English

• 1. INTRODUCTION

  Inorganic borate and phosphate compounds possess a lot of interesting characteristics including low price, steady chemical property, various chemical structures and optically transparent in visible region^[1-3]. The basic building block of phosphate compounds is PO₄ tetrahedron which is flexible and can inhibit various coordination environments by altering the P–O bond distances. The crystal structure of borate consists of BO₄ and BO₃ units, which can be linked together via common O atoms to give many structural species. It is conceivable that mixed borate-phosphate and polymerized borophosphate compounds possess more complicated and multitudinous structure types associated with interesting properties. So far, a large number of borate-phosphates and borophosphates have been reported, extending from isolated species, oligomers, rings, and chains to layers and frameworks^[4-6]. These compounds known to date are systemically classified in terms of reviews by Kniep et al^[7].

  Rare-earth ions can be excited resonantly through f→f or f→d transitions or non-directly in charge transfer process or dipole-dipole energy transfer. If doping rare-earth ions into suitable host materials, promising phosphors would be given, which have become a hot topic in the recent twenty years^[8-13]. As a typical representative, terbium is an old acquaintance to researchers for producing green-emitting phosphors. As is known to all, Tb³⁺ ion serves as an efficient green-emitting activator in a huge number of photoluminescence materials due to bright emission colors and high luminescent efficiencies when excited by charge transfer transitions in ultraviolet region^{[14, 15]}. In this work, we started a searching for new borate-phosphate compound using high-temperature solution growth (HTSG) method, which is usually an effective method to prepare 0.2×millimeter-level crystals for structure determination by single-crystal X-ray diffraction (SC-XRD) method. We selected the mixture of Cs₂O−Na₂O−B₂O₃−P₂O₅ as flux and Lu₂O₃ as solvent. Surprisingly, a new borophosphate CsNa₂Lu₂(BO₃)(PO₄)₂ was successfully obtained. Meanwhile, we prepared Tb³⁺ doped phosphors CsNa₂Lu_2(1-x) Tb_2x(BO₃)(PO₄)₂ and studied the photoluminescence (PL) properties.

  2. EXPERIMENTAL

  2.1 Materials and instrumentation

  The synthesized materials Na₂CO₃ (AR ≥ 99.0%), Cs₂CO₃ (AR ≥ 99.0%), Lu₂O₃ (AR ≥ 99.9%), Tb₄O₇ (AR ≥ 99.9%), H₃BO₃ (AR ≥ 99.0%) and NH₄H₂PO₄ (AR ≥ 99.0%) were purchased from Aladdin Reagent Ltd. Co. (China). Phase structure analysis was performed with X-ray diffraction (XRD) equipment SmartLab 9KW diffractometer (Rigaku Co.) at room temperature. The 2θ range, scanning speed and step width for XRD measurement were set at 5~75°, 5 °/min, and 0.02 °/step. Solid-state ultraviolet-visible absorption spectra (UV-Vis) were measured using a spectrophotometer Hitachi UH4150 in the range of 240~780 nm. Photoluminescence performance was measured using an Edinburgh FLS1000 Fluorescence Spectrometer. The excitation source for steady-state emission and exciting spectra is a 500 W xenon lamp. The transient state fluorescence was measured using an EPL 365 nm laser. The electroluminescent (EL) properties of the prepared LED lamp were also measured with a FLS1000 testing system.

  2.2 Preparation

  The high-temperature molten salt method, that is, flux method, was used to prepare small single crystals of compound CNLBP. Additional reactants Na₂CO₃, Cs₂CO₃, H₃BO₃ and NH₄H₂PO₄ in proper molar ratios were used as the flux to make sure melting of refractory Lu₂O₃. The raw materials, Cs₂CO₃ (1.303 g, 4 mmol), Na₂CO₃ (1.060 g, 10 mmol), Lu₂O₃ (0.0796 g, 0.2 mmol), H₃BO₃ (0.4177 g, 10 mmol) and NH₄H₂PO₄ (1.150 g, 10 mmol) were mixed and put into an arc platinum crucible after carefully grinding in an agate mortar. It was pre-treated in muffle furnace at 500 ℃ for 6 hours to release volatile gas (CO₂, NH₃, and H₂O). A necessary regrinding was performed to ensure the homogeneity of mixture. After that, the temperature was slowly increased to 850 ℃ to melt the mixture completely. After holding the temperature at 850 ℃ for 15 hours, the solution was cooled to 550 ℃ at a rate of 2 ℃·h⁻¹ to grow small single crystals. Finally, the production was washed by hot water to get rid of the addition flux. A few small single crystals of compound CNLBP can be carefully selected using a light microscope.

  After proper structural analysis, pure powder samples of compounds CsNa₂Lu_2(1-x)Tb_x(BO₃)(PO₄)₂ (CNLBP: xTb; x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0) were obtained quantitatively from the solid state reaction of Na₂CO₃, Cs₂CO₃, Lu₂O₃, Tb₄O₇, H₃BO₃ and NH₄H₂PO₄ in the stoichiometry ratio. The mixture was ground thoroughly in an agate mortar and pressed into a pellet. It was then calcined in a platinum crucible for 48 h at 760 ℃, with several intermediate grinding stages ensuring a complete solid state reaction. Through XRD powder diffraction studies, it was proven that all seven samples were obtained successfully in a single phase (Fig. 1). The samples used for spectral measurements were polycry-stalline powders synthesized by solid-state reactions.

  Figure 1

  

  Figure 1.  XRD patterns of CsNa₂Lu_2(1-x)Eu_2x(BO₃)(PO₄)₂ (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0) samples

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  2.3 Single-crystal X-ray diffraction

  Single-crystal X-ray diffraction (SC-XRD) analysis was performed using the Bruker Smart Apex2 CCD device under the homeothermic condition of 20 ℃. The data were collected in the range from 3.27° to 28.25°, the exposure time was set as 10 second per deg and the scan width was set as 0.5°. Using this strategy, 1466 frames were collected in all for about six hours, and then the data were integrated with the Bruker Apex2 software package^[16] using a narrow-frame integration algorithm. The unit cells were determined and refined by least-squares upon the refinement of XYZ-centroids of reflections above 3 times of σ(I). Then the data were scaled for absorption using the SADABS programme of Apex2 package. Intensities of all measured reflections were corrected for Lp and multi-scan crystal absorption effects. The crystal structure of the title complex was solved by the Shelx-2017 crystallographic computing system^[17]. All atoms were refined with anisotropic thermal parameters. The atomic coordinates and thermal parameters are given as Supporting information (Table S1, S2), and some important bond distances are summarized in Table S3.

  3. RESULTS AND DISCUSSION

  3.1 Crystal structure description

  As shown in Fig. 2a, CNLBP can be described as a three-chain framework of [Lu₂(BO₃)(PO₄)₂]_∞ that delimits the 1D tunnels filled by Na⁺ and Cs⁺ ions. There is one unique caesium (I) atom, one unique sodium (I) atom, one unique lutecium (III) atom, one unique boron (III) atom, and one unique phosphorus (V) atom in each asymmetry unit. The B and P atoms form BO₃ and PO₄ coordination, respectively, and both BO₃ and PO₄ groups are isolated with each other. The B–O and P–O band distances are given in Table S3 (see supporting information), which are the common values within borate and phosphate compounds^{[18, 19]}. The Lu atoms coordinate BO₃ and PO₄ to form a 3D open framework of [Lu₂(BO₃)(PO₄)₂]_∞. All Lu atoms in this structure are surrounded by seven O atoms to form LuO₇ pentagonal bipyramids. Then each LuO₇ group connects with two adjacent LuO₇ via corner- and edge-sharing O atoms to form an infinite zig-zig [Lu₂O₁₃]_∞ chain running along the c-axis, as shown in Fig. 2b. What is noticeable is that the B atom locates in the BO₃ plane with zero eccentricity ratio, and the Lu atoms exactly locate in the plane of LuO₇ pentagonal bipyramid. The Lu−O bond lengths fall in the range of 2.205(3)~2.449(4) Å. In this chain, each PO₄ group connects three LuO₇ groups.

  Figure 2

  

  Figure 2.  (a) View of the crystal structure of CsNa₂Lu₂(BO₃)(PO₄)₂; (b) Chain framework of [Y(BO₃)(PO₄)₂]_∞

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  Furthermore, large Cs and Na atoms locate among [Lu₂(BO₃)(PO₄)₂]_∞ chains, connecting them and keeping up charge balance. Cs is surrounded by ten O atoms with the Cs–O distances of 3.236(3)~3.6556(1) Å, and the Na connects with seven O atoms with the Na–O distances of 2.286(3)~2.804(4) Å. These values are common within Na and Cs oxysalts^[20]. Results of bond-valence calculations indicate that Cs, Na, Lu, B and P atoms are in reasonable oxidation states of +1, +1, +3, +3 and +5^[21]. The calculated total bond valences are 0.842, 1.143, 3.018, 2.949 and 5.195 for Cs, Na, Lu, B and P atoms, respectively. This conclusion also supports the reasonability of our structure model for CNLBP.

  3.2 Luminescent properties

  Fig. S1 (see supporting information) shows the UV-Vis absorbance spectrum of CsNa₂Lu₂(BO₃)(PO₄)₂. There is no absorbance above 400 nm, suggesting that the host material is optically transparent in the visible region. And thus, CsNa₂Lu₂(BO₃)(PO₄)₂ is suitable to be used as host lattice for rare-earth ion doping.

  Fig. 3a shows the photoluminescence excitation (PLE) spectrum of CsNa₂Lu_1.2Tb_0.8(BO₃)(PO₄)₂. The PLE spectrum, which is recorded by monitoring with green emission peak at 541 nm, processes a series of spectral bands in the range of 200~450 nm. The broad excitation band at 250~290 nm is due to the band-to-band electronic transitions of CNLBP host material with further excitation transfer to Tb³⁺ dopant. The sharp peaks could be appropriately attributed to Tb³⁺: 4f → 4f forbidden transitions, i.e. ⁷F₆→ ⁵H₆ (304 nm), ⁷F₆ → ⁵H₇ (319 nm), ⁷F₆ → ⁵D₂ (354 and 360 nm), ⁷F₆ → ⁵G₆ (370 and 378 nm), and ⁷F₆ → ⁵D₄ at 487 nm^[22]. Among these excitation peaks, the ⁷F₆ → ⁵G₆ transition at 378 nm shows the highest intensity. The photoluminescence emission (PL) spectrum of CsNa₂Lu_1.2Tb_0.8(BO₃)(PO₄)₂, which is excited by 378 nm light, is shown in Fig. 3b. Herein, the peaks arising in the green region at 487, 550, 582 and 628 nm are attributed to ⁵D₄ → ⁷F_j (j = 6, 5, 4, 3) transition^{[23, 24]} in turn. Among them, the ⁵D₄ → ⁷F₅ transition at 544 nm is the highest intensity, which suggests that the CsNa₂Lu_1.2Tb_0.8(BO₃)(PO₄)₂ can emit blue light under near-UV light excitation.

  Figure 3

  

  Figure 3.  PLE (a) and PL (b) spectra of CsNa₂Lu_1.2Tb_0.8(BO₃)(PO₄)₂ phosphor

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  To achieve the best concentration of Tb₃₊ activator, a series of powder samples CsNa₂Lu_2(1-x)Tb_x(BO₃)(PO₄)₂ (CNLBP: xTb; x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0) were prepared. As shown in Fig. 4, there was no obvious peak shift but the intensity changes with the variation of Tb³⁺ concentration. With increasing the concentration, the emission intensity increases and then reaches to the maximum at optimum concentration x = 0.8. For most phosphors, the luminous efficiency of Tb³⁺-doped luminescent materials suffers greatly from the negative concentration quenching effect, leading to a low optimized concentration of Tb³⁺ (< 20%). However, this is negligibility in CNLBP: xTb phosphor. The optimal concentration of Tb³⁺ is 80%, and the concentration quenching occurs only for the full Tb³⁺ concentration due to the large separation of Lu³⁺ ions in CNLBP structure lattice. As mentioned above, the Lu³⁺ ions are in a linear array, and the nearest distance between neighbouring interline Lu³⁺ ions is 3.5492(2) Å, whereas the nearest intraline Lu³⁺–Lu³⁺ distance is 5.9664(4) Å. Even doping large concentration of Tb³⁺ in Lu³⁺ sites, energy migration between Tb³⁺ ions will no longer occur freely for the large separation of intraline Tb³⁺ ions. This will greatly reduce the possibility of the effective energy to be captured by quenching centres, even for 80% Tb³⁺ concentration. Hence, we suppose that the CNLBP host can accommodate high concentration of Tb³⁺ ions with neglectable concentration quenching.

  Figure 4

  

  Figure 4.  PL spectra of CNLBP: xTb (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0) phosphors by 378 nm excitation

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  In order to further prove the minor concentration quenching in phosphors CNLBP: xTb, the room temperature decay curves of CNLBP: xTb phosphors with respect to the ⁵D₄→⁷F₅ emission of Tb³⁺ by 378 nm excitation was studied. All average decay curves can be well fitted with the mono-exponential Eq. (1)^{[25, 26]}:

  $ {I_{(t)}} = {I_0}\exp ( - t/\tau ) $

  (1)

  where t is the time, τ presents the decay time, I_(t) and I₀ are the emission intensity at time t and 0. Typically, the specified fitting lifetime values of CNLBP: xTb are 2.21, 2.12, 1.97, 1.82, 1.76 and 1.14 ms for x = 0.1, 0.2, 0.4, 0.6, 0.8 and 1.0 correspondingly (Fig. 5). It is evident that the decay time decreases very slowly with increasing x from 0.1 to 0.8, owing to the increase of non-radiative decay rate of internal Tb³⁺ and the absence of concentration quenching. When x is 1.0, larger than 0.8, the decay time drops quickly from 1.76 to 1.14 ms, suggesting the appearance of concentration quenching.

  Figure 5

  

  Figure 5.  Comparison of the fluorescent decay curves of CNLBP: xTb (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0)

  DownLoad: Full-Size Img PowerPoint

  3.3 Chromaticity coordinates

  It is well-known that three main colors recognized by the human vision system are red, green and blue^[27-29]. These three colors are usually referred to as the Commission Internationale de l´Eclairage (CIE) 1931 color coordinates, which is the current standard for lighting specifications on the market. In general, the color of any light source in this color space can be represented as an (x, y) coordinate. The location of the color coordinates of CsNa₂Lu_1.2Tb_0.8(BO₃)(PO₄)₂ phosphor on the CIE chromaticity diagram is presented in Fig. 6. Under excitation at 378 nm, the calculated CIE chromaticity coordinate is (0.3117, 0.6033), falling in the green region. Thus, we may expect that compound CsNa₂Lu_1.2Tb_0.8(BO₃)(PO₄)₂ can be used as a good green phosphor for white LED.

  Figure 6

  

  Figure 6.  Chromaticity coordinates of CsNa₂Lu_1.2Tb_0.8(BO₃)(PO₄)₂ phosphor in the CIE 1931 chromaticity diagram

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  4. CONCLUSION

  For the first time, a new borate phosphate CsNa₂Lu₂(BO₃)(PO₄)₂ was discovered by high temperature flux method, and its crystal structure was determined by SC-XRD method. The structure can be described as a chain framework of [Lu₂(BO₃)(PO₄)₂]_∞ that delimits 1D tunnels filled by Na⁺ and Cs⁺ ions. Then Tb³⁺ ion was introduced to prepare a series of phosphors CNLBP: xTb (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0), and it can emit bright green light under near-UV excitation due to the ⁵D₄ →⁷F_j (j = 6, 5, 4, 3) transition of Tb³⁺. Owing to the large separation of Lu³⁺ ions in CNLBP structure lattice, the optimal concentration of Tb³⁺ is 80%, and concentration quenching occurs only for the full Tb³⁺ concentration. Therefore, we can say that CNLBP host can accommodate high concentration of Tb³⁺ ions with neglect-table concentration quenching.

  
  
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• 

  Figure 1  XRD patterns of CsNa₂Lu_2(1-x)Eu_2x(BO₃)(PO₄)₂ (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0) samples

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  Figure 2  (a) View of the crystal structure of CsNa₂Lu₂(BO₃)(PO₄)₂; (b) Chain framework of [Y(BO₃)(PO₄)₂]_∞

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  Figure 3  PLE (a) and PL (b) spectra of CsNa₂Lu_1.2Tb_0.8(BO₃)(PO₄)₂ phosphor

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  Figure 4  PL spectra of CNLBP: xTb (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0) phosphors by 378 nm excitation

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  Figure 5  Comparison of the fluorescent decay curves of CNLBP: xTb (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0)

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  Figure 6  Chromaticity coordinates of CsNa₂Lu_1.2Tb_0.8(BO₃)(PO₄)₂ phosphor in the CIE 1931 chromaticity diagram

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