VUV and X-ray excitation

Optical Materials 45 (2015) 13–21

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Optical Materials journal homepage: www.elsevier.com/locate/optmat

The electronic structure and luminescence properties of Ce3+ doped Sr10[(PO4)5.5(BO4)0.5]BO2 under UV/VUV and X-ray excitation Ang Feng a,b, Zhi-Jun Zhang c,⇑, Lin-Lin Zhu a,b, Ri-Hua Mao d, Jing-Tai Zhao c,⇑ a

Key Laboratory of Transparent Opto-Functional Inorganic Materials of Chinese Academy of Sciences, Shanghai Institute of Ceramics, Shanghai 200050, PR China University of Chinese Academy of Sciences, Beijing 100039, PR China c School of Materials Science and Engineering, Shanghai University, Shanghai 200072, PR China d Laboratory for Advanced Scintillation Materials & Performance, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, PR China b

a r t i c l e

i n f o

Article history: Received 12 January 2015 Received in revised form 9 February 2015 Accepted 24 February 2015 Available online 23 March 2015 Keywords: Sr10[(PO4)5.5(BO4)0.5]BO2 VUV Electronic structure Luminescence

a b s t r a c t The apatite related compound Sr10[(PO4)5.5(BO4)0.5]BO2 (SrBPO) doped with Ce3+ was synthesized via solid state reaction method. Undoped SrBPO shows blue-green emission under ultraviolet (UV) and X-ray excitation due to the defects in the host. When excited by vacuum ultraviolet–ultraviolet (VUV–UV) light or X-ray, Ce3+ doped SrBPO shows a broad emission band peaking at 450 nm originating from 5d–4f transition of Ce3+ and defects in the host. The phosphor exhibits strong excitation bands in UV range and a weak broad excitation band in VUV region. The site occupation of Ce3+ was proposed based on fluorescence decay curves. Electronic structure shows the compound is an indirect semiconductor with a band gap of 3.04 eV. The extremely small density of states of [PO4]3 or [BO4]5 group near Fermi level or in the conduction band is a possible origin of the weak excitation band in the VUV range. A possible mechanism was proposed to explain the luminescence properties observed. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Apatites, represented by general formula M10(ZO4)6X2 (M = Ca2+, Sr2+, Ba2+, Pb2+, La3+, Na+, etc., Z = P5+, Si4+, V5+, As5+, etc., X = F, Cl, I, OH, etc.), have shown a wide range of miscibility to chemical substitutions. They have been explored as phosphors [1], biocompatible materials [2], chemical catalysts [3], electrolytes for solid oxide fuel cells [4], and optical laser hosts [5,6]. In particular, phosphate apatites, M10(PO4)6X2, have found applications in phosphors and solid state lighting industry, due to their excellent properties, such as low preparation temperature, high thermal stability, high valence stability of dopant in such hosts. Among many dopants, Ce3+ carries huge importance because of its simple electronic configuration ([Xe]4f1) for theoretical investigation and wide utilization as blue-green light emitter or sensitizer for other lanthanides. Chen et al. synthesized SrBPO and solved its crystallographic structure to be a derivative of the apatite Sr10(PO4)6F2 [7]. About 8.33% of [PO4]3 tetrahedra are replaced by [BO4]5, and the displacement of [PO4]3/[BO4]5 groups destroys the characteristic mirror plane for apatite  Sr ions take two structure and render the space group P3. ⇑ Corresponding authors. E-mail addresses: [email protected] (Z.-J. Zhang), [email protected] (J.-T. Zhao). http://dx.doi.org/10.1016/j.optmat.2015.02.031 0925-3467/Ó 2015 Elsevier B.V. All rights reserved.

Wyckoff sites: 2d (C3 symmetry, Sr1 and Sr2) and 6g (C1 symmetry, Sr3). Sr1 and Sr2 are nine-fold coordinated with oxygen atoms that belong to [PO4]3/[BO4]5 tetrahedra, leading to average interatomic distances dSr–O of 2.720 and 2.676 Å, respectively. A Sr3 ion is surrounded by six oxygen from [PO4]3/[BO4]5 units and one oxygen from [BO2] unit, and the Sr–O distance dSr–O equals 2.620 Å. The O–B–O trimer (B atom at (0, 0, 1/2)), parallel to c-axis, lies between two adjacent triangles constituted by Sr3 ions, which is unlike to its Sr10(PO4)6F2 counterpart where F ions (at (0, 0, 1/4)) are exactly in triangle planes. Recently, the compound was developed as excellent phosphor for white light light-emitting diodes application when doped with Eu2+/Ce3+, Tb3+, Mn2+ ions [8,9]. For Ce3+ doped SrBPO, the author observed a very large full width at half maximum (FWHM) of emission band, 5030 cm1, and ascribed it to the luminescence of Ce3+ occupying two different Wyckoff sites in the compound [9]. However, in many strontium apatite hosts, such as Sr5(PO4)3F [10], Sr5(SiO4)(PO4)2 [11], Ce3+ is supposed to only occupy the compact 6g site with smaller coordination number and shorter average Sr–O distances than that of 4f site. Furthermore, there have been reports on defect related luminescence of phosphate apatites [12,13], which may shed light upon the source of the large FWHM of emission band in SrBPO: 0.03Ce3+ phosphor. This raises some confusion about the origin of large FWHM of emission band for this material, which apparently needs further investigation.

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Host excitation of phosphate apatites are often strong as the case of Sr5(PO4)5F [10], and the origin of very weak excitation band in phosphate compound is of huge interest. The size and shape of anion species, X, has a great influence on its electronic [14] and luminescence [15] properties of apatite Ca10(PO4)6X2; therefore, the impact of inclusion of [BO2] unit in SrBPO on the electronic and luminescence properties apparently needs investigation urgently. For the reasons aforementioned, we investigated the luminescence properties of Ce3+ doped in SrBPO using VUV–UV and X-ray excitation and proposed possible site occupancy of Ce3+ in the compound based on luminescence decay and computational results. In addition, the electronic structure was calculated by first principle calculations and its possible influence on the luminescence properties has been proposed.

2. Experimental section Sr10(1–2x)[(PO4)5.5(BO4)0.5](BO2): xCe3+, xLi+ (x = 0, 0.008, 0.01, 0.02, 0.03, 0.05) powder samples were prepared according to literature [7] via solid state reaction method. The starting materials were analytical grade SrCO3, NH4H2PO4, H3BO3, Li2CO3 and high purity CeO2 (99.99%). Appropriate amounts of reactants were weighted, thoroughly mixed and grounded in an agate mortar. The mixture was placed in a closed alumina crucible and heated at 773 K for 3 h. The preheated powders were ground again and subsequently fired at 1523 K for 8 h in a horizontal tube furnace under H2 (5%)–Ar (95%) atmosphere. After cooling down to room temperature, the final products were crashed and ground into fine powders for characterization. The phase formation was verified by powder X-ray diffraction (XRD) method through using a HUBER Guinier Imaging Plate G670 (Cu Ka1 radiation (k = 1.54056 Å), Ge monochromatic), which was operated at 30 mA and 40 kV in air. The 2h ranges of all data sets are from 10° to 100°, with a step size of 0.005°. The lattice parameters were obtained by least square fit of diffraction data using WinCSD program [16]. Morphology observation and chemical composition examination of SrBPO: 0.03Ce3+, 0.03Li+ were performed on a field emission scanning electron microscope (Magellan 400) equipped with an energy-dispersive spectrometer (EDS) attachment. The diffuse reflectance spectrum was recorded on a Varian Cary 5000 spectrophotometer, and the data was calibrated with the reflection of white barium sulphate (BaSO4, reflection 100%) in the wavelength range from 200 to 700 nm. The emission and excitation spectra, as well as luminescence decay profile were measured on an Edinburgh FSL980 spectrometer which equips with a continuous Xeon lamp, an nF920 lamp, and a lF2 lamp as excitation sources. Excitation and emission spectra of pure SrPBO and SrBPO: 0.03Ce3+, 0.03Li+ in the vacuum ultraviolet were collected at the Beijing Synchrotron Radiation Facility (BSRF), and the details of experimental setting can be found in a specific Ref. [17]. X-ray excited luminescence (XEL) spectra were measured on a home-made spectrometer (SicOmin-X) in which an F-50 tube (W anticathode target) operating at 50 mA and 70 kV was used as the X-ray source at room temperature [18]. For the purpose of investigating the electronic structure of the title compound, a 1  1  2 super cell was constructed as the structure model where one of the twelve equivalent [PO4]3 group was replaced by [BO4]5 group, resulting in the right chemical formula.  Accordingly, the space group of the supercell turned out to be P 1 (No. 2). The crystal structure relaxation, band structure and density of states (DOS) calculations were performed by the plane wavepseudopotential CASTEP code [19]. Perdew–Burke–Ernzerhof (PBE) functional in general gradient approximation (GGA) [20] was adopted as the framework to solve Kohn–Sham equation

iteratively based on density mixing technique, while ultra-soft format of potentials suggested by Vanderbilt [21] were selected from a database provided along with CASTEP. The plane wave cut-off energy was chosen at 500 eV and kept unchanged throughout. The Broyden–Fletcher–Goldfarb–Shanno (BFGS) method [22] was used to relax the crystal structure to its ground state. In this stage, a Monkhorst–Pack k point mesh [23] of 4  4  2 was used, and the convergence threshold were 1.0  106 eV/atom for total energy, 0.01 eV/Å for forces, 0.01 GPa for stress, and 5  104 Å for ionic displacement, respectively. The band structure and DOS (including partial DOS) were calculated after the structure relaxation, and the total energy convergence criterion was 5  107 eV/atom. A much denser k point mesh of 6  6  4 was used to calculate DOS and partial DOS accurately. The Mulliken charges and bond overlap populations were calculated by projecting the plane waves onto a localized linear combination of atomic orbitals (LCAO) [24,25]. For a comparison purpose, the band structure and DOS of Sr10(PO4)6X2 (X = F, Cl, Br, OH) were also calculated using similar parameters for SrBPO. To figure out the site occupancy of Ce3+ in SrBPO, cohesive energies of virtual Sr10(PO4)6(BO2)+: Ce3+ and SrBPO: Ce3+ (1  1  2 supercell) where Ce3+ occupy different crystallographic sites of Sr ions were calculated using spin polarized DFT (PBE functional). All parameters have been tested against convergence. 3. Results and discussion 3.1. Phase formation The XRD patterns and cell parameters of samples SrBPO: xCe3+, xLi (x = 0, 0.008, 0.01, 0.02, 0.03 and 0.05) were shown in Fig. 1(a) and (b), respectively. The Li+ ions has been added as charge compensators for Ce3+ in the host. It can be clearly seen that all diffraction peaks agree well with the result reported by Chen et al. [7] (ICSD No. 421389), and can be indexed into space group  (No. 147). As depicted in Fig. 1(b), the lattice parameter a P3 increases while parameter c decreases but the cell volume remains almost constant with increasing Ce3+ concentration from x = 0 to 0.05. The typical scanning electron microscopy image of a selected sample (x = 0.03) is shown in Fig. 2 (a). It is clearly seen that a crystal of hexagonal prismatic shape with 60 lm in length is at present together with many particles in irregular shapes. Fig. 2(b) shows the EDS results of two selected areas (shown in the insect) of the crystal and several peaks of Sr, P, O, Ce elements are distinctive from the background. It is noticeable that peaks of boron is absent because of limitation of the apparatus and that the peak of aluminum is from the sample holder in the examination process. The ratio of elements P, O, Ce with respect to Sr element in Table 1 are close to theoretical calculations, confirming the composition of the targeted sample. +

3.2. Electronic structure of SrBPO A 1  1  2 super-cell was applied to construct the model of the random 8.33% replacement of [PO4]3 by [BO4]5, hence the starting parameter was adopted from experimental values [7] a = b = 9.7973 Å, c0 = 2c = 14.6112 Å, a = b = 90°, and c = 120°. After fully relaxing the cell and ions positions, the parameters were a = 9.8339(7) Å, b = 9.8626(6) Å, c0 = 14.7332(6) Å, a = b = 90°, and c = 120°, which deviated within 4% from the starting values. The total DOS, partial DOS of anion groups, partial DOS of Sr atom were shown in Fig. 3(a)–(c). The conduction band was dominated by B 2p and O 2p from [BO2] near the bottom and Sr 4d states and tiny amount of [BO4]5 or [PO4]3 states at higher energy. This differs dramatically from Sr5(PO4)3F (Fig. S1 (a) in

A. Feng et al. / Optical Materials 45 (2015) 13–21

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Fig. 1. (a) XRD patterns and (b) cell parameters of SrBPO: xCe3+, xLi+ (x = 0, 0.008, 0.01, 0.02, 0.03 and 0.05).

Fig. 2. (a) Typical SEM image and (b) EDS spectra of SrBPO: 0.03Ce3+, 0.03Li+ (the insect shows two different areas for which EDS spectra were measured).

Table 1 Atomic ratios of P, O, Ce relative to Sr in SrBPO measured from EDS results.

Position 1 Position 2 Theoretical

P/Sr

O/Sr

Ce/Sr

0.478 0.504 0.585

2.725 2.709 2.76

0.027 0.028 0.032

Supporting Information) or Ca5(PO4)3F [14] where the bottom of conduction band is mainly occupied by d states of metal ions with considerable content of [PO4]3 states. The DOS of valence band can be divided into 3 regions: (i) 10 to 0 eV; (ii) 23 to 13 eV; (iii) 35 to 30 eV. In region (i), peaks E, F and shoulder of peak D are composed of O 2p states of [BO4]5 group, as can be seen from Fig. 3(b). It is clear that B atoms of [BO4]5 do not make a contribution to DOS near Fermi level but they interact with O atoms through s/p hybridization, which is very similar to that of P in [PO4]3 (Fig. S2 in Supporting Information). The peaks A–D are mainly composed of DOS of [PO4]3 and their shoulders stem from a hybridization with [BO4]5 or with [BO2]. Further, peak 3 and peak 30 in region (ii) are mainly composed of Sr 3p states, with little contribution from [BO4]5 and [PO4]3, which are responsible for the shoulder peaks. This is in accordance with the case of Sr5(PO4)3F, in which Sr ions show considerable interaction with O atoms from [PO4]3 group at about 13 eV (Fig. S1 (a) in Supporting Information). Region (iii) is dominated by Sr 2s/3s states which have little interaction with other atoms (Fig. S3 in Supporting Information).

Selected bond lengths and Mulliken bond overlap population of Sr–O bonds, B–O bonds were listed in Table S1 (Supporting Information); all these atoms labeled were depicted in Fig. S4 (Supporting Information). Apparently, O atoms which belong to [BO4]5 group are more strongly bonded to nearby Sr atoms as manifested by shorter Sr–O distances and larger overlap populations, compared to O atoms that belong to [PO4]3. This is in consistent with the reality that Sr ions hybridize with [BO4]5 through O atoms as shown by DOS analysis. Compared to [PO4]3, the smaller size and higher charge state of [BO4]5 create stronger electric field and attracts Sr ions more violently to its neighbourhood and lead to stronger Sr–O interaction. As B and P possess similar electronegativity (2.0, 2.1 for B and P, respectively), they show similar covalent bonding to O, as verified by close overlap population (0.60, 0.59 respectively). Interestingly, B–O bond in [BO2] group has large overlap population meanwhile the Sr–O bond (O from [BO2]) shows a population of 0.03, which is smaller than that of Sr–F bond in Sr5(PO4)3F (0.11). This suggests huge covalent content of B–O bond and ionic Sr–O bonding. As is known, F ions are at the center of triangles formed by Sr2 of Sr5(PO4)3F and have short distances to nearest Sr atom and thus show considerable amount of bond overlap population, which is also true for Ca5(PO4)3X (X = F, Cl, Br, OH) [14]. In our case, the O–B–O unit run parallel to c axis and perpendicular to triangles formed by Sr3 ions, and its big size and linear shape may allow distributing charges that does not participated in Sr bonding further from the plane via O atoms. Consequently, B–O bond show very high bond population and that of Sr–O bond remains quite low.

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Fig. 3. (a) Full DOS of SrBPO and partial DOS of [PO4]3, [BO4]5, [BO2] and Sr; (b) DOS of [BO4]5 group; and (c) DOS of [BO2] group.

Judging from the band structure shown in Fig. 4(a), the crystal is an indirect band gap semiconductor with conduction band minimum at C(0 0 0) and the valence band maximum at F(0 1/2 0) (insect of Fig. 4(a)) and the band gap is 3.04 eV, which is far less than that of Sr5(PO4)3F (5.35 eV, Fig. S1 (b) in Supporting Information). The band near Fermi level is very flat and is mainly composed of O 2p states from [BO4]5, and this probably results from localized states introduced by [BO4]5. The replacement of P with element of lower electronegativity tends to reduce the band gap of phosphate apatite, as shown in the case of Ca10xLax(PO4)6x(SiO4)xF2 (0 < x < 6) [26]. Since the electronegativity of B (2.0, Pauling scale) is slightly lower than that of P (2.1, Pauling scale), the contraction of band gap caused by introducing [BO4]5 should be very limited when we notice the fact that only 8.33% [BO4]5 are incorporated into one unit cell. As shown in Table S2 (Supporting Information), the band gap of apatite Sr5(PO4)3X (X = F, Cl, Br, OH) decrease from 5.35 to 4.93 eV as X changes from small ionic F to big covalent OH group. Such trend has also been observed for calcium apatite [14]. Compared to [OH], [BO2] is bigger and possesses more content of covalency, and it is reasonable to expect the band gap of SrBPO to be lower than that of Sr5(PO4)3OH. Therefore, it is reasonable to conclude

that, the inclusion of [BO4]5, as well as the big size and covalent nature of [BO2] are responsible for the low band gap value of SrBPO. The diffuse reflectance spectrum of pure SrBPO was shown in Fig. 4(b). To calculate the band gap, the diffuse reflectance data was first converted into absorption F(R) according to Kubelka– Munk function F(R) = (1  R)2/(2R), where R is the reflectance. As can be seen from the insect of Fig. 4(b), the band gap, Eg, was then estimated to be 3.46 eV using the Tauc relation: [F(R)hm]1/2 / (hm  Eg) for indirect band gap insulator. Clearly, the band gap calculated by first principle calculation is smaller than the experimental one, which is ascribed to the inability of standard DFT to describe the exact exchange–correlation interactions in real materials. 3.3. Luminescence properties of undoped SrBPO The excitation, emission spectra, as well as fluorescence decay curves of undoped SrBPO are shown in Fig. 5(a) and (b), respectively. The sample shows a broad emission band in the wavelength range of 380–580 nm with a maximum at about 450 nm, while the excitation spectrum monitored at 450 nm includes two bands: a strong band peaking at 225 nm and a weak shoulder around

Fig. 4. (a) The band structure and (b) diffuse reflectance spectrum of undoped SrBPO. Insect of (b) displays the [F(R)hm]1/2 / hm relation.

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Fig. 5. (a) The excitation and emission spectra and (b) fluorescence decay curves of undoped SrBPO.

250 nm. No luminescence was detected for samples sintered in air. Additional experiments were performed as a function of time or gas flow rate, and the results indicated that the change of reducing time and gas flow rate have no effect on the luminescence property. Since neither Sr2+ nor [BO4]5/[PO4]3 is able to give out any luminescence, the luminescence from the sample sintered under reducing atmosphere may be related to defects or impurities in the host. The luminescence decay curve in Fig. 5(b) shows a biexponential decay profile (I(t) = A1exp(t/s1) + A2exp(t/s2) + I0), and the life time is fitted to be 4.35 and 16.9 ls, respectively. This points out that there exist two separate defects that give out the luminescence, and thus the emission spectrum can be decomposed into two Gaussian peaks at 411 and 463 nm, respectively (Fig. S5 in Supporting Information). As shown by previous calculations, introduction of B atoms creates strong local electric field and lattice distortion near [BO4]5 group. Further, the reducing atmosphere under which samples were prepared is likely to produce oxygen vacancies in undoped SrBPO. As Sr3 site have quite different coordination environment from that of Sr1/Sr2 site, the combined influence of local distortion and oxygen vacancies will lead to two types of luminescent defects, which may be absent in SrBPO sintered in air. However, the exact physical nature of these defects is beyond the scope of this paper.

3.4. Luminescence of SrBPO: 0.03Ce3+, 0.03Li+ A typical emission under excitation of UV light is presented in Fig. 6(a). Under excitation at 301 nm, the sample shows an asymmetric broad emission band from 330 to 650 nm with the peak at about 440 nm. The emission peak is shifted toward lower energy compared to that in Sr5(PO4)3F: Ce3+ (425 nm) [10] or Sr5(PO4)2(SiO4): Ce3+ (343 nm) [11]. This can be explained with the structure disorder created by the replacement of [PO4]3 for [BO4]5 and the substitution of [BO2] for the more ionic counterpart-F [27]. Emission profiles obtained by using excitation light in VUV–UV range also show asymmetric emission peaking at about 440 nm and can be decomposed into two bands, but the relative intensity of the sub bands varies as shown in Fig. S6 (Supporting Information). Similar to the case reported by Zhu et al. [9], the FWHM of emission is as large as 6200 cm1 which deviates much from 4000 cm1, a typical value where Ce3+ occupies only one crystallographic site in the host [28]. We failed to decompose the emission band into four Gaussian components which were supposed to originate from doublet emissions of Ce3+ at 2d site and 6g site in SrBPO. Though the emission band can be decomposed into Gaussian peaks at 464 and 414 nm, the energy difference of

Fig. 6. (a) The emission spectrum (kex = 302 nm) and (b) excitation spectra (kem = 460 nm) of SrBPO: 0.03Ce3+, 0.03Li+.

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them is about 2410 cm1, far larger than that of the two ground state levels (2F5/2, 2F7/2) of free Ce3+ ion due to spin–orbit splitting (2000 cm1). The origin of the large FWHM of emission band will be discussed in following text. The excitation spectra were collected separately in UV region (blue line) and VUV range (red line, VUV station at BSRF) respectively, and the intensity were normalized at 287 nm for comparison. In Fig. 6(b), the excitation spectra involve seven peaks at about 360, 330, 301, 276, 225, 202 and 167 nm (insect of Fig. 6(b)). As a Ce3+ ion in one local environment possess at most five excitation peaks due to splitting of 5d1 state by crystal field, the 225 nm peak could be ascribed to excitation band from defects in the host, which is consistent with the excitation peak of undoped SrBPO. Then peak at 202 nm could be the highest 4f–5d excitation band, and peaks at 276, 302, 330, 360 nm are also 4f–5d excitation bands, which is very similar to the case of Sr5(PO4)3F: Ce3+ [10]. The band at 167 nm can be ascribed to transitions within energy levels of [PO4]3 or [BO4]5 group, which has been observed in many phosphate and borate [29], and confirmed by theoretical calculation [30]. It is known that the relative energy of the first f–d transitions for different rare earth ions is almost independent of host, and the 5d level in a defined host show lower energies than that of free ions due to crystal field splitting. Dorenbos has proposed that the crystal field decrease D(Ln, A) for the energy of the 4fn15d levels of lanthanide ion in compound A relative to the same level energies of free ion can be calculated by the formula D(Ln, A) = E(Ln, free)  E(Ln, A) [31]. Here E(Ln, free) and E(Ln, A) are the energy of the first f–d transition for lanthanide ions in free state and in a defined compound A, respectively. The lowest 4f–5d excitation of Ce3+ in SrBPO was found to be 360 nm (i.e., 27,778 cm1). The 5d level of free Ce3+ lies at 49,340 cm1. Thus the f–d transition energy of Ce3+ in SrBPO was decreased by 21,562 cm1, which implies D(Ce3+, SrBPO) is 21,562 cm1. In many phosphate apatite, the valence band near Fermi level is mainly composed of O 2p states from [PO4]3 while the conduction band is composed of d orbital from metal ions and considerable amount of DOS of [PO4]3. The large amount of DOS of [PO4]3 group in both valence band and conduction band facilitates band transitions and often results in strong excitation band in VUV region. In our case, the top of valence band is mainly composed of [BO4]5 states, while the conduction band is mainly composed of DOS of [BO2] and DOS of Sr 4d with tiny contribution from [PO4]3 or [BO4]5 groups. This fact leads to small probabilities of intra-configuration transition within [PO4]3 or [BO4]5 groups, and thus excitation intensity in VUV range is quite low. In a borophosphate compound 10CaO(6  x)PO2.5xBO1.5tH2O doped with Ce3+ in which [BO2] partially substituted OH, very weak or even no luminescence was found by using VUV photon as excitation source [32], which confirms our explanation. 3.5. Possible site occupancy of Ce3+ in SrBPO When a dopant occupies more than one crystallographic site, its luminescence decay curve is often multi-exponential due to different radiation rates of emitting centers. The decay curves of SrBPO: 0.03Ce3+, 0.03Li+ using 230 and 301 nm as excitation source are shown in Fig. 7(a)–(c). Apparently, in the nanosecond range, the decay curve could be described by single exponential function I(t) = I0 + Aexp(t/s), and the life time is fitted to be 52.2 ns, which is close to that of Ce3+ doped in YAG (60 ns) [33]. The fact that decay curves show single exponential characteristics indicates that the Ce3+ occupies only one local environment in SrBPO. Furthermore, in microsecond region, the emission under excitation of 320 and 230 nm show bi-exponential character and the life time are of several microseconds. Since the life time of defect

luminescence is also bi-exponential and shows life time of 16.5 and 5.4 ls, it is reasonable to believe that the emission of SrBPO: 0.03Ce3+, 0.03Li+ has some component of emission from defects in the host. This defect-related emission in the host materials leads to a large FWHM as 6200 cm1. Fluorescence decay results suggest that Ce3+ experiences only one type of local environment, which can be further confirmed by X-ray excited luminescence in the following section. Otherwise, the decay curve will not maintain the single exponential characteristics. In our case, we proposed that Ce3+ takes the Sr3 site in SrBPO. The reasons are as follows: (i) compared to Sr1 or Sr2 site, Sr3 site has smaller coordination number (CN = 7) and shorter bond length (2.620 Å) and possess only C1 symmetry, thus the crystal field is much stronger than that of Sr1/Sr2 site. This is the origin of splitting of 5d states of Ce3+ into 5 components in the excitation spectra. Among the seven O atoms bonded to Sr3 ions, there is one O that belongs to the [BO2] unit, and thus further move emission and excitation energy into low energy; (ii) Sr3 ions situated in the large tunnel formed by [PO4]3/[BO4]5 group are more distorted than other Sr ions, hence it is easier for bigger Ce3+ to occupy Sr3+ ions in the host; (iii) previous research reported that trivalent ions tend to be ordered at 6h site in apatite structure. For example, Nd3+ only occupy Ca2 in Ca5(PO4)3F [34] and in Ca10xEux(PO4)6S1+x/2 (0 < x < 1.6) solid solution and Eu3+ was thought to exclusively take the Ca2 position [35]. Cohesive energy, which is defined as the energy difference between a crystal (Ecrystal) and all its constituting P P atoms ( E(Ai), A stands for atom species), Ec = Ecrystal  E(Ai), provides an indication for the site occupation preference of Ce3+ in SrBPO. From Table 2, in both charged cell Sr10(PO4)6(BO2)+ and 1  1  2 super cell of Sr10[(PO4)5.5(BO4)0.5](BO2), the cohesive energy of crystal where Ce3+ occupies Sr3 site is 1–2 eV lower than that where Ce3+ occupies Sr1 or Sr2 site. Therefore, Ce3+ are most likely to occupy Sr3 site, confirming our reasoning above. 3.6. The effect of dopant concentration on the luminescence properties Fig. 8(a) and (b) shows emission spectra and life time of SrBPO: xCe3+, xLi+ (x = 0.008, 0.01, 0.02, 0.03 and 0.05) under excitation of 301 nm. The fluorescence decay curves are presented in Fig. S7 in Supporting Information. The emission peak does not move with respect to dopant concentration. It implies that the crystal splitting in our case is not important because the cell volume is almost constant for different dopant concentrations. The intensity climbs to top at x = 0.02 and then goes down gradually afterwards. Meanwhile, fluorescence life time increased from 50.1 ns (x = 0.008), reached the peak at 52.2 ns (x = 0.03), and then fall down to 49.5 ns (x = 0.05). As is well known, the life time can be expressed as s = 1/(kr + knr), where kr and knr are the radiative and non-radiative decay rates, respectively. Before optimal dopant concentration, kr drops as a result of shortening distance between dopants since kr / (Rc/R)n (where n = 6, 8, 10 for dipole–dipole, dipole–quadrupole, quadrupole–quadrupole interactions respectively, R is the distance between adjacent dopants, and Rc is the critical distance at which kr = knr) and luminescence life time shows an upward trend accordingly. Life time drops after optimal dopant concentration because of increasing knr caused by defects in the host. Therefore, the optimal dopant concentration can be estimated to be about 0.03 mol. The critical distance for Ce3+ in SrBPO host can be estimated according to the formula [36] Rc = 2(3V/ (4pxN))1/3, in which V is the volume of unit cell, x is the critical concentration and N is the number of crystallographic sites occupied by activators. For SrBPO, V = 602.40 Å3, the critical concentration x = 0.03, and N = 7, and thus the critical distance Rc was calculated to be 17.62 Å, which is slightly larger than that of Ce3+ in other apatite as Ca3Gd7(PO4)(SiO4)5O2 (14.23 Å) [37].

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Fig. 7. The fluorescence decay curves of SrBPO: 0.03Ce3+, 0.03Li+ monitoring 450 nm emission under excitation of 302 nm light in the (a) nanosecond range, (b) microsecond range, and (c) under excitation of 230 nm light in the microsecond range.

Table 2 Cohesive energy of Sr10(PO4)6(BO2)+, and Sr10[(PO4)5.5(BO4)0.5](BO2) with one Ce atom replacing one Sr atom at different crystallographic sites. Cohesive energy/eV

Ce@Sr1 (1/3, 2/3, 0.0069)

Ce@Sr2 (1/3, 2/3, 50644)

Ce@Sr3 (0.24911, 0.0158, 0.24588)

Sr10(PO4)6(BO2)+ Sr10[(PO4)5.5(BO4)0.5](BO2) (1  1  2 super-cell)

309.323 620.146

309.319 620.214

311.542 621.235

Fig. 8. (a) The emission spectra and (b) fluorescence life time of SrBPO: xCe3+, xLi+ (x = 0.008, 0.01, 0.02, 0.03 and 0.05) under 302 nm excitation.

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A. Feng et al. / Optical Materials 45 (2015) 13–21

3.7. X-ray excited luminescence properties The X-ray excited luminescence spectra of undoped SrBPO and SrBPO: 0.03Ce3+, 0.03Li+ were shown in Fig. 9(a) and (b), respectively. Different from the asymmetric emission band at about 450 nm when excited at 226 nm, the undoped SrBPO exhibits only one emission band centering at about 407 nm under the radiation of X-ray. Note that the 407 nm emission band is very close to the 411 nm component of the emission band under 226 nm excitation. It is then reasonable to assign the 407 nm emission band to the emission from one type defect in the host material. Therefore the energy level of this defect must lie near the top of the valence band and is easier for the recombination of holes with electrons, while the energy level of the other defect should be far away from the conduction band maximum and hence prohibits the recombination process. For SrBPO: 0.03Ce3+, 0.03Li+, doublet emission band peaking at 435 nm is observed and the band can be further decomposed into two bands with an energy difference about 1893 cm1, which is close to the theoretical value of the energy difference of 2F5/2 and 2 F7/2, ~2000 cm1. This points out that the emission originates from the 5d–4f transition of Ce3+ ion occupying only one site in the SrBPO host, which has been verified in previous section. The absence of broadening of emission band may be due to the much lower luminescence intensity of defects than that of 5d–4f transition of Ce3+ ion. 3.8. An illustrative mechanism diagram Based on the results aforementioned, a mechanism diagram is illustrated in Fig. 10. The energy levels of 5d and 4f states of Ce3+, as well as the ground and excited states of two types of defect structure (labeled as I and II, respectively) are constructed properly in the diagram. Under the excitation of UV light, as denoted by arrow a, Ce3+ ion is excited from 4f ground state into excited state, undergoes relaxation to the lowest states, and then emits bluegreen emission light (arrow 1). Meanwhile, defects in the host can be excited by both 250 and 225 nm UV light as shown by arrow b and c, respectively. Their excited levels are so close that energy migration is possible and the compound gives out asymmetric band with sub-peaks at 460 (arrow 2) and 411 nm (arrow 3). The excitation spectrum of SrBPO: 0.03Ce3+, 0.03Li+ exhibits a peak at 225 nm, which coincide with that of the defects of host. Furthermore, the fluorescence life time of SrBPO: 0.03Ce3+,

Fig. 10. The illustrative mechanism diagram for the luminescence of SrBPO: 0.03Ce3+, 0.03Li+.

0.03Li+ monitored in microsecond region was shorter than that of undoped SrBPO. Therefore, we tend to believe there exists energy transfer between Ce3+ and luminescent defects in the host (labeled as ET in the diagram). Broad band emission with FWHM of 6200 cm1 is obtained mainly due to the mixture of emission from Ce3+ and two types of defects in the host. Electrons and holes were created under X-ray radiation, as shown by dark and white filled circles in the diagram. The electrons could be captured by 5d states levels of Ce3+ and subsequent recombination with holes render 5d–4f transition of Ce3+ possible. Meanwhile, as the ground state energy level of defect I is far from the top of valence band, it is quite difficult for holes to recombine with electrons trapped by excited states of the defects. Rather, recombination mainly occurs through defect II whose ground state energy level is quite close to top of valence band. Thus, only one Gaussian band was detected for undoped SrBPO under the radiation of X-ray. Noting emission of SrBPO: 0.03Ce3+, 0.03Li+ is much stronger than that of the defects in the host (Fig. 9.), we were unable to observe broadening of emission band of Ce3+ doped SrBPO. Consequently, only one doublet emission with an energy difference of 1893 cm1 was detected in SrBPO: 0.03Ce3+, 0.03Li+.

Fig. 9. The X-ray excited luminescence spectra for (a) undoped SrBPO and (b) SrBPO: 0.03Ce3+, 0.03Li+.

A. Feng et al. / Optical Materials 45 (2015) 13–21

4. Conclusions Undoped and Ce3+ doped Sr10[(PO4)5.5(BO4)0.5]BO2 phosphor were successfully synthesized by solid state reaction method and their phase formation was confirmed by XRD and EDS techniques. The undoped sample prepared under reducing atmosphere shows luminescence with emission band at about 450 nm, originating from two kinds of defects in the host. Ce3+ doped SrBPO shows broad band emission with FWHM being about 6200 cm1. It is ascribed to the defect luminescence and luminescence from Ce3+ occupying only Sr3 site (6g, C1) of the host. The excitation band of Ce3+ is splitted into five components and defect excitation band and weak host excitations are also observed. Electronic calculations reveal that relative small amount of DOS of [PO4]3 or [BO4]5 group in the conduction is the cause of the weak excitation in VUV region. Acknowledgements We would like to thank the High Performance Computing Centre of Shanghai University for providing the computation resources. This work was supported by the National Natural Science Foundation of China under Grant Nos. 50990304 and 11104298, and Innovation Program of Shanghai Institute of Ceramics under Grant No. Y34ZC130G. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.optmat.2015.02. 031. References [1] T.S. Davis, E.R. Kreidler, J.A. Seoules, J. Lumin. 4 (1971) 48–62. [2] L.C. Palmer, C.J. Newcomb, S.R. Kaltz, E.D. Spoerke, S.I. Stupp, Chem. Rev. 108 (2008) 4754–4783. [3] B.M. Choudary, C. Sridhar, M.L. Kantam, G.T. Venkanna, B. Sreedhar, J. Am. Chem. Soc. 127 (2005) 9948–9949. [4] H. Yoshioka, Y. Nojiri, S. Tanase, Solid State Ionics 179 (2008) 2165–2169.

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