Structure Determination and Luminescent Property Studies of the Single Crystal Na₃Sm(BO₃)₂

Abstract

Sodium samarium borate Na₃Sm(BO₃)₂, was prepared by a flux method and structurally characterized by single-crystal structure analysis for the first time. The results show that it crystallizes in the monoclinic system P2₁/n, with a = 6.5667(3) Å, b = 8.7675(4) Å, c = 10.1850(5), β = 90.86°, V = 586.32(5) ų and Z = 4. The structure contains NaO₇, NaO₆, NaO₅, SmO₈, and BO₃ units, which are interconnected via corner- or edge-sharing O atoms into a three-dimensional structure. The excitation spectra, emission spectra, decay time, and Commission International de l’Éclairage (CIE) chromaticity index of Na₃Sm(BO₃)₂ were studied. Under near light excitation (406 nm), the powdered Na₃Sm(BO₃)₂ shows the orange-red emission, which originates from the ⁴G_5/2→⁶H_9/2 and ⁴G_5/2→⁶H_7/2 transformation of Sm³⁺ ion.

1. Introduction

In recent years, developing new luminescent materials has become a hot topic for new lighting and display technology such as phosphor-converted white light emitting diodes (LED) because of their excellent advantages of eco-friendliness, high efficiency, high power efficiency, long lifetime, and low cost [1,2,3]. The most commercially utilized kind of white LED is the combination of a blue LED chip with a yellow phosphor (Y₃Al₅O₁₂: Ce³⁺), which blends the blue light from the chip and yellow light from the phosphor resulting in white light. However, this kind of white LED has the poor color rendering index and high Correlated Color Temperature (CCT) for the lack of red component. Another way is to directly use a near-ultraviolet (NUV) LED to excite red, green, and blue (RGB) multiphase phosphors to create warm-white light, which results in a great interest in searching for novel phosphors for white LEDs. The crystal matrix of host materials is one of the most important roles to determine the performance of phosphors. In recent years, a large amount of research work has been devoted to explore new phosphors in various host materials with high performance, good stability, and easily preparation [4,5].

Among various host materials, rare earth borates have been paid intense attention for a wide range of applications due to their significant advantages, such as a low sintering temperature, low cost, broad band gap, high luminous efficiency, and high chemical stability [6,7]. The boron atoms can coordinate to three and four oxygen atoms forming a BO₃ triangle and a BO₄ tetrahedron, respectively, and these units can further polymerize to complicated B_xO_y architectures. Meanwhile, the rare earth ions within these borates possess a unique optical behavior and have paved the way for the development of optical phosphors. For these compounds, the electronic transitions originate from the partially filled 4f energy shell of rare earth ions, which are not influenced by the 5s and 5p electrons. These transitions lead to narrow and intense emission bands, which are useful sources of individual colors in multicolor light emitting devices.

The crystal structure of sodium rare-earth borates Na₃Ln(BO₃)₂ (Ln = rare-earth metals) was first reported by Mascetti et al. for La and Nd species through single-crystal X-ray diffraction (SC-XRD), revealing the monoclinic space group P2₁/c of this family [8]. Later on, the structure of three isotype compounds Na₃Ln(BO₃)₂ (Ln = Pr, Sm Eu) were studied by Wang et al., through powder XRD [9]. However, no single crystal data and luminescent properties were given for the ternary compound Na₃Sm(BO₃)₂ until now. We deem that single crystals of Na₃Sm(BO₃)₂ can be obtained by using a facile high temperature flux method, which is thought to be an effective and powerful tool in solid-state chemistry. In this method, the reactant materials are completely dissolved in molten salt to obtain a uniform liquid, and a single crystal with large size can then be obtained by limiting the number of nuclei formed during cooling. Up to now, many mixed-metal oxides, sulfides, and complex intermetallics have been successfully obtained using this method [10,11,12,13]. We think the Na₂O‒B₂O₃ system can be used as a good flux for the low melting point (700~900 °C) and powerful dissolving capacity for many oxides, including refractory rare-earth oxides. Moreover, Na₂O‒B₂O₃ mixed salt can easily be removed by washing with water after the reaction. Herein, we report the powder synthesis, single crystal preparation, and luminescent properties of Na₃Sm(BO₃)₂.

2. Experimental Section

2.1. Material and Methods

All of the chemicals Na₂CO₃ (≥99.0%, Sinopharm Chemical Reagent Co., Ltd, Shanghai, China), Sm₂O₃ (≥99.9%, Sinopharm Chemical Reagent Co.,Ltd, Shanghai, China) and H₃BO₃ (≥99.0%, Sinopharm Chemical Reagent Co., Ltd, Shanghai, China) were used without further purification. X-ray powder diffraction (XRD) patterns were collected on a Rigaku DMax-2500 diffractometer (Rigaku Corporation, Tokyo, Japan) by using graphite-monochromated Cu Kα radiation in the angular range 2θ = 5−75° with a step size of 0.02°. TGA studies were all carried out with NETZSCH STA 449C instruments (NETZSCH-Gerätebau GmbH, Selb, Germany). The sample and reference (Al₂O₃) were enclosed in a platinum crucible and heated at a rate of 15 °C·min⁻¹ from room temperature to 1000 °C under a nitrogen atmosphere. Photoluminescence (PL) spectra and the lifetime test were carried out using an FLS920 Edinburgh Analytical Instrument (Edinburgh Instruments Company, Edinburgh, Britain) apparatus. A standard Xe900 continuous-wave xenon lamp (450 W) was used as the excitation source for steady-state measurements (stimulation slit width: 2.0 nm, emission slit width: 2.0 nm). The PL excitation and emission spectra was recorded within 350–450 nm and 475–750 nm, respectively, with a step width of 1 nm and an integration time of 0.2 s. A standard microsecond flash lamp μF920H was used for excitation in lifetime measurements using the time-correlated single-photon counting (TCSPC) technique. The flash lamp operated at a pulse frequency of 2000 Hz with a pulse width of 2 μs. The decay was measured with a range of 10 μs, and counts at the maximum were 2000.

2.2. Synthetic Procedures

Powder sample of Na₃Sm(BO₃)₂ can be obtained in quantitative yield by the solid-state reaction of a mixture of Na₂CO₃ (2.000 g, 18.87 mmol), Sm₂O₃ (2.193 g, 6.289 mmol), and H₃BO₃ (1.555 g, 25.16 mmol) in a molar ratio of 3:1:4, which is similar to Wang’s method [9]. The mixture was ground with an agate mortar and then pressed into a pellet to ensure optimal homogeneity and reactivity. The paste was transferred to a 20 mL platinum crucible, which was placed into a muffle furnace heated to 830 °C in the open air for 40 h. In this stage, intermediate grindings were performed every 10 h to improve the completeness of reaction. The purity of the powder pattern was confirmed by powder XRD studies on a Rigaku DMax-2500/PC powder diffractometer (Rigaku Corporation, Tokyo, Japan) (Figure 1). The syntheses can be expressed by the following equation:

3Na₂CO₃ + Sm₂O₃ + 4H₃BO₃ →2Na₃Sm(BO₃)₂ + 3CO₂ + 6H₂O.

Single crystal of Na₃Sm(BO₃)₂ can be prepared using a high-temperature molten salt method using excess mixture of Na₂O(Na₂CO₃)‒B₂O₃ as flux, which is the component of the compound Na₃Sm(BO₃)₂ to avoid dopant contamination. The mixture of initial reagents Na₂CO₃ (2.000 g, 18.87 mmol), Sm₂O₃ (0.4390 g, 1.258 mmol), and H₃BO₃ (1.944 g, 31.45 mmol), with a molar ratio of 15:1:25, was thoroughly ground in an agate mortar. The mixture was placed in a 20 mL platinum crucible that was heated in a programmable temperature muffle furnace at 900 °C and held at this temperature for 40 h in the open air until the solution became transparent and clear. The homogenized solution was then slowly cooled to 700 °C at a rate of 4 °C∙h⁻¹ to induce the growth of single crystals. Surprisingly, colorless prism-shaped crystals of Na₃Sm(BO₃)₂ were obtained in a yield of about 55% (based on Sm₂O₃), and a high quality crystal with dimensions 0.20 × 0.10 × 0.03 mm was selected for SC-XRD analysis. Some millimeter-sized single crystals with a maximum size of 1.50 × 0.30 × 0.20 mm can be manually selected, washed with hot water, and ground into powder for luminescent property studies.

2.3. Structure Solution

A suitable single crystal with dimensions of 0.20 × 0.10 × 0.03 mm was selected for the single-crystal X-ray diffraction (SC-XRD) experiments. A set of intensity data was collected using a Bruker Smart Apex2 CCD single-crystal diffractometer system equipped with a graphite-monochromated Mo Kα radiation source (λ = 0.71073 Å) with a tube power of 50 kV and 30 mA. The frames were collected at an ambient temperature of 296 K with a scan width of 0.5° in ω and integrated with the Bruker Saint software package using a narrow-frame integration algorithm. The unit cell was determined and refined by the least square method upon the refinement of the XYZ centroid of reflections above 20 σ(I). No weak satellite reflections were observed and thus no structure modulation was considered in the structure model. Then, the date was scaled for absorption using the SADABS program of Apex2 package [14]. Intensities of all measured reflections were corrected for Lorentz–polarization (Lp) and crystal absorption effects. The crystal structure of Na₃Sm(BO₃)₂ was solved by the charge-flipping method using the Superflip program [15] and subsequently refined by the Jana2006 crystallographic computing system [16]. All atoms in the structure were refined using harmonic anisotropic atomic displacement parameters (ADPs). The details of the data collection and structure refinement are summarized in

Table 1. Summary of crystal data and structure refinement of Na₃Sm(BO₃)₂.

Chemical Formula	B2Na3O6Sm
Mr	336.94
Crystal system, space group	Monoclinic, P21/n
Temperature (K)	296
a, b, c (Å)	6.5667 (3), 8.7675 (4), 10.1850 (5)
β (°)	90.86
V (Å3)	586.32 (5)
Z	4
Radiation type	Mo Kα
µ (mm−1)	10.20
Crystal size (mm)	0.20 × 0.10 × 0.03
Diffractometer	Bruker Apex2 CCD
Absorption correction	multi-scan
No. of measured, independent and observed [I > 2σ(I)] reflections	7164, 1449, 1403
Rint	0.025
(sin θ/λ)max (Å−1)	0.666
R[F2 > 2σ(F2)], wR(F2), S	0.018, 0.044, 1.14
No. of reflections/ parameters	1449/110
Δρmax, Δρmin (e∙Å−3)	1.67, −1.43

, important bond lengths and angles are listed in

Table 2. Selected bond distances (Å) and angles (°) of Na₃Sm(BO₃)₂.

Bonds	Distances (Å)	Bonds	Distances (Å)
Sm1—O4	2.338(3)	Na1—O6	2.547(3)
Sm1—O2	2.411(3)	Na1—O5 x	2.354(3)
Sm1—O5 i	2.418(2)	Na1—O3 ii	2.428(3)
Sm1—O1 ii	2.425(3)	Na1—O1 xi	2.440(3)
Sm1—O6 iii	2.471(2)	Na1—O4 iii	2.497(3)
Sm1—O3 ii	2.485(2)	Na1—O2 iii	2.658(3)
Sm1—O6 i	2.510(2)	Na1—O4	2.573(3)
Sm1—O3	2.518(2)	Na2—O5	2.485(3)
Na3—O1	2.301(3)	Na2—O2 xii	2.225(3)
Na3—O2	2.375(3)	Na2—O6 viii	2.467(3)
Na3—O5 vi	2.350(3)	Na2—O1 ii	2.666(3)
Na3—O1 vi	2.893(4)	Na2—O3	2.261(3)
Na3—O5 vii	3.071(3)	Na2—O4	2.470(3)
B1—O4	1.352(4)	B2—O1	1.372(4)
B1—O5	1.382(4)	B2—O2	1.373(4)
B1—O6	1.399(4)	B2—O3	1.387(4)
Bonds	Angles (°)	Bonds	Angles (°)
O4—B1—O5	121.2(3)	O1—B2—O2	123.2(3)
O4—B1—O6	122.2(3)	O1—B2—O3	117.9(3)
O5—B1—O6	116.5(3)	O2—B2—O3	118.9(3)

Symmetry codes: (i) x − 1, y, z; (ii) −x + 1/2, y + 1/2, −z + 1/2; (iii) −x + 1, − y + 2, −z; (iv) −x + 1/2, y − 1/2, −z + 1/2; (v) x, y − 1, z; (vi) −x + 1, −y + 1, −z; (vii) x − 1/2, −y + 3/2, z − 1/2; (viii) −x + 3/2, y − 1/2, −z + 1/2; (ix) x + 1, y, z; (x) −x + 3/2, y + 1/2, −z + 1/2; (xi) x, y + 1, z; (xii) x + 1/2, −y + 3/2, z + 1/2.

, and atomic coordinates and ADPs are given as supporting information (Tables S1 and S2). Further details of the crystal structure investigations can be obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (e-mail: [email protected]), on quoting the depository number of CSD-432527.

3. Results and Discussion

3.1. Single Crystal Structure

SC-XRD analysis reveals that the compound Na₃Sm(BO₃)₂ crystallizes in the Monoclinic system with space group P2₁/n. The crystallographic asymmetric unit contains three unique sodium atoms, one unique samarium atom, two unique boron atoms, and six unique oxygen atoms. Each B atom is coordinated by three oxygen atoms into nearly flat BO₃ tetrahedron geometry with three B–O bond lengths of 1.352(4)–1.399(4) Å and bond angles of 116.5(3)–123.2(3)°, which is the common value in borate compounds [17,18]. All BO₃ triangles are isolated in two types of linear array, whereas Na⁺ and Sm³⁺ atoms reside among the BO₃ groups and connect them through the columbic action of Na−O and Sm−O to form the structure of Na₃Sm(BO₃)₂, as shown in Figure 2. The coordination environments of three Na atoms are different. Na1 atom is coordinated by eight O atoms into Na1O₈ polyhedron with Na1−O bond distances ranging from 2.428(3) Å to 2.658(3) Å, whereas Na2 atom connects with six O atoms to form Na2O₆ octahedron with Na2−O bond distances ranging from 2.225(3) Å to 2.666(3) Å. The coordination environment Na3 atom is surrounded by five O atoms into a Na3O₅ polyhedron with three shorter Na3−O bond distances (2.301(3) Å, 2.350(3) Å, and 2.375(3) Å) and two longer Na3−O bond distances (2.893(4) Å and 3.071(3) Å), which can be considered as secondary coordination bonds to complete the extended coordination spheres of the large Na⁺ ion. The coordination mode of Na3O₅ shows a big difference from those of Na1O₇ and Na2O₆, and is an uncommon coordination geometry compared with other reported Na(I) oxide compounds [19]. Although the ionic radii of Sm³⁺ (0.964 Å) is comparable with that of Na⁺ (0.950 Å), it adopts a Sm1O₈ coordination mode with a Sm1−O bond length of 2.338(3)–2.518(2) Å. In detail, each SmO₈ polyhedron connects with three BO₃ groups via edge-sharing manner, and two BO₃ groups via corner-sharing manner.

The coordination environment of B1O₃ and B2O₃ groups are shown in Figure 3a,b, respectively. Each B1O₃ planar triangle connects to three units (Na1O₇, Na2O₆, and Sm1O₈) via edge-sharing O atoms, and seven units (two Na1O₇, one Na2O₆, two Na3O₅, and two Sm1O₈) via corner-sharing O atoms. The coordination environment of the B2O₃ unit is similar to that of B1O₃ unit; each B2O₃ unit coordinates with three units (one Na3O₅ and two Sm1O₈) via edge-sharing O atoms, and seven units (three Na1O₇, three Na2O₆ and one Na3O₅) via corner-sharing O atoms. Hence, the structure of the compound Na₃Sm(BO₃)₂ can be considered a 3D framework that is constructed by NaO₇, NaO₆, NaO₅, SmO₈, and BO₃ units.

3.2. Thermal Stability Study

The analysis of TG/DTA illustrates that the compound Na₃Sm(BO₃)₂ shows a high stable temperature, as shown in Figure 4. There is no obvious exothermic peak below 900 °C. Upon further heating, Na₃Sm(BO₃)₂ started to decompose at 960 °C, and no weight lost was observed until 1000 °C. This result demonstrates a good thermal stability of the compound Na₃Sm(BO₃)₂.

3.3. UV-Vis Spectroscopic

Figure 5 shows the UV-Vis absorption spectrum of Na₃Sm(BO₃)₂ in the range from 240 to 800 nm. The broad absorption band from 240 to 330 nm may correspond to the band-gap transition of Na₃Sm(BO₃)₂, whereas the rest sharp absorption from 340 to 500 nm corresponds to typical intra-4f forbidden transitions of the Sm³⁺ ions.

3.4. Luminescent Properties

Although with 100% concentration of Sm³⁺, the compound Na₃Sm(BO₃)₂ exhibits an orange-red emission of Sm³⁺ ion upon excitation with a xenon lamp at near-ultraviolet light. Figure 6a shows the excitation spectrum monitored at the ⁴G_5/2→⁶H_7/2 transition of Sm³⁺ with a 605 nm red emission in the range of 350–450 nm. The excitation spectrum consists of a series of peaks centered at 364, 377, 392, 406, 419, and 442 nm, corresponding to the ⁶H_5/2→⁴L_15/2, ⁶H_5/2→⁶P_7/2, ⁶H_5/2→⁴K_11/2, ⁶H_5/2→⁴F_7/2, ⁶H_5/2→⁴F_7/2, and ⁶H_5/2→(⁴G_9/2+⁴I_15/2) intra-configurational 4f→4f transition of Sm³⁺ ions, respectively [20,21]. Among these, the ⁶H_5/2→⁴F_7/2 transition at 406 nm was the highest intensity, indicating that phosphor Na₃Sm(BO₃)₂ can be effectively activated by near-ultraviolet light.

Upon excitation at 406 nm, the emission spectrum is composed of several distinct groups of peaks in the range of 475–750 nm (Figure 6b). There are two strong emissions in the orange-red region coming from positions at 605 nm and 650 nm, corresponding to the ⁴G_5/2→⁶H_7/2 and ⁴G_5/2→⁶H_9/2 transitions of Sm³⁺, respectively [22]. Besides, there are two weak emissions centered at 566 nm and 714 nm, which can be assigned to the ⁴G_5/2→⁶H_5/2 and ⁴G_5/2→⁶H_11/2 transitions of Sm³⁺, respectively. It is well-known that the emission spectrum of Sm³⁺ activated phosphor strongly depends on the symmetry of the Sm³⁺ site in the host lattice [23,24]. The ⁴G_5/2→⁶H_5/2 transition is the magnetic dipole transition, which scarcely changes the crystal field strength around the Sm³⁺ ions, leading to the independence of the symmetry, whereas the transition of ⁴G_5/2→⁶H_9/2 is the electric dipole transition, whose intensity is very sensitive to the site symmetry of the Sm³⁺ activators in the host matrix. Although the ⁴G_5/2→⁶H_7/2 emission corresponds to the magnetic dipole transition, its magnetic dipole character is usually very low and dominated by the electric dipole (ED) transition. Hence, if the Sm³⁺ occupies the site in a low symmetry, the ⁴G_5/2→⁶H_9/2 or ⁴G_5/2→⁶H_7/2 emission is frequently the strongest, but in a site with an inversion center, the ⁴G_5/2→⁶H_5/2 transition becomes dominant. For Na₃Sm(BO₃)₂, it is obviously that the intensity of ⁴G_5/2→⁶H_9/2 and ⁴G_5/2→⁶H_7/2 emissions is stronger than the ⁴G_5/2→⁶H_5/2 emission, which is in good agreement with the crystallographic study that all Sm³⁺ atoms occupy a general site without an inversion center. The quantum efficiency (QE) of a phosphor is an important parameter to be considered for practical applications. The QE can be measured and calculated according to the equation: ${\eta }_{QE}=\frac{{\displaystyle \int }{L}_S}{{\displaystyle \int }{E}_R-{\displaystyle \int }{E}_S}$, in which the L_S represents the emission spectrum, E_S represents the excitation spectrum, and E_R represents the background. Upon excitation at 406 nm, the corresponding QE of Na₃Sm(BO₃)₂ is calculated to be a low value, about 12%. For the most part, the concentration quenching occurs in 100% Sm³⁺ phosphor Na₃Sm(BO₃)₂, and the other affecting factors include preparing temperature, the morphology, and the size of single crystals. Therefore, doping La³⁺ or Gd³⁺ ions into Na₃Sm(BO₃)₂ may be an effective method to improve the luminescent QE of Na₃Sm(BO₃)₂. This work is still in process.

These three colors are usually referred to as the 1931 color coordinates, the current standard for lighting specifications on the market [25]. 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 phosphor Na₃Sm(BO₃)₂ on the Commission International de l’Éclairage (CIE) chromaticity diagram is presented in Figure 7. Under the excitation at 406 nm, the calculated CIE chromaticity coordinate is (0.660, 0.340), falling in the orange-red region. We may expect that the compound Na₃Sm(BO₃)₂ can be used as a good orange-red phosphor.

Moreover, we studied the luminescence decay curve for the ⁴G_5/2→⁶H_7/2 emission of Sm³⁺, as shown in Figure 8. The decay curve cannot be well fitted with a single-exponential but can be well fitted with a bi-exponential function according to the equation [26,27]:

I_(t) = A₁^* exp(−t/τ₁) + A₂^* exp(−t/τ₂) + I₍₀₎

where I_(t) and I₍₀₎ are the luminescence intensities at time 0 and t; A₁^* and A₂^* are the fitting parameters; t is the time; τ₁ and τ₁ are the rapid and slow lifetimes for exponential components, respectively. The double exponential behavior of Na₃Sm(BO₃)₂ may originate from energy loss by ion–ion interactions. Then, the average decay time can be calculated to be 1.654 μs to represent the lifetime by the equation [28]:

$$\tau =\frac{A_1{\tau }_1^2+A_2{\tau }_2^2}{A_1{\tau }_1+A_2{\tau }_2}.$$

4. Conclusions

A new type of sodium samarium borate Na₃Sm(BO₃)₂ was prepared using a high temperature molten salt (flux) method and its structure was determined by SC-XRD analyses. Compound Na₃Sm(BO₃)₂ crystallizes in the monoclinic P-centered space group P2₁/n. The structure of the compound Na₃Sm(BO₃)₂ can be considered as a 3D framework that is constructed by NaO₇, NaO₆, NaO₅, SmO₈, and BO₃ units. Even with the 100% Sm³⁺ ion concentration, Na₃Sm(BO₃)₂ shows intense luminescent emission behavior. The excitation spectra cover a wide range from 350 to 450 nm, which suggests that the phosphor Na₃Sm(BO₃)₂ can be effectively excited by a near-UV light source. The emission spectrum excited at 406 nm shows two strong emission bands in the orange-red region: 605 nm (⁴G_5/2→⁶H_9/2) and 650 nm (⁴G_5/2→⁶H_7/2). The CIE chromaticity color coordinates (x = 0.660, y = 0.340) were found to be in the orange-red region of the chromaticity diagram. The decay curve excited by 406 nm was measured and the decay time was fitted to be 1.654 μs. We think Na₃Sm(BO₃)₂ can be potentially used as an orange-red phosphor under near-UV light excitation.