Electronic Properties of Ce3+-Doped Sr3Al2O5Cl2:A Combined Spectroscopic and Theoretical Study
Lixin Ning,*,?Cuicui Zhou,?Wanping Chen,?Yucheng Huang,?Changkui Duan,§Pieter Dorenbos,∥Ye Tao,⊥and Hongbin Liang*,?
?Center for Nano Science and Technology,Department of Physics,Anhui Normal University,Wuhu,Anhui241000,China
?MOE Laboratory of Bioinorganic and Synthetic Chemistry,KLGHEI of Environment and Energy Chemistry,School of Chemistry and Chemical Engineering,Sun Yat-sen University,Guangzhou510275,China
§Department of Physics,University of Science and Technology of China,Hefei,Anhui230026,China
∥Faculty of Applied Sciences,Delft University of Technology,Mekelweg15,2629JB Delft,The Netherlands
⊥Beijing Synchrotron Radiation Facility,Institute of High Energy Physics,Chinese Academy of Sciences,Beijing100039,China *Supporting Information
states and the lowest5d states of all trivalent and
discussed in association with experimental?ndings.
1.INTRODUCTION
Oxychloride phosphors activated by Ce3+and Eu2+with parity-and spin-allowed5d→4f transitions have been of considerable interest for potential applications in solid-state lighting, displays,and scintillators.1?8Compared to pure oxide phosphors,the energy of the emitting5d state relative to that of the4f ground state is generally lower in oxychlorides,which is desirable for many practical applications.This is due to the greater downward shift of the5d centroid energy relative to that of the free ion owing to the larger polarizability of Cl?than O2?.9This feature,as well as the large structure?composition space for mixed-ligand systems,makes it possible to fabricate oxychloride phosphors that exhibit luminescence properties suitable for target applications.In this process,theoretical calculations of atomic and electronic structures can give important contributions,by interpreting experimental results and furthermore by developing a fundamental understanding of structure?property relationships.
Many experimental studies have recently been reported on photoluminescence properties of the Sr3Al2O5Cl2crystal doped with Ce3+,Eu2+,and other trivalent lanthanide(Ln)ions for their potential applications as white light-emitting diode(LED)and persistent luminescent phosphors.The atomic structure of the host crystal,which has an orthorhombic symmetry with the space group P212121(No.19),contains three crystallographically independent Sr2+sites of C1 symmetry.Each Sr2+site is coordinated by?ve O2?and four Cl?ligands with the di?erences in the average Sr?O and Sr?Cl bond lengths less than0.025and0.005?,respectively.16When doped with Ce3+at low concentrations,the room temperature excitation spectra of the material exhibited?ve band maxima in the UV spectral region.11The shortest wavelength band at215 nm was ascribed to the host-related absorption and the other four at247,282,312,and330nm to the4f→5d transitions of Ce3+,11which was supposed to be located on the Sr2+sites due to their similar ionic sizes.17Since the5d1con?guration of Ce3+ located on a site of C1symmetry should split into?ve levels by the crystal?eld and spin?orbit interactions,one level is missing in the UV excitation spectra,even if the spectra of Ce3+in the Received:December11,2014
Revised:February8,2015
Published:March10,2015
three distinct Sr2+sites are assumed to overlap completely with each other.
Knowledge of all the?ve5d levels of Ce3+is important also from a theoretical viewpoint.The shift of the5d centroid energy with respect to the free ion value was found to be related with the energy-level di?erence between the4f ground states of Eu2+and Eu3+located at the same chemical environment.18This energy-level di?erence is a crucial parameter in the construction of the energy-level diagram for the4f ground states of all Ln3+and Ln2+ions relative to the valence and conduction bands of the host by using theoretical models developed by Dorenbos.19In the present work,we extend previous photoluminescence studies of Ce-doped Sr3Al2O5Cl2by using VUV-UV excitation with synchrotron radiation at a low temperature of15K.The band maxima observed in the excitation spectrum are assigned by comparing their energies with calculated Ce3+4f→5d transition energies using the wave function-based CASSCF/CASPT2method at the spin?orbit level.On the basis of our experimental and calculated results,the energy-level diagram for the4f ground states and the lowest5d states of all Ln3+and Ln2+ions relative to the valence and conduction bands of the host is constructed and discussed.
2.METHODOLOGY
2.1.Experimental Details.The power samples with nominal compositions Sr3?2x Ce x Na x Al2O5Cl2(x=0.01,0.03, 0.06,0.10,0.15,0.20,and0.30)were synthesized by a solid-state reaction technique.The reactants included analytical reagents SrCO3,SrCl2·6H2O,Al2O3,Na2CO3,and99.99% purity CeO2and Gd2O3,among which Na2CO3was used to introduce Na Sr point defects to compensate for the excess charge of Ce3+on the Sr2+site.Stoichiometric amounts of the starting materials were thoroughly mixed in an agate mortar and then pre?red in air at500°C for3h.After being slowly cooled down to room temperature,the sample was thoroughly reground and sintered at1000°C for5h in a CO atmosphere. The crystal structures of?nal products were examined by powder X-ray di?raction(XRD)using Cu Kαradiation on a Rigaku D/max2200vpc X-ray di?ractometer.The VUV-UV excitation and the corresponding emission spectra were measured at the VUV spectroscopy experimental station on beamline4B8of Beijing Synchrotron Radiation Facility (BSRF).20The UV?vis excitation and emission spectra were recorded on a Jobin Yvon FL3-21spectro?uorometer and an Edinburgh FLS920instrument at room temperature with a450 W xenon lamp as the excitation source.The luminescence decay curves were measured by the Edinburgh FLS920 combined?uorescence lifetime and steady-state spectrometer. The X-ray excited luminescence spectra were recorded with a Philips PW2253/20X-ray tube with a Cu anode operating at60 kV and25mA.Further details of the measurement can be found in the Supporting Information of ref21.
https://www.doczj.com/doc/5e1271647.html,putational Details.The Ce-doped Sr3Al2O5Cl2 crystal was modeled by a2×2×2supercell containing384 atoms,in which one of the96Sr2+ions was replaced by a Ce3+, corresponding to the chemical formula Sr3?x Ce x Al2O5Cl2(x≈0.03).The lattice parameters and atomic coordinates of the supercells were?rst optimized by periodic DFT calculations with the pure PBE functional.22,23The electronic properties were then calculated with a hybrid functional in the PBE0 scheme24that admixes a fraction of Hartree?Fock(HF) exchange with PBE exchange.The excess charge of Ce3+on the Sr2+site was compensated by introducing a uniform back-ground charge density,and the spurious Coulomb interaction in charged supercells is expected to be small due to the large size of the supercells.25For comparison,we have also studied the model systems Sr3?2x Ce x Na x Al2O5Cl2(x≈0.03)with the excess charge compensated by a Na+on a nearest-neighbor Sr2+ site.The Sr(4s24p65s2),Al(3s23p1),O(2s22p4),Cl(3s23p5), Ce(5s25p64f15d16s2),and Na(2p63s1)were treated as valence electrons,and their interactions with the cores were described by the projected augmented wave(PAW)method.26The convergence criteria for total energies and atomic forces were set to10?6eV and0.01eV??1,respectively,with a cuto?energy of550eV for the plane wave basis.One k-pointΓwas used to sample the Brillouin zone.All DFT calculations were carried out using the Vienna ab initio simulation package (VASP).27,28
Based on the DFT-optimized supercell structures,the Ce-centered embedded clusters were constructed,which contain the central Ce3+and the coordinating O2?and Cl?ions.Their immediate surroundings within a sphere of radius10.0?were simulated by470?472embedding ab initio model potentials (AIMPs)29at lattice sites,and the remainders of the surroundings were represented by about131230point charges at lattice sites,as generated using Lepetit’s method30which produces the same electrostatic potentials as Ewald’s method.31 For these embedded clusters,the wave-function-based CASSCF/CASPT2calculations with spin?orbit coupling were carried out to obtain the4f1and5d1energy levels of Ce3+by using the program MOLCAS.32In the CASSCF calculations,a[4f,5d,6s]complete active space was adopted, and in the CASPT2calculations the dynamic correlation e?ects of the Ce3+(5s,5p,4f,5d),O2?(2s,2p),and Cl?(3s,3p) electrons were taken into consideration.More details on the calculation can be found in refs33and34.A relativistic e?ective core potential([Kr]core)with a(14s10p10d8f3g)/ [6s5p6d4f1g]Gaussian valence basis set from ref35was used for cerium,and a[He]core e?ective core potential with a (5s6p1d)/[2s4p1d]valence basis set from ref36was used for oxygen and chlorine.Extra basis set(11s8p)/[1s1p]were added to the four Al atoms closest to the embedded clusters in order to improve the orthogonality of the cluster orbitals with the embedding environments.
3.RESULTS AND DISCUSSION
3.1.XRD Patterns and Photoluminescence Spectra. Figure1shows the XRD patterns of the synthesized samples Sr3?2x Ce x Na x Al2O5Cl2(x=0and0.01),along with the standard pattern(JCPDS80-0564)as a reference.The XRD patterns of all the other samples are https://www.doczj.com/doc/5e1271647.html,parison of the patterns in the?gure shows that the samples under investigation are of single phase,with no evident in?uence of Ce3+and Na+on the crystal structure of the host.
Figure2displays the VUV-UV excitation and the corresponding emission spectra of Sr2.98Ce0.01Na0.01Al2O5Cl2 by employing synchrotron radiation at15K.In the excitation spectrum(curve a),there are six bands with their maxima at about330,309,280,243,215,and185nm(labeled A?F, respectively).The former four bands are assigned to4f→5d vibronic transitions of Ce3+,similar to the assignment made in ref11.The latter two bands are ascribed to host-or defect-related absorptions,as will be evident below from a comparison with the excitation spectrum of the Gd-doped sample.In the emission spectrum(curve b),three band maxima are observed
at 460(I),420(II),and 372nm (III).The former two emission bands with an energy separation of ～2070cm ?1can be assigned to the transitions from the excited 5d 1level to the 2
F 5/2and 2F 7/2multiplet terms of the 4f 1con ?guration,which are usually split by around 2000cm ?1due to spin ?orbit interaction of the 4f 1electron.For the emission band III,its maximum energy is larger by ～3070cm ?1than that of band II.This band,as well as the emission tail between 500and 600nm,are ascribed to host-or defect-related emission.From the maximum energies of the lowest excitation band A and the emission band II,the Stokes shift of the 5d 1emission is estimated to be ～6490cm ?1but with a slight overestimation.This is due to the fact that the emission band maximum is a result of superposition of the three 5d 1→(7F 5/2)4f 1?3transitions rather than a result of the single 5d 1→(7F 5/2)4f 1transition.This value of the Stokes shift is larger than those (in the range of 2000?5000cm ?1)estimated for many of the Ce-doped aluminate compounds.37
In the inset of Figure 2,we depict the VUV-UV excitation spectrum (curve c)of a Gd-doped sample at 15K by monitoring the characteristic 6P 7/2→8S 7/2emission of Gd 3+at 314nm (curve d).The sharp lines around 275nm in the excitation spectrum are typical of Gd-doped systems,resulting from intracon ?gurational 4f 7→4f 7transitions of Gd 3+.In addition,in the spectral range of 150?250nm,two band maxima are observed at about 208(G)and 189nm (H),which
coincide with those of bands E (215nm)and F (185nm)in the excitation spectrum of the Ce-doped sample (curve a).The higher energy band (F or H)is assigned to the host excitonic absorption,corresponding to the excitation of an electron from the host valence band maximum (VBM)to the energetically lowest bound excitonic state.By taking into account the electron ?hole binding energy of the exciton,38the band gap of the host is estimated to be 7.1?7.2eV.The lower energy band (E or G)could be due to the absorption of excitons trapped near some lattice defects,which is usually observed at several tenths of an electronvolt lower in energy than the host excitonic absorption band.39
The UV excitation spectrum of the 440nm emission in Sr 3?2x Ce x Na x Al 2O 5Cl 2(x =0.01)at room temperature is shown on the left side of Figure 3.It contains two broad bands
peaking at about 338and 282nm,consistent with the pro ?le of the low-temperature excitation spectrum in Figure 2.On the right side of Figure 3are the normalized emission spectra of Sr 3?2x Ce x Na x Al 2O 5Cl 2(x =0.01,0.06,0.15,and 0.20)at room temperature under UV excitation at 340nm.It shows a broad emission band extending from 380to 580nm and peaking at ～440nm,similar to the emission spectrum measured at 15K (curve b in Figure 2).The overlap of the excitation and emission spectra occurs at about 383nm,which is supposed to be the ZPL position of the 4f 1?5d 1transition.The ?gure also shows that the emission maximum shifts slightly to longer wavelengths with the increase of Ce 3+concentration.The inset of Figure 3plots the luminescence decay curve of the 440nm emission in Sr 2.98Ce 0.01Na 0.01Al 2O 5Cl 2,upon excitation at 327nm at room temperature.The curve can be fairly well ?tted by a single-exponential function with a decay time of 33.8ns.For this sample,the blue luminescence has a CIE chromaticity coordinate of x =0.280and y =0.492.
3.2.X-ray Excited Luminescence.In view of the strong absorption in the VUV region and the resultant blue luminescence,we have further measured the X-ray excited luminescence of the samples at room temperature.Figure 4s h o w s t h e X -r a y e x c i t e d e mi s s i o n s p e c t r a o f
Figure 1.Powder XRD patterns of samples of Sr 3Al 2O 5Cl 2and Sr 2.98Ce 0.01Na 0.01Al 2O 5Cl 2and the comparison with JCPDS standard data.
Figure 2.(a)VUV-UV excitation and (b)the corresponding emission spectra of Sr 2.98Ce 0.01Na 0.01Al 2O 5Cl 2at 15K.The inset shows the VUV-UV excitation (c)and the related emission (d)spectra of Sr 2.98Gd 0.01Na 0.01Al 2O 5Cl 2at 15K.
Figure 3.(a)UV excitation spectrum (λem =440nm)of Sr 3?2x Ce x Na x Al 2O 5Cl 2(x =0.01)and (b)the normalized emission spectra (λex =340nm)at di ?erent doping levels with x =0.01?0.20at room temperature.The inset shows the luminescence decay curve of the emission at 440nm (λex =327nm)for the sample with x =0.01at room temperature.
Sr 3?2x Ce x Na x Al 2O 5Cl 2(x =0.01,0.06,and 0.10),together with those of the powder pellet and crystalline BaF 2samples as a reference to estimate the absolute light yield.The emission spectra of Ce 3+samples exhibit a broad band centered at ～440nm in line with the position observed in Figure 3b,and the emission intensity decreases with increasing Ce 3+due to concentration quenching.The absolute light yield were calculated to be 26300,13300,and 9000photons/MeV of absorbed X-ray energy for the x =0.01,0.06,and 0.10samples,respectively.The light yield of the x =0.01sample is comparable to that of Lu 2SiO 5:Ce 3+(26000photons/MeV)40and thus could have potential applications as a scintillation phosphor.
3.3.Ab Initio Calculations. 3.3.1.Structural and Electronic Properties of Pure Sr 3Al 2O 5Cl 2.The crystal structure of pure Sr 3Al 2O 5Cl 2was ?rst optimized using periodic DFT with the PBE functional,and the calculated (experimental 16)lattice parameters are a =9.504(9.422)?,b =9.510(9.422)?,and c =9.513(9.422)?,with a slight overestimation of 0.9?1.0%.The calculated internal parameters also agree well with the experimental data,as shown by a comparison of the data in Table S1of the Supporting Information.
On the basis of the PBE-optimized geometry,we calculated the total and orbital projected densities of states (DOSs)of Sr 3Al 2O 5Cl 2,using a modi ?ed PBE0hybrid functional with 35%HF exchange.This modi ?ed PBE0functional gives a band gap of 7.2eV for Sr 3Al 2O 5Cl 2,in agreement with the experimentally estimated value.In Figure 5,the orbital projected DOS shows that the top of valence band is formed primarily by O 2p and Cl 3p states,with dispersions of ～6.0and ～3.2eV,respectively.The conduction band edge is constituted by a small peak at 7.2eV above the top of the valence band,mainly composed of s-character states of Sr,O,and Cl atoms.Right above this peak,the conduction band is mainly composed of Sr 4d states with small contributions from the p-character states of other atoms.It is noted that,using the standard PBE0hybrid functional with 25%HF exchange,the calculated orbital characters for the valence and conduction band states are basically the same as those derived using the modi ?ed PBE0with 35%HF exchange,except that the predicted band gap is 0.7eV smaller (see Figure S1in the Supporting Information).
3.3.2.Local Structures and 4f →5d Transition Energies of Ce 3+in Sr 3Al 2O 5Cl 2.The lattice parameters calculated for the Ce-doped Sr 3Al 2O 5Cl 2supercells with the DFT-PBE method are listed in Table S2of the Supporting Information,along with those of the undoped supercell for comparison.One can see
that the substitution of a Ce 3+into the Sr 2+site introduces a very small decrease (by 0.260?0.263%)of the supercell volume and slightly distorts the orthorhombic phase of the undoped crystal into lower symmetry phases,with the deviations of angles within ±0.05°.Thus,the deformation of the crystallo-graphic phase is negligible when a small amount of Ce 3+is substituted into the Sr 2+sites of Sr 3Al 2O 5Cl 2,in agreement with results from XRD measurements.
The DFT-optimized local structures of Ce 3+at the three di ?erent Sr sites of Sr 3Al 2O 5Cl 2are depicted in Figure S2of the Supporting Information along with those of Sr 2+in the undoped system and of Ce 3+with a Na +on a nearest-neighbor Sr 2+site for comparison.The values of selected bond lengths are indicated in the ?gure.It shows that the average Ce ?O and Ce ?Cl bond lengths are similar for the three substitutions (Figure S2a ?c),in the ranges of 2.649?2.654and 3.124?3.133?,https://www.doczj.com/doc/5e1271647.html,pared to the undoped system (Figures S2d ?f),the average distances to the surrounding O and Cl atoms are decreased by 0.106?0.109and 0.086?0.089?,respectively,consistent with the smaller ionic radius of Ce 3+(1.143?)than that of Sr 2+(1.26?)in the 9-fold coordination.17When a nearest-neighbor Na +ion is present,the coordination structures of Ce 3+are slightly distorted,as shown by comparison of the corresponding structures in Figures S2g ?i and S2a ?c.The Ce ?O and Ce ?Cl bond lengths change by the values of 0.014and 0.017?on average,with maximum deviations of 0.170and 0.157?,respectively.
Based on the PBE-optimized supercell structures,the Ce-centered embedded clusters,(Ce Sr i O 5Cl 4)11?(i =1?3),were constructed,with their lattice environments simulated by AIMPs and point charges at lattice sites.Wave-function-based CASSCF/CASPT2calculations with spin ?orbit coupling were then conducted to obtain 4f 1and 5d 1energy levels of Ce 3+,and the results are shown in Table 1.We see that the derived energy levels for the three clusters are very close to each other,with the maximum deviations of 181and 728cm ?1in absolute value for 4f 1and 5d 1levels,respectively,in line with the similar local structures of Ce 3+at the three Sr 2+sites.The calculated 4f 1levels fall into two groups (i.e.,4f 1?3and 4f 4?7)with a separation of about 2300cm ?1due to the spin ?orbit splitting of 4f 1con ?guration into 2F 5/2and 2F 7/2multiplet terms.
Figure 4.X-ray excited emission spectra of BaF 2and Sr 3?2x Ce x Na x Al 2O 5Cl 2(x =0.01,0.06,and 0.10)at room temperature.
Figure 5.Total and orbital-projected DOSs for the Sr 3Al 2O 5Cl 2unit cell calculated by DFT with the PBE0hybrid functional containing 35%HF exchange and a 3×3×3k-point grid to sample the Brillouin zone.An enlarged view of the DOS for the s-character states of Sr atoms is shown in the inset.The Fermi level is set at zero energy.
Figure 6depicts the calculated energies and relative intensities for the 4f 1→5d i (i =1?5)transitions,where the
relative intensities were computed using the wave functions and energies obtained at the spin ?orbit level.41By comparing the calculated transition energies with those estimated from the band maxima in the VUV-UV excitation spectrum at 15K (Figure 6a),the observed bands A ?D can be unambiguously assigned to the 4f 1→5d i (i =1?4)transitions of Ce 3+located on the three Sr 2+sites in Sr 3Al 2O 5Cl 2,with an average deviation of ～660cm ?1.The band E,which has been attributed to an excitonic absorption near a lattice defect,may also contain the contributions from the 4f 1→5d 5transitions of Ce 3+.The band F has been assigned to the host excitonic absorption (see section 3.2).Furthermore,the lower intensity of the band B with respect to the band A in the experimental excitation spectrum could be mainly attributed to the lower intensity of 4f 1→5d 2transition compared to the 4f 1→5d 1transition of Ce 3+located on Sr2sites.
The above comparison between calculated and experimental results indicates that the 5d level that was not observed in the experimental excitation spectrum is the highest (5d 5)level,and the transition energy to this level is close to a defect-related
excitonic absorption band (band E).The absorption intensity to this level is very small,only about 0.06×that to the lowest 5d 1level,and thus is masked by the much stronger adjacent excitonic absorption and not observed in the excitation spectrum.
The 4f 1and 5d 1energy levels of Ce 3+with Na +on a nearest-neighbor Sr 2+site have also been calculated,and the results are listed in Table S3of the Supporting Information.It shows that the presence of the nearby Na +causes downward shifts of the 5d 1?4levels by 17?1551cm ?1.For the 5d 5level,upward shifts of 163and 387cm ?1are observed for Ce 3+on the Sr1and Sr2sites,respectively,and a downward shift of 109cm ?1is found for Ce 3+on the Sr3site.These variations in the 5d 1level positions lead to an increase of the 5d crystal-?eld splitting by 404?877cm ?1and a decrease of the 5d centroid energy by 461?501cm ?1.The calculated energies and relative intensities for the 4f 1→5d i (i =1?5)transitions are also schematically represented in Figure S3of the Supporting Information.It shows that the 5d 1level shifts as caused by the presence of the nearest-neighbor Na +ions do not a ?ect the assignment of experimental excitation bands.
3.3.3.Electronic Properties of Ce-Doped Sr 3Al 2O 5Cl 2.For the three Ce Sr i -doped Sr 3Al 2O 5Cl 2(i =1?3)supercells,the calculated DFT total energies are also very similar,with the deviations less than 20meV,showing that the three Sr 2+sites are nearly equally preferred by the dopant Ce 3+.In view of this result as well as those for local structures and 4f →5d transition energies,in the following only the results for the Ce Sr1substitution are presented and discussed,and the relevant results for the other two substitutions are included in the Supporting Information.Figure 7depicts the total and partial
DOSs for the Ce Sr1-doped supercell in the ground state by using the modi ?ed PBE0functional with 35%HF exchange.It shows that the incorporation of a Ce 3+into the Sr 3Al 2O 5Cl 2supercell leads to the formation of an occupied 4f state deep inside the band gap (indicated by the dashed line in Figure 7b),corresponding to a lone 4f electron of Ce 3+.This occupied 4f state is located to be ～1.0eV above the host VBM.The unoccupied 4f states lie well above the conduction band edge,hybriding with the cation states of the host with a band broadening.The DOSs for the Ce Sr2and Ce Sr3substitutions are almost identical,as shown in Figure S4of the Supporting Information.
Table 1.Calculated Energy Levels of 4f 1and 5d 1Con ?gurations for the Ce Sr -Centered Embedded in
Sr 3Al 2O 5Cl 2Using the CASSCF/CASPT2Method at the Spin ?Orbit Level
Ce Sr1
Ce Sr2Ce Sr34f 10004f 25273465104f 31147110111334f 42203223121894f 52642247726354f 63080295030384f 73559362435955d 12930529681294675d 23160431700319435d 33614736269368755d 44171741386417925d 5
48745
48367
48448
Figure 6.Schematic representation of the calculated energies and relative intensities of the 4f 1→5d i (i =1?5)transitions of Ce 3+on the three di ?erent Sr 2+sites of Sr 3Al 2O 5Cl 2.The VUV-UV excitation spectrum of Sr 2.98Ce 0.01Na 0.01Al 2O 5Cl 2(λem =440nm)at 15K is also shown for comparison.
Figure 7.(a)Total DOS of Ce Sr1-doped Sr 3Al 2O 5Cl 2and (b)partial DOS of the 4f and 5d states of Ce 3+calculated using DFT with the PBE0hybrid functional containing 35%HF exchange.The occupied Ce 3+4f states are indicated by the dashed lines.
3.4.Energy-Level Diagram for Ln 3+and Ln 2+Ions in Sr 3Al 2O 5Cl 2.Above ab initio calculations of 4f 1and 5d 1energy levels of Ce 3+yield a value of 4.66eV for the 5d centroid energy.This value may provide a good approximation to within 0.1eV of the experimental 5d centroid energy based on the comparison between calculated and experimental 4f →5d transition energies.Thus,the 5d centroid shift (ΔE c )of Ce 3+in Sr 3Al 2O 5Cl 2with respect to the free ion value (6.35eV)42is estimated to be 1.69eV.As mentioned in the Introduction,a relationship exists between ΔE c of Ce 3+and the energy-level di ?erence (U (6))between the 4f ground states of Eu 2+and Eu 3+located at the same chemical environment,on the basis of many compounds with known values of the two quantities.18This relationship is expressed as
=+?ΔU (6) 5.44 2.834e eV
E /2.2c (1)
from which we obtain U (6)=6.75eV.This is a crucial parameter in deriving the 4f energy level of Eu 3+from that of Eu 2+or vise versa.
We start to construct the energy-level diagram for Ln 2+and Ln 3+in Sr 3Al 2O 5Cl 2with the 4f ground-state level of Eu 2+.Our measurements of excitation and emission spectra of Eu 3+-doped Sr 3Al 2O 5Cl 2(Figure S5of the Supporting Information)display a broad band with maximum at 275nm (4.51eV)in the excitation spectrum,which is the charge transfer (CT)band of Eu 3+.This is due to the process that an electron is transferred from the host VBM to the 4f-shell of Eu 3+,thus creating the ground state of Eu 2+.The energy of 4.51eV places the 4f ground state of Eu 2+above the host VBM.43With U (6)=6.75eV,the 4f ground state of Eu 3+is then placed at 2.24eV below the host VBM.Now,given the energy positions for the 4f ground states of Eu 3+and Eu 2+relative to the host VBM,we can derive such positions for the 4f ground states of the other Ln 3+and Ln 2+ions located at the same site of the host.This is done by utilizing the characteristic energy di ?erences between the 4f ground states of di ?erent Ln 3+or Ln 2+ions,which are largely independent of the host compound.19The results are depicted by the double zigzag curves 1and 2in Figure 8.Note that the Ce 3+4f ground state is located at 3.0eV above the host VBM,which is set as zero energy on the left-hand energy scale of the ?gure.This ground-state level is discriminated from the position of the occupied Ce 3+4f state in Figure 7,in that the
latter was calculated by hybrid DFT in the single-electron approximation.It should be noted that the energy position of curve 1would be shifted upward by about 0.03eV if the 5d 1energy level data for the Ce 3+with a nearest-neighbor Na +ion were used for the calculation of the curve.From curve 2,the 4f ground states of Dy 2+and Tm 2+are predicted to be around 0.42and 0.97eV below the host CBM,thus forming a shallow and a deeper electron trap,respectively.These are consistent with the experimental ?ndings that the persistent luminescence of Sr 3Al 2O 5Cl 2:Eu 2+,Tm 3+persists for a longer time than that of Sr 3Al 2O 5Cl 2:Eu 2+,Dy 3+at room temperature.10,14
On the basis of the energy positions for the 4f ground states of Ln 3+and Ln 2+ions,we can further derive the energy positions for the lowest 5d 1states,by employing the red-shift model developed by Dorenbos.19This model states that the lowering of the ?rst 4f 1?5d 1transition energy with respect to the free ion value is the same for each Ln 3+(or Ln 2+)ion,when situated at the same site of the same compound.Accordingly,from the known position of the zero-phonon line of the Ce 3+4f 1?5d 1transition at ～383nm (3.24eV),the energy levels for the 5d 1states of Ce 3+and hence of the other Ln 3+ions are obtained and plotted as curve 3in Figure 8.This curve would be displaced upward by about 0.03eV in the case of Ce 3+with a nearest-neighbor Na +ion.As for Ln 2+,the positions of their lowest 5d 1states may be estimated from the relationship 44
=?++D D (Ln )0.64(Ln )0.233eV
23(2)
where D (Ln 3+)and D (Ln 2+)stand for the red-shift of the 4f 1?5d 1transition of Ln 3+and Ln 2+ions in Sr 3Al 2O 5Cl 2(with the value of 2.88and 1.61eV),respectively,when compared to the case of the free ion.The resultant positions for the Ln 2+5d 1levels are depicted as curve 4in Figure 8.From this curve,we see that the Eu 2+5d 1level is predicted to be only 0.08eV below the host CBM,which implies that the normal 5d →4f emission is not possible for Eu 2+in Sr 3Al 2O 5Cl 2.45This is because the 5d orbital will hybridize strongly with conduction band states and a Eu 3+trapped excitonic state will be created,i.e.,Eu 3+plus an electron in the conduction band states orbiting around https://www.doczj.com/doc/5e1271647.html,ually,the emission from such an excitonic state is much Stokes shifted and anomalously broad,and the decay time is strongly di ?erent from those observed for the normal Eu 2+5d →4f emission (about 0.6?1.0μs).45This is indeed what was observed experimentally in ref 14,where the Eu 2+emission at room temperature exhibited a large Stokes shift (～7000cm ?1),a very broad band (～4100cm ?1full width at half-maximum),and a much longer decay time (3.1μs)than the normal Eu 2+5d →4f emission.
With the chemical shift model recently developed by Dorenbos,46the value of U (6)also provides the binding energy of an electron in Eu 2+4f ground state relative to the vacuum level.By using eq 10in ref 45,the Eu 2+4f binding energy is predicted to be ?3.94eV,and this pins the energy-level scheme for Ln 3+and Ln 2+in Sr 3Al 2O 5Cl 2relative to the vacuum level,as indicated by the right-hand energy scale in Figure 8.We note that the binding energy of an electron at the host VBM is ?8.45eV.
4.CONCLUSIONS
We have ?rst measured the VUV-UV excitation spectra of Ce 3+emission in Ce-doped Sr 3Al 2O 5Cl 2crystals by using synchro-tron radiation at a low temperature of 15K.Six excitation bands have been identi ?ed and ascribed to host-related
Figure 8.Host referred (left-hand energy scale)and vacuum referred (right-hand energy scale)binding energies for the 4f ground states and the lowest energy 5d 1states of Ln 3+and Ln 2+ions in Sr 3Al 2O 5Cl 2,with the values of selected energy di ?erences indicated.
excitonic absorptions and localized4f→5d transitions of Ce3+. The band gap of the host was estimated to be around7.2eV. On the basis of the wave function-based CASSCF/CASPT2 calculations on Ce-centered embedded clusters at the spin?orbit level,the observed localized transitions have been assigned to the4f1→5d1?4transitions of Ce3+occupying the three distinct Sr2+sites in Sr3Al2O5Cl2with nearly equal preference.It was found that the4f1→5d5transition intensity was so weak that it was masked by a much stronger adjacent defect-related excitonic absorption band.Finally,starting from present experimental and theoretical results,we have derived the energy positions for the4f ground states and the lowest5d state of all Ln3+and Ln2+ions in Sr3Al2O5Cl2,by using various empirical models developed by Dorenbos.The present work demonstrates that,by combining experiments,elaborate ab initio calculations,and reliable theoretical models,we are able to gain valuable new insights into luminescence and electronic properties of lanthanide-activated complex phosphors,which is
indispensable for their practical applications.
■ASSOCIATED CONTENT
*Supporting Information
Calculated internal parameters for pure Sr3Al2O5Cl2(Table S1),lattice parameters for Ce-doped Sr3Al2O5Cl2(Table S2), 4f1and5d1energy levels of Ce3+with a nearest-neighbor Na Sr+ ion(Table S3),total and partial DOSs for pure Sr3Al2O5Cl2 with the standard DFT-PBE0method(Figure S1),local structures of Sr2+and Ce3+in the undoped and Ce-doped Sr3Al2O5Cl2(Figure S2),schematic representation of4f→5d transitions of Ce3+with a nearest-neighbor Na Sr+ion(Figure S3),total DOSs for Ce-doped Sr3Al2O5Cl2supercells with a modi?ed DFT-PBE0method(Figure S4),excitation and emission spectra of Sr3Al2O5Cl2:Eu3+(Figure S5).This material
is available free of charge via the Internet at https://www.doczj.com/doc/5e1271647.html,.■AUTHOR INFORMATION
Corresponding Authors
*(L.N.)E-mail ninglx@https://www.doczj.com/doc/5e1271647.html,;tel+865533869748;fax +865533869748.
*(H.L.)E-mail cesbin@https://www.doczj.com/doc/5e1271647.html,;tel+862084113695; fax+862084111038.
Notes
The authors declare no competing?nancial interest.■ACKNOWLEDGMENTS
This work has been supported by the National Natural Science Foundation of China(Nos.11174005,21171176,51102106, 11274299,U1232108,and U1432249),Guangzhou Science and Technology Project(2013Y2-00118),Science and T e c h n o l o g y P r o j e c t o f G u a n g d o n g P r o v i n c e (2013B010403010),and Natural Science Foundation of Guangdong Province(S2013030012842).L.N.acknowledges support from the Special and Excellent Research Fund of Anhui
Normal University.
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