Site Occupation of Eu2+ in Ba2–xSrxSiO4 (x = 0 ... - ACS Publications

Article Cite This: Inorg. Chem. 2018, 57, 7090−7096

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Site Occupation of Eu2+ in Ba2−xSrxSiO4 (x = 0−1.9) and Origin of Improved Luminescence Thermal Stability in the Intermediate Composition Litian Lin,† Lixin Ning,*,‡ Rongfu Zhou,† Chunyan Jiang,§ Mingying Peng,§ Yucheng Huang,‡ Jun Chen,† Yan Huang,∥ Ye Tao,∥ and Hongbin Liang*,†

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†

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of Environment and Energy Chemistry, School of Chemistry, School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou 510275, China ‡ Anhui Key Laboratory of Optoelectric Materials Science and Technology, Department of Physics, Anhui Normal University, Wuhu, Anhui 241000, China § The China-Germany Research Center for Photonic Materials and Device, State Key Laboratory of Luminescent Materials and Devices, and Guangdong Provincial Key Laboratory of Fiber Laser Materials and Applied Techniques, School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, China ∥ Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China S Supporting Information *

ABSTRACT: Knowledge of site occupation of activators in phosphors is of essential importance for understanding and tailoring their luminescence properties by modifying the host composition. Relative site preference of Eu2+ for the two d i s t i n c t t y p e s o f a l k a l i n e e a r t h ( A E ) s i t e s in Ba1.9995−xSrxEu0.0005SiO4 (x = 0−1.9) is investigated based on photoluminescence measurements at low temperature. We found that Eu2+ prefers to be at the 9-coordinated AE2 site at x = 0, 0.5, and 1.0, while at x = 1.5 and 1.9, it also occupies the 10-coordinated AE1 site with comparable preference, which is verified by density functional theory (DFT) calculations. Moreover, by combining low-temperature measurements of the heat capacity, the host band gap, and the Eu2+ 4f7 ground level position, the improved thermal stability of Eu 2+ luminescence in the intermediate composition (x = 1.0) is interpreted as due to an enlarged energy gap between the emitting 5d level and the bottom of the host conduction band (CB), which results in a decreased nonradiative probability of thermal ionization of the 5d electron into the host CB. Radioluminescence properties of the samples under X-ray excitation are finally evaluated, suggesting a great potential scintillator application of the compound in the intermediate composition. increasing the fraction of the Sr component.12,13 Moreover, the intermediate composition (x ≈ 1.0) exhibits higher luminescence thermal stability than the two end members.8,9,16 Although the variation of the emission color with the AE cation composition can be understood by the sensitivity of Eu2+ 5d energy levels to the crystalline environment, the latter variation of the thermal stability remains to be elucidated. Denault et al.8 correlated the improved thermal stability with the optimal bonding in the intermediate composition that results in a more rigid crystal structure as established by measurements of Debye temperatures. However, this correlation is not completely certain because only the Debye temperatures of one intermediate composition (x = 0.92) and

1. INTRODUCTION Lanthanide-activated luminescent materials have been widely investigated for applications as phosphors in white lightemitting diodes (wLEDs).1−4 As the operating temperature of wLEDs increases above a certain threshold (typically between 100 and 200 °C), thermal quenching of the luminescence from phosphors will occur and result in a decrease in the emission intensity and a change in the emission color.5−7 The thermal stability of luminescence is thus an important issue to be understood and optimized in terms of host composition. Recently, alkaline earth (AE) orthosilicate phosphors, AE2SiO4:Eu2+ (AE = Sr, Ba), first reported in 1968, have received renewed attention for their potential applications in wLEDs.8−15 The Eu2+ emission in Ba2−xSrxSiO4 can be efficiently excited under both blue and near-UV radiations, and the emission color can be tuned from green to yellow by © 2018 American Chemical Society

Received: March 22, 2018 Published: June 4, 2018 7090

DOI: 10.1021/acs.inorgchem.8b00773 Inorg. Chem. 2018, 57, 7090−7096

Article

Inorganic Chemistry

Figure 1. (a) XRD patterns for Ba2−xSrxSiO4 (x = 0−1.9) and (b) their enlarged views in the range from 28° to 33° at room temperature. (c) Refined unit cell parameters (a, b, and c), volume (V), and the occupancy factors (Occ.) for Ba and Sr atoms at the AE1 and AE2 sites. (d) 29Si MAS solid-state NMR spectra of the samples. Ba2−xSrxSiO4:Eu2+ hereafter) were synthesized by a solid-state reaction at high temperature. The starting materials included analytical grade BaCO3, SrCO3, and 99.99% purity SiO2 and Eu2O3. Stoichiometric amounts of the materials were thoroughly mixed in an agate mortar using alcohol as the mixing medium and then calcined in 10% H2/90% N2 reducing atmosphere at 1350 °C for 5 h. Powder samples of Ba1.999−xSrxCe0.0005Na0.0005SiO4 (denoted as Ba2−xSrxSiO4:Ce3+,Na+ hereafter) and Ba 1.999−x Sr x Eu 0.0005 Na 0.0005 SiO 4 (denoted as Ba2−xSrxSiO4:Eu3+,Na+ hereafter) with x = 0−1.9 were synthesized with similar methods by including Na2CO3 and CeO2 in the starting materials, where Na2CO3 was used to compensate for the excess charge of Ce3+/Eu3+ on the Ba2+/Sr2+ site. The mixtures of the starting materials for Ba2−xSrxSiO4:Eu3+,Na+ were calcined in air. High quality powder X-ray diffraction (XRD) data of Ba2−xSrxSiO4 (x = 0, 0.5, 1.0, 1.5, 1.9) were collected over a 2θ range from 10° to 110° using a step size of 0.02° and a dwell time-per-step of 3 s at Bruker D8 advance X-ray diffractometer with a Cu Kα X-ray tube operated at 40 kV and 40 mA. Rietveld refinements were performed using TOPAS-Academic V4.1.18 The 29Si magic-angle spinning (MAS) solid-state nuclear magnetic resonance (NMR) spectra of Ba2−xSrxSiO4 were measured at 9.4 T using a Bruker AVANCE III HD 400 instrument with tetramethylsilane (TMS) as a reference. The UV−vis excitation and emission spectra as well as the luminescence decay curves were recorded at an Edinburgh FLS920 combined with a steady-state and fluorescence lifetime spectrometer. The luminescence spectra were excited by a 450 W Xe900 continuous xenon lamp. The luminescence decay curves were measured using a 150 W nF920 ns flash-lamp (hydrogen filler gas) with a pulse width of less than 2 ns and a pulse repetition rate of ∼40 kHz. The excitation and emission spectra in the VUV−UV range were measured on the beamline 4B8 of BSRF.19 The X-ray excited luminescence was excited by an X-ray tube with a tungsten anode operated at 90 kV and 5 mA, and the spectra were recorded via Ocean Optics QE65000 spectrometer. 2.2. Computational Details. The Ba2−xSrxSiO4:Eu2+ crystals were modeled by 2 × 1 × 2 supercells containing 112 atoms, in which one of 32 Ba2+/Sr2+ ions was substituted by a Eu2+. For x = 0.5, 1.0, and 1.5, the supercells were constructed from the most stable structures of the unit cells, generated by using the methodology developed by GrauCrespo et al.20 The nearest distance between Eu2+ ions in the periodic supercell systems is larger than 9.8 Å, which is large enough to avoid their mutual influence in view of the localized 4f electronic states of

the two end members were measured for comparison. He and co-workers9 related the thermal stability to the degree of socalled cation ordering that characterizes the preferential occupations of AE cations. They found that the cation ordering reached its maximum at x = 1.0 at which the sample exhibits the best thermal stability of photoluminescence. In the orthorhombic structure of Ba2−xSrxSiO4, there are two crystallographically distinct Ba/Sr sites (AE1 and AE2 sites) of Cs symmetry, coordinated by 10 and 9 oxygen atoms, respectively. 17 At room temperature, Eu 2+ -activated Ba2−xSrxSiO4 displayed a single emission band under UV−vis excitations, which were ascribed to a combination of 5d → 4f transitions of Eu2+ at the two AE sites.8 In view of complexity of the host structures, knowledge of Eu2+ occupations at the AE1 and AE2 sites as a function of the Sr content is essential for further understanding of the photoluminescence tuning and the improved thermal stability in the intermediate composition. In the present work, we report photoluminescence properties of Eu-doped Ba2−xSrxSiO4 (x = 0−1.9) measured at low temperature. By analyzing the dependence of the excitation (emission) spectrum on the monitoring emission (excitation) wavelength, the site occupation of Eu2+ as a function of the Sr content is elucidated and verified by density functional theory (DFT) calculations. Moreover, on the basis of low-temperature measurements of the heat capacity, the host band gap, and the Eu2+ 4f7 ground level position, the improved thermal stability of Eu2+ luminescence in the intermediate composition (x = 1.0) is explained as due to an enlarged energy gap between the emitting 5d level and the bottom of the host conduction band (CB). This results in a decreased nonradiative probability of thermal ionization of the 5d electron into the host CB. Finally, the scintillation performances of the materials are evaluated under X-ray excitation at room temperature.

2. METHODOLOGY 2.1. Experimental Details. Powder samples of Ba 1 . 9 9 9 5 − x Sr x Eu 0 . 0 0 0 5 SiO 4 (x = 0−1.9) (abbreviated to 7091

DOI: 10.1021/acs.inorgchem.8b00773 Inorg. Chem. 2018, 57, 7090−7096

Article

Inorganic Chemistry

Figure 2. (a) UV−vis excitation and (b) emission spectra of Ba2−xSrxSiO4:Eu2+ (x = 0−1.9) recorded at 4 K. Eu2+. Periodic DFT calculations were performed using the PBE+U approach with U = 2.5 eV for the Eu 4f electrons,21,22 as implemented in the VASP code.23,24 The Ba(5s25p66s2), Sr(4s24p65s2), Si(3s23p6), O(2s22p4), and Eu(5s25p64f76s2) were treated as valence electrons, and their interactions with the cores were described by the projected augmented wave (PAW) approach.25 The atomic structures of the supercells were fully optimized until the total energies and the forces on the atoms were converged to 10−6 eV and 0.01 eV Å−1. One k-point Γ was used to sample the Brillouin zone, and the cutoff energy of the plane-wave basis was set to 530 eV.

and c are reduced by 2.3, 4.6, and 5.4%, respectively. Moreover, the variations of the unit cell parameters with Sr content are nonlinear, agreeing well with previous reports.9 It is most probably due to the preferential occupation of Sr on the AE2 site, as manifested by the refined occupancies listed in Tables S1−S5 of the SI and shown in panel v of Figure 1c. Only after the AE2 sites are almost filled by Sr at x = 1.0, Sr atoms start to occupy AE1 sites (at x = 1.5 and 1.9). This can be understood by the fact that the ionic radius of Sr2+ is smaller (by ∼0.16 Å) than that of Ba2+ in the 9- or 10-fold coordinations,26 and the size of the AE2 site is smaller than that of the AE1 site in Ba2SiO4. Thus, the size mismatch between the dopant and host ions exerts an evident effect on the site occupation of Sr in Ba2SiO4. Figure 1d displays the 29Si MAS solid-state NMR spectra of Ba2−xSrxSiO4. Each spectrum consists of a single resonant line at around −70 ppm, coinciding with the existence of only one type of SiO4 (Q0) tetrahedra in the unit cell. The resonance line moves very slightly toward the lower-field side with increasing Sr content, reflecting a small decrease of the mean bond length of Si−O,27 consistent with results from Rietveld refinements (Tables S1−S5 the SI). In addition, the line widths of the 29Si signals at x = 0.5−1.9 are broader than that at x = 0, which may be related to an increased long-range disorder of the 29Si environment.28 3.2. Site Occupation of Eu2+. Figure 2a,b displays the UV−vis excitation and emission spectra of Ba2−xSrxSiO4:Eu2+ recorded at 4 K. For the samples with x = 0, 0.5, and 1.0, the positions and profiles of the spectra are independent of the monitoring emission wavelength (λem) or the excitation wavelength (λex) (Figure S2 of the SI), and thus only one representative spectrum in each case is presented (panels i−iii of Figure 2). The emission spectra each consist of a single band, which can be related to 5d → 4f transitions of Eu2+ with similar coordination environments. These Eu2+ are supposed to be located at smaller AE2 (rather than larger AE1) sites, in view of the smaller ionic radius of Eu2+ than that of Ba2+. However, for the samples with x = 1.5, and 1.9, the spectral position and profile depend on λem or λex (Figure S2 of the SI), and thus, in each case two representative spectra are displayed (panels iv and v of Figure 2). A Gaussian deconvolution of the emission

3. RESULTS AND DISCUSSION 3.1. Structure Characterization. Figure 1a displays the XRD patterns of Ba2−xSrxSiO4 (x = 0−1.9) at room temperature. The diffraction profiles and peak positions are similar, and thus, the as-prepared samples formed continuous solid solutions. Figure 1b depicts enlarged views of the most intense diffraction peaks in the range from 28° to 33°. These peaks systematically shift toward higher-angle side from x = 0 to 1.9, showing a contraction of the unit cell with increasing Sr content. Moreover, the diffraction peak of (130) crystallographic plane merges with the adjacent peak of (200) plane at x = 0.5, and their ordering is interchanged at x = 1.0−1.9. Similar interchanges are also observed for other adjacent diffraction peaks, such as those of (022) and (112) reflections. These observations are correlated with different extents of the decreases in the interplanar spacing of (hkl) planes with the increase of Sr content. To further illustrate the dependence of crystal structure on Sr concentration, Rietveld refinements of the XRD patterns were performed with the orthorhombic structure (ICSD no. 6246) of Ba2SiO417 in Pmcn space group as the initial model. The results are presented in Tables S1−S5 and Figure S1 of the Supporting Information (SI), and the variations of the refined lattice parameters and the site occupancy factors are schematically shown in Figure 1c. The goodness of fit parameters (Rfactors and GOF in Tables S1−S5 of the SI) indicate that the samples are of single phase without detectable impurities. The unit cell parameters (a, b, c) and volume (V) decrease monotonously with increasing Sr content. The contraction of unit cell is anisotropic since, from x = 0 to 1.9, the values of a, b, 7092

DOI: 10.1021/acs.inorgchem.8b00773 Inorg. Chem. 2018, 57, 7090−7096

Article

Inorganic Chemistry

Figure 3. (a) Temperature-dependent luminescence lifetimes of Eu2+ at the AE2 site of Ba2−xSrxSiO4 (x = 0−1.9) in the range of 300−550 K. The temperature (T95%) for the onset of thermal quenching and the fitted values of the activation energy (Ea) are indicated in the legends. (b) Low temperature heat capacities and extracted Debye temperatures (ΘD) of Ba2−xSrxSiO4 (x = 0−1.9).

intermediate compositions of Ba2−xSrxSiO4. DFT calculations were thus performed for Eu2+ doping in Ba2−xSrxSiO4 (x = 0.5 and 1.5) supercells. It is shown that, in Ba1.5Sr0.5SiO4, Eu2+ still prefers to occupy the Ba2 (AE2) site over the Ba1 (AE1) site (by 216 meV), but not as strongly as in the case of Ba2SiO4 (with the energy difference of 326 meV). By contrast, in Ba0.5Sr1.5SiO4, the occupations of Eu at the Sr1 (AE1) and Sr2 (AE2) sites lead to very similar stability, with an energy difference of only 4 meV. To summarize, DFT calculations indicate that Eu2+ prefers to be at AE2 sites in Ba2SiO4:xSr2+ if these sites are not filled with Sr, like in the cases of x = 0, 0.5, 1.0 (Tables S1−S5 of the SI). If, however, the AE2 sites are almost completely occupied by Sr, as in Ba2−xSrxSiO4 (x = 1.5 and 1.9), Eu2+ tends to occupy the AE1 and AE2 sites with comparable preferences. These results are in support of the site preference of Eu2+ suggested on the basis of photoluminescence properties. 3.3. Origin of Improved Luminescence Thermal Stability. The decay curves of the Eu2+ emission at the AE2 site of Ba2−xSrxSiO4 (x = 0−1.9) are displayed in Figure S4 of the SI as a function of temperature from 300 to 550 K. It shows that the luminescence decay for each sample is strongly dependent on the temperature. The corresponding average luminescence lifetimes (τ), which were estimated by ∞ ∞ ∫0 tI(t ) dt /∫0 I(t ) dt , were plotted in Figure 3a. Also indicated in the figure are the temperatures (T95%) for the onset of thermal quenching at which the lifetime drops to 95% of its initial value at 300 K. The thermal quenching behaviors may be described by the equation

spectra under 335 or 340 nm excitation (Figure S3 of the SI) suggests the presence of two types of Eu2+ centers in the samples, which are supposed to be Eu2+ located at the AE1 and AE2 sites. Thus, the substitution of Sr for Ba in Ba2SiO4 has a strong influence on the luminescence of the dopant Eu2+, by inducing its redistribution in the material. Moreover, the excitation spectrum of Eu2+ emission at the AE2 site displays a slight redshift with increasing Sr content (Figure 2a), which is consistent with the decrease in the average AE2−O bond length as observed from Rietveld refinements of the XRD patterns (Tables S1−S5 of the SI). However, the redshift of the Eu2+ emission at the AE2 site is relatively large (Figure 2b), from 503 nm at x = 0 to 564 nm at x = 1.9 in the maximum emission wavelength, along with an increase of the full width at half maxima (fwhm) from 42 to 100 nm. This can be attributed to an enlargement of the Stokes shift with the increase of Sr content. To better understand the site occupation of Eu2+ in Ba2−xSrxSiO4, DFT calculations within the supercell model were carried out to evaluate the site preference of Eu2+ as a function of x. For Ba2SiO4:Eu2+, the DFT total energies of the relaxed EuBa1- and EuBa2-doped supercells reveal that Eu2+ strongly prefers to be at the Ba2 (AE2) site over the Ba1 (AE1) site (by 326 meV). Thus, the single emission band and the corresponding excitation spectrum in panels i−iii of Figure 2 can be ascribed to Eu2+ located at Ba2 sites of Ba2SiO4. It is noted that a similar preference for the Ba2 site (by 348 meV) was also observed for Sr2+ in Ba2SiO4, as expected from the similar ionic sizes of Eu2+ and Sr2+. Since now both Eu2+ and Sr2+, taken separately, preferentially occupy the Ba2 site, it is logical to see if the site preference of Eu2+ will change when SrBa substitutions are present in Ba2SiO4. To address this question, calculations for Eu-doped orthorhombic Sr2SiO4 supercells were carried out to determine if the site preference of Eu2+ changes from Ba2SiO4 to Sr2SiO4. Results show that the Sr1 (AE1) and Sr2 (AE2) sites are nearly equally favored by Eu2+ with a marginal energy difference of 21 meV. This means that from Ba2SiO4 to Sr2SiO4, Eu2+ loses its tendency to occupy the AE2 site, which may be related to the close ionic radii of Eu2+ and Sr2+. One may wonder whether this trend applies to

τ = τ0/[1 + τ0 Γ0 exp( −Ea /kT )]

(1)

where Ea is the activation energy for thermal quenching, τ0 is the radiative lifetime, Γ0 is the attempt rate, and k is the Boltzmann constant. A least-squares fitting of the formula to the experimental lifetime data yields the parameter values listed in Table S6 of the SI, with the values of Ea also indicated in Figure 3a. These results show that the sample with x = 1.0 exhibits the best thermal stability of Eu2+ luminescence with the highest activation energy, in agreement with previous reports.8 7093

DOI: 10.1021/acs.inorgchem.8b00773 Inorg. Chem. 2018, 57, 7090−7096

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Inorganic Chemistry

Figure 4. (a) Host excitonic absorption bands observed in the synchrotron radiation VUV−UV excitation spectra of Ba2−xSrxSiO4:Ce3+,Na+ (x = 0− 1.9) at 25 K. (b) Excitation and emission spectra of Ba2−xSrxSiO4:Eu2+ (x = 0−1.9) at 4 K, and the intersections denote the corresponding zerophonon lines. (c) Variations of Egap, Evf, Efd, and Edc as a function of Sr content in Ba2−xSrxSiO4:Eu2+ (x = 0−1.9).

In order to investigate if the thermal stability of Eu2+ luminescence is correlated with the structural rigidity of the host, as claimed in ref 8, we measured the low temperature heat capacities (Cp) of the samples Ba2−xSrxSiO4 (x = 0−1.9). The Debye temperatures (ΘD), usually taken as a proxy for structural rigidity,29 were determined by fitting the data to the Debye model in the low temperature limit30 3 12Nkπ 4 ⎛ T ⎞ Cp ≈ ⎜ ⎟ 5 ⎝ ΘD ⎠

First, to study the dependence of Egap on Sr content, VUV− UV excitation spectra of Ce3+ emission at the AE2 site of Ba2−xSrxSiO4:Ce3+,Na+ were measured by using synchrotron radiation at 25 K. We mention that the emission of Ce3+ at the AE2 site can be easily discriminated on the basis of its strong tendency to occupy this site.31 The results are shown in Figure S5 of the SI, where the excitation bands with peaks at about 175 nm are ascribed to the host excitonic absorption. An enlarged view of them are given in Figure 4a, showing a slight blue shift of the band maximum with increasing Sr content. By taking into account the electron−hole binding energy of the exciton,32 the host band gaps can be estimated and are listed in the second row of Table 1. Second, the values of Evf for Eu2+ at the

(2)

where N is the Avogadro’s number multiplied by the number of atoms per formula unit, k is the Boltzmann constant, and T is the temperature. The results are shown in Figure 3b. One sees that the sample with x = 1.5 has the largest ΘD value, but not the sample (x = 1.0), which displayed the best thermal stability of Eu2+ luminescence. Thus, caution should be exercised when correlating the thermal stability of Eu2+ luminescence with the structural rigidity of the host. The thermal quenching of Eu2+ luminescence in inorganic compounds is usually explained as due to thermal ionization of the 5d electron into the host CB.16 In this mechanism, the thermal quenching behavior may also be described by eq 1, where the activation energy (Ea) represents now the energy difference (Edc) between the emitting Eu2+ 5d level and the bottom of the host CB. To confirm this mechanism for the present case, the variation of Edc as a function of the Sr content were investigated by using the following relationship: Edc = Egap − Evf − Efd

Table 1. Energy of the Host Band Gap (Egap), Energy Difference (Evf) between the EuAE22+ 4f Ground Level and the Top of the Host VB, Zero-Phonon Line Energy of EuAE22+ 4f−5d Transition, and Energy Difference (Edc) between the Emitting EuAE22+ 5d Level and the Bottom of the Host CB, as a Function of Sr Content in Ba2−xSrxSiO4:Eu2+ (x = 0−1.9)a

a

(3)

x

0

0.5

1.0

1.5

1.9

Egap Evf Efd Edc

7.51 4.45 2.69 0.37

7.53 4.49 2.65 0.39

7.58 4.55 2.61 0.42

7.63 4.91 2.48 0.24

7.64 4.99 2.42 0.23

All values are in units of eV.

AE2 site of Ba2−xSrxSiO4 were approximated by the energies of charge transfer (CT) transition energies of Eu3+ at the same site.33 Accordingly, synchrotron radiation VUV−UV excitation spectra of the 5D0 → 7F2 emission (at ∼609 nm) of Eu3+ at the AE2 site of Ba2−xSrxSiO4:Eu3+,Na+ were recorded at 25 K (see Figure S6 the SI). It shows that the CT transition band shifted gradually to the shorter wavelength side with increasing Sr content, as expected from the decrease of the AE2 site size, which should produce an enlarged CT transition energy of Eu3+.34 From the maxima of the CT transition bands, the Evf

where Egap is the band gap energy of the host, Evf denotes the energy difference between the Eu2+ 4f7 ground level and the top of the host valence band (VB), and Efd represents the energy of the zero-phonon line (ZPL) of Eu2+ 4f−5d transition. By monitoring the changes of Egap, Evf, and Efd with Sr content, the variation of Edc was determined. It is important to note that although the absolute values of these quantities are subject to systematic estimation errors in experiments (shown below), the trends of changes with increasing Sr content are more accurate. 7094

DOI: 10.1021/acs.inorgchem.8b00773 Inorg. Chem. 2018, 57, 7090−7096

Article

Inorganic Chemistry values for Eu2+ at the AE2 site were estimated and are listed in the third row of Table 1. Third, the ZPL energies Efd were determined from the excitation and emission spectra of Ba2−xSrxSiO4:Eu2+ (x = 0−1.9) at 4 K by employing the mirror image method, as shown in Figure 4b, and the results are listed in the fourth row of Table 1. It shows that the ZPL gradually shifts toward lower energies with increasing Sr content as a result of an enhanced 5d crystal field interaction with the local environment due to the shrinkage of the AE2 coordination polyhedron. Finally, by combining the values of Egap, Evf, and Efd, the values of Edc at various Sr contents were derived from eq 3 and are listed in the bottom row of Table 1. The variations of these quantities as a function of the Sr content are also plotted in Figure 4c. It clearly shows that at x = 1.0 the value of Edc reaches a maximum, in agreement with the trend observed for the activation energy (Ea) of thermal quenching (Figure 3a). This implies that the improved thermal stability of Eu2+ luminescence in the x = 1.0 composition is due to an enlarged energy gap between the 5d level and the bottom of the host CB, which leads to a decreased probability of thermal ionization of the 5d electron into the host CB. 3.4. Radioluminescence Properties. In a previous study,35 it was shown that the light yield of Eu2+-activated Ba2SiO4 under X-ray excitation is 1.7 times larger than a reference scintillator Bi4Ge3O12. It is thus timely to evaluate the scintillation performance of Ba2−xSrxSiO4:Eu2+ as a function of Sr content. Figure 5 shows the X-ray excited luminescence

Lu3Al5O12:Ce3+, under X-ray excitation and has potential as a promising scintillation material.

4. CONCLUSIONS Photoluminescence properties of Eu2+ in Ba2−xSrxSiO4 (x = 0− 1.9) have been investigated at low temperature. From the dependence of the excitation (emission) spectrum on the monitoring emission (excitation) wavelength, the site occupation of Eu2+ as a function of Sr content has been elucidated. It was found that Eu2+ tends to occupy the 9-coordinated AE2 site at x = 0, 0.5, and 1.0, but at x = 1.5 and 1.9, it also occupies the 10-coordinated AE1 site with preference comparable to the AE2 site. This finding was also confirmed by DFT calculations on the relative stabilities of Eu2+ occupations at the AE1 and AE2 sites. Furthermore, on the basis of low-temperature measurements of the heat capacity, the host band gap, and the Eu2+ 4f7 ground level position, the improved thermal stability of Eu2+ luminescence in the intermediate composition (x = 1.0) was explained as due to an enlarged energy gap between the emitting 5d level and the bottom of the host CB. This enlarged energy gap leads to a decreased nonradiative probability of thermal ionization of the 5d electron into the host CB. Finally, X-ray excited luminescence properties of the compounds were measured, and the results suggest a great potential application of Ba2−xSrxSiO4:Eu2+ (x = 1.0) as a scintillation material. This work demonstrates the usefulness of low-temperature spectral measurements to enhance our understanding of luminescence properties of Eu2+ in complex host compounds and also a systematic approach to elucidate the variation of thermal quenching behavior when the composition engineering procedure is adopted to optimize luminescence properties of phosphors.

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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.inorgchem.8b00773. Refined values of structure parameters and bond distances (Tables S1−S5), least-squares fitting parameter values of the experimental Eu2+ lifetimes (Table S6), Rietveld refinements of XRD data (Figure S1), UV−vis excitation and emission spectra of Eu2+ (Figure S2), Gaussian deconvolution of the emission spectra of Eu2+ (Figure S3), temperature-dependent (300−550 K) decay curves of Eu2+ (Figure S4), and VUV−UV excitation spectra of Ce3+ and Eu3+ (Figures S5 and S6) (PDF)

Figure 5. X-ray excited luminescence spectra of Ba2−xSrxSiO4:Eu2+ (x = 0−1.9) and Lu3Al5O12:Ce3+ (LuAG:Ce) in single-crystal and powder forms for comparison.

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spectra of these samples and those of the commercial scintillator Lu3Al5O12:Ce3+ in single-crystal and powder forms for comparison, measured at the same experimental conditions. It is interesting to see that, under X-ray excitation, the asprepared samples show only typical Eu2+ emissions from AE2 sites. The reason for no observation of Eu2+ emissions from AE1 sites at x = 1.5 and 1.9 is possibly due to an efficient energy transfer from Eu2+ at AE1 sites to those at the AE2 site at room temperature, in spite of the low doping concentration of Eu2+. The light yields of the x = 0, 0.5, 1.0, 1.5, and 1.9 samples were estimated to be 12 200, 27 000, 30 000, 20 600, and 22 000 photon/MeV, respectively, when using the commercial Lu3Al5O12:Ce3+ powder (18 000 photon/MeV)36 as a reference. Thus, the sample with x = 1.0 exhibits the highest light yield, around 1.7 times that of the commercial

AUTHOR INFORMATION

Corresponding Authors

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

Litian Lin: 0000-0003-3241-2651 Lixin Ning: 0000-0003-2311-568X Mingying Peng: 0000-0002-0053-2774 Yucheng Huang: 0000-0002-7818-8811 Jun Chen: 0000-0001-7397-2714 Hongbin Liang: 0000-0002-3972-2049 Notes

The authors declare no competing financial interest. 7095

DOI: 10.1021/acs.inorgchem.8b00773 Inorg. Chem. 2018, 57, 7090−7096

Article

Inorganic Chemistry

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ACKNOWLEDGMENTS The work is financially supported by the National Natural Science Foundation of China (21671201, 11574003, U1432249, and U1632101) and the Science and Technology Project of Guangdong Province (2017A010103034).

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DOI: 10.1021/acs.inorgchem.8b00773 Inorg. Chem. 2018, 57, 7090−7096