KMnO₄ (99.5%), KF (99%), HF (40 wt%), H₂O₂ (30%), MoO₃ (99.5%), KOH (85%), NH₄HF₂ and HCl. All reagents were purchased from the Shanghai Macklin Biochemical Co. Ltd. and were used directly without further purification. K₂MnF₆ was prepared as a manganese source by the strategy shown in Figure 1 [15].

The K₃MoO₂F₅∙2H₂O:Mn⁴⁺ phosphor was prepared by a simple co-precipitation method with low toxicity (NH₄HF₂ + HCl) instead of highly toxic HF. Typically, the preparation details of K₃MoO₂F₅∙2H₂O:Mn⁴⁺ phosphor are as follows (Figure 2). Firstly, 0.2822 g MoO₃ is completely dissolved in the 5 mL prepared KOH

(1.6 mol∙L⁻¹) solution. Subsequently, 0.4563 g NH₄HF₂ was added into 10 mL HCl solution (4 mol∙L⁻¹), and this solution was added dropwise to the above solution. After 30 min of magnetic stirring, 0.0099 g K₂MnF₆ was added to the reacted solution and continued magnetic stirring for 30 min, a yellow precipitate was formed. Finally, the target product was obtained by centrifugation, washing with ethanol three times and drying at 60˚C for 6 h.

The warm white LED device was fabricated by mixing the as-synthesized red phosphor, yellow (YAG:Ce³⁺) emitting commercial phosphor and organic silica gel A and B (a mass ratio of 1:4) with a blue LED chip (0.4 W, 460 nm), the mass ratio of yellow phosphor to red phosphor is 1:8. The integrating sphere LED photoelectric parameter comprehensive test system (SSP6612) was used to measure the photoelectric performances including the color rendering index, correlation color temperature electroluminescence (EL) spectra and CIE color coordinates of the fabricated LED devices.

The crystal structure of the phosphor was characterized via RigakuD/max - RA X-ray diffraction (XRD) with Cu Kα radiation (λ = 0.15406 nm) in the scanning range from 10˚ to 90˚ and the scanning speed was 6˚ min⁻¹. The morphology and composition of the sample was identified via JEOL JSM-7610F field emission scanning electron microscope (FE-SEM) and OXFORD ISIS-300 energy dispersive spectrometer (EDS). Using barium sulfate as the substrate, the UV-visible diffuse reflectance spectrum (DRS) of the phosphor was measured by Shimadzu UV-2550 spectrophotometer, in which the mode of the integrating sphere was set to external. Using the light source of 150 W xenon lamp as excitation source, the photoluminescence excitation (PLE) spectra, photoluminescence (PL) spectra of the sample was collected by HITACHI F-7000 fluorescence spectrophotometer in natural environment.

The crystal phase structure and purity of the sample can be examined by XRD. Figure 3 shows the XRD pattern of the as-prepared K₃MoO₂F₅∙2H₂O:Mn⁴⁺ red

phosphor by the co-precipitation method. It can be seen that the as-obtained diffraction peaks can be better corresponded with the standard card (PDF# 31-1117). The Mo⁶⁺ ion and Mn⁴⁺ ion have similar ionic radius and the same coordination number ((R_Mo⁶⁺ = 0.59 Å, R_Mn⁴⁺ = 0.53 Å and CN = 6), thus rendering it possible the doping of Mn⁴⁺ does not influence significantly on the crystal structure of K₃MoO₂F₅∙2H₂O. SEM image (a) and EDS spectrum (b) of K₃MoO₂F₅∙2H₂O:Mn⁴⁺ can be seen from Figure 4. The image shows that the phosphor is irregular lump with an average particle size of 70 - 100 μm and has a rough surface. As show in Figure 4(b), the presence of K, Mo, O, F, Mn elements can be clearly observed. The above results show that the K₃MoO₂F₅∙2H₂O:Mn⁴⁺ phosphor was successfully synthesized by the co-precipitation method.

Figure 5(a) shows the excitation and emission spectra of phosphor respectively. When 632 nm was used as the monitoring wavelength, there were two obvious wide absorption bands in the wavelength range of 300 - 550 nm, and the centers of the absorption bands were located at 374 nm (UV region) and 470 nm (blue region), respectively. These two absorption bands are derived from the spin-allowing ⁴A_2g → ⁴T_1g and ⁴A_2g → ⁴T_2g energy level transitions of Mn⁴⁺, respectively. The DRS of K₃MoO₂F₅∙2H₂O:Mn⁴⁺ (Figure 5(b)) shows that the phosphor has strong absorption in the blue region. In addition, the absorption peak also appears in the ultraviolet region at 270 nm, which can be attributed to the O/F → Mo charge transfer band in the matrix. At 470 nm excitation, the emission spectrum of K₃MoO₂F₅∙2H₂O:Mn⁴⁺ phosphor consists of seven typical emission peaks of Mn⁴⁺, 600 nm, 611 nm and 615 nm correspond to anti-Stokes v₃ (t_1u), v₄ (t_1u) and v₆ (t_2u) vibration models of Mn⁴⁺, respectively. Stokes vibration model v₆ (t_2u), v₄ (t_1u) and v₃ (t_1u) of Mn⁴⁺ correspond to emission peaks at 632 nm, 636 nm and 649 nm in the spectrum, among which 632 nm is the strongest emission peak. The characteristic emission peak at 624 nm is strong zero phonon line (ZPL) emission. This is due to the octahedral distortion caused by the non-equivalent substitution and the low symmetry of the crystal structure, so the strength of the ZPL is strong. Figure 5(c) is the CIE coordinate diagram of K₃MoO₂F₅∙2H₂O:Mn⁴⁺ phosphor excited by 470 nm blue light. It can be seen that its coordinate is located at (0.6808, 0.3190), close to the NTSC ideal red coordinate point (0.67, 0.33). The illustration shows the phosphor under sunlight and UV-light irradiation respectively. It can be seen from the figure that the phosphor emits bright red light under UV-light irradiation. The color purity of phosphor is an important parameter to evaluate its color characteristics. The color purity of K₃MoO₂F₅∙2H₂O:Mn⁴⁺ red phosphor is calculated as 98% by Equation (1) [16].

Colorpurity = ( x − x i ) 2 + ( y − y i ) 2 ( x d − x i ) 2 + ( y d − y i ) 2 (1)

where, (x_i, y_i) is the color coordinate of equal energy white light (0.33, 0.33), and (x_d, y_d) is the color coordinate of the strongest emission wavelength of the light source (0.6851, 0.3148). (x, y) represents the color coordinate of K₃MoO₂F₅∙2H₂O: Mn⁴⁺ sample (0.6808, 0.3190). Obviously, all the emission peaks of oxyfluorides are located in sensitive areas that can be observed by the human eye. The excellent

optical properties of phosphor indicate that K₃MoO₂F₅∙2H₂O:Mn⁴⁺ red phosphor has potential application value in warm white LED.

In order to explore the value of K₃MoO₂F₅∙2H₂O:Mn⁴⁺ red phosphor in practical applications, a series of WLED devices were packaged by combining blue chip (a) with yellow YAG:Ce³⁺ phosphor (b), red K₃MoO₂F₅∙2H₂O:Mn⁴⁺ phosphor (c) yellow YAG:Ce³⁺ phosphor + red K₃MoO₂F₅∙2H₂O:Mn⁴⁺ phosphor (d) mixed phosphor. Figures 6(a)-(d) show the electroluminescence spectra of the corresponding LED devices at 20 mA driving current. The characteristic emission at 430 - 470 nm is that of blue chip. The 470 - 590 nm range belongs to the emission peak of YAG:Ce³⁺ yellow phosphor. In the range of 590 - 660 nm, there is obvious red emission, indicating that the K₃MoO₂F₅∙2H₂O:Mn⁴⁺ phosphor can be well excited by blue chip and emit strong red light. Due to the lack of red light component, the cold WLED device has a higher relative color temperature (CCT = 5190 K) and a lower color rendering index (Ra = 70.4), and its LE is 120.07 lm∙W⁻¹. In order to improve this problem, the red light component was introduced into the WLED device, and the CCT of the device was reduced to 3608 K, the CRI was increased to 90.1, and the LE was 50.17 lm∙W⁻¹. At the same time, CIE coordinate of the cold WLED was transferred from (0.3536, 0.3960) to (0.4069, 0.3681) in the warm WLED region (Figure 6(e)). The above results show that the device obtained is more suitable for the application of warm WLED device in indoor lighting field. Table 1 lists the photoelectric performance parameters of each device in detail.