NUV-pumped luminescence of thermally stable samarium-activated alkali metal borophosphate phosphor

Exploring outstanding rare-earth activated inorganic phosphors with good thermostability has always been a research focus for high-power white light-emitting diodes (LEDs). In this study, we report a Sm3+-activated KNa4B2P3O13 (KNBP) powder phase. Its particle morphology, photoluminescence properties, concentration quenching mechanism, thermal quenching mechanism, and chromatic properties are demonstrated. Upon the near-ultraviolet (NUV) irradiation of 402 nm, the powder phase exhibits orange-red visible luminescence performance, originating from typical 4G5/2→6HJ/2 (J = 5, 7, 9) transitions of Sm3+. Importantly, the photoluminescence performance has good thermostability, low correlated color temperature (CCT), and high color purity (CP), indicating its promising application in the NUV-pumped warm white LEDs.


Introduction 
To support the field of solid-state lighting and display, developing a variety of inorganic phosphors suitable for white light-emitting diodes (LEDs) has increasingly attracted a tremendous attention [1][2][3]. However, currently phosphor-converted white LEDs technology is facing the urgent challenge. The increase of operating-temperature generally gives rise to the drop of luminescence efficiency of phosphor due to the long-time work of white LEDs. In addition, excellent red phosphor is urgently needed for reducing correlated color temperature and improving color rending index The 4f transitions of Sm 3+ dominate its visible light emission. The symmetry of the coordination field for Sm 3+ ions closely affects the luminescence efficiency and emission light color of inorganic phosphors.
Borophosphate materials have been proven to have good optical, physical, and chemical properties correlated with the diversified anionic frameworks, which originate from the various connections of boron-oxygen tetrahedron (BO 4 ), boron-oxygen triangle (BO 3 ), and phosphorus-oxygen tetrahedron (PO 4 ) structural groups [17]. So, the diversified structures of borophosphate materials give a great probability for exhibiting the different emission light colors of Sm 3+ related to the diversity of its coordination fields. In 2019, Yang et al. [18] firstly reported borophosphate KNa 4 B 2 P 3 O 13 (KNBP) with the non-centrosymmetric Pna2 1 space group and its nonlinear optical properties. The (B 2 P 3 O 13 ) 5anionic framework of the borophosphate is composed of BO 4 and PO 4 tetrahedra. The borophosphate KNBP exhibits a good transmittance in the whole visible light region. Besides, it has the characteristics of stable physicochemical properties, facile preparation, and nontoxicity. The previous investigation indicates that the inorganic KNBP borophosphate could be a prospective host of phosphor materials. However, KNBP-based phosphor doped with Sm 3+ activators has not been proposed.

1 Material synthesis
The reagents used in the experiment were Sm 2 O 3 (99.99%), H 3 BO 3 (99.9%), NH 4 H 2 PO 4 (99.9%), K 2 CO 3 (99.9%), and Na 2 CO 3 (99.9%). According to the stoichiometric ratio of the KNBP host, the required reagents were weighed precisely. After mixing and grinding these reagents, the mixture was preheated at 673 K for 5 h in a high-temperature furnace. Then the temperature was increased to 873 K. The heat preservation period was 168 h in Ref. [18]. To optimize the synthesis procedure, we carried out a series of solid-state reactions with different heat preservation periods (24,48,72,120, and 168 h). The sintering time-dependent X-ray diffraction (XRD) patterns of the KNBP host powder phase are shown in Fig. S1 in the Electronic Supplementary Material (ESM). The pure KNBP powder with good crystallinity can be prepared in only 24 h. Thus, a series of KNBP:xSm 3+ (x = 0, 0.25, 0.5, 0.75, 1, 2, 3 mol%) polycrystalline phases were synthesized by the more simplified solid-state synthesis procedure, in comparison to the preparation reported in Ref. [18].

2 Characterizations
Powder XRD patterns were recorded via a Rigaku D/Max-3B diffractometer with Cu Kα radiation. A Nicolet iS10 spectrometer was used for recording Fourier transform infrared (FTIR) spectra. The morphology observation was carried out via a scanning electron microscope (SEM, FEI QUANTA200) and a transmission electron microscope (TEM, JEOL 2100F), and the elemental analysis was made by the scanning electron microscope with an energy dispersive spectroscopy (EDS) detector. The UV-Vis diffuse reflectance spectra (DRS) were performed via a SHIMADZU UV2600 spectrophotometer. A Hitachi F7100 spectrophotometer equipped with an integrating sphere was used to measure photoluminescence excitation (PLE), photoluminescence (PL), and internal quantum efficiency (IQE). An Edinburgh FLS920 spectrophotometer with xenon lamp excitation was employed to measure decay lifetime.

1 XRD and FTIR analysis
The powder XRD reflections for the prepared KNBP:xSm 3+ (x = 0, 0.25, 0.5, 0.75, 1, 2, 3 mol%) phases are displayed in Fig. 1(a). Based on the crystallographic information file (CIF) of the KNBP orthorhombic phase reported in 2019 [18], we have used the Diamond software to simulate its XRD reflection. The XRD patterns for all Sm 3+ -doped samples have the same peak positions and relative intensities as the simulated one, revealing that all the prepared powder phases are isostructural with the KNBP orthorhombic matrix and belong to the non-centrosymmetric space group Pna2 1 . There are crystallographically independent four sodium (Na), one potassium (K), two boron (B), three phosphorus (P), and thirteen oxygen (O) atoms in its asymmetric unit. K, B, and P atoms are connected with eight, four, and four O atoms, respectively. Figure 1(b) shows the 5, 6, and 10-coordinated environments of Na atoms. The coordination number (CN)-dependent radius values for K + , Na + , B 3+ , P 5+ , and Sm 3+ ions are listed in Table S1 in the ESM [19]. There is a good approximation for the ionic radius values of Na + and Sm 3+ ions. The dopant Sm 3+ ion prefers to occupy the lattice position of the Na + ion, and the structure of the KNBP host is not altered. To discuss the influence of the dopant Sm 3+ ion on the shift of diffraction peak, the enlarged XRD patterns in the 29°-30.5° range are shown in Fig. 1(a). The strongest peak at around 29.7° slightly shifts toward a higher 2θ angle with the increase of the dopant Sm 3+ concentration, supporting the fact that Sm 3+ ion is doped into the host lattice and substitute Na + ion. The main reason for the peak-shift phenomenon is that the radius of the dopant Sm 3+ ion is slightly smaller than that of the substituted Na + ion (Table S1 in the ESM). However, increasing the dopant concentration of Sm 3+ ions to 3 mol% gives rise to a weak diffraction peak at 2θ = 31.7°, corresponding to a small amount of impurity phase KBO 2 (JCPDS 03-0729) due to the non-equivalent replacement of Sm 3+ for Na + and the solid solubility limitation, which also resulted in the appearance of the second phase in Ba 3 (VO 4 ) 2 :Sm 3+ and LiNa 2 B 5 P 2 O 14 :Eu 3+ phosphors reported recently [15,20]. Besides, to verify the purity of the as-synthesized samples, the XRD data of the KNBP phase and KNBP:0.75%Sm 3+ powder was refined by the Rietveld method, as shown in Fig. 2. Here, the basis refinement was derived from the KNBP initial structure parameters (orthorhombic system with space group Pna2 1 ). The refinement results of profile residual factor (R p ) and weighted profile R-factor (R wp ) are reliable and reasonable owing to R p = 7.42%, R wp =  10.99% for the KNBP phase and R p = 7.21%, R wp = 9.55% for the KNBP:0.75%Sm 3+ powder, indicating that the samples have good phase purity. Figure 3 displays the FTIR curves of the KNBP:xSm 3+ (x = 0, 0.25, 0.5, 0.75, 1, 2, 3 mol%) powders. All the FTIR spectra with dopant Sm 3+ ions keep the same shapes and locations of vibration peaks as that of the pure KNBP phase. These absorption vibrations are caused by BO 4 and PO 4 tetrahedra [21,22], which are basic structural groups in the KNBP matrix.

2 Particle morphology and elemental analysis
Particle morphology and size of a phosphor is closely related to photoluminescence performance. The comparative investigation on the particle morphologies of the KNBP host and KNBP:0.75%Sm 3+ powder was carried out (Figs. 4(a) and 4(b)). Both powder samples consist of block-like micron particles with irregular shape and inhomogeneous size. During the hightemperature sintering process, adjacent small-sized particles have aggregated to attain micron bulks. The aggregation of the KNBP:0.75%Sm 3+ particles ( Fig. 4(b)) is more serious than that of the KNBP particles ( Fig. 4(a)). The TEM image of the KNBP:0.75%Sm 3+ powder (Fig. 4(c)) shows single particle with a micronlevel size. The inset of Fig. 4(c) shows the corresponding high-resolution TEM (HRTEM) image. The lattice fringe with 0.424 nm corresponds to the (112) crystal plane of the host phase. The clear lattice fringe verifies that the sample has good crystallization and stability under high energy electron beam. Figure 4(d) depicts the measured EDS profile of the KNBP:0.75%Sm 3+ sample, which manifests the presence of the elements Sm, K, Na, B, P, and O. The elemental distribution map for the KNBP:0.75%Sm 3+ sample is shown in Fig. 4(e), confirming the uniform distribution of these chemical components.
The PLE curve of the KNBP:0.75%Sm 3+ powder recorded at the monitoring wavelength of 598 nm is depicted in Fig. 6(a). We can observe excitation peaks at 344 ( 6 H 5/2 → 4 H 9/2 ), 361 ( 6 H 5/2 → 4 D 3/2 ), 374 ( 6 H 5/2 → 4 D 1/2 ), 402 ( 6 H 5/2 → 4 F 7/2 ), 415 ( 6 H 5/2 → 6 P 5/2 ), 438 ( 6 H 5/2 → 4 G 9/2 ), and 469 nm ( 6 H 5/2 → 4 I 13/2 ) [24]. The strongest transition appears at 402 nm ( 6 H 5/2 → 4 F 7/2 ), which keeps consistent with the obtained diffuse reflectance result in Fig. 5. The comparison of photoluminescence under various excitations was investigated. Figure 6  powder upon excitations at 344, 361, 374, 402, 415, and 469 nm. All the PL curves are mainly composed of three typical 4f-4f transitions of Sm 3+ , which are located at 645 nm ( 4 G 5/2 → 6 H 9/2 ), 598 ( 4 G 5/2 → 6 H 7/2 ), and 562 ( 4 G 5/2 → 6 H 5/2 ) [16]. The dominant 4 G 5/2 → 6 H 7/2 (598 nm) transition belongs to visible orange-red light. Differently, the integrated emission intensity obtained by 402 nm excitation is higher than others, indicating that an NUV LED chip is suitable for NUV-pumped photoluminescence of the sample. As a rule, the purely magnetic dipole transition (MDT) and purely electric dipole transition (EDT) are known as 4 G 5/2 → 6 H 5/2 and 4 G 5/2 → 6 H 9/2 transition in the same order. The combination of MDT and EDT results in the 4 G 5/2 → 6 H 7/2 transition. The asymmetry of the local coordination of Sm 3+ can be used to analyze the EDT/MDT intensity ratio. The www.springer.com/journal/40145 asymmetric coordination of Sm 3+ corresponds to the intensity ratio of EDT/MDT > 1. In this case, the coordination environment of Na + is distorted ( Fig. 1(b)). The local coordination of Sm 3+ has a slight asymmetry due to the substitution of Sm 3+ for Na + . Hence, the MDT intensity is slightly less than the EDT one ( Fig. 6(b)) [15].  Figure 7(b) illustrates the Sm 3+ concentration-dependent emission intensity. As the Sm 3+ concentration increases from 0.25 to 0.75 mol%, the PL intensity increases. Subsequently, the non-radiative energy transfer among Sm 3+ ions occurs with the further increase of the Sm 3+ concentration, leading to the decrease of the emission intensity. The Sm 3+ concentration of 0.75 mol% is determined to be the optimization for concentration quenching. As a rule, the critical energy transfer distance (R c ) for concentration quenching is greater than 5 Å, meaning that the multipole-multipole interaction is the cause of concentration quenching. In order to discuss the mechanism of concentration quenching, the R c value of Sm 3+ in the KNBP matrix could be estimated using Eq. (S1) in the ESM. The calculated R c value of Sm 3+ is 26.1 Å. Hence, the concentration quenching for KNBP:xSm 3+ comes from the multipole-multipole interaction. The multipolar energy transfer process was further analyzed via Eqs. (S2) and (S3) in the ESM. The obtained result is illustrated in Fig. 7(c), which stands for the linear correlation of log(I/χ) versus log(χ). On the basis of the slope −θ/3 = −1.73, the obtained θ value equals 5.19, which is near to 6. The result manifests that the electric dipole-dipole interaction dominates the concentration quenching of the prepared KNBP:xSm 3+ powder samples. Figure 8 shows the lifetime for the KNBP:xSm 3+ (x = 0.25, 0.5, 0.75, 1, 2, 3 mol%) powder samples, which were recorded under 6 H 5/2 → 4 F 7/2 (402 nm) excitation and 4 G 5/2 → 6 H 7/2 (598 nm) emission. All decay lifetime curves were fitted using a typical double exponential function, which is shown as Eq. (S4) in the ESM. The average decay lifetime (τ ave ) was calculated via Eq. (S5) in the ESM. As the Sm 3+ doping concentration increases, the τ ave value shortens from 1.49 to 1.29 ms. The sample with the optimized doping concentration of 0.75 mol% possesses the τ ave value of 1.43 ms. The downtrend of decay lifetime arises from the increased probability of the non-radiative transitions among dopant Sm 3+ ions [25,26]. Figure 9(a) shows the CIE chromaticity diagram for the KNBP:xSm 3+ (x = 0.25, 0.5, 0.75, 1, 2, 3 mol%) powder samples. The inset of Fig. 9(a) lists the values of color coordinates (x, y), correlated color temperature (CCT), and color purity (CP), which were calculated using Eqs. (S6)-(S9) in the ESM, respectively. The emission colors of all the samples upon 402 nm NUV irradiation are situated in the orange-red area. The CCT parameters range from 2050 to 2890 K, which are far lower than 5000 K. The low CCT indicates the potential application of the title phosphor in warm white LEDs. Furthermore, the CP values of all the samples are higher than 80%. Figure 9(b) illustrates the comparison of the digital photos of the KNBP: 0.75%Sm 3+ powder under daylight and a 365 nm UV lamp. When the KNBP:0.75%Sm 3+ white powder was excited by a 365 nm UV lamp, the obtained emission www.springer.com/journal/40145 color is comparable to the above color coordinate data. Besides, Eq. (S10) in the ESM was employed for assessing the IQE value. Under 402 nm excitation, the IQE value of the KNBP:0.75%Sm 3+ powder is 26.2%, as depicted in Fig. 9(c). Figure 10(a) illustrates the correlation between operating temperature and PL intensity for the KNBP:0.75%Sm 3+ powder upon 402 nm NUV irradiation. As the operating temperature increases to 528 K, there is no obvious change in the positions and peak shapes for the 4f-4f transitions of Sm 3+ . However, the integrated PL intensity slowly drops due to the increased non-radiative transitions stimulated at high temperatures. Figure 10(b) illustrates clearly the normalized emission intensities depending on different temperatures. When the operating temperature rises to 428 K, the loss of the integrated PL intensity is 15% of the initial intensity, which is comparable with the 14% loss of the commercially available phosphor Sr 2 Si 5 N 8 :Eu 2+ [4]. To further explore the operating temperature-dependent PL performance of the phosphor, the Arrhenius equation (Eq. (S11) in the ESM) was employed for assessing the activation energy (E a ). Figure 10(c) shows the influence of 1/kT on ln[(I 0 /I(T)) -1]. The E a value of the KNBP: 0.75%Sm 3+ powder is around 0.31 eV via fitting the experimental data. The synthesized phosphor has good thermostability comparing with reported inorganic phosphors doped with Sm 3+ , such as SrBi 2 Ta 2 O 9 :Sm 3+ (E a = 0.23 eV) [27], Ca 19 Mg 2 (PO 4 ) 14 :Sm 3+ (E a = 0.13 eV) [28], and KLaSr 3 (PO 4 ) 3 F:Sm 3+ (E a = 0.163 eV) [29]. Equation (S12) in the ESM was employed for analyzing color stability. The obtained parameter of chromaticity shift (ΔE) is around 0.029 at 503 K, which is smaller than ΔE = 0.044 at 500 K for commercial red CaAlSiN 3 :Eu 2+ product [30].
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