mn⁴⁺ luminescence under UV photoexcitation.

Fig. 2: Mn⁴⁺ luminescence of K₂TiF₆:Mn⁴⁺.

a Photographic image of K₂TiF₆:Mn⁴⁺ (x%) phosphors with x = 0.01, 0.1, 0.8, 1.3, 3.8, 5.4, 9.4, and 15.7. The phosphors have a white to yellow body color under ambient light (top) and show red Mn⁴⁺ luminescence under 365 nm UV illumination (bottom). b Tanabe−Sugano energy level diagram of the d³ electron configuration in an octahedral crystal field. The ⁴A₂ → ⁴T₁, ⁴A₂ → ⁴T₂, and ²E → ⁴A₂ transitions of Mn⁴⁺ are indicated by the purple, blue and red arrows, respectively. Note that the excitation transitions are displaced for clarity. For a specific coordination all transitions take place around the same crystal field Δ_O. c Emission spectrum of K₂TiF₆:Mn⁴⁺ (0.8%) upon excitation with blue light (λ_exc = 450 nm). d Excitation spectrum of the red Mn⁴⁺ luminescence (λ_em = 630 nm) from K₂TiF₆:Mn⁴⁺ (0.8%). Spectra are recorded at ambient temperature

Figure 2b depicts the Tanabe–Sugano energy level diagram of Mn⁴⁺ (3d³ electron configuration) in an octahedral crystal field^39,40. The diagram gives the d³ energy levels as a function of the crystal field splitting Δ_O. Due to its high effective positive charge, Mn⁴⁺ experiences a strong crystal field and therefore the ²E state is the lowest energy excited state. Hence, the emission spectrum of K₂TiF₆:Mn⁴⁺ (0.8%) is dominated by narrow red emission lines due to spin- and parity-forbidden ²E → ⁴A₂ transitions, as can be seen in Fig. 2c. The other K₂TiF₆:Mn⁴⁺ (x%) phosphors exhibit similar emission spectra. As the potential energy curves of the ²E and ⁴A₂ states are at the same equilibrium position, the ²E → ⁴A₂ emission is characterized by narrow zero-phonon and vibronic emission lines. The potential energy curves of the ²E and ⁴A₂ states are at the same equilibrium position because the ²E and ⁴A₂ states originate from the same $t_{{\mathrm{2g}}}^3$ electron configuration⁴¹.

The ²E → ⁴A₂ emission spectrum consists of a weak zero-phonon line (ZPL) at ~622 nm and more intense anti-Stokes and Stokes vibronic emissions (labeled ν₃, ν₄, and ν₆) on the high and low energy sides of the ZPL, respectively^13,15. The ZPL is very weak because Mn⁴⁺ is located on a site with inversion symmetry in K₂TiF₆:Mn⁴⁺. Due to the inversion symmetry, there are no odd-parity crystal field components to admix opposite parity states into the ⁴A₂ and ²E states and, as a result, the ²E → ⁴A₂ transition is electric dipole forbidden. The ²E → ⁴A₂ transition can become partly allowed, however, by coupling with asymmetric vibrations that induce odd-parity crystal field components. The most intense lines in Fig. 2c are assigned to ²E → ⁴A₂ transitions coupling with the asymmetric ν₃, ν₄, and ν₆ vibrational modes (phonons) of the ${\mathrm{MnF}}_6^{2 - }$ group. Thermal population of phonons at room temperature allows coupling with ν₃, ν₄, and ν₆ phonon modes in the ²E excited state (giving rise to the anti-Stokes lines), while transitions to these phonon modes in the ⁴A₂ ground state can occur at all temperatures (Stokes lines).

Figure 2d displays the excitation spectrum of the red Mn⁴⁺ luminescence from K₂TiF₆:Mn⁴⁺. The two broad excitation bands correspond to spin-allowed ⁴A₂ → ⁴T₁ and ⁴A₂ → ⁴T₂ transitions (violet and blue arrows in Fig. 2b). In addition, some weak peaks are visible around 600 nm. These peaks are assigned to ⁴A₂ → ²E and ⁴A₂ → ²T₁ transitions. The ⁴A₂ → ²T₁,²E transitions are spin-forbidden and therefore low in intensity compared to the spin-allowed ⁴A₂ → ⁴T₁,⁴T₂ transitions.

Temperature dependence of the Mn⁴⁺ luminescence

To study the thermal quenching of the Mn⁴⁺ emission, we measure the PL intensity and Mn⁴⁺ emission lifetime of K₂TiF₆:Mn⁴⁺ (0.01%) as a function of temperature between 4 and 600 K. We use a very low Mn⁴⁺ doping concentration of 0.01%, as for higher Mn⁴⁺ concentrations reabsorption of emission and energy transfer between Mn⁴⁺ ions can occur. These processes will influence (the temperature dependence of) the Mn⁴⁺ luminescence spectra and decay curves⁶. As a result, with a high concentration of Mn⁴⁺ ions, the observations may not reflect the intrinsic thermal quenching properties of Mn⁴⁺.

Figure 3a shows emission spectra of K₂TiF₆:Mn⁴⁺ (0.01%) at various temperatures between 4 and 600 K. At 4 K the Mn^{4+ 2}E → ⁴A₂ emission spectrum consists of zero-phonon and Stokes vibronic lines. Upon raising the temperature, phonon modes are thermally populated and anti-Stokes emission lines appear (solid arrow in Fig. 3a). With the appearance of anti-Stokes lines, the relative intensity of the Stokes emission decreases between 4 and 300 K. Above 400 K the intensities of both the anti-Stokes and Stokes emission lines begin to decrease (dashed arrow in Fig. 3a), which indicates the onset of non-radiative transitions from the ²E excited state. The luminescence is quenched at 600 K. From the measurements, we obtain the temperature dependence of the integrated PL intensity (I_PL) relative to the integrated PL intensity at room temperature (I_RT) (Fig. 3b). The PL intensity of K₂TiF₆:Mn⁴⁺ (0.01%) gradually increases between 4 and 350 K but then rapidly drops due to the onset of non-radiative transitions (luminescence quenching).

Fig. 3: Temperature dependence of the Mn⁴⁺ luminescence from K₂TiF₆:Mn⁴⁺ (0.01%).

a Emission spectra (λ_exc = 450 nm) of K₂TiF₆:Mn⁴⁺ (0.01%) at various temperatures between 0 and 600 K. b Integrated PL intensity of K₂TiF₆:Mn⁴⁺ (0.01%) as a function of temperature. The integrated PL intensity I_PL is scaled to the integrated PL intensity at room temperature I_RT. The red and green lines represent fits to Eqs. 6 and 7, respectively. c PL decay curves of the Mn⁴⁺ emission from K₂TiF₆:Mn⁴⁺ (0.01%) at various temperatures between 0 and 600 K (λ_exc = 450 nm and λ_em = 631 nm). d Temperature dependence of the Mn⁴⁺ emission lifetime for K₂TiF₆:Mn⁴⁺ (0.01%). The red and green lines represent fits to Eqs. 4 and 8, respectively. The cyan line gives the fit for Eq. 4 (red line) divided by two

An alternative method to determine the luminescence quenching temperature is by measuring luminescence decay times. Figure 3c shows a selection of PL decay curves of K₂TiF₆:Mn⁴⁺ (0.01%) measured between 4 and 600 K. The decay of the Mn⁴⁺ emission is single exponential and becomes faster with increasing temperature. The PL decay time is on the order of milliseconds, which is expected as the transition between the ²E and ⁴A₂ states is both parity- and spin-forbidden. In Fig. 3d, the Mn⁴⁺ emission lifetime (determined from single exponential fitting) is plotted as a function of temperature. The lifetime shows a steady decrease, starting above 50 K. The decrease levels off between 300 and 400 K but then shows a rapid decrease above 400 K.

The temperature dependences observed in Fig. 3b and d are quite exceptional. For most luminescent materials, the PL intensity and lifetime are relatively constant with temperature and both begin to decrease once thermal quenching sets in^6,42,43. The PL intensity of K₂TiF₆:Mn⁴⁺, however, rises by 40% between 4 and 350 K while the lifetime decreases before thermal quenching takes place. To understand this peculiar temperature dependence, we first discuss how the radiative decay rate of the ²E state changes with temperature. The ²E → ⁴A₂ emission of K₂TiF₆:Mn⁴⁺ mainly consists of anti-Stokes and Stokes vibronic emissions (Fig. 2c). Their transition probabilities increase with phonon population. The population of phonon modes is given by the phonon occupation number n, which increases with temperature according to⁴¹:

$$n = \frac{1}{{\exp \left( {hv/k_{\mathrm{B}}T} \right) - 1}}$$

(1)

where k_B is the Boltzmann constant and hν is the energy of the phonon coupling to the ²E → ⁴A₂ transition. The transition probabilities P_R of the anti-Stokes and Stokes vibronics scale with n by:

$${\mathrm{Anti-Stokes}}:\; P_{\mathrm{R}}\left( T \right) = P_{\mathrm{R}}\left( 0 \right)\left[ n \right]$$

(2)

$${\mathrm{Stokes}}:\;P_{\mathrm{R}}\left( T \right) = P_{\mathrm{R}}\left( 0 \right)\left[ {n + 1} \right]$$

(3)

where P_R(0) is the transition probability at T = 0 K. As the radiative lifetime τ_R is proportional to 1/[P_R(anti-Stokes) + P_R (Stokes)], it follows from Eqs. 1–3 that:

$$\tau _{\mathrm{R}}\left( T \right) = \frac{{\tau _{\mathrm{R}}\left( 0 \right)}}{{{\mathrm{coth}}(hv/2k_{\mathrm{B}}T)}}$$

(4)

Here, τ_R(0) is the radiative lifetime at T = 0 K. In Fig. 3d,Eq. 4 (red line) has been plotted for τ_R(0) = 12.3 ms and hν = 216 cm⁻¹ (phonon energy of the intense ν₆ mode emission). Equation 4 accurately describes the measured temperature dependence of the Mn⁴⁺ emission lifetime up to 375 K, confirming that the decay of the ²E state is mainly radiative up to this temperature. The radiative lifetime of the Mn⁴⁺ emission shortens with temperature due to thermal population of odd-parity vibrational modes at higher temperatures.

Next, we investigate the increase in PL intensity between 4 and 350 K. The PL intensity I_PL equals the product of the PL QE and number of absorbed photons (as I_PL scales with the number of absorbed photons, the excitation wavelength can have a large influence on the temperature dependence observed for I_PL; see Supplementary Information). The PL QE η of K₂TiF₆:Mn⁴⁺ can be expressed as:

$$\eta = \frac{{\gamma _{\mathrm{R}}}}{{\gamma _{\mathrm{R}} + \gamma _{{\mathrm{NR}}}}}$$

(5)

where γ_R and γ_NR are the radiative and non-radiative decay rates of the emitting ²E state, respectively. The results in Fig. 3d show that the decay of the ²E state is mainly radiative up to 375 K, so we can assume that γ_NR is negligible between 0 and 350 K. The value for η is therefore approximated as a constant close to unity between 0 and 350 K. On the other hand, the ⁴A₂ → ⁴T₂ absorption will change with temperature. Like the ²E → ⁴A₂ transition, the ⁴A₂ → ⁴T₂ transition is electric dipole (parity) forbidden and gains intensity by coupling with vibrations (for more details on the vibronic structure of the ⁴A₂ → ⁴T₂ excitation band, see refs. ^15,16,44). As a result, the PL intensity I_PL will scale with temperature as^20,41,45:

$$I_{{\mathrm{PL}}}\left( T \right) = I\left( 0 \right){\mathrm{coth}}\left( {\frac{{hv}}{{2k_{\mathrm{B}}T}}} \right)$$

(6)

with I(0) being the PL intensity at T = 0 K. The results in Fig. 3b show that the increase in PL intensity between 4 and 350 K follows the temperature dependence given by Eq. 6. This confirms that the higher PL intensity at 350 K is due to a stronger absorption of excitation light. An increase in PL intensity between 4 and 350 K due to enhanced absorption is observed for all investigated Mn⁴⁺ doping concentrations (see Supplementary Information). Although the temperature dependence of the PL intensity follows Eq. 6, there is deviation between the fit of Eq. 6 and the measured data (see red line in Fig. 3b). The model of Eq. 6 is simple and does not take into account the shift and broadening of the ⁴A₂ → ⁴T₂ absorption band with temperature. Both these effects also influence the temperature dependence of the PL intensity, and this can explain the deviation between the model and the experimental data. Including the effect of a shift and broadening of the ⁴A₂ → ⁴T₂ band on the absorption strength is complex and will not aid a more accurate determination of T_½.

Above 400 K the PL intensity of K₂TiF₆:Mn⁴⁺ (0.01%) begins to decrease due to the onset of non-radiative transitions (Fig. 3a, b). The non-radiative decay probability rapidly increases with temperature above 400 K and as a result the luminescence is quenched, with no emission intensity remaining at 600 K. The quenching temperature T_½ is determined to be 462 K. The Mn⁴⁺ emission lifetime also rapidly decreases once thermal quenching sets in (Fig. 3d). Above 400 K the Mn⁴⁺ emission lifetime is shorter than the radiative lifetime τ_R predicted by Eq. 4 (red line). The lifetime shortens because of an additional thermally activated non-radiative contribution to the decay of the ²E state. From the temperature dependence of the lifetime, T_½ can be determined by locating the temperature at which the lifetime has decreased to half of its radiative lifetime value. To estimate T_½, we divide the value from the fit of Eq. 4 for τ_R by a factor of 2 (Fig. 3d, cyan line). The cyan line crosses the data points at 457 K. This value for T_½ is very close to the T_½ of 462 K obtained from the PL intensity measurements.

Thermal quenching can be described as a thermally activated process with an activation energy ΔE. The activation energy is obtained by fitting a modified Arrhenius equation to the temperature dependence of the PL intensity I_PL between 350 and 600 K^43,46:

$$I_{{\mathrm{PL}}}\left( T \right) = \frac{{I\left( 0 \right)}}{{1 + A \times {\mathrm{exp}}\left( { - \Delta E/k_{\mathrm{B}}T} \right)}}$$

(7)

In Eq. 7, I(0) is the maximum PL intensity, k_B is the Boltzmann constant and A is a rate constant for the thermal quenching process. The best fit to Eq. 7 (green line in Fig. 3b) gives an activation energy ∆E of 9143 cm⁻¹ and a rate constant A of 2.5 × 10¹². We can also determine ∆E by fitting the temperature dependence of the Mn⁴⁺ emission lifetime τ(T) to the following expression⁴⁷:

$$\tau \left( T \right) = \frac{{\tau _{\mathrm{R}}\left( T \right)}}{{{\mathrm{1}} + \left( {\frac{{\tau _{\mathrm{R}}\left( T \right)}}{{\tau _{{\mathrm{NR}}}}}} \right){\mathrm{exp}}\left( { - \Delta E/k_{\mathrm{B}}T} \right)}}$$

(8)

Here, 1/τ_NR is the non-radiative decay rate and τ_R(T) is the radiative lifetime as described by Eq. 4 with τ_R(0) = 12.3 ms and hν = 216 cm⁻¹. We fit Eq. 8 to the Mn⁴⁺ emission lifetimes (green line in Fig. 3d) and find an activation energy ∆E of 7100 cm⁻¹ and a prefactor 1/τ_NR of 1.5 × 10¹² s⁻¹. On the basis of the two similar values for ∆E, we conclude that the activation energy of the thermal quenching process is ~8000 cm⁻¹. The rate constants A and 1/τ_NR should be approximately equal to the vibrational frequencies of the ${\mathrm{MnF}}_6^{2 - }$ group. The ν₆ vibrational mode has a frequency of 6.5 × 10¹² s⁻¹, close to the rate constants found by fitting the data to Eqs. 7 and 8. The variation in activation energy values and prefactors can be explained by the fact that thermal quenching is not a simple thermally activated process. Struck and Fonger have shown that the temperature dependence of a non-radiative process is accurately described by considering ground and excited state vibrational wave function overlap^46,48. According to the Struck–Fonger model, the non-radiative process occurs through tunneling (crossover) from a vibrational level of the excited state to a high vibrational level of the ground state. The tunneling rate, i.e., the non-radiative decay rate, depends on the wave function overlap of the vibrational levels involved. The tunneling rate will be faster for a larger overlap between the wave functions and when the vibrational levels are in resonance. For the present discussion, analysis of the data using complex models such as the Struck–Fonger model is not relevant, but it is important to realize that the Struck–Fonger model gives a more correct description of the actual quenching process.

Thermal quenching in Mn⁴⁺-doped fluorides

To obtain insight into the thermal quenching of Mn⁴⁺ luminescence, we will discuss four possible quenching processes: (1) multi-phonon relaxation, (2) thermally activated photoionization, (3) thermally activated crossover via the F⁻ → Mn⁴⁺ charge-transfer (CT) state, and (4) thermally activated crossover via the Mn^{4+ 4}T₂ excited state.

In the configurational coordinate diagram, the parabolas of the Mn^{4+ 2}E and ⁴A₂ states do not cross and luminescence quenching by crossover from the ²E to the ⁴A₂ states is not possible (Fig. 4a). The ⁴A₂ ground state may however be reached by multi-phonon relaxation. In Mn⁴⁺-doped fluorides more than 30 phonons of ~500 cm⁻¹ are needed to bridge the energy gap between the ²E and ⁴A₂ states⁴⁹. For such high numbers of phonons (p > 30), it is unrealistic that non-radiative multi-phonon relaxation is responsible for thermal quenching (see Supplementary Information for a more detailed discussion). Alternatively, the thermal quenching can be due to thermally activated photoionization of an electron from the Mn^{4+ 2}E state to the fluoride host conduction band. Thermally activated photoionization typically quenches the emission from a luminescent center if the emitting state is close in energy to the host conduction band^26,50. In density functional theory (DFT) calculations, large band gaps of around 8 eV have been found for fluoride hosts like K₂SiF₆ and K₂TiF₆^51,52. It is therefore expected that the Mn^{4+ 2}E state is well below the host conduction band levels. Based on this, we conclude that thermal quenching in Mn⁴⁺-doped fluorides is not caused by thermally activated photoionization. However, more evidence is necessary to exclude this quenching mechanism. Photoconductivity measurements on Mn⁴⁺ phosphors at elevated temperatures need to be performed to provide convincing evidence for a possible role of photoionization in the thermal quenching of Mn⁴⁺ emission.

Fig. 4: Thermal quenching in Mn⁴⁺-doped fluorides.

a, b Configuration coordinate diagrams showing luminescence quenching due to a thermally activated crossover via the F⁻ → Mn⁴⁺ charge-transfer (CT) state and b thermally activated crossover via the Mn^{4+ 4}T₂ excited state. c Quenching temperature T_½ of Mn⁴⁺-doped fluoride phosphors as a function of the ⁴A₂ → ⁴T₂ transition energy. The red dashed line is a linear fit to the data points. d Quenching temperature T_½ of Mn⁴⁺-doped fluorides (blue dots) and Mn⁴⁺-doped oxides (red dots) as a function of the ⁴A₂ → ⁴T₂ transition energy

Thermal quenching in Mn⁴⁺-doped fluorides has been suggested to occur by thermally activated crossover via the Mn^{4+ 4}T₂ state or the F⁻ → Mn⁴⁺ charge-transfer (CT) state^15,24,26. Both these states are displaced relative to the potential curve of the ⁴A₂ ground state (Fig. 4a, b). Hence, the ⁴T₂ and CT state parabolas cross the ⁴A₂ ground state parabola. The difference between the potential curve equilibrium positions is given by the offset ∆R = R₀′ − R₀. By using the energies of the ⁴A₂ → ²E, ⁴A₂ → ⁴T₂ and ⁴A₂ → CT transitions in K₂TiF₆:Mn⁴⁺ (Fig. 2d and ref. ¹³) and assuming specific offsets ∆R for the ⁴T₂ and CT states, we can construct the diagrams in Fig. 4a and b, where non-radiative relaxation occurs either via (a) the crossing of the CT and ⁴A₂ states or (b) the crossing of the ⁴T₂ and ⁴A₂ states. The offset of the CT state is typically larger than the offset of the ⁴T₂ state. Note that the diagrams in Fig. 4a and b are schematic configuration coordinate diagrams to illustrate the different quenching mechanisms.

In Fig. 4a, the CT state has a larger offset ∆R than the ⁴T₂ state, which causes the CT parabola to cross the ⁴A₂ parabola at lower energies than the ⁴T₂ parabola. Thermal activation over the energy barrier ∆E will allow crossover from the ²E state into the CT state followed by non-radiative relaxation to the ground state via the crossing of the CT and ⁴A₂ parabolas. Alternatively, thermal quenching of the Mn⁴⁺ luminescence may be due to the mechanism depicted in Fig. 4b. Here, the CT state has a smaller offset ∆R compared to that shown in Fig. 4a, and its potential curve is therefore at higher energies. In addition, the ⁴T₂ state has a slightly larger offset. As a result, the crossing of the ⁴T₂ and ⁴A₂ parabolas is now at a lower energy and non-radiative relaxation will proceed via the crossing of the ⁴T₂ and ⁴A₂ parabolas.

The activation energies ∆E in the configuration coordinate diagrams are ~8000 cm⁻¹, similar to the ∆E values obtained from the temperature-dependent measurements. This indicates that both mechanisms in Fig. 4a, b can explain the thermal quenching of Mn⁴⁺ luminescence. To determine which of these two mechanisms is responsible for the luminescence quenching, we compare the quenching temperature T_½ of K₂TiF₆:Mn⁴⁺ to the T_½ of other Mn⁴⁺-doped materials. A relation between the quenching temperature and the energy of either the CT or ⁴T₂ state in a variety of hosts will give insight. If quenching occurs by crossover from the CT state to the ⁴A₂ state, T_½ will be higher for Mn⁴⁺-doped solids with higher CT transition energies. In K₂TiF₆:Mn⁴⁺ and other Mn⁴⁺-doped fluorides the F⁻ → Mn⁴⁺ CT transition is at ~40,000 cm^−113,15. Mn⁴⁺-doped oxides have lower O²⁻ → Mn⁴⁺ CT transition energies of 30,000–35,000 cm⁻¹ and are therefore expected to have lower T_½ values than fluorides if quenching occurs by the mechanism in Fig. 4a^26,27,53,54. Some Mn⁴⁺-doped oxides, however, have much higher quenching temperatures than Mn⁴⁺-doped fluorides. For example, Mg₄GeO₆:Mn⁴⁺, Mg₂₈Ge_7.5O₃₈F₁₀:Mn⁴⁺, and Mg₆As₂O₁₁:Mn⁴⁺ have a T_½ of ~700 K^55,56,57, while K₂TiF₆:Mn⁴⁺ and other Mn⁴⁺-doped fluorides have a T_½ of 400–500 K (see also Tables 1 and 2). No correlation is found between the Mn⁴⁺ luminescence quenching temperature and the energy of the CT transition (see Supplementary Information for an overview and a plot of quenching temperatures and CT energies). From this we conclude that thermal quenching in Mn⁴⁺-doped fluorides is not caused by thermally activated crossover from the F⁻ → Mn⁴⁺ CT state to the ⁴A₂ ground state.

Table 1 Quenching temperature T_½ (K) and ⁴A₂ → ⁴T₂ energy (cm⁻¹) for Mn⁴⁺-doped fluoride materials

Table 2 Quenching temperature T_½ (K) and ⁴A₂ → ⁴T₂ energy (cm⁻¹) for Mn⁴⁺-doped oxide materials

Alternatively, thermal quenching of the Mn⁴⁺ luminescence can be caused by thermally activated crossover via the Mn^{4+ 4}T₂ excited state (Fig. 4b). To investigate the validity of this mechanism, we compare the T_½ and ⁴A₂ → ⁴T₂ transition energies for K₂TiF₆:Mn⁴⁺ and a variety of other Mn⁴⁺-doped fluorides. From the literature and measurements on Mn⁴⁺ luminescence we have collected quenching temperatures and luminescence spectra, preferably for systems with low doping concentrations. Figures 2d and 3b show that K₂TiF₆:Mn⁴⁺ has a ⁴A₂ → ⁴T₂ energy of 21,459 cm⁻¹ (maximum of the excitation band) and a T_½ of 462 K. For K₂SiF₆:Mn⁴⁺, we measured a ⁴A₂ → ⁴T₂ energy of 22,099 cm⁻¹ and a T_½ of 518 K (Supplementary Figure S6, K₂SiF₆:Mn⁴⁺ BR301-C commercial phosphor from Mitsubishi Chemical, Japan). In Fig. 4c we plot the quenching temperature T_½ against the ⁴A₂ → ⁴T₂ energy for K₂TiF₆:Mn⁴⁺, K₂SiF₆:Mn⁴⁺ and many other Mn⁴⁺-doped fluoride phosphors reported in the literature (displayed data also listed in Table 1). The data show that the T_½ increases with the energy of the ⁴T₂ state. The clear trend shows that the thermal quenching in Mn⁴⁺-doped fluorides is due to thermally activated crossover from the ⁴T₂ excited state to the ⁴A₂ ground state. Further confirmation for this quenching mechanism is provided by Mn⁴⁺ spectra measured at elevated temperatures (see Supplementary Information). Supplementary Figure S7 shows emission spectra of K₂SiF₆:Mn⁴⁺ at T = 573 and 673 K. At 573 K a broad ⁴T₂ → ⁴A₂ emission band is observed, which is almost completely quenched at 673 K. The initial rise of the ⁴T₂ → ⁴A₂ emission at elevated temperatures confirms thermal population of the ⁴T₂ level, which eventually leads to thermal quenching of all Mn⁴⁺ emission via this state.

To investigate whether thermally activated crossing via the ⁴T₂ state is also responsible for temperature quenching in Mn⁴⁺-doped oxides, we extend the data set of Fig. 4c with quenching temperatures reported for Mn⁴⁺-doped oxides. Figure 4d shows the quenching temperature T_½ as a function of the ⁴A₂ → ⁴T₂ energy for the Mn⁴⁺-doped fluorides and oxides listed in Tables 1 and 2. The results show that T_½ increases with the energy of the ⁴A₂ → ⁴T₂ transition. This indicates that the Mn⁴⁺ emission in fluorides and oxides are both quenched due to thermally activated crossover from the ⁴T₂ excited state, and not the CT state as previously suggested in some reports^24,25,26,27. The present results and analysis provide strong evidence that in many Mn⁴⁺ phosphors the thermal quenching mechanism involves thermally activated crossover via the ⁴T₂ excited state. A contribution from other mechanisms cannot be ruled out and further research, for example, photoconductivity measurements and high pressure studies, can give additional information on the role of alternative quenching mechanisms.

As quenching occurs by thermally activated crossover via the ⁴T₂ excited state, the quenching temperature T_½ of the Mn⁴⁺ luminescence is controlled by the energy of the Mn^{4+ 4}T₂ state (the dependence of T_½ on the energy of the ⁴T₂ state is shown in Fig. 4c,d). In addition, the T_½ of the Mn⁴⁺ luminescence depends on the offset ∆R between the ⁴T₂ and ⁴A₂ states, as ∆R also determines where the ⁴T₂ and ⁴A₂ states cross in the configuration coordinate diagram (Fig. 4a,b). The horizontal displacement of the ⁴T₂ parabola will influence the quenching temperature. A variation in ∆R can explain the spread observed in the data of Fig. 4c and d. To investigate the variation in the offset ∆R for Mn⁴⁺-doped fluorides, we compare the bandwidth of the ⁴A₂ → ⁴T₂ excitation band in K₂TiF₆:Mn⁴⁺, K₂SiF₆:Mn⁴⁺ and Cs₂HfF₆:Mn⁴⁺ (see Supplementary Figure S9). The width of the ⁴A₂ → ⁴T₂ excitation band is controlled by the displacement of the ⁴T₂ state and therefore gives a good indication of ∆R. Comparison of the ⁴A₂ → ⁴T₂ bandwidths shows that there is a variation in ∆R for Mn⁴⁺-doped fluorides. The variation in ∆R is small, however, compared to the differences in the ⁴T₂ energy, and no correlation is observed between the spectral width and quenching temperatures. This indicates that the ⁴T₂ level energy has the largest influence on the quenching temperature of Mn⁴⁺-doped fluorides.

Finally, in view of applications, it is interesting to see how we can control the ⁴T₂ level energy (and thereby T_½) through the choice of the host lattice. The energy of the Mn^{4+ 4}T₂ state depends on the crystal field splitting Δ_O (Fig. 2b), where Δ_O is typically larger for shorter Mn–F distances^44,58. For Mn⁴⁺-doped fluorides the luminescence quenching temperature can therefore be raised by selecting host lattices with short M⁴⁺–F⁻ distances (see Supplementary Figure S10a). This is consistent with findings that T_½ increases if the radius of the M⁴⁺ host cation decreases, as expected based on crystal field theory^11,18. If, however, T_½ is plotted against the M⁴⁺-ligand distance for both Mn⁴⁺-doped fluorides and Mn⁴⁺-doped oxides (see Supplementary Figure S10b), no correlation between T_½ and the M⁴⁺-ligand distance is found. This shows that the crystal field splitting and ⁴T₂ energy give a better indication of the quenching temperature for Mn⁴⁺-doped phosphors.

Concentration quenching

In addition to insight into thermal quenching, concentration quenching in Mn⁴⁺-doped fluorides is important for application in w-LEDs. The weak parity-forbidden ⁴A₂ → ⁴T₂ absorption requires that commercial phosphors have high Mn⁴⁺ concentrations. If there is effective concentration quenching, the PL decay time and QE will decrease when the Mn⁴⁺ doping concentration is raised^26,28. We therefore investigate concentration quenching in K₂TiF₆:Mn⁴⁺ by measuring the PL decay times and QEs of K₂TiF₆:Mn⁴⁺ phosphors with Mn⁴⁺ concentrations ranging from 0.01 to 15.7% Mn⁴⁺.

Figure 5a presents room-temperature PL decay curves of the Mn⁴⁺ emission from K₂TiF₆:Mn⁴⁺ with increasing Mn⁴⁺ doping concentration x. It can be seen that the PL decay becomes slightly faster as the Mn⁴⁺ concentration increases. We analyze the decay dynamics by single exponential fitting of the PL decay curves. The fit for K₂TiF₆:Mn⁴⁺ (0.8%) is shown in Fig. 5b. The fit residuals (bottom panel) are random and the PL decay thus resembles a single exponential. This indicates that the decay of the ²E state is mainly radiative. Consequently, the K₂TiF₆:Mn⁴⁺ (0.8%) phosphor has a very high QE of 90%. Figure 5c gives an overview of the fitted decay times (blue squares) and QEs (red dots) of K₂TiF₆:Mn⁴⁺ with different Mn⁴⁺ concentrations. The emission lifetime barely shortens if the Mn⁴⁺ concentration is increased (5.7 ms for 0.01% Mn⁴⁺ to 5.4 ms for 15.7% Mn⁴⁺). This suggests that energy migration to quenching sites is inefficient in K₂TiF₆:Mn⁴⁺. To verify this, we look at the QE values obtained for the K₂TiF₆:Mn⁴⁺ (x%) phosphors. The QE remains above 80% for Mn⁴⁺ doping concentrations of 5% or less, which shows that concentration quenching is indeed limited up to a concentration of 5% Mn⁴⁺ ions. This result is important for applications in w-LEDs, as these high Mn⁴⁺ doping concentrations (e.g., 5 mol%) are required for sufficient absorption of the blue LED light in the parity-forbidden d–d transitions¹².

Fig. 5: Luminescence decay and quantum efficiency of K₂TiF₆:Mn⁴⁺ as a function of the Mn⁴⁺ doping concentration.

a Room-temperature PL decay curves of the Mn⁴⁺ emission from K₂TiF₆:Mn⁴⁺ (x%) for 0.01% (pink), 0.1% (blue), 0.8% (green), 1.3% (orange), 3.8% (purple), 5.4% (cyan), 9.4% (yellow), and 15.7% (red) Mn⁴⁺ (λ_exc = 450 nm and λ_em = 631 nm). b PL decay curve of K₂TiF₆:Mn⁴⁺ (0.8%) at T = 298 K. The decay time corresponding to the mono-exponential fit (red line) is 5.6 ms. The bottom panel shows the fit residuals. c Mn⁴⁺ emission lifetime (blue squares) and PL quantum efficiency (red dots) of K₂TiF₆:Mn⁴⁺ with different Mn⁴⁺ doping concentrations. d, e PL decay curves of K₂TiF₆:Mn⁴⁺ (15.7%) at d T = 298 K and e T = 4 K. The decay times corresponding to the mono-exponential fits (red lines) are 5.4 and 10.6 ms, respectively. The bottom panels show the fit residuals

For higher Mn⁴⁺ concentrations (x > 10%), non-radiative decay from the ²E excited state becomes stronger, however, and as a result the QE of K₂TiF₆:Mn⁴⁺ falls below 60% (Fig. 5c). The non-radiative decay is also visible in the PL decay curve of K₂TiF₆:Mn⁴⁺ (15.7%), shown in Fig. 5d. The decay is multi-exponential, which proves that with 15.7% Mn⁴⁺ the ²E state decays both radiatively and non-radiatively. The faster initial decay indicates that there is enhanced quenching by single-step energy transfer for Mn⁴⁺ ions close to a quencher. In case of energy migration, a faster decay is also expected for longer times after the excitation pulse. As this is not observed, the contribution of energy migration via many Mn⁴⁺ ions to quenching sites seems to be small.

To further investigate the role of energy migration in the concentration quenching of the Mn⁴⁺ emission, we measure a PL decay curve of K₂TiF₆:Mn⁴⁺ (15.7%) at T = 4 K, which is displayed in Fig. 5e. At T = 4 K energy migration among the Mn⁴⁺ ions (blue arrows in Fig. 1) will be hampered, as there is almost no spectral overlap between the Mn^{4+ 2}E → ⁴A₂ emission and ⁴A₂ → ²E excitation lines (see Supplementary Figure S11). Hence, at 4 K non-radiative decay due to energy migration to quenching sites will be suppressed. The Mn⁴⁺ decay dynamics in Fig. 5e, however, show that the non-radiative decay is not suppressed at 4 K. The deviation from single exponential behavior is similar to that at 300 K. There is an initial faster decay (single-step energy transfer to quenching sites) followed by an exponential decay with a decay time very close to that measured for Mn⁴⁺ at low doping concentrations. This suggests that the decrease in QE at higher Mn⁴⁺ concentrations is not due to energy migration. The absence of strong concentration quenching by energy migration is confirmed by the thermal quenching behavior measured for the different Mn⁴⁺ concentrations. In Supplementary Figure S4, it can be seen that the luminescence quenching temperature is approximately the same for doping concentrations of 0.01% and 15.7% Mn⁴⁺, which shows that effects due to thermally activated energy migration (i.e., concentration quenching) are weak. Hence, we conclude that the non-radiative decay at high Mn⁴⁺ concentrations is not caused by energy migration. Inefficient energy migration can be understood based on the strongly forbidden character of the ²E → ⁴A₂ transition. This allows only Mn⁴⁺–Mn⁴⁺ energy transfer via short range exchange interaction (see Supplementary Information for details).

We instead assign the non-radiative decay to direct transfer of excitation energy from Mn⁴⁺ ions to quenchers (green arrow in Fig. 1). This process can occur at all temperatures and becomes more efficient at higher Mn⁴⁺ dopant concentrations. With an increasing Mn⁴⁺ dopant concentration, the stress on the K₂TiF₆ lattice grows and as a result more crystal defects (i.e., quenchers) may be formed. In addition, Mn in different valence states (Mn²⁺ and Mn³⁺) may be incorporated at higher Mn⁴⁺ concentrations. Even if a very small fraction of Mn⁴⁺ ions has a different valence state than 4+, effective quenching can occur via metal-to-metal charge-transfer states or direct energy transfer. Consequently, the probability for energy transfer to quenchers increases, resulting in faster initial PL decay and lower QEs for K₂TiF₆:Mn⁴⁺ at high Mn⁴⁺ dopant concentrations. Optimized synthesis procedures to reduce quenchers (defects and impurity ions) are thus crucial for obtaining highly luminescent Mn⁴⁺-doped fluoride phosphors (see also recent work of Garcia-Santamaria et al.⁵⁹ on concentration quenching in K₂SiF₆:Mn⁴⁺).

Conclusions

Narrow-band red-emitting Mn⁴⁺ phosphors form an important new class of materials for LED lighting and displays. For these applications, it is important to understand and control the luminescence efficiency. We have therefore investigated quenching of the Mn⁴⁺ luminescence in Mn⁴⁺-doped fluorides by measuring the PL intensity and luminescence lifetimes of K₂TiF₆:Mn⁴⁺ between 4 and 600 K and for Mn⁴⁺ concentrations from 0.01 to 15.7%. Temperature-dependent measurements of the Mn⁴⁺ emission intensity and lifetime for K₂TiF₆:Mn⁴⁺ and other Mn⁴⁺-doped phosphors show that thermal quenching is caused by thermally activated crossover via the Mn^{4+ 4}T₂ excited state. As a result, the quenching temperature is higher in Mn⁴⁺-doped materials with higher ⁴T₂ state energies. These findings can be used to engineer Mn⁴⁺-doped fluoride phosphors with higher quenching temperatures for application in high-power w-LEDs.

Furthermore, quantum efficiency and luminescence decay measurements for a wide range of Mn⁴⁺ doping concentrations show that no concentration quenching occurs up to 5% Mn⁴⁺ in K₂TiF₆:Mn⁴⁺. This is important for the application of Mn⁴⁺-doped materials in w-LEDs, as high Mn⁴⁺ doping concentrations (e.g., 5 mol%) are required for sufficient absorption of the blue LED light in the parity-forbidden Mn⁴⁺ d–d transitions. At very high Mn⁴⁺ doping concentrations (>10 mol%) the quantum efficiency of K₂TiF₆:Mn⁴⁺ decreases due to enhanced direct energy transfer from Mn⁴⁺ to quenching sites. Concentration quenching by Mn⁴⁺–Mn⁴⁺ energy migration is limited. To optimize the efficiency in highly doped Mn⁴⁺ phosphors, a synthesis procedure aimed at reducing quenching sites (defects, impurity ions, Mn²⁺, and Mn³⁺) will be crucial.