Dependence of Charge Transfer Energy on Crystal Structure and

21438

J. Phys. Chem. B 2006, 110, 21438-21443

Dependence of Charge Transfer Energy on Crystal Structure and Composition in Eu3+-Doped Compounds Ling Li†,‡ and Siyuan Zhang*,† Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China, and Graduate School of the Chinese Academy of Sciences, Changchun 130022, China ReceiVed: September 15, 2005; In Final Form: August 26, 2006

We report a method for estimating the positions of charge transfer (CT) bands in Eu3+-doped complex crystals. The environmental factor (he) influencing the CT energy is presented. he consists of four chemical bond parameters: the covalency, the bond volume polarization, the presented charge of the ligand in the chemical bond, and the coordination number of the central ion. These parameters are calculated with the dielectric theory of complex crystals. The relationship between the experimental CT energies and calculated environmental factors was established by an empirical formula. The calculated values are in good agreement with the experimental results. Such a relationship was confirmed by detailed analysis. In addition, our method is also useful to predict the charge-transfer position of any other rare earth ion.

Introduction In recent years, charge transfer (CT) spectra in various Ln3+doped (Ln ) lanthanide element) complex crystals have attracted much attention due to their potential application in the design of new displays and lamps.1-11 CT transitions occur when a valence electron is transferred from the ligand to an empty or incompletely filled orbital of the lanthanide ions.12 Generally, the CT bands are very broad absorption bands in the UV and VUV energy domain.13 Since Jo¨rgensen14 assigned the broad and strong absorption band in the spectra of the trivalent lanthanides to CT transitions, many workers have applied such assignments in the solution and solids.15-17 There has been much experimental1,4,16,18,19 and theoretical3,12,14-17,20-22 work regarding CT bands in Ln3+-doped compounds. In 1962, a systematic change in the CT energy, with different types of lanthanides, was revealed for bromides of Sm3+, Eu3+, Tm3+, and Yb3+ in ethanol by Jo¨rgensen.14 Blasse and Brill19 found that the CT energy of Sm3+ in the compounds always appears 1.1 ( 0.1 eV higher compared to the CT energy of Eu3+. Van Pieterson and co-workers1,18 observed the CT energies of Yb3+ are 3000 cm-1 higher in comparison with Eu3+. Recently, Dorenbos21,22 did systematic statistical research on CT energies of available lanthanide ions in various compounds. He found that the CT energy of Ln3+ can be predicted from that of Eu3+ according to his model. That is, if the CT energy of Eu3+ is known, the CT position of any other Ln3+ can be obtained.21 The relationship between the type of compounds and the CT energy of Eu3+ has been extensively studied and discussed. Jo¨rgensen23 gave the CT energy as the following empirical formula: Ect ) 30[χopt(x) - χuncorr(M)] (kcm-1), where χopt(x) is the Pauling electronegativity of ligand x and χuncorr(M) is the optical electronegativity of the metal M. According to Jo¨rgen* Address correspondence to this author. E-mail: [email protected]. Fax: 86-431-569804. † Key Laboratory of Rare Earth Chemistry and Physics. ‡ Graduate School of the Chinese Academy of Sciences.

sen’s formula, the CT energy is the same when the anion remains unchanged for Eu3+-doped compounds. However, especially for the oxides,17 their CT energies are different, so Jo¨rgensen’s equation cannot be used to predict CT energies. Recently more and more Eu3+-doped nanophosphors2,5,6,10,24-27 have been prepared. A small shift of the CT excitation spectra could be observed in the nanocrystal compared with that in the bulk crystal.24,28 The shift partly originates from the change in the bond length Eu-L between the central ion (Eu) and the ligands (L).24 Hoefdraad12,29 also reported the shorter the bond length Eu-L, the shorter the wavelength of the CT band position will be. Except for the bond length, some other research groups studied other possible factors influencing the position of the CT band, such as the covalency or ionicity,30 the coordinate number,29 the potential field of the coordinate ions,31 the standard cation oxidation potential,16 and so forth. All of these studies help to reveal the origin of CT, but a quantitative explanation of the CT energy is still lacking. Our interest is focused on the quantitative relationship between the CT energy of Eu3+ and the structure of the host. It is well-known that the CT energy depends strongly on the nature of the surrounding ions.17 For example, in the sequence of the ligands F, Cl, Br, and I, these CT bands shift to lower energies.14 This was attributed to the nephelauxetic effect.17,32 It is known that the factor h comes from the factorization of the nephelauxetic ratio β as proposed by Jo¨rgensen: 1 - β ) hk, where h relates to the ligands and k to the central metal.32 Blasse tried to find the linear relationship between the CT energy and h, but he was not successful because the factor h was not given quantitatively.17 Gao et al. studied the mechanism of the nephelauxetic effect for the electronic structure of 3d elements and identified the main factors responsible for the effect.33 Recently, the environmental factor (he) was revised for the lanthanide ions by Shi et al.34 They successfully explained the dependence of the crystal field splitting of 5d levels shifts on hosts in the halide crystals as well as the barycenter of energy of lanthanide 4fN-15d configuration in inorganic crystals.35

10.1021/jp0552224 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/10/2006

Positions of CT Bands in Eu3+-Doped Complex Crystals

J. Phys. Chem. B, Vol. 110, No. 43, 2006 21439

However, the CT energy of lanthanide ions has not been studied quantitatively. In this paper, we will study the dependence of CT energy on the crystal structure and composition in Eu3+-doped compounds, based on the dielectric theory of complex crystals.36 The theory has been successfully applied in a variety of research fields, such as nonlinear optical crystals,37,38 high Tc oxides,39 hardness of covalent crystals,40,41 and the barycenter of energy of lanthanide 4fN-15d configuration35 and so forth. The environmental factor was presented. Twenty-nine Eu3+-doped compounds were studied. The relationship between the charge transfer energies and environmental factors was successfully established by an empirical formula. The calculated values are in good agreement with the experimental results. Our method is also useful to predict the charge-transfer positions of any other rare earth ions. Theoretical Methods

From these results we can find that the presented charge of the O ion is different for different bonds. According to PV42 and Levine’s43 theory, the macroscopic linear susceptibility χ of crystals can be obtained by the following equation:

χ ) ( - 1)/4π )

∑µ Fµχµ ) ∑µ Nµb χµb

µ ) 1 + 4πχµ

(3) (4)

where  is the crystal dielectric constant, obtained from the index of refraction n ( ) n2). µ is the dielectric constant of a µ type chemical bond, and Fµ is the fraction of bonds of type µ composing the actual crystal, χµb is the susceptibility of a single bond, and Nνb is the number of bonds per cubic centimeter. χµ is the total macroscopic susceptibility which a crystal composed entirely of bonds of type µ would have, which can be written as:

The dielectric theory of complex crystals is based on the theory developed by Phillips and Van Vechten42 (PV). It is known that the PV theory can only deal with binary crystals. Later, some workers36,43 extended the theory to the compounds with a complex structure. In this theory, the complex crystals with crystal formula Aa11Aa22...Aai iBb11Bb22...Bbj j can be written as a linear combination of the subformula of various binary crystals when the crystal structure is known.36,44 The subformula of any type of chemical bond A-B in the multibond crystal Aa11Aa22... Aai iBb11Bb22...Bbj j... can be expressed by the following formula:

where Eµg is the average energy gap for µ type bonds (in eV), Ωµp is the plasma frequency obtained from the number of valence electrons of type µ per cubic centimeter Nµe , using the following equation:

N(Bj-Ai) × ai iN(Ai-Bj) × bj j A B ) Ami i Bjnj NCAi NCBj

Dµ(A,B) ) ∆µA ∆µB - (δµA δµB - 1)[(ZµA)* - (ZµB)*]2

(7)

Aµ ) 1 - (Eµg /4EµF) + 1/3(Eµg /4EµF)2

(8)

(1)

where

N(Bj-Ai) × ai , mi ) NCAi

nj )

NCBj

Ami Bnj ∑ i,j i

j

(pΩµp )2 ) (4π(Nµe )*e2/m)DµAµ

(5)

(6)

Here, Dµ and Aµ are correction factors, their expressions are

where (ZµA)* and (ZµB)* are the numbers of effective valence electrons on the A and B atoms of the µth bond, and ∆ and δ are the periodic dependent constants.43 The Fermi energy EµF (in eV) is given in terms of the Fermi wave vector kµF by

N(Ai-Bj) × bj

And the bond subformula equation is given by

A1a1A2a2...Aai iB1b1B2b2...Bjbj )

χµ ) (4π)-1[(pΩµp )2/(Eµg )2]

(2)

where Aai i, Bbj j stands for the different constituent elements or different sites of the same element in the crystal formula, and ai, bj represents the number of the corresponding element. N(Bj-Ai) is the number of Bj ions in the coordination group of a Ai ion, and NCAi represents the nearest coordination number of Ai ion. This means that the complex crystal is decomposed into the sum of different binary crystals like Ami i Bnj j. Therefore, the presented charge Q of each ion can be obtained according to the neutral principle of the binary crystals. For any binaryAmBn, QA is the normal valence of the cation A, and QB is that obtained from QB ) mQA/n. Taking YPO4 for an example, each Yttrium atom is surrounded by eight oxygen atoms and each phosphorus atom has four neighboring oxygen atoms, while each oxygen atom is surrounded by two yttrium atoms and one phosphorus atom. Therefore, in terms of eqs 1 and 2, YPO4 can be decomposed into YO8/3 and PO4/3, and the bond subformula equation can be obtained as YPO4 ) YO8/3 + PO4/3, i.e., decomposing the complex crystal into different kinds of binary crystals such as AmBn. For YO8/3, let QY ) 3.0, then the presented charge of the O ion is QO ) 3.0 × 3/8 ) 1.125 in the Y-O chemical bond. For PO4/3, let QP ) 5.0, the presented charge of the O ion is OO ) 5.0 × 3/4 ) 3.75 in the P-O bond.

EµF ) (pkµF)2/2m

(9)

(kµF)3 ) 3π2(Nµe )*

(10)

where (Nµe )* is the effective valence electron density for the µ bond per cubic centimeter and it is given by

(Nµe )* ) (nµe )*/Vµb

(11)

(nµe )* ) (ZµA)*/NµCA + (ZµB)*/NµCB

(12)

Vµb ) (dµ)3/

∑v (dV)3NVb

(13)

Here, (nµe )* is the number of effective valence electrons per µ bond. The bond volume Vµb for the bonds of type µ (Å3) is proportional to (dµ)3 (Vµb ∝ (dµ)3), where dµ is the bond distance (in Å). NVb is the number of bonds of type ν per cubic centimeter. The average energy gap Eµg (in eV) can be separated into homopolar energy Eµh (in eV) and heteropolar energy Cµ (in eV) as shown in the equation below:

(Eµg )2 ) (Eµh )2 + (Cµ)2

(14)

21440 J. Phys. Chem. B, Vol. 110, No. 43, 2006

Li and Zhang

TABLE 1: Charge Transfer Energies in Eu3+-Doped Compounds and Chemical Bond Parameters Relating to the Environmental Factorsn crystals

n a

a

β

bond

dµ (Å)

f µc

Rµb (Å3)

QµB

C.N.

he

Ect,exp l,m (eV)

Ect,cal (eV)

Y-F Gd-F La-F Sc-O Y-O Lu-O Y-O Lu-O La-Cl Y-O Y-O Gd-O Gd-O Y-O La-O Gd-O Y-O Y-Cl Gd-O Gd-Cl Y-O Y-Br La-O La-Cl Gd-O Gd-Br La-O La-Br La-O La-I Lu-O Lu-S Gd-O Gd-S Y-O Y-S La-O La-S Y-O Gd-O

2.270 2.313 2.495 2.151 2.240 2.218 2.368 2.330 2.951 2.383 2.469 2.487 2.548 2.371 2.489 2.398 2.284 3.010 2.312 3.046 2.347 3.320 2.396 3.177 2.359 3.325 2.398 3.320 2.411 3.614 2.209 2.877 2.287 2.947 2.247 2.896 2.399 3.086 2.356 2.359

0.0226 0.0225 0.0440 0.1074 0.1048 0.1054 0.0783 0.0800 0.0577 0.0860 0.0817 0.0819 0.0813 0.1584 0.1552 0.1576 0.1613 0.0564 0.1612 0.0566 0.1683 0.0614 0.1618 0.0575 0.1673 0.0609 0.1647 0.0597 0.1632 0.0612 0.1565 0.0956 0.1533 0.0941 0.1709 0.1054 0.1521 0.094 0.1308 0.1307

0.173 0.195 0.477 0.315 0.396 0.404 0.382 0.382 1.234 0.426 0.393 0.423 0.467 0.715 0.913 0.786 0.503 0.746 0.539 0.799 0.545 1.148 0.613 0.958 0.570 1.172 0.610 1.205 0.624 1.657 0.505 0.879 0.578 0.968 0.550 0.935 0.692 1.169 0.784 0.829

9/8 9/8 1 9/8 9/8 9/8 3/2 3/2 1 3/2 5/3 5/3 5/3 9/8 9/8 9/8 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 2 2 2 2 2 2 2 3/2 3/2

8 8 9 8 8 8 8 8 9 8 9 9 9 8 8 8 4 5 4 5 4 5 4 5 4 5 4 5 4 5 4 3 4 3 4 3 4 3 6 6

0.199 0.211 0.435 0.585 0.648 0.656 0.733 0.742 0.800 0.812 0.895 0.931 0.974 1.071 1.198 1.119 1.229

8.16 8.00 7.42 6.05 5.66 5.74 5.54 5.53 5.15 5.28 5.02 4.73 5.00 4.44 3.72 4.28 4.40

8.20 8.12 6.82 6.12 5.87 5.84 5.56 5.53 5.34 5.30 5.05 4.95 4.84 4.61 4.34 4.52 4.28

1.271

4.34

4.21

1.349

4.2

4.08

1.365

4.28

4.05

1.372

4.14

4.04

1.402

4.08

3.99

1.462

3.70

3.91

1.508

3.72

3.85

1.584

3.60

3.75

1.640

3.60

3.69

1.733

3.58

3.59

1.176 1.209

4.96 4.88

4.38 4.32

LiYF4 LiGdF4 b LaF3 a ScPO4 c YPO4 a LuPO4 c Y3Al5O12 a Lu3Al5O12 a LaCl3 a Y3Ga5O12 a YAlO3 a GdAlO3 d GdGaO3 d YVO4 a LaVO4 e GdVO4 f YOClg

1.450 1.464* 1.570a 1.670* 1.720a 1.730* 1.832a 1.842a 1.830a 1.940a 1.926a 1.961* 2.017* 2.078a 2.167* 2.114* 2.159*

0.116 0.116 0.089 0.112 0.112 0.112 0.100 0.099 0.072 0.094 0.078 0.078 0.078 0.088 0.088 0.088 0.059

GdOClg

2.224* i

0.059

YOBrg

2.365* i

0.059

LaOClg

2.318* i

0.059

GdOBrg

2.416* i

0.059

LaOBrg

2.424* i

0.059

LaOIg

2.462* i

0.059

Lu2O2Sd

2.115* i

0.084

Gd2O2Sd

2.130* i

0.084

2.150i

0.079

La2O2Sd

2.210i

0.084

YAl3(BO3)4 h GdAl3(BO3)4 h

1.739* j 1.746* j

0.121 0.121

Y2O2S

d

a Reference 50. b References 51 and 52. c Reference 53. d Reference 46. e Reference 54. f Reference 55. g Reference 56. h References 57 and 58. Reference 59. j The n value is deduced from the isostructural crystal NdAl3(BO3)4. k Reference 60. l Reference 22. m Reference 8. n Indexes of refraction are listed in column 2. Some compounds for which n values are unobtainable and estimated from the isostructural compounds are marked with an asterisk. β is the preceding factor of the average antisymmetrical potential Cµ. Ect,exp stands for the position of the charge transfer band obtained experimentally, Ect,cal is the value of the charge transfer energy obtained theoretically. Both values are in electronvolts.

i

The ionicity and covalency of any µ type bond are defined as:

fµi ) (Cµ)2/(Eµg )2,

fµc ) (Eµh )2/(Eµg )2

(15)

where

Eµh ) 39.74/(dµ)2.48 (eV)

(16)

Cµ ) 14.4bµ exp(-kµs rµ0 )[(ZµA)* - (n/m)(ZµB)*]/rµ0 (n g m) (eV) (17) Cµ ) 14.4bµ exp(-kµs rµ0 )[(m/n)(ZµA)* - (ZµB)*]/rµ0 (n e m) (eV) (17′) With

rµ0

) d /2 µ

kµs ) (4kµF/πaB)1/2

(18) (19)

Here, kµs is the Thomas-Fermis screening wavenumber of

valence electrons and RB is the Bohr radius. bµ is proportional to the square of the average coordination number Nµc

bµ ) β(Nµc )2

(20)

Nµc ) mNµCA/(m + n) + nNµCB/(m + n)

(21)

where bµ depends on the crystal structure. Once the dielectric constant of the crystal is known, the value of β can be deduced from the above equations. When the dielectric constant is unknown, it may also be estimated by using the β value of its isostructural crystals. The environmental factor designated by the symbol he 33,34 can be expressed as:

∑µ fµc Rµb Qµ2B )1/2

he ) (

(22)

where QµB stands for the presented charge of the nearest anion in the chemical bond, and Rµb is the polarizability of the chemical bond volume in the µ type of chemical bonds.

Positions of CT Bands in Eu3+-Doped Complex Crystals

J. Phys. Chem. B, Vol. 110, No. 43, 2006 21441

Figure 1. The charge-transfer energy for Eu3+ (Ect) in lanthanide compounds against the environmental factor (he).

Figure 2. Schematic diagram of the valence band and the CT energy in Ln3+-doped compounds. The horizontal lines indicate the barycenter of the energy levels for Ln3+ and Ln2+. The vertical double arrows indicate charge-transfer energy to the trivalent lanthanide. The vertical single arrow stands for the average energy gap of the compound.

For the chemical bond of type µ, the polarizable coefficient Rµ0 can be obtained from the Lorentz-Lorenz equation

(µ - 1)/(µ - 2) ) (4π/3)Rµ0

(23)

Hence, the polarizability of the chemical bond volume (Å3) is given by

Rµb ) Rµ0 νµb

(24)

Results and Discussion CT energy depends strongly on the nature of the surrounding ions. The environment is governed by a number of factors that are related to the bond character, such as the coordination number, the covalency, the bond polarizability, and the presented charge of the anions. By using the theory mentioned above, we calculated the chemical bond parameters and environmental factors for 29 lanthanide compounds. The results are compiled in Table 1. In the 29 selected compounds (column 1), various types of compounds are included, such as fluorides, chlorides,

phosphates, borates, aluminates, and oxysulfides. Table 1 gives some useful parameters, such as the coordination number of the central ion (C.N.), the index of refraction of the crystal (n), the average nearest distance to the surrounding anions (d), the covalency of the chemical bond (f µc ), the polarizability of the chemical bond volume (Rµb ), and the environmental factor (he). The relationship between the CT energy and the environmental factor is shown in Figure 1. The experimental and calculated results of the CT energies are listed in Table 1. From Figure 1 we see that the CT energy (Ect) decreases with the increase of the environmental factor (he). By fitting the curve, we obtained the following empirical formula

Ect ) A + Be-khe

(25)

where A ) 2.804, B ) 6.924, and k ) 1.256 for the Eu3+ ion. These constants only relate to the type of rare earth ion. The calculated CT energies are listed in Table 1 (column 12). Compared with the experimental values (column 11), the maximum error is 0.56 eV for GdAl3 (BO3)4.

21442 J. Phys. Chem. B, Vol. 110, No. 43, 2006

Li and Zhang rare earth ion and the valence electrons orbital of ligands. On the left of Figure 2, the horizontal lines indicate the barycenter of the energy levels for Ln3+ and Ln2+. The orbital energy levels of the valence electron of the ligands are labeled on the right side. The hybridization between the 5d orbital of the rare earth ions and the valence electron orbital of the ligands forms the valence band and conduction band. This is shown by the rectangles in Figure 2. The double arrow stands for the chargetransfer energy (Ect). It starts from the top of the valence band and ends in the Ln2+ ground state.22 Therefore, the CT energy can be expressed as:

Ect(Ln3+:A) ) Egr(4fN+1) - EVB(A)

Figure 3. Charge-transfer energy level diagram of Eu3+-doped ScPO4 and YPO4. The left horizontal lines stand for the relative energy level position of the 5d orbital in ScPO4, the 5d orbital in YPO4, the 4f7 configurations for Eu2+, and the 4f6 configurations for Eu3+, respectively. The right horizontal line is the energy level position of the coordination anions for O. Eground(4f7) is the Eu2+ ground-state energy and Eground(4f6) is the ground-state energy for Eu3+.

It can be concluded that the CT energy has a direct relationship with the environmental factor. From Figure 1 it can be seen that the CT energy tends to decrease with the increase of the environmental factor, in the sequence of fluorides, oxides, oxyhalides, and oxysulfides. This sequence has the same trends as the nephelauxetic series studied by Jo¨rgensen.23 Further efforts can be made to explain how the change of the environmental factor influences the CT energy. Figure 2 gives the schematic diagram for the form of the valence band and CT energy in Ln3+-doped compounds. It has been reported that the 4f orbital is hardly involved in the bonding with the ligands.45 The ground states of the 4f configuration for Ln3+ and Ln2+ lie within the energy gap (Eg) of the host crystals. The HOMO bonding and LUMO antibonding energy levels are mainly formed by the interaction between the 5d orbital of the

(26)

where Ect(A) stands for the CT energy of Ln3+ in the host A, Egr(4fN+1) is the ground state of Ln2+, and EVB(A) represents the top of the valence band. It is well-known that 4f orbitals are shielded from the crystalline environment by the outer filled 5s2 and 5p6 orbitals. As a result, the position of the valence band (EVB(A)) plays a decisive role in the CT energy. In fact, the form of the valence band comes from two factors: the 5d energy level of Ln3+ and the valence electron energy level of the ligand. The CT energy level diagram in Eu3+-doped YPO4 and ScPO4 is shown in Figure 3. The compounds have the same ligands.46 Experimentally the CT energy in ScPO4 is higher compared to the CT energy in YPO4.18 Compared with YPO4 (he ) 0.648), the environmental factor he of ScPO4 (he ) 0.585) is small; therefore, the 5d energy level in ScPO4 is higher compared to the 5d energy level in YPO4.35 As a result, the hybridization of the 5d orbital of Eu3+ ion with the orbital of the anion is smaller in ScPO4 than in YPO4, as shown in Figure 3. That is to say, the position of the valence band formed in YPO4 is higher than that in ScPO4 (EVB(YPO4) > EVB(ScPO4)). From eq 26 it can be concluded that the CT energy in Eu3+-doped ScPO4 is higher than that in Eu3+-doped YPO4. Therefore, the higher the value of he, the lower the CT energy is. This is in good agreement with eq 25. In this situation, the change of the CT position for Eu3+ mainly comes from the shift of the 5d orbital when the ligands of the hosts are the same.

Figure 4. Charge-transfer energy level diagram of Eu3+-doped LaF3 and LaCl3. The left horizontal lines stand for the relative barycenter position of the 5d orbital in LaF3, the 5d orbital in LaCl3, the 4f7 configurations for Eu2+, and the 4f6 configurations for Eu3+, respectively. The right horizontal lines indicate the energy level position of the coordination anions for Cl and F. Eground(4f7) is the ground-state energy for Eu2+ and Eground(4f6) is the ground-state energy for Eu3+.

Positions of CT Bands in Eu3+-Doped Complex Crystals Figure 4 shows the CT energy level diagram in Eu3+-doped LaF3 and LaCl3. The compounds have different ligands. Experimentally, the CT energy in LaF3 is higher compared to the CT energy in LaCl3.21,22,47 In this situation there are two reasons responsible for the CT-energy difference. First, with respect to the ligands, it is known that the 3p energy level of the Cl orbital is much higher than the 2p energy level of the F orbital (∆E ) 4.66 eV).48 Second, with respect to the 5d orbital, from Table 1 we know that the environmental factor of LaCl3 (he ) 0.800) is larger compared to the environmental factor of LaF3 (he ) 0.435). Therefore, the 5d barycenter of Eu in LaCl3 is lower than that in LaF3.35 Both of the factors make the valence band in LaCl3 higher than that in LaF3 (EVB(LaCl3) > EVB(LaF3)). Thus the CT energy in LaCl3 is less than that in LaF3. This is in good agreement with eq 25. In this situation we can notice that the difference between the valence energy levels of the two ligands (∆E ) 4.66 eV) is much higher compared to that of the two 5d energy levels (0.45 eV).49 Accordingly, the difference of the CT energy mainly comes from the ligand. Therefore, according to the type of ligands of the hosts the sequence of CT energy can be expressed as follows: fluorides > chlorides > oxyhalides > oxysulfides. From the analysis above, we know that the shift of the CT energy of Eu3+ in different compounds comes from two factors: the change of the 5d energy level and the change in the type of ligand. However, the environmental factor includes the two influencing factors and can directly reflect the CT energy. In conclusion, for any lanthanide compound, if the structure and the refractive index are known, the charge-transfer energy of Eu3+ in the compound can be predicted by the presented formula. Some microcosmic factors should be met to confirm the CT energy: these factors are the chemical bond volume polarizability, the fractional covalency of the chemical bond between the central ion and the nearest neighboring ligands, the coordination number, and the presented charge of the nearest anion in the chemical bond. Our method has successfully built a link between the CT energy and the environmental factor (he). This method, which requires detailed crystallographic information and elaborate computation, can provide a tool for us to understand the relationship between the CT energy and the structure of the host. In addition, our method is also useful to predict the CT position of any other rare earth ion. Acknowledgment. We wish to thank Dr. A. Matthew of University of Hull (England) for helping to revise the English in the manuscript. References and Notes (1) van Pieterson, L.; Heeroma, M.; de Heer, E.; Meijerink, A. J. Lumin. 2000, 91, 177. (2) Dhanaraj, J.; Jagannathan, R.; Kutty, T. R. N.; Lu, C.-H. J. Phys. Chem. B 2001, 105, 11098. (3) Nakazawa, E. J. Lumin. 2002, 100, 89. (4) Jubera, V.; Chaminade, J. P.; Garcia, A.; Guillen, F.; Fouassier, C. J. Lumin. 2003, 101, 1. (5) Dhanaraj, J.; Geethalakshmi, M.; Jagannathan, R.; Kutty, T. R. N. Chem. Phys. Lett. 2004, 387, 23. (6) Jia, C.-J.; Sun, L.-D.; Luo, F.; Jiang, X.-C.; Wei, L.-H.; Yan, C.H. Appl. Phys. Lett. 2004, 84, 5305. (7) Wang, Y.; Guo, X.; Endo, T.; Murakami, Y.; Ushirozawa, M. J. Solid State Chem. 2004, 177, 2242. (8) Chen, X. Y.; Liu, G. K. J. Solid State Chem. 2005, 178, 419. (9) Peng, H.; Huang, S.; You, F.; Chang, J.; Lu, S.; Cao, L. J. Phys. Chem. B 2005, 109, 5774.

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