Location of the Energy Levels of the Rare-Earth Ion in BaF2 and CdF2
IISSN 1063-7834, Physics of the Solid State, 2008, Vol. 50, No. 9, pp. 1639–1643. © Pleiades Publishing, Ltd., 2008.Original Russian Text © P.A. Rodny œ , I.V. Khodyuk, G.B. Stryganyuk, 2008, published in Fizika Tverdogo Tela, 2008, Vol. 50, No. 9, pp. 1578–1581.
1. INTRODUCTION The location of energy levels of rare-earth ( RE ) ele-ments in various matrices plays an important role in thephysical processes occurring in crystals. In recentyears, owing to the generalization of a great amount ofdata available in the literature on different characteris-tics of rare-earth elements in ionic compounds [1–3], ithas become possible to determine the location of theground and excited levels of these rare-earth elementsin the energy level diagram of some crystals. In thisstudy, we determined the location of energy levels ofrare-earth elements in the band diagram for crystals oftwo scintillators BaF and CdF , which exhibit intrinsicluminescence, and evaluated the effect of rare-earthelements on some physical properties of these objects,specifically on the charge carrier capture. The location of the ground and excited energy levelsof rare-earth states exerts a noticeable effect on theluminescence properties of crystals. For example, theCdF crystal exhibit intrinsic luminescence, which issubstantially quenched at room temperature [4].Repeated attempts to increase the intensity of lumines-cence of the CdF crystal by means of the introductionof an activator, including Ce , have failed. In thisstudy, we attempted to answer the question as to whythis rare-earth element serves as a good activator, i.e.,produces intense luminescence, in one matrix and doesnot luminesce at all in another matrix. Rare-earth elements embedded in halides andoxides can affect their radiation resistance, which isparticularly important for scintillation crystals. In somecases, a small amount of impurities substantially increases the radiation resistance (the permissible dose)of the crystal. For example, the permissible dose for theGd SiO : Ce (0.5 at %) scintillator is several orders ofmagnitude higher than that for pure Gd SiO [5]. Thephysical mechanism of enhancement of the radiationresistance is poorly understood, and the existing mod-els of the process have a number of contradictions. Itwas assumed that the Ce and Pr ions, which exhibita tendency toward transformation into the tetravalentstate under irradiation, exert a negative effect on theradiation resistance of crystals. At the same time, theEu, Sm, and Yb ions, which change the charge from +3to +2, should suppress the formation of radiationdefects [6]. The above change in the charge state of therare-earth elements should undoubtedly be taken intoaccount; however, this change cannot be considered asa basic phenomenon, if for no other reason than the factthat an increase in the radiation resistance upon intro-duction of Ce ions has been observed for a number ofcrystals and glasses [5, 7]. Another idea is that the intro-duction of trivalent rare-earth elements into crystalswith divalent cations of the host matrix (BaF , CdF )decreases the amount of anion vacancies in the crystal.This circumstance, in turn, decreases the concentrationof F centers [8]. However, it is known that the initialvacancies bring about the effective formation of F cen-ters only at the early stage of the process. In this paper,we propose the use of a new approach to solving theproblem of radiation resistance of crystals, which isbased on the location of the energy levels of RE ionsin BaF and CdF . Location of the Energy Levels of the Rare-Earth Ionin BaF and CdF P. A. Rodny œ a , I. V. Khodyuk a , and G. B. Stryganyuk a a St. Petersburg State Polytechnical University, Politekhnicheskaya ul. 29, St. Petersburg, 195251 Russia e-mail: [email protected] b Franko Lviv National University, ul. Dragomanova 50, Lviv, 79005 Ukraine
Abstract —The location of the energy levels of rare-earth ( RE ) elements in the energy band diagram of BaF and CdF crystals is determined. The role of RE and RE ions in the capture of charge carriers, luminescence,and the formation of radiation defects is evaluated. It is shown that the substantial difference in the lumines-cence properties of BaF : RE and CdF : RE is associated with the location of the excited energy levels in theband diagram of the crystals. PACS numbers: 61.72.Ww, 76.30.Kg, 73.20.Hb DOI:
PROCEEDINGS OF THE XIII FEOFILOV SYMPOSIUM“SPECTROSCOPY OF CRYSTALS DOPED BY RARE-EARTH AND TRANSITION-METAL IONS” (Irkutsk, July 9–13, 2007)
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2. SAMPLE PREPARATION AND EXPERIMENTAL TECHNIQUE The excitation and emission spectra of crystals weremeasured at room temperature at the HASYLAB Syn-chrotron Research Laboratory (DESY, Hamburg) withthe use of experimental equipment of the SUPERLUMIstation. The measurement of the luminescence spectrawas performed under continuous x-ray excitation, andthe kinetics was measured under pulsed x-ray excita-tion. The experimental technique was described inmore details in [4, 9]. Crystals of the BaF : RE andCdF : RE were grown by the Stepanov–Stockbargermethod at the Vavilov State Optical Institute (St. Peters-burg, Russia). 3. RESULTS AND DISCUSSION Figure 1 shows the excitation spectrum of the UV(250-nm) luminescence band of the BaF : Pr crystal.This luminescence band can undoubtedly be attributedto the d – f interconfiguration transitions of the Pr ionboth in the spectral position (inset to Fig. 1) and in thedecay time (22 ns). The excitation spectrum involvestwo characteristic bands with maxima at energies of 6.1and 7.5 eV. The low-energy absorption edge (5.5 eV)allows one to determine the location of the 5 d level withrespect to the 4 f electron configuration in the groundstate of the Pr ion. The corresponding absorption edgefor Ce and BaF : Ce is located at an energy of4.1 eV. Figures 2 and 3 present the energy level diagramsfor the BaF : RE and CdF : RE crystals, respectively.When constructing these diagrams, we used the follow-ing data: the luminescence excitation spectra of the Pr and Ce ions in the BaF crystal, the previously mea-sured excitation spectra of rare-earth elements in BaF [9–11], the data available in the literature on the photo-conductivity and photoionization of rare-earth elementsin crystals [12–14], and a number of model–theoreticalconsiderations [1, 2]. The results obtained from thegeneralization of a large amount of data available in theliterature for rare-earth elements in crystals [1–3] canbe summarized as follows. (1) The relative energy position of the ground (4 f )states of the RE and RE ions remains almostunchanged in the series of lanthanides and weaklydepends on the nature of the matrix into which theseions were embedded. (2) The energy position of the excited 5 d levels ofthe RE and RE ions is determined by the environ-ment (the crystal field) and weakly depends on the typeof the ion itself. I n t e n s it y , a r b . un it s
300 400Wavelength, nm0400 I n t e n s it y , a r b . un it s Fig. 1.
Excitation spectrum of the luminescence band at260 nm for the BaF : Pr (0.3 at %) crystal at room tem-perature. The inset shows the luminescence spectrum of thecrystal upon excitation by photons with an energy of 6.1 eV. E n e r gy , e V VB 5 d , RE CB 5 d , RE f , RE f , RE Ce Pr Nd SmEu GdCTTb DyHo Er TmYbPC fd PC fd Fig. 2.
Energy level diagram of the rare-earth ions in the BaF crystal ( RE f and RE f are the ground states, and RE d and RE d are the lower excited states). The dash-dotted line indicates the Fermi level E F . Designations of the electron transitions: PCis photoconductivity, CT is the charge transfer, and fd is the low-energy edge of the absorption band for the 4 f –5 d transitions. PHYSICS OF THE SOLID STATE Vol. 50
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LOCATION OF THE ENERGY LEVELS 1641 (3) The energy gap between the RE d and RE d levels for the particular matrix (crystal) remains nearlyconstant. (4) In charge-transfer transitions of the RE ions,the initial state is associated with the top of the valenceband, whereas the final state coincides with the groundstate of the corresponding RE ion. As a result, themaximum of the absorption band upon charge transferof the RE ion unambiguously determines the positionof the 4 f level of the corresponding RE ion withrespect to the valence band. Therefore, it is sufficient to know the energy param-eters of one rare-earth element in order to construct theenergy level diagram for all rare-earth elements. Sincethe particularly large amount of data are available onthe energy position of the charge-transfer band of Eu ions in different matrices [3], europium usually servesas a reference point in the construction of energy leveldiagrams similar to those presented in Figs. 2 and 3.When constructing the scheme depicted in Fig. 2, weused the data on the photoconductivity of the BaF :Eu [12], BaF : Tm [13], and BaF : Ce [14] crys-tals. According to the data reported in [12], the lower 5 d level of the Eu ion is located ~0.4 eV above the bot-tom of the conduction band. For the BaF : Ce crystal,we took into account the approximate value of theenergy of the charge-transfer band, which was indi-rectly estimated in [15]. The location of the fundamen-tal 4 f level of the Ce ion should be discussed sepa-rately. As follows from the theory, this level should belocated in the conduction band of the BaF crystal [1].However, according to [14], the photoconductivitythreshold for Ce ions in BaF is equal to 1.1 eV. Thepoint is that the ground state of the Ce ion is 4 f d rather than 4 f ; in addition, Ce is the largest ionamong the RE ions, which increases the lattice distor-tion near the ion. When constructing the energy level diagram of theCdF : RE crystal, the energy position of the Eu f level (0.35 eV) with respect to the bottom of the con-duction band [16] and a number of spectral characteris-tics [17, 18] served as the main reference point. The energy levels of the rare-earth elements deter-mine the state of the electron and hole traps in the crys-tal. Trivalent ions with the ground (4 f ) states locatedabove the top of the valence band can capture holesfrom the valence band. Correspondingly, the RE ionswith the 4 f levels located below the bottom of the con-duction band can capture electrons from the conductionband. The energy levels of the rare-earth ions, whichare located near the bottom of the conduction band ( ∆ E < 1 eV), serve as shallow-level electron traps. Thesecenters are thermally unstable and responsible for theundesirable persistent luminescence of scintillators.The impurities with ∆ E > 1 eV serve as deep (stable) carrier traps; moreover, they are important for phos-phors with optical memory [19]. In barium fluoride, the Eu , Sm , and Yb ions aredeep electron traps, because, after the electron capture,they are transformed into Eu , Sm , and Yb ionswith the ground levels located 1–3 eV below the con-duction band (Fig. 2). The Nd , Ho , and Er ions areshallow electron traps, whereas the Pr , Nd , Tb ,and Dy ions should be considered to be deep holetraps. For the CdF : RE crystal, the situation differs fromthat for the BaF : RE crystal. Trivalent ions (Eu, Sm,and Yb), which exhibit a tendency toward a transition tothe divalent state, cannot be electron traps in the CdF crystal because the corresponding RE f states arelocated in the conduction band (Fig. 3). In this case,hole traps dominate; in particular, the Nd and Dy ions should be considered to be deep hole traps. It isknown that the CdF compound containing rare-earthions (except for europium) can be transformed into thesemiconductor state by means of its annealing in cad-mium vapors [20]. We assume that this property of theCdF : RE compound is determined by the energylocation of the RE f levels (except for the Eu f level) in the conduction band of the crystal (Fig. 3). The energy diagrams constructed in this study arealso very useful in evaluating the luminescence proper-ties of rare-earth elements in the crystal under consid-eration. It can be seen from Fig. 2 that all the excited 5dstates of divalent ions lie in the conduction band of theBaF crystal. The states localized inside the conductionband effectively interact (intermix) with band states;consequently, the d – f luminescence of RE ions isimpossible in BaF . This inference was confirmed inthe experiment: the d – f luminescence in BaF : RE was not observed (BaF : Eu exhibits a so-calledanomalous luminescence, whereas BaF : Sm is char-acterized by transitions from the Sm D , 4 f level E n e r gy , e V VB 5 d , RE CB 4 f , RE f , RE Ce Pr Nd Sm Eu GdTbDy Ho Er Tm Yb fd d , RE fd E F Fig. 3.
The same as in Fig. 2 for the CdF crystal. PHYSICS OF THE SOLID STATE
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RODNYŒ et al. which is located below the 5 d levels and slightly belowthe bottom of the conduction band). In the CdF crystal, all the excited levels of themixed 4 f d configuration of the RE and RE ions arelocated in the conduction band. The exclusion is theCdF : Eu crystal, which luminesces (Fig. 4) as result-ing from the fact that the charge-transfer band is par-tially overlapped with the conduction band. As can beseen from Fig. 4, the f – f luminescence of the Eu ion(the D F j transitions) suppresses intrinsic emis-sion (the broad band at 400 nm) of the CdF crystal. The shift of the 5 d levels to the conduction band inthe series of CaF [2], BaF , and CdF is associated pri-marily with an increase in the covalency of the crystals.It seems likely that the absence of luminescence of therare-earth elements in the PbF crystal (with a band gapof 5.84 eV) is also associated with the arrangement ofthe excited 5 d levels of the RE and RE ions in theconduction band. The location of the energy levels of the rare-earthelements should be taken into account when analyzingthe mechanism of defect formation in crystals. It isknown that halides are characterized by the effectiveaxial relaxation of anions, which leads to the formationof V k centers. Since the crystal structure is distorted inthe vicinity of the V k center, this state has a tendencytoward the formation of point defects. It is clear that theretardation of defect formation in the crystal can beachieved by means of the introduction of impurities(including rare-earth elements) involved in the processof capture of valence holes, which competes with theprocess of the formation of V k centers. According to the energy level diagram depicted inFig. 3, virtually any RE ion (except for Cd ) in theCdF crystal can be a hole trap and can prevent the for-mation of V k centers in the crystal. This inference cor-responds to the experimental data: upon introduction of a small amount (0.5 at %) of RE ions into the CdF crystal, the permissible dose of irradiation of the crystalincreased, in particular, by a factor of 10 for the Nd and Sm ions and by a factor of 10 for the Ce andTb ions [21]. In the BaF crystal, the deep donors are Ce , Pr ,and Tb ions, which could improve the radiation resis-tance of the crystals. However, the experiments showedthat Ce ions deteriorate the radiative properties of theBaF crystal [21]. The Ce ions effectively captureconduction electrons and return to the trivalent stateafter irradiation. A somewhat increase in the radiationresistance is observed upon introduction of ytterbium[21], which can serve as a trap for both holes and elec-trons. 4. CONCLUSIONS Thus, the energy level diagram of rare-earth ele-ments in BaF and CaF was constructed using theobtained results and data available in the literature. Therole of RE and RE ions in the capture of charge car-riers, luminescence, and the generation of radiationdefects was determined. The substantial difference inthe luminescence properties of the BaF : RE and CdF : RE crystals is associated with the location of theenergy levels in the conduction band of the crystals.The radiation resistance of the crystals depends on thelocation of the RE f levels in the band gap of thesematerials. Energy level diagrams similar to those presented inFigs. 2 and 3 can be constructed for any crystal with asufficient number of known parameters. These dia-grams are useful both for understanding the mecha-nisms of capture of electrons and holes and for investi-gating the emission and absorption spectra of crystals.To date, the energy level diagrams of rare-earth ele-ments have been constructed for a number of oxidesand sulfides [22]. REFERENCES
1. C. W. Thiel, H. Cruguel, H. Wu, Y. Sun, G. J. Lapeyre,R. L. Cone, R. W. Equall, and R. M. Macfarlane, Phys.Rev. B: Condens. Matter , 085107 (2001). 2. P. Dorenbos, J. Lumin. , 301 (2004). 3. P. Dorenbos, J. Phys.: Condens. Matter , 8417 (2003). 4. P. A. Rodnyi, Radiat. Meas. , 605 (2001). 5. M. Kobayashi and M. Ishii, Nucl. Instrum. MethodsPhys. Res., Sect. B , 85 (1992). 6. S. Ren, G. Chen, P. Zhang, and Y. Zheng, Mater. Res.Soc. Symp. Proc. , 435 (1994). 7. E. Auffray, I. Dafinei, P. Lecoq, and M. Schneegans,Mater. Res. Soc. Symp. Proc. , 111 (1994). 8. S. I. Kuptsov, A. S. Solov’ev, V. G. Vasil’chenko,A. A. Bistrova, I. I. Buchinskaya, E. A. Krivandina,B. P. Sobolev, Z. I. Zhmurova, Yu. A. Krechko, I n t e n s it y , a r b . un it s CdF Emission CdF :Eu Fig. 4.
Luminescence spectra of the pure CdF crystal andthe CdF : Eu crystal upon x-ray excitation. T = 300 K. HYSICS OF THE SOLID STATE Vol. 50
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