Site Occupancy and VUV-UV-Vis Photoluminescence of the

Article Cite This: J. Phys. Chem. C 2018, 122, 7421−7431

pubs.acs.org/JPCC

Site Occupancy and VUV−UV−Vis Photoluminescence of the Lanthanide Ions in BaY2Si3O10 Rui Shi,† Xiaojun Wang,*,‡ Yan Huang,§ Ye Tao,§ Lirong Zheng,§ and Hongbin Liang*,† †

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of Environment and Energy Chemistry, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, P. R. China ‡ Department of Physics, Georgia Southern University, Statesboro, Georgia 30460, United States § Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, P. R. China S Supporting Information *

ABSTRACT: This work provides a scheme to investigate the site occupancy and the luminescence properties of lanthanide ions in BaY2Si3O10. The Rietveld refinement of the samples indicates that the lanthanide ions preferably occupy the Y3+ sites in BaY2Si3O10. It is confirmed that valence state of lanthanide ions is stable at +3 in all doping samples. The sitedependent spectroscopic properties of Ce3+ and Eu3+ are studied in VUV−UV−vis spectral region at low temperatures, and the polarization effect on Ce3+ luminescence decay is evaluated. The results indicate that the lanthanide ions experience the similar polarization effect when substituting the Y3+ sites in BaY2Si3O10. The Stokes shift of Ce3+ luminescence becomes larger as Ce3+ doping content increases. Eu3+ f−f line-shape change has not been observed in the spectra as Eu3+ content increases. It demonstrates that the change of the electrostatic binding effect in the lattice has little effect on the ligand polarization of the central lanthanide ion. Finally, a mechanism is proposed to explain why the thermal-quenching of Ce3+ luminescence is negligible in BaY2Si3O10 even if the temperature increases up to 500 K. The influence of dynamic ion−lattice interaction on luminescence properties of the lanthanide ions in BaY2Si3O10 is discussed in detail. environment of the activator.13,14 Accordingly, investigations of Ce3+ and Eu3+ luminescence properties are of significance for both fundamental research and material applications. BaY2Si3O10 is an ideal host compound because of its excellent properties, such as good chemical stability, easy preparation, and the rigid crystal structure, similar to those of the other silicates.15,16 In addition, the appropriate energy band gap of the silicates provides a great convenience for studying the sophisticated energy levels of Ln ions in the spectroscopy. In this article, the Rietveld refinement and X-ray absorption near edge structure (XANES) technique are employed to determine the site occupancy preference and valence state of the dopants in Ln3+-doped BaY2Si3O10 materials. The site-dependent luminescence properties of Ce3+ and Eu3+ are investigated in VUV−UV−vis regions at low temperatures. The dependencies of Ln3+ luminescence properties on doping concentration and temperature are discussed in detail. Finally, a scheme is provided to understand the internal connection between the luminescence properties of Ln3+ and the involved ion−lattice interaction by employing the Judd−Ofelt (J−O) analysis in

1. INTRODUCTION Luminescence properties of lanthanide (Ln)-doped compounds are generally determined by electronic configuration of the dopant and dynamic coupling between the dopant and host medium.1,2 The studies on luminescence properties of different Ln ions deliver an explicit comprehension on the internal correlations between them on the basis of the 4f electronic configuration.3−5 Meanwhile, variation of the local crystal structure around the Ln in the lattice also plays a key role in controlling its luminescence performance, especially the 4f−5d transitions.6 These analyses will help to understand the relationship of the coordination environment and luminescence property of the Ln ions and to further explore the potential applications of the materials.7 Ce3+, with a unique 4f1 electronic configuration, often serves as a “probe” to elucidate the crystal field strength, ligand polarization, and vibration coupling for the Ln ions in the lattice, and its parity-allowed 4f−5d luminescence is tunable in the UV−vis regions.8−10 Eu3+ is one of the most fascinating Ln ions because of the bright red emission arising from its 5D0 multiplet.11,12 More fundamentally, its f−f line splitting characteristic follows the transition selection rules and the experimental results as well as the Eu3+−O2− charge transfer (CT) energy in the spectra reflect the specific coordination © 2018 American Chemical Society

Received: January 31, 2018 Revised: February 21, 2018 Published: March 1, 2018 7421

DOI: 10.1021/acs.jpcc.8b01118 J. Phys. Chem. C 2018, 122, 7421−7431

Article

The Journal of Physical Chemistry C

Figure 1. (a) Rietveld refinement results of BaY2Si3O10 high-quality XRD pattern by the TOPAS program; (b) the variations of unit cell parameters (a, b, and c) of Ba(Y1−xCex)2Si3O10 with different Ce3+ contents; (c) the temperature-dependent variations of unit cell parameters of BaY2Si3O10.

Ba2+, Y3+, and the Ln3+ with the specific coordination number (CN). There is a larger ionic radius difference between Ba2+ and Ln3+ when the CN is equal to 8. Besides, the wedged-shape coordination environment would largely alter the outer 5d orbital shape of the Ln3+ when substituting the Ba2+ sites and this kind of replacement would cause a considerable distortion of the crystal structure even without considering the nonequivalent ion substitution issue. Accordingly, the Ln3+ ions would preferably occupy the Y3+ sites because of the similarity of their ionic radii and valence states. The dependence of unit cell parameters of Ba(Y1−xCex)2Si3O10 on Ce3+ concentration is given in Figure 1b. The values of a, b, and c increase when Ce3+ content increases. In consideration of the ionic radius difference (0.11 Å) of Y3+ and Ce3+, the results confirm that the Ce3+ prefer to enter the Y3+ sites. Figure 1c displays the temperaturedependent unit cell parameters of BaY2Si3O10. The results indicate that the increase of temperature causes the expansion of the unit cell. XRD patterns of representative samples Ba(Y0.99Ln0.01)2Si3O10 (Ln3+ = Eu3+, Pr3+, and Tb3+) at room temperature (RT) are shown in Figure S1a. They are in good agreement with the refined diffraction pattern of BaY2Si3O10, and there is no second phase observed. Ce L3-edge XANES spectra of Ba(Y1−xCex)2Si3O10 (x = 0.03, 0.05, and 0.09) samples are shown in Figure S1b. The results indicate that cerium ions exist predominantly in the form of Ce3+ in all doping samples and the influence of intervalence charge transition (Ce3+−Ce4+) on luminescence properties of Ce3+ is avoided in BaY2Si3O10.18 3.2. Luminescence of Ce3+ in BaY2Si3O10. To further confirm the site occupancy of Ce3+, the height-normalized excitation and emission spectra of Ba(Y0.99Ce0.01)2Si3O10 with different emission and excitation wavelengths at RT are measured, as shown in Figure S2. All excitation spectra overlap with each other, and the similar feature is seen in emission spectra. It indicates that the luminescence of the sample

combination with the vacuum referred binding energy (VRBE) diagram.

2. EXPERIMENTAL SECTION Several series of powder samples with Ln3+-doped BaY2Si3O10 (Ln3+ = Ce3+, Eu3+, Gd3+, Pr3+, Sm3+, and Tb3+) were prepared by a high-temperature solid-state reaction technique. The details of the sample preparation and characterization are described in the Supporting Information (SI). 3. RESULTS AND DISCUSSION 3.1. Crystal Structure. Figure 1a gives the Rietveld refinement results of high-quality powder X-ray diffraction (XRD) pattern of BaY2Si3O10 that were performed using the TOPAS program.17 The values of the refinement parameters Rp, Rwp, and Rexp are 3.447, 5.625, and 2.084%, respectively, which are within the rational range, demonstrating that the refinement results are reliable. BaY2Si3O10 has a layer structure arrangement on a-axis direction. Three [SiO4]4− polyhedra connect with each other by corner-sharing and forming an isolated [Si3O10]8− cluster in bc plane. Another two different types of cation sites exist in the structure: (A) Ba2+ sites coordinated by eight oxygen atoms with Cs symmetry and (B) distorted Y3+ sites with 6-fold coordination with C1 symmetry. As displayed in the inset of the figure, the Ba2+ site is located in the cavity with the half-surrounding of a [Si3O10]8− cluster and owns a wedge-shaped coordination polyhedron when considering the coordination effect by another back-forward [Si3O10]8− cluster. The Y3+ site is placed in the interval between different [Si3O10]8− clusters and follows a zigzag arrangement on b-axis direction, as marked by the dashed black arrow in the figure. Refined atom positions and unit cell parameters are listed in Table S1 and the refined bond lengths in Table S2. The distance between two nearest neighbor Ba2+ ions is 5.394 Å, and the closest distance between two Y3+ ions is 3.697 Å. The average Ba2+−O2− and Y3+−O2− bond lengths are 2.873 and 2.268 Å, respectively. Table S3 lists the ionic radii of 7422


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Figure 2. VUV−UV excitation spectrum of Ba(Y0.999Ce0.001)2Si3O10 with 400 nm emission at 15 K; the inset denotes the enlargement in 6.3−10 eV region of the curve.

Figure 3. Emission spectrum of Ba(Y0.999Ce0.001)2Si3O10 under 330 nm excitation; the inset represents the decay curve of Ce3+ emission in BaY2Si3O10 at 3 K.

from 4f ground state to the first (5d1), second (5d2), and third (5d3) 5d excited states, respectively. Moreover, the band at 5.59 eV (D, 222 nm, 45.1 × 103 cm−1) can be attributed to another 4f−5d transition of Ce3+ when substituting the Y3+ sites. Usually, Ce3+ possesses five 4f−5d excitation bands in the spectrum when replacing a low-symmetry site. The crystal field splitting (CFS) of Ce3+ 5d states corresponds to the energy difference between the lowest (4f−5d1) and the highest (4f− 5d5) f−d transitions. Its value is closely associated with the CN, the shape of coordination polyhedron, and the average distance between central Ce3+ and ligands.19 The CFS values of Ce3+ in the materials with 6-fold coordinated sites have been reported

originates from the Ce3+ ions at the same lattice site. Referring to the previous discussion, we consider that the Ce3+ would only enter the Y3+ sites but not the Ba2+ sites. Figure 2 represents the VUV−UV excitation spectrum of Ba(Y0.999Ce0.001)2Si3O10 with 400 nm emission monitored at 15 K. Two intense excitation bands (A and C) are clearly observed with a shoulder (band B) at the left of A. The peak energies of these bands are approximately 3.73 eV (A, 332 nm, 30.1 × 103 cm−1), 3.92 eV (B, 316 nm, 31.6 × 103 cm−1), and 4.25 eV (C, 292 nm, 34.2 × 103 cm−1), respectively, by deconvoluting the experimental curve into three Gaussian functions. Evidently, these bands correspond to the electronic transitions of Ce3+ 7423


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Figure 4. (a) Normalized emission spectra of Ba(Y0.999Ce0.001)2Si3O10 under 330 nm excitation at different temperatures; the dependence of Ce3+ integral emission intensity on temperature is plotted in the inset; (b) the decay curves of Ce3+ emission in Ba(Y0.999Ce0.001)2Si3O10 at different temperatures.

to be 18.9 × 103 cm−1 (Cs2NaErCl6),20 18.6 × 103 cm−1 (Cs2NaYCl6),21 23.3 × 103 cm−1 (Lu2Si2O7),22 22.0 × 103 cm−1 (YBO3),23 and 26.0 × 103 cm−1 (LiYSiO4),24 respectively. These CFS data cover a range of (18.6−26.0) × 103 cm−1, with an average value of 21.8 × 103 cm−1. In our case, the average Y−O bond length is shorter in comparison with most of these compounds.22 Therefore, the Ce3+ would experience a much stronger crystal field in BaY2Si3O10. Consequently, the band D should be the Ce3+ 4f−5d4 excitation band and the fifth band, 4f−5d5, is still missing. In the higher energy side, there are three broad bands observed from 6.3 to 10 eV in the excitation spectrum and their energies are estimated, respectively, to be 6.74 eV (E, 184 nm, 54.3 × 103 cm−1), 7.13 eV (F, 174 nm, 57.5 × 103 cm−1), and 8.29 eV (G, 150 nm, 66.7 × 103 cm−1) by fitting the curve with a sum of three Gaussian functions, as displayed in the inset of Figure 2. To assist in identifying the nature of these bands, the VUV excitation spectrum of Gd 3+ 313 nm emission (6P7/2−8S7/2) in Ba(Y0.99Gd0.01)2Si3O10 was measured at 15 K, as shown in Figure S3a. Two broad bands with maxima at 6.84 and 7.90 eV are seen and attributed to be host-related absorptions.25 Both band E and the highest-energy band G in Figure 2 are assumed to follow the origin similar to those in Figure S3a; therefore, the band F is confirmed as the Ce3+ 4f− 5d5 transition. Accordingly, the experimental CFS of Ce3+ in BaY2Si3O10 is 3.40 eV (27.4 × 103 cm−1). In addition, a semiempirical CFS estimation of Ce3+ in a 6-fold coordinated site has been reported.23 The estimated CFS value of Ce3+ in BaY2Si3O10 is 3.52 eV (28.4 × 103 cm−1), and the 4f−5d5 excitation energy is predicted to be 7.25 eV by adopting the value of Ce3+ 4f−5d1 excitation energy (3.73 eV). The results are consistent with the experimental values and further confirm the above assignments. Besides, the Ce3+ 5d energy centroid shift (εc) in BaY2Si3O10 is 1.43 eV (11.5 × 103 cm−1) with respect to that of the gaseous state (6.35 eV, 51.2 × 103 cm−1),

which is consistent with the typical region of 10 500−12 650 cm−1 of most silicates.23 Figure 3 shows the emission spectrum of Ba(Y0.999Ce0.001)2Si3O10 under 330 nm (3.76 eV, 30.3 × 103 cm−1) excitation at 3 K. A broad band with the maximum at 390 nm (3.18 eV, 25.6 × 103 cm−1) is observed, corresponding to transitions from Ce3+ 5d1 state to its 2F5/2,7/2 spin−orbit multiplets. The spectrum is deconvoluted to two Gaussian functions peaked at 3.32 eV (373 nm, 26.8 × 103 cm−1) and 3.06 eV (405 nm, 24.7 × 103 cm−1), respectively. The Stokes shift (ΔES) of Ce3+ luminescence is evaluated to be 3300 cm−1 (0.37 eV) using the energy difference between the maxima of 4f(2F5/2)−5d1 excitation and 5d1−4f(2F5/2) emission,9 which is slightly larger than those in most silicates, implying the stronger electron-vibrational interaction (EVI) of Ce 3 + in BaY2Si3O10.10,26 The inset represents the decay curve of Ce3+ emission in Ba(Y0.999Ce0.001)2Si3O10 at 3 K. It follows the single exponential characteristic with the decay time of 33.5 ns or the transition rate (AAR) of 2.99 × 107 s−1 from the reciprocal of decay time. Generally, the spontaneous radiative transition rate (ASR) of the activator in a dielectric medium is sensitive to the polarization effect being affected by its coordination environment and the ASR value for an electric-dipole (ED) transition is controlled by the polarization electric field strength (Epol) around the activator and usually termed the local-field effect.27,28 Herein, the ASR value of Ce3+ f−d transition can be simulated by the model29 ASR = 4.34 × 10−4 |⟨5d|r |4f⟩eff |2 ⟨υ ̅ ⟩3fcf f 2 n

(1)

where |⟨5d|r|4f⟩eff| represents the effective radial integral between Ce3+ 4f and 5d orbits and its value is 0.025 nm when Ce3+ remains in the gaseous state in vacuum.30 The term ⟨υ ̅ ⟩fcf is the Franck−Condon weighted emission wavenumber, f is the local-field effect factor equal to [(n2 + 2)/3], and n is the refractive index of lattice.27,31 Here, the n value is evaluated to be 1.97 by the J−O analysis, as shown in the following section. 7424


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Figure 5. Gaussian fit results of (a) excitation and (b) emission spectra of the representative Ba(Y1−xCex)2Si3O10 (x = 0.001, 0.01, and 0.05) samples at RT.

Thus, the simulated ASR value of Ce3+ f−d transition is 3.16 × 107 s−1, which corresponds to the experimental result, as given above. This result demonstrates that this kind of cation substitution (Y3+ → Ce3+) has little effect on the Epol in BaY2Si3O10 because of the similar electronegativity values (χ) of Y3+ (χ = 1.22) and Ce3+ (χ = 1.12).29 Accordingly, the Ln3+ ions would experience the similar polarization effect when substituting the Y3+ sites in BaY2Si3O10 because of the similar χ values (from χ = 1.10 of La to χ = 1.27 of Lu). The luminescence properties of Ba(Y0.999Ce0.001)2Si3O10 at different temperatures should be assessed. As displayed in Figure S4, the excitation spectra of Ce3+ at different temperatures were measured and normalized to the peaks. The relative intensity of Ce3+ 4f−5d3 excitation band decreases with the increase of temperature due to the more efficient thermal-ionization effect. Figure 4a represents the normalized emission spectra of the studied material under 330 nm excitation at different temperatures. When the temperature rises, the emission maximum shows a slight blueshift that may relate to the coordination polyhedron distortion. Also, the dependence of Ce3+ integral emission intensity on temperature is plotted in the inset of the figure. When the temperature increases to 500 K, the intensity of Ce3+ emission decreases to 75% of that at RT. The decay curves of Ce3+ emission in Ba(Y0.999Ce0.001)2Si3O10 at different temperatures are plotted in Figure 4b. Usually, luminescence intensity of the sample is easily influenced by distributed scattering centers and temperature-dependent absorption oscillator strength;32,33 therefore, the luminescence decay variation presents a more reliable characterization of the thermal-quenching behavior of Ce3+ luminescence. All decay curves overlap with each other, indicating that the thermal-quenching of Ce3+ luminescence is negligible in BaY2Si3O10 in the investigated temperature region.

Figure 5 provides the representative excitation and emission spectra of Ce3+ in Ba(Y1−xCex)2Si3O10 (x = 0.001, 0.01, and 0.05) at RT and the Gaussian fit results. When Ce3+ content increases, the peak of 4f−5d1 excitation shows a redshift (0.037 eV). This shift behavior is mainly controlled by the three contributions: (1) the CFS, (2) the ligand polarization effect, and (3) the lattice distortion. When more Ce3+ ions are doped into the lattice and substitute the Y3+ sites with a smaller ionic size, it indeed induces an expansion of the unit cell, as displayed in Figure 1b, but squeezes the Ce−O coordination, provided that the shape of coordination polyhedron is invariant. Therefore, the CFS should increase with the increase of Ce3+ content to some extent.34 Also, the change of the electrostatic binding effect from other neighboring cations (Y3+ → Ce3+) to the O2− ligands may impact the ligand polarization effect of central Ce3+ ion.22,35 Moreover, the substitution-induced lattice distortion makes some contributions to the peak shift. Figure 5b represents the emission spectra of the studied samples with different Ce3+ contents. The peak maxima of Ce3+ 5d1−2F5/2,7.2 emissions show a more significant redshift (0.071 and 0.081 eV, respectively) in comparison to that of the 4f−5d1 excitation band, leading to the increase of the ΔES value (approximately 300 cm−1) of Ce3+ luminescence. Usually, the variation of EVI makes an additional contribution to the peak shift of Ce3+ luminescence in the emission spectrum. The reported line shape modification functions indicate that the EVI is primarily determined by the vibronic coupling constant and the frequency of the vibrational mode in the electronic state of Ce3+.10,36 Here, the gradual incorporation of Ce3+ ions into smaller Y3+ sites causes a large lattice relaxation, resulting in the strong EVI in the system. Therefore, this effect further pushes down the lowest energy of Ce3+ 5d state. In addition, the reabsorption of Ce3+ luminescence makes its contribution to the emission peak shift to some extent.9 7425


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Figure 6. VUV−UV−vis excitation spectra of Eu3+ with 607 nm emission in Ba(Y0.99Eu0.01)2Si3O10 at low temperatures.

Figure 7. High-resolution emission spectrum of Ba(Y0.99Eu0.01)2Si3O10 under 393 nm excitation at 3 K, the red labeling numbers (1−5) denote the numbers of stark splitting levels of 7F0‑2 multiplets; the inset represents the decay curve and single exponential fitting of Eu3+ 5D0−7F2 emission at 3 K.

The decay curves of Ce3+ emission in Ba(Y1−xCex)2Si3O10 (x = 0.001−0.05) upon 330 nm excitation are shown in Figure S5. All curves follow the same single exponential and overlap with each other, demonstrating that the concentration-quenching of Ce3+ luminescence does not occur in this doping range. 3.3. Luminescence of Eu3+ in BaY2Si3O10. Figure 6 represents the VUV−UV−vis excitation spectra of Ba(Y0.99Eu0.01)2Si3O10 with 607 nm emission at low temperatures. The spectra mainly consist of a broad band below 250 nm and several weak sharp lines in the 300−550 nm region. The strong broad excitation band with the maximum at 225 nm (5.51 eV) is the Eu3+−O2− CT band. Meanwhile, a weak host-related absorption band is detected around 170 nm and no Eu3+ f−d excitation signal is observed in the spectrum. The sharp f−f

lines at 393, 414.1, 463.7, and 526.4 nm are attributed to transitions from the 7F0 ground state to the 5L6, 5D3, 5D2, and 5 D1 multiplets of Eu3+, respectively. Figure 7 shows the high-resolution emission spectrum of Ba(Y0.99Eu0.01)2Si3O10 under 393 nm excitation at 3 K. It can be distinguished that there is one 5D0−7F0 transition at 577.7 nm, three 5D0−7F1 transitions at 586.1, 589.6, and 595.9 nm, and five 5D0−7F2 transitions at 607.3, 610.7, 612.3, 620.2, and 623.6 nm, respectively. The strongest one is the 5D0−7F2-induced ED transition at 607.3 nm. These results are consistent with the fact that only the Y3+ sites are occupied by the Ce3+ in Ba(Y1−xCex)2Si3O10.5 The decay curve of Eu3+ 5D0−7F2 emission in Ba(Y0.99Eu0.01)2Si3O10 at 3 K is exhibited in the inset, and the curve follows the single exponential characteristic. 7426


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Figure 8. (a) Emission spectra of Ba(Y1−xEux)2Si3O10 (x = 0.01, 0.03, and 0.09) under 393 nm excitation at RT; (b) the decay curves of Eu3+ D0−7F2 emission with different contents at RT.

5

Table 1. Wavenumbers, Transition Rates, and U(λ) of Eu3+ f− f Transitions in BaY2Si3O10a

Because of the relatively low doping concentration, Eu3+ is regarded as an isolated ion in Ba(Y0.99Eu0.01)2Si3O10 and relevant energy transfer between different Eu3+ ions is negligible. Besides, considering a large energy gap (∼12 257 cm−1) between the 5D0 and high-lying 7F6 multiplets of Eu3+, the multiphonon relaxation from the 5D0 state is inefficient at 3 K. The measured decay time (τ0) of the 5D0−7F2 emission, 2.09 ms, should be intrinsic. With the increase of Eu3+ contents in Ba(Y1−xEux)2Si3O10 (from x = 0.01 to 0.09), the emission spectra under 393 nm excitation are exhibited in Figure 8a. All spectra have an identical emission line shape. Here, the J−O analysis is adopted to investigate the spectral properties and emission rates of Eu3+ 4f−4f transitions.37,38 The radiative Eu3+ 5D0−7F1 emission is the pure magnetic dipole (MD) transition, and its transition rate, which is free from the change of the surrounding crystal field, can be simulated by AJMD −J′ =

64π 4ν 3 3 n SMD 3h(2J + 1)

squared reduced matrix element43

64π 4e 2ν 3 2 f n 3h(2J + 1)

D0−7F1 D0−7F2 5 D0−7F3 5 D0−7F4 5 D0−7F5 5 D0−7F6

16 931 16 280 15 327 14 310 13 458 12 181

91.6 300.2 24.2 54.0 4.2 4.0

U(2)

U(4)

U(6)

0.0032

0

0

0

0.0023

0

0

0

0.000211,43

Ω2 = 3.95 × 10−20 cm2, Ω4 = 1.46 × 10−20 cm2, and Ω6 = 2.02 × 10−20 cm2. a

where AED J−J′ represents the ED transition rate from J to J′ multiplets; e is the electron charge (e = 4.80 × 10−10 esu); f is the local-field effect, as defined in eq 1; Ωλ (λ = 2, 4, and 6) is J−O parameters; and ⟨ΨJ||Uλ||Ψ′J′⟩ is the reduced matrix element for J−J′ transition. The squared reduced matrix elements ⟨ΨJ||Uλ||Ψ′J′⟩2 are often abbreviated to U(λ), and specific values have been previously reported.43−45 Consequently, the simulative 5D0−7F2,4,6 ED transition rates in Ba(Y0.99Eu0.01)2Si3O10 are listed in Table 1. The Ωλ (λ = 2, 4, and 6) values are derived via a least square fitting and listed in the last row of the table. When more Eu3+ ions substitute the Y3+ sites, the electrostatic binding effect from the cations (Y3+ → Eu3+; χ(Y) = 1.22, χ(Eu) = 1.20) to the O2− ligands decrease, which may slightly change the Epol in the lattice. As displayed in Figure 8a, the relative intensities of Eu3+ 5D0−7FJ (J = 0−6) emissions remain constant when Eu3+ contents increase. This result indicates that the Ω2 value is less affected by the amount of doping content. Usually, the Ω2 value is closely associated with the ligand polarization effect of the central Eu3+ ion.46,47 Accordingly, the change of the electrostatic binding effect in the lattice has little effect on the ligand polarization effect of the central Eu3+ ion. Also, this result

λ

λ = 2,4,6

transition rate (s−1)

5

(2)

∑

wavenumber (cm−1)

5

where AMD J−J′ represents the MD transition rate from J to J′ multiplets; ν is the average wavenumber of the transition; h is Planck’s constant; 2J + 1 is the degeneracy of the initial state J; n is the refractive index, as defined in eq 1; and SMD is the dipole strength of the MD transition (7.83 × 10−42 esu2·cm2) independent of the lattice.39 Usually, the MD radiative rate of Eu3+ 5D0−7F1 transition can be evaluated from its relative emission intensity in the spectrum.40,41 Accordingly, the experimental 5D0−7F1 MD radiative rate is 91.6 s−1, as listed in Table 1, and the n value of BaY2Si3O10 is deduced to be 1.97 within the typical region for most silicates.42 On the other hand, the Eu3+ 5D0−7F2,4,6 ED radiative transition rates can be simulated as follows AJED −J′ =

transition

Ωλ⟨ΨJ || U λ || Ψ′J ′⟩2 (3) 7427


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Figure 9. (a) VRBE scheme of Ln2+/3+ 4f and 5d states in BaY2Si3O10; (b) the VUV−UV excitation spectra of Ba(Y0.999Ce0.001)2Si3O10; (c−e) the VUV−UV excitation spectra of Ba(Y0.99Ln0.01)2Si3O10 (Ln3+ = Pr3+, Sm3+, and Tb3+), with the characteristic emission wavelengths (Pr3+: 5d1−3H4‑6, 280 nm; Sm3+: 4G5/2−6H5/2, 564 nm; and Tb3+: 5D4−7F5, 541 nm) at 15 K; the predicted 4f−5d1‑5 transition energies of Pr3+, Sm3+, and Tb3+ are depicted as red bars and marked in the figure.

confirms that the shift of Ce3+ 4f−5d1 excitation peak, as depicted in Figure 5a, is not affected by the change of ligand polarization in the studied materials. Figure 8b represents the decay curves of Eu3+ 5D0−7F2 emission with different contents at RT. The decay lifetime shortens and the curves deviate gradually from the exponential characteristic, demonstrating that the concentration-quenching of Eu3+ luminescence through the relevant de-excitation channel occurs in the sample with higher doping contents. On the basis of substantial discussions of luminescence properties of Ce3+ and Eu3+, the VRBE scheme of Ln 4f and 5d states in BaY2Si3O10 is constructed and shown in Figure 9a. The construction procedure of the VRBE scheme has been reported elsewhere.48,49 Several experimental data are employed: (1) the εc value (1.43 eV) of Ce3+ 5d states, (2) the Eu3+−O2− CT energy (5.51 eV), (3) the host absorption energy (Eex = 8.29 eV), and (4) Ce3+ 4f−5d1 transition energy (3.73 eV) in BaY2Si3O10. First, the Coulomb repulsion energy U(6) of Eu3+ in the Y3+ site is evaluated to be 6.92 eV by an empirical relationship, which is expressed as U(6) = 5.44 + 2.834 e−εc /2.2 (eV)

Ln3+ E4f(Ln3+) (or Ln2+, E4f(Ln2+)) are largely independent of the host lattice, the values of E4f(Ln3+), E4f(Ln2+) are obtained. Subsequently, the Eu3+−O2− CT energy is usually ascribed as the energy difference between the top of the valence band (VB) and the 4f ground states of Eu2+ because the top of the valence band (VB) of silicates is primarily formed by the O2− ligand 2p orbit, as confirmed by ab initio calculations.50 In addition, the energy difference between the top of the VB and the bottom of the conduction band (CB) is described as the energy band gap and its value is approximately 1.08 times larger than the Eex value.48 Consequently, the binding energies of the top of the VB, the bottom of the CB, and the exciton level are −9.53, −0.57, and −1.24 eV, respectively. Then, the binding energies of the 5d1 states of Ln3+ E5d1(Ln3+) (or Ln2+, E5d1(Ln2+)) can be derived by employing the redshift model.23 On account of the similar CFS and ligand polarization of different Ln3+ (or Ln2+) ions in the same site, the descending of 4f−5d1 transition energy relative to that of Ln3+ ions in gaseous state D(Ln3+) (or Ln2+, D(Ln2+)) is alike. In our case, the value of D(Ln3+) is 2.39 eV and that of D(Ln2+) can be deduced from the relationship51 D(Ln 2 +) = 0.64D(Ln 3 +) − 0.233 (eV)

(4)

3+

The values of E5d1(Ln ) and E5d1(Ln ) are then obtained. Finally, the exchange interactions between 5d electron spin and the total spin of the 4fn−1 electrons are considered when n > 7; therefore, the E5d1(Ln3+) in high spin [HS] and low spin [LS] states of Ln3+ [4fn (n > 7)] are evaluated.48 From the scheme, the extensive electron transfer processes can be predicted not only within f and d orbits of the Ln ions but also between the dopant and the host. Specifically, the thermal-quenching mechanism of Ce3+ luminescence can be studied by the assistance of this scheme. Thermal ionization of electrons in Ce3+ 5d1 state to the CB should be the most potential quenching pathway when the temperature in-

The binding energy of Eu2+ 4f ground state E4f(Eu2+) relative to the vacuum level (0 eV) is predicted to be −4.03 eV using the relation23 E4f (Eu 2 +) = − 24.92 +

18.05 − U (6) (eV) 0.777 − 0.0353U (6)

(6)

2+

(5)

Because U(6) = 6.92 eV equals the energy difference between the 4f ground states of Eu2+ and Eu3+, the binding energy of Eu3+ 4f ground state E4f(Eu3+) is evaluated to be −10.95 eV. Considering that the energy differences between E4f(Eu3+) (or E4f(Eu2+)) and the binding energies of 4f ground states of other 7428


Article

The Journal of Physical Chemistry C creases.24,52,53 However, in our case, the energy gap between Ce3+ 5d1 state and the bottom of the CB is 1.41 eV, as depicted in the scheme. This large energy gap restrains the potential of the electron thermal-ionization process even at higher temperatures; therefore, the Ce3+ luminescence intensity in BaY2Si3O10 is stable at different temperatures. Also, the 4f−5d1‑5 transition energies of other Ln3+ ions in BaY2Si3O10 can be predicted because of the similar CFS and ligand polarization effect. To facilitate the discussion, five Ce3+ f−d transition energies are plotted in Figure 9b as black bars. The VUV−UV excitation spectra of Ba(Y0.99Ln0.01)2Si3O10 (Ln3+ = Pr3+, Sm3+, and Tb3+) with the corresponding emission wavelengths (Pr3+: 5d1−3H4‑6, 280 nm; Sm3+: 4G5/2−6H5/2, 564 nm; Tb3+: 5D4−7F5, 541 nm) at 15 K are given in Figure 9c−e, respectively. Several excitation bands are observed in the spectra, which are attributed to the f−d transitions of Ln3+ when substituting the Y3+ sites. The predicted 4f−5d1‑5 transition energies of the Pr3+, Sm3+, and Tb3+ ions are depicted as red bars and marked in the figure. These predictions are well in accord with the experimental results. Besides, the Sm3+−O2− CT energy in Ba(Y0.99Sm0.01)2Si3O10 can be predicted to be 6.75 eV on the basis of the scheme, which is essentially consistent with the experimental result, as exhibited in Figure 9d. Moreover, the intense f−d excitation bands are observed in the VUV−UV excitation spectra of Sm3+ and Tb3+ when monitoring the f−f emission wavelengths. However, this phenomenon is absent in the VUV−UV excitation spectrum of Eu3+ with 607 nm emission monitored, as exhibited in Figure 6. An explanation for this distinction is that the energy difference between the 5d1 state and the highest 4f multiplet of Sm3+ (5d1 ↔ 2H9/2: 0.94 eV) and that of Tb3+ (5d1[LS] ↔ 5K7: 0.24 eV) is significantly smaller than that of Eu3+ (5d1 ↔ 5H5: 2.94 eV), as shown in the scheme. Consequently, electrons in Sm3+/Tb3+ 5d states can populate their respective 4f excited multiplet more efficiently and finally give rise to the corresponding f−f luminescence.54

Besides, the spectroscopic study of different Ln3+ ions in VUV−UV−vis regions gives the explicit information to predict the electron transfer processes within f and d orbits. The internal correlations of luminescence properties between different Ln3+ ions are emphasized when substituting the identical crystallographic site in the lattice. On the basis of the fundamental understanding of the information, the development of advanced phosphors with excellent properties can be achieved.

4. CONCLUSIONS Prepared by a high-temperature solid-state reaction routine, Ln3+-doped BaY2Si3O10 (Ln3+ = Ce3+, Eu3+, Gd3+, Pr3+, Sm3+, and Tb3+) phosphors exhibit the fascinating luminescence properties. The Rietveld refinement results indicate that the Ln3+ ions preferably occupy the Y3+ sites in BaY2Si3O10. The Ce3+ 4f−5d1‑5 transition energies, energy band gap of the lattice, are determined in VUV−UV−vis spectra at low temperatures, and the Ln3+ ions experience the similar polarization effect when substituting the Y3+ sites. With the increase of Ce3+ doping content, the ΔES of Ce3+ luminescence becomes larger, which is largely caused by the strong lattice relaxation in samples. The high-resolution emission spectrum of Eu3+ confirms that the ion only occupies the Y3+ sites. No Eu3+ f−f line-shape change is observed in the spectra when the Eu3+ content increases. It indicates that the change of the electrostatic binding effect in the lattice has little effect on the ligand polarization of the central Ln3+ ion, which is consistent with the result of the J−O analysis. On the other hand, the strong ion− ion interaction affects the Eu3+ luminescence decay process when Eu3+ doping content increases. With the assistance of the VRBE scheme, it is confirmed that the larger energy gap (∼1.41 eV) between Ce3+ 5d1 state and the bottom of the CB restrains the potential of the electron thermal-ionization process even at higher temperatures.

Rui Shi: 0000-0002-3120-0596 Xiaojun Wang: 0000-0003-1506-0762 Hongbin Liang: 0000-0002-3972-2049

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b01118. Details of sample preparations and measurements, refined unit cell parameters and refined positions of all atoms (Table S1), interatomic distances between Ba1/Y1 and O atoms (Table S2), ionic radii of Ba2+, Y3+, and Ln3+ with different coordination numbers (Table S3), XRD patterns and Ce3+ L3-edge XANES spectra of representative samples (Figure S1), height-normalized excitation and emission spectra of Ba(Y0.99Ce0.01)2Si3O10 with different emission and excitation wavelengths (Figure S2), VUV excitation and emission spectra of Ba(Y0.99Gd0.01)2Si3O10 (Figure S3), height-normalized excitation spectra of Ba(Y0.999Ce0.001)2Si3O10 at different temperatures (Figure S4), and decay curves of Ce3+ emission in Ba(Y1−xCex)2Si3O10 (x = 0.001−0.05) upon 330 nm excitation at RT (Figure S5) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.W.). *E-mail: [email protected] (H.L.). ORCID

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is financially supported by the National Natural Science Foundation of China (U1432249, 21671201, and U1632101) and the Science and Technology Project of Guangdong Province (2017A010103034).

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