Crystal structure, morphology and enhancement of luminescence properties of Cr3+-doped KZnF3 phosphor by using a HF-free hydrothermal method with different chromium sources

In this study, a series of near-infrared (NIR) KZnF3:Cr3+ phosphors was prepared by different chromium sources using a HF-free hydrothermal method. The influence of different chromium sources on the crystal structure, morphology, and luminescence properties of Cr3+-doped KZnF3 phosphors were systematically investigated. The results showed that the chromium source changed from CrF3·xH2O to (NH4)3CrF6 and leaded to an increased crystal field, resulting in a blue shift of the emission peak position from 803 nm to 753 nm, accompanied by the full-width half maximum (FWHM) reduced from 140 nm to 122 nm. Moreover, KZnF3:0.03Cr3+ (CrF3·xH2O) and KZnF3:0.03Cr3+ ((NH4)3CrF6) phosphors maintained 58% and 87% of their initial room-temperature intensity at 423 K, respectively. These results indicated that altering the synthetic raw materials provided new insights for designing NIR phosphors with highly thermal stability.


Introduction
Near-infrared (NIR) light has drawn great attention as a non-destructive and convenient technique for various applications, including medical diagnostics, information encryption, food quality inspection and night vision [1][2][3].NIR (700-1400 nm) is invisible light and its unique characteristics, such as chemical absorption, high penetration depth and invisibility to the human eyes, offer immense potential [4][5][6][7].However, traditional NIR light sources, such as incandescent bulbs and halogen lamps, which exhibit a broad spectrum ranging from visible to NIR wavelengths, fail to meet the requirements of commercial applications due to their low efficiency, large physical dimensions and short lifetime [8][9][10].In contrast, NIR-emitting phosphor-converted lightemitting diodes (pc-LEDs) have gained significant attention as a novel NIR light source, addressing some of the limitations of NIR LEDs by improving efficiency and compactness [11][12][13][14].The performance of pc-LEDs crucially depends on the quantum efficiency and thermal stability of the used NIR luminescent materials.Therefore, there is an urgent need to develop high-performance broadband NIR phosphors that exhibit exceptional efficiency as well as excellent thermal stability.Thermal stability is of paramount importance for the long-term stable operation and performance retention of NIR light sources.Hence, the development of broadband NIR luminescent materials with exceptional thermal stability holds great significance in advancing the field of pc-LEDs and facilitating their widespread applications in various domains.
In this work, a series of KZnF 3 :Cr 3+ NIR phosphors was prepared by different chromium sources through a HF-free hydrothermal method.The crystal structures, morphologies and luminescence properties were systematically investigated.The small size of KZnF 3 :Cr 3+ phosphors made them suitable for application in NIR phosphor converted micro-LEDs [28].These two compounds exhibited a broad NIR emission ascribed to the Cr 3+ : 4 T 2 → 4 A 2 transition under blue light excitation.With the change of the chromium source from CrF 3 •xH 2 O to (NH 4 ) 3 CrF 6 , the emission spectra showed a blue shift from 803 to 753 nm, accompanied by FWHM decreased from 140 to 122 nm.The thermal stability of KZnF 3 :0.03Cr3+ ((NH 4 ) 3 CrF 6 ) was significantly improved with the narrower FWHM.KZnF 3 :0.03Cr3+ ((NH 4 ) 3 CrF 6 ) exhibited high thermal stability at 423 K with an intensity of 87% compared with that at room temperature.The replacement of CrF 3 •xH 2 O with (NH 4 ) 3 CrF 6 presented promising industrial prospects for mass-producing NIR phosphors with reduced thermal quenching effects.

Synthesis
A series of KZnF 3 :Cr 3+ samples was synthesized by a typical hydrothermal method.Cr 3+ -doped KZnF 3 phosphors were obtained by varying chromium sources.
2.2.1.Preparation of KZnF 3 :Cr 3+ using CrF 3 •xH 2 O as the chromium sources.Taking Cr 3+ -doped KZnF 3 :0.03Cr3+ as an example, 2 * 10 −2 mol of KF, 2.5 * 10 −3 mol of C 4 H 6 O 4 Zn and 7.5 * 10 −5 mol of CrF 3 •xH 2 O were weighed and dissolved in 10 ml of water with the assistance of stirring to form a uniform solution.Afterward, the mixture was transferred to a 30 ml Teflon-lined autoclave and heated for 8 h at 220 ℃.Then, the system was allowed to cool down naturally to room temperature.The precipitates were then collected and washed several times with ethanol by centrifugation, followed by drying at 70 ℃ for 5 h in an oven to obtain the final products.The Cr 3+ -doped KZnF 3 samples with different concentrations can be obtained by varying the mole ratios of CrF 3 •xH 2 O and KZnF 3 .
2.2.2.Preparation of KZnF 3 :Cr 3+ using (NH 4 ) 3 CrF 6 as the chromium sources The first step is to prepare the precursor Cr 3+ source ((NH 4 ) 3 CrF 6 which was synthesized by the Song's method [24].In a specific synthesis, 20 mmol of Cr(NO 3 ) 3 •9H 2 O and 240 mmol of NH 4 HF 2 were thoroughly dissolved in 10 ml of deionized water in a plastic beaker, and then the system was stirred for 2 h at 200 ℃.Afterward, the products were collected and washed with ethanol for several times.Finally, the obtained precipitates were dried at 70 °C for 8 h to obtain (NH 4 ) 3 CrF 6 .The following step was just like 2.2.1, using (NH 4 ) 3 CrF 6 instead of CrF 3 •xH 2 O.

Characterization
Powder x-ray diffraction (XRD) patterns of the samples were collected in a 2θ range of 10 to 80 °by an x-ray diffractometer (Miniflex 600, Rigaku, Cu Kα1 radiation, λ = 0.154187 nm) operated at 40 kV, 40 mA.The sample morphologies were analyzed by a field-emission scanning electron microscopy (SEM) (Thermo Scientific, Apreo S) equipped with an energy dispersive spectroscopy (EDS) analyzer.Photoluminescence excitation (PLE) spectra of these phosphors were recorded by a steady-state fluorescence spectrometer (Edinburgh Instruments FS5).The photoluminescence emission (PL) spectra were collected using a spectrometer (Ocean Optics, QE-Pro).The diffuse reflection spectra (DRS) were collected by an UV-vis-NIR Spectrophotometer (Hitachi UH4150).The temperature dependent PL spectra were measured from 298 K to 523 K by Ocean Optics in conjunction with a temperature-controlled sample stage (HZ instruments, RT600).S1(b)).To investigate the variation of XRD patterns of the prepared samples with different chromium sources, XRD patterns of KZnF 3 :0.03Cr3+ were compared in figure S2.With the exception of their positions, all diffraction peaks of KZnF 3 :0.03Cr3+ with different chromium sources exhibited notable similarity, as illustrated in figure S2.This similarity implied that these two compounds possess comparable crystal structures.The subtle distinctions observed in the diffraction peaks were attributed to the varying crystal field environments influenced by the Cr ions.It is worth noting that when the chromium source was (NH 4 ) 3 CrF 6 , the magnified peak location shifted towards larger diffraction angles.This indicates that the crystal structure of the KZnF 3 :Cr 3+ phosphor samples prepared with (NH 4 ) 3 CrF 6 distorted, possibly due to changes in the crystal field environment.The refined results, as shown in table 1, revealed a decrease in lattice parameters and volume for the precursor, indicating its more compact and rigid structure.

Results and discussion
It is widely recognized that the luminescence properties of nanocrystals are greatly influenced by their morphology.Even if the phases are the same, a minor alteration of the synthesis conditions can result in significant changes of the morphology of the phosphors [12].To study the evolution of KZnF 3 morphology with different chromium sources, SEM images had been taken.Figures 2(a)-(f) showed the SEM images of KZnF 3 :0.03Cr3+ with different chromium sources at 200 nm, 500 nm, and 1 μm, respectively, indicating that the particles were in the micron level with varying average dimensions and morphologies.When the chromium source was CrF 3 •xH 2 O, most nanocrystals present octahedron as shown in figures 2(a)-(c).When chromium source was changed, the morphology of nanocrystals changes from octahedron to cubic morphology (figures 2(d)-(f)).This indicated that the (NH 4 ) 3 CrF 6 made the crystals growth more sufficient.Some cavities formed on the KZnF 3 :0.03Cr3+ using (NH 4 ) 3 CrF 6 as the chromium source was mainly ascribed to the mutual restricted growth of the aggregated or contacted crystals [24].However, the size ( ∼ 400 nm) of the KZnF 3 :0.03Cr3+ particles were still much smaller than that of the micro-LEDs ( ∼ 100 μm) or mini-LEDs (100-200 μm), which may find application in NIR pc micro-LEDs or mini-LEDs [22].In addition, the EDS results of KZnF 3 :0.03Cr3+ with different chromium sources (figures S3(a)-(b)) showed the existence of the composition elements, including K, Zn, F, and Cr.The identification of the Au element observed in the EDS analysis due to the utilization of spray-gold treatment.

Luminescence properties
Figures 3(a) and (b) showed the normalized PLE and PL spectra of the KZnF 3 :Cr 3+ samples with different chromium sources at room temperature.The excitation spectra were obtained by monitored at 803 nm (KZnF 3 :0.03Cr3+ (CrF 3 •xH 2 O)) and 753 nm (KZnF 3 :0.03Cr3+ ((NH 4 ) 3 CrF 6 )).The excitation spectrum of KZnF 3 :0.03Cr3+ ((NH 4 ) 3 CrF 6 ) consisted of two excitation peaks (figure 3(a)), which were located in the blue and red regions, respectively, belonging to 4 A 2 → 4 T 1 ( 4 F) and 4 A 2 → 4 T 2 ( 4 F) transitions of Cr 3+ ions in the octahedral crystal field [29].The peak positions were observed at 432 nm and 628 nm, respectively.Interestingly, when CrF 3 •xH 2 O was used as chromium source, the additional excitation was observed in the ultraviolet region, belonging to 4 A 2 → 4 T 1 ( 4 P).The PLE bands peaking at 294 nm, 454 nm, and 680 nm of KZnF 3 :0.03Cr3+ (CrF 3 •xH 2 O), exhibited a red-shift, compared with KZnF 3 :0.03Cr3+ ((NH 4 ) 3 CrF 6 ). Figure 3(c) presented the observed positions of these excitation bands in the range of 250 ∼ 1200 nm, as indicated by the corresponding DR spectra.Notably, the 4 A 2 → 4 T 2 band included extra two dips related to the Fano antiresonance, which was also consistent with the DRS results [23].Both excitation peaks demonstrated the large full width at half maximum (FWHM), and the strongest excitation peaks were in the blue region, indicating that the broadband NIR emitting phosphors KZnF 3 :Cr 3+ were well-matched with the light emitted by commercial high-efficiency blue LED chips.The emission spectra were obtained by monitored at 454 nm (KZnF 3 :0.03Cr3+ (CrF 3 •xH 2 O)) and 432 nm (KZnF 3 :0.03Cr3+ ((NH 4 ) 3 CrF 6 )).The emission spectra of KZnF 3 :Cr 3+ with different chromium sources featured a single broadband (figure 3(b)), corresponding to the 4 T 2 → 4 A 2 ( 4 F) transitions of Cr 3+ in the weak octahedral environment.The peak positions of KZnF 3 :0.03Cr3+ (CrF 3 •xH 2 O) and KZnF 3 :0.03Cr3+ ((NH 4 ) 3 CrF 6 ) were at 803 nm and 753 nm, and the corresponding FWHM values were 140 nm and 122 nm, respectively.Compared the excitation and emission spectra of the two NIR phosphors, it can be found that the peak positions presented a blue-shift, accompanied by the decrease of the FWHM with the variation on the chromium sources from CrF 3 •xH 2 O to (NH 4 ) 3 CrF 6 .In the investigation of the PLE and PL spectra variations in KZnF 3 :Cr 3+ with different chromium sources, it was found that with the substitution of (NH 4 ) 3 CrF 6 for CrF 3 •xH 2 O of the phosphor, the unit cell volume reduced from 65.0896 Å 3 to 64.9968 Å 3 and the average bond length of Zn-F decreased from 2.0113 Å to 2.0000 Å (as shown in table 1).These changes resulted in a more compact arrangement of the unit cell, leading to increased local pressure between the metal cations and fluorine anions, and subsequently caused an increased crystal field strength.In this regard, the crystal field strength can be calculated using the following formula [30,31]: In this formula, Dq represents the CFS, Z is the charge or valence of the anion, e is the charge of an electron, r is the radius of the d-orbital wave function, and R is the distance between the central ion and the surrounding ligands.Dq is inversely proportional to the bond length between the activating ion and the anions, indicating that Dq decreases as the bond length increases.According to crystal field theory, crystal field splitting is related to the bond length between the activating ion and the ligands, represented by R in the formula mentioned above.A shorter bond length leads to a larger crystal field splitting and a stronger crystal field strength.As the substitution of (NH 4 ) 3 CrF 6 for CrF 3 •xH 2 O occurred, resulting in a decrease in the average bond length of Zn-F, and then the crystal field strength of KZnF 3 :0.03Cr3+ ((NH 4 ) 3 CrF 6 ) increased.
The blue shift in the spectra was attributed to the strong crystal field [32].To elucidate the energy levels of Cr 3+ in octahedral coordination, we employed the Tanabe-Sugano diagram (figure 3(d)).The crystal field parameters of Cr 3+ were determined uising the following equations [33]: Here, Dq is the crystal field parameter, B is the Racah parameter, E( 4 T 1g ) and E( 4 T 2g ) are consistent with the peak locations of the 4 A 2 to 4 T 1 and 4 T 2 transitions, respectively.The crystal field can be represented by Dq/B.Generally, the emission is narrowband in strong crystal field (Dq/B > 2.3) and broadband in weak crystal field (Dq/B < 2.3).According to the PLE spectra of the KZnF 3 :Cr 3+ with different chromium sources and the above equations, the calculated Dq/B values of KZnF 3 :Cr 3+ are 1.55 for KZnF 3 :Cr 3+ (CrF 3 •xH 2 O) and 1.83 for KZnF 3 :Cr 3+ ((NH 4 ) 3 CrF 6 ), indicating that Cr 3+ is in generally weak crystal field in the KZnF 3 :Cr 3+ systems.The variation of the chromium sources from CrF 3 •xH 2 O to (NH 4 ) 3 CrF 6 decreased the unit cell extension, increased the crystal field strength for the substitution of Cr 3+ ions.The higher the Dq/B ratio (<2.3), the shorter the NIR emission position.These results agreed with the PL spectral features of the KZnF 3 :Cr 3+ NIR phosphors [34].
The influence of Cr 3+ concentration on the PL properties of KZnF 3 :Cr 3+ with different chromium sources was studied via the concentration-dependent PL spectra monitored at room temperature, as shown in figures 4(a)-(d).Different broadband emission peaks and similar spectral shapes can be observed from KZnF 3 :Cr 3+ using different chromium sources with different contents of Cr 3+ .As the x value increased, the luminescence intensity gradually increased and reached the maximum at x = 0.03, where the KZnF 3 :Cr 3+ (CrF 3 •xH 2 O) sample exhibited the strongest PL intensity (figure 4(a) and the inset of figure 4(a)).Further increasing in the dopant content ineluctably resulted in decreasing of PL intensities, indicating that the nonradiative energy transfer causes the concentration quenching behavior of Cr 3+ -doped phosphors [35].Consequently, the critical distance R c can judge the non-radiative energy transfer, the value can be calculated according to the following equation [36].
where V is the volume of the crystallographic unit cells, X c is the critical value of the concentration, and N refers to the number of sites in the crystallographic unit cells that can be doped.By the equation (5), the R c of Cr 3+ ions of KZnF 3 :0.03Cr3+ (CrF 3 •xH 2 O) and KZnF 3 :0.03Cr3+ ((NH 4 ) 3 CrF 6 ) were calculated to be 16.06 Å and 16.05 Å, respectively, much greater than 5 Å.Therefore, the interaction between the activator ions can be eliminated.The quenching mechanism of KZnF 3 :xCr 3+ may be dominated by multipole-multipole interactions, including dipole-dipole (d-d), dipole-quadrupole (d-q) and quadrupolequadrupole (q-q) interactions, which are respectively equal to the θ of 6, 8 and 10.The value can be determined by the following equation [36,37].
where I and x mean the emission intensity and concentration of Cr 3+ dopant, K and β are the certain excitation condition and specific matrix crystal.From the linear fitting of log(I/x) versus log(x) in the inset in figure S4, θ can be calculated to be 2.91.The calculation outcome, which approached a value of 6, indicated that the dominant quenching mechanism of KZnF 3 :xCr 3+ using different chromium sources is dipole-dipole interaction.
Meanwhile, with increasing Cr 3+ concentration, FWHM expanded slightly, whereas the emission peak positions remained almost unchanged as shown in figure 4(b).The same optimal doping concentration can be found in the KZnF 3 :Cr 3+ ((NH 4 ) 3 CrF 6 ) (figure 4(c)).It is shown from figure 4(d) that the emission peak positions had a slight red shift and FWHM expanded.
Figure S5 is the fluorescence decay curves of KZnF 3 :Cr 3+ using different chromium sources at the optimal concentration.The fluorescence decay curve can be well fitted by double exponential equation [37]: Where I(t) represents the emission intensity, A 1 and A 2 are constants, and τ 1 and τ 2 are the decay time for the exponential components, respectively.The lifetimes at the optimal doping concentration of Cr 3+ are 0.16 ms for KZnF 3 :0.03Cr3+ (CrF  Obviously, the calculated microsecond lifetimes revealed the spin-allowed 4 T 2 → 4 A 2 transitions of Cr 3+ activators [33,37].Figure S6 is the band gap calculations of KZnF 3 :0.03Cr3+ with different sources.Based on the calculated band gaps, it can be inferred that the KZnF 3 matrix is suitable for Cr ion doping shown in figure S7.

Temperature-dependent luminescence
The temperature of the LED chip coated with phosphors can easily reach 423 K, often cause the luminescence quenching of the phosphors due to thermal effects.Selecting the optimal KZnF 3 :0.03Cr3+ with different chromium sources to investigate their thermal stability and evaluate their potential application in pc-NIR LEDs.Figures 5(a)-(d) showed the temperature-dependent PL spectra of KZnF 3 :Cr 3+ with different chromium sources in the temperature range of 298-523 K.As the temperature increased, the peak position of KZnF 3 :0.03Cr3+ (CrF 3 •xH 2 O) red shifted slightly from 803 nm to 810 nm.This is due to the fact that the crystal lattice expanded with the increase of temperature, resulting in a decrease in CFS.Meanwhile, the FWHM of the emission bands significantly extended from 139 nm to 167 nm (figure 5(b)).Although the integrated emission intensity at 423 K remained 58% of the initial value at room temperature (figure 5(e)), the thermal stability of the phosphor was not very satisfactory.The reason for the deteriorated thermal stability was explicated by two aspects.One is that the non-radiative transition increased as the temperature raised.
In general, the thermal quenching of Cr 3+ in the weak octahedral crystal field is mainly due to non-radiative relaxation from the 4 T 2 to the 4 A 2 energy levels.As mentioned previously, the crystal field of KZnF 3 ((NH 4 ) 3 CrF 6 ) is stronger than that of KZnF 3 (CrF 3 •xH 2 O), leading to a greater vertical displacement, which contributes to enhanced thermal stability [40].In addition, the varying electron-phonon coupling effect influences the thermal stability, with a weaker coupling effect potentially reducing the thermal quenching effect.By calculating the Huang Rhys factor (S), the electron-phonon coupling strength of KZnF 3 :0.03Cr3+ with different chromium sources phosphors can be determined.The S can be obtained by fitting the FWHM with temperatures using the following equation [29,41]: where ω stands for the phonon frequency, ћω means the phonon energy, and k means the Boltzmann constant.Figure 5(f) illustrated the fitting results of FWHM 2 as a function of 2kT, based on the temperaturedependent emission spectra of KZnF 3 :0.03Cr3+ with different chromium sources.The ћω and S for KZnF 3 (CrF 3 •xH 2 O) are 0.05082 eV and 2.30705, and those for KZnF 3 ((NH 4 ) 3 CrF 6 ) are 0.05743 eV and 1.69522, respectively.It is worth mentioning that a larger S value indicates a stronger electron-phonon coupling effect in KZnF 3 :0.03Cr3+ (CrF 3 •xH 2 O) compared to KZnF 3 :0.03Cr3+ ((NH 4 ) 3 CrF 6 ).As previously mentioned, a weaker coupling effect reduced the thermal quenching effect.Hence, the better thermal stability of KZnF 3 :0.03Cr3+ ((NH 4 ) 3 CrF 6 ) compared to KZnF 3 :0.03Cr3+ (CrF 3 •xH 2 O ) can be attributed to the fact that the weaker electronphonon coupling effect.

Conclusion
In summary, KZnF 3 :0.03Cr3+ with different chromium sources with high purity and good crystallinity were successfully synthesized by a HF-free hydrothermal method.A wide NIR emission band in the range of 650-1000 nm under blue light excitation was obtained.Specifically, the KZnF 3 :0.03Cr3+ (CrF 3 •xH 2 O) exhibited a broadband NIR emission (FWHM = 140 nm, peaked at 803 nm) under 454 nm excitation.The KZnF 3 :0.03Cr3+ ((NH 4 ) 3 CrF 6 ) exhibited a broadband NIR emission (FWHM = 122 nm, peaked at 753 nm) under 432 nm excitation, accompanied by the blue shift of the peak position in the variation of the chromium sources.This was mainly due to the stronger crystal field caused by shorter distance between the activating cations and fluorine anions.Moreover, the thermal stability of KZnF 3 :0.03Cr3+ was also significantly improved.KZnF 3 :0.03Cr3+ ((NH 4 ) 3 CrF 6 ) exhibited highly thermal stability at 423 K with an intensity of 87% of that at room temperature, which was due to stronger structural rigidity and weaker electron-phonon coupling effect.The green synthesis strategy using different chromium sources offers promising industrial prospects for the mass production of NIR phosphors with good thermal stability.

3. 1 .
Structure and morphologyFigures1(a) and (b) show the XRD patterns of KZnF 3 :xCr 3+ (x = 0.01-0.06)with different chromium sources.All diffraction peaks of the samples matched well with the standard card of KZnF 3 (PDF#06-0439) and no additional impurity phase was observed, indicating pure-phased products had been achieved with low doping concentrations.As for changing the chromium source, the XRD patterns of samples well fitted with KZnF 3 without introducing any new impurities.KZnF 3 crystal structure is presented in figure 1(c).KZnF 3 is a cubic structure with a space group of Pm-3m and lattice constants are a = b = c = 4.06110 Å, α = β = γ = 90 °, V = 66.9778Å 3 and Z = 1.In this structure, Zn 2+ is located in the centers of the octahedron coordinated with six F − ions to form a [ZnF 6 ] octahedron.The ionic radius are 0.615 Å for Cr 3+ (CN = 6, CN: coordination number) and 0.74 Å for Zn 2+ (CN = 6).Due to the similar radius of these two ions, Zn 2+ sites are easily occupied by Cr 3+ in the KZnF 3 host.The structural parameters and detailed crystallographic information of KZnF 3 :Cr 3+ with different chromium sources are obtained through XRD Rietveld refinement (figures 1(d) and S1(a)).In these cases, the refined reliability factors Rp and Rwp values are 5.97% and 8.86% for the chromium sources were CrF 3 •xH 2 O.When the chromium sources were (NH 4 ) 3 CrF 6 , the refined reliability factors Rp and Rwp values are 5.42% and 12.04%, respectively, which proved that the refined results are reliable.The Rietveld refinements of KZnF 3 :Cr 3+ ((NH 4 ) 3 CrF 6 ) were performed, and the results were shown in figureS1(b).With the increase of x content, the lattice parameters (a) and volume (V) of the unit cell decreased, confirming the successful replacement of larger Zn 2+ ions (r = 0.74 Å, CN = 6) with smaller Cr 3+ ions (r = 0.615 Å, CN = 6) (see figure

Table 1 .
The main refinement parameters of the rietveld refinement.

Table 2 .
Thermal stability parameters of Cr 3+ doped wideband NIR phosphor materials and photoelectric properties of NIR pc-LED devices.