Current Applied Physics 10 (2010) 351–354

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Current Applied Physics

journal homepage: www.elsevier .com/locate /cap

Study on the doping mechanism and electronic structure for Nb5+ dopingPbWO4 crystals q

Teng Chen a,*, Kaiyu Xu a, Dongli Shi a, Mingjie Wang a, Tingyu Liu b

a Department of Mechanics, Shanghai University, 99 Shangda Road, Shanghai 200444, Chinab College of Science, University of Shanghai for Science and Technology, 516 Jungong Road, Shanghai 200093, China

a r t i c l e i n f o

Article history:Received 13 April 2009Received in revised form 26 May 2009Accepted 18 June 2009Available online 23 June 2009

PACS:71.20.�b78.20.Ci61.72.Ji61.72.Bb

Keywords:PbWO4

DopingCharge-compensating mechanismComputer simulationElectronic structure

1567-1739/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.cap.2009.06.026

q Supported by China National Natural Science Foun* Corresponding author.

E-mail address: chen-teng@163.com (T. Chen).

a b s t r a c t

The existing forms of the impurity Nb5+ in the Nb5+:PbWO4 (PWO) crystals are simulated by computertechnology. The possible various kinds of defects in the Nb5+:PWO crystals are also simulated. By analyz-ing the calculation results of defect formation energies and binding energies, the optimal substitutionpositions of the Nb5+ ions and the charge compensating mechanism [Nb�w+V2þ

O +Nb�w] in the Nb5+:PWOcrystals were obtained. The electronic structures for Nb5+:PWO were calculated with density functionaltheory code CASTEP. It shows that the doping of Nb5+ ions could suppress the 350 nm absorption band.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Since Kobayashi et al. [1–3] found doping La3+ ions can enhancethe radiation hardness and transmittance near UV regions of PWOcrystals, the doping effects for various kinds of ions with differentvalence in PWO crystals have been studied in order to obtain theoptimal scintillation PWO crystals, especially the trivalent, tetrava-lent and pentavalent ions. So far, people have reached a consensuson the doping effects of the doping mechanism for trivalent andtetravalent ions in PWO crystals. However, for the doping mecha-nism of pentavalent ions, there still exists considerable contro-versy. The emergence of Nb5+:PWO crystals has arousedextensive attentions [4–6], because through the experiment itwas found doping niobium can enhance the transmittance nearUV regions, suppress the 350 nm absorption band and enhancethe radiation hardness [4]. However, there is no clear explanationon the Nb5+ doping mechanism as yet; even the same group pub-lished different views at different times [5,6]. Korzhik et al. [4] pro-

ll rights reserved.

dation (10802044).

posed that Nb5+ ions occupy the W6+ positions in the (WO3 + VO)(one oxygen atom escapes from the WO2�

4 ) forming hole center(NbO3 + F+) (one oxygen vacancy traps one electron thus formingthe F+ center)in the Nb5+:PWO crystals, yet, the author did not clar-ify that Nb5+ ions occupy the W6+ positions in the (WO3 + VO) in-stead of the normal W6+ positions in the (WO4)2�. In order toexpound the charge compensating mechanism in Nb5+:PWO crys-tals, in this paper, the possible various defect models were simu-lated using the free-software GULP program [7–9]. The electronicstructure of Nb5+:PWO crystal was calculated using CASTEP code[10,11] with the lattice structure optimized.

2. The simulation methods and cluster models

2.1. The simulation methods

The lattice simulations were performed by using the free-soft-ware GULP program that is based upon the Mott–Littleton method-ology for accurate modeling of defective lattices. The programGULP optimizes the structure, for the asymmetric unit fractionalcoordinates and cell strains, using analytical symmetry-adaptedfirst and second derivatives within a Newton–Raphson procedure

Table 1Empirically derived Potential parameters used in PWO crystal.

Short-range potential parameters Shell parameters

Interactions A/eV q/10�10m C/eV(10�10m)6 Reference Ions Y/e2 k/eV (10�10m)- Reference

O2––O2 9547.960 0.219200 32.000 [8–10] Pb2+ �0.09 21006.539 [8–10]Pb2+–Pb2+ 18912.114 0.313781 2.600 [8–10] W6+ 5.89 7.690 [8–10]Pb2+–O2– 8086.804 0.264866 3.564 [8–10] O2� �2.04 6.300 [8–10]W6+–O2– 767.430 0.438600 0.000 [9,10] Nb5+ 5.000 0.000 [11]Nb5+–O2– 3023.184 0.300000 0.000 [11]

Table 2Calculated properties of perfect crystal of PbWO4.

Properties Calculated Experimental [9,10] Difference (%)

Lattice energy (eV/formula)PbWO4 �247.67 – –PbO �37.89 – –a 5.441 5.456 �0.27c 12.024 12.020 0.03Volume (Å3) 355.978 357.811 �0.51

Bond distance (Å)Pb–O 2.576 2.58 �0.15

2.637 2.64 �0.07W–O 1.765 1.77 �0.28Refractive index (z) 2.19 2.2 �0.04

352 T. Chen et al. / Current Applied Physics 10 (2010) 351–354

starting from the exact Hessian matrix [7,8]. The potential param-eters used in the simulation are listed in Table 1. The calculationresults of the lattice parameters and bond distance after using po-tential parameters from Table 1 and their experimental values arelisted in Table 2. Through comparing, we can see that simulationresults vary within an average of 0.51% from the experimental val-ues. Such a good consistency between the simulation results andthe experimental values indicates that the potential parametersare reasonable.

The lattice optimization for the PWO containing impurity ionsNb5+ is performed using CASTEP code. In this paper, CASTEP codehas been used for the ground-state electronic structure calcula-tions under the density functional theory (DFT) method theoreti-cally based on the generalized gradient approximations (GGA).The basic theory of DFT is to solve the Kohn–sham equation[10,11]. The detailed parameters were chosen as follows: kineticenergy cut-off = 340 eV, fast Fourier transform (FFT) grid dimen-sions = 60 � 60 � 64, and space representation = reciprocal, SCFtolerance = 1.0 � 10�6 eV/atom [12].

Table 3Energies of isolated point defects and defect clusters.

Lattice Energy (eV) Lattice Energy (eV)

(a) Lattice Energy (eV)PbWO4 �247.67 WO3 �213.38PbO �37.89 Nb2O5 �327.35

Defect Energy (eV) Defect Energy (eV)

(b) Formation energyV2þ

O 18.72 Nb�W 34.12

V2�Pb 25.21 Nb3þ

Pb1****

O�i �9.83 Nb5�i

****

Nb�W1–V2þO 50.879 Nb�W2

-–V2þO 51.181

Nb�W3–V2þO 56.102

Defect clusters Energy (eV) Defect clusters Energy (eV)

(c) Binding energyNb�W1+V2þ

O1 +Nb�W1- 8.179 Nb�W2+V2þ

O +Nb�W2 7.453Nb�W1+V2þ

O1 +NbW1 8.362 Nb�W+F+ 1.234

2.2. The possible various defect models in Nb5+: PWO crystals

In theory, there may exist many different compensating mech-anisms in Nb5+ doping PbWO4 crystals which correspond to differ-ent defect clusters.

Firstly, for the purpose of proving that Nb5+ ions occupy the W6+

position in the (WO3 + VO) [4], the formation energies of three de-fect clusters related to oxygen vacancies were calculated: (1). Nb5+

ions occupy the W6+ position in the (WO3 + VO) forming defectcluster (a) Nb�1

W1–V2þO ; (2). Nb5+ ions occupy the W6+ position in

the (WO4)2� nearest to the (WO3 + VO) forming defect cluster (b)Nb�W2–V2þ

O ; (3). Nb5+ ions occupy the W6+ position in the (WO4)2�

a little far away from the (WO3 + VO) forming defect cluster (c)Nb�W3

--V2þO .

Secondly, the binding energies of defect clusters [Nb�W1� + V2þ

O +

Nb�W1], [Nb�W1 + V2þO + Nb�w2] and [Nb�w1 + F+] were calculated. The

three defect clusters all have reached local charge balance.

3. Results and discussion

3.1. The main charge compensating mechanism in the crystal

From the calculation results listed in Table 3, it can be found theformation energies of point defect Nb3þ

Pb and interstitial ions Nb5þi

were not convergent (in Table 1 denoted with ***), which meansthere is no possibility for Nb5+ ions to occupy the Pb2+ lattice or ex-ist in the form of interstitial ions in the crystal. Nb5+ should occupyW6+ lattice in the crystal.

The formation energies of cluster (a), (b), (c) are listed in Table3. With regard to defect cluster (c), thirty cases were calculated(the minimum of these thirty cases is listed in Table 1). As the cal-culation results show, Nb5+ should preferentially occupy the W6+

lattice in (WO3 + VO) in the Nb5+: PWO crystal. Nevertheless, wecan also find the formation energy of defect cluster (b) and (a)are very close to each other, less than that of cluster (c). Sincethe formation energy of (b) is so close to that of (a), we can spec-ulate that the W6+ ions nearest to the oxygen vacancies and secondnearest to the oxygen vacancies could possibly all be replaced by

T. Chen et al. / Current Applied Physics 10 (2010) 351–354 353

Nb5+ ions, therefore, the charge compensating mechanism can beexpressed as cluster defect: [Nb�w+V2þ

O +Nb�w] or [Nb�w1+V2þO +Nb�w2]

or [Nb�w2+V2þO +Nb�w2], Fig. 1 shows its microscopic structure.

The cluster binding energy is defined as the difference betweenthe sum of the formation energy of the constituent point defectsand the formation energy of the appropriate cluster energy. Asit is shown in Table 3, we can also see the binding energy of[Nb�w1

-+V2þO +Nb�w2] is higher than that of [Nb�w1+V2þ

O +Nb�w1],[Nb�w2+V2þ

O +Nb�w2-] and Nb�w + F+, it can be concluded that the main

compensating mechanism in the Nb5+ doping PWO crystals is[Nb�w+V2þ

O +Nb�w] will not be compensated by F+ color center. Lianget al. [13] adopted the positron annihilation lifetime spectrum andX-ray photoelectron spectra(XPS) as the experimental means onthe study of the micro-mechanism of Nb-doped PbWO4 and the re-sults turned out that Nb5+ will occupy the lattice of W 5+ and makesome (WO4)2� change to (NbO3 + VO)�. Hence we can see ourcalculating results are basically consistent with the experimentalresults, nevertheless, we obtained a more precise charge compen-sation mechanism through computer simulation.

-2 0 -1 5 -1 0 -5 0 5 1 00

50

100

150

200

250

300 Pb5d

Pb6s

O2p

W5d

Fermi level

TDO

S

Energy [eV]

Fig. 2. The total density of states for perfect PWO crystal.

VO2+

NbW-

NbW-WO4

2-

c

b

aPb2+

Fig. 1. Micro-structural models of V2þO and Nb�W color center in the as-grown PWO

crystal.

3.2. The densities of states

The electronic structure of perfect PWO crystal, PWO crystalcontaining oxygen vacancy and PWO crystal with defect cluster[Nb�w+V2þ

O +Nb�w] were also calculated. The total densities of statesare given in Figs. 2–4.

Comparing the total density of states of perfect PWO crystalwith PWO crystal containing oxygen vacancy V2þ

O , a small peak oc-curs within the forbidden band peaking at about 1.3 eV for the lat-ter. The real existence of the small peak means the 350 nmabsorption band has close relation with the oxygen vacancies forthe reason that oxygen vacancy V2þ

O forms a F+ color center whencatching an electron, while it will form F color center if catchingtwo electrons, in addition, the energy level of F and F+ color centerdo locate in the forbidden band. This has already been detailed dis-cussed in our previous work [14,15].

The total density of states of PWO crystal with defect cluster[Nb�w+V2þ

O +Nb�w] in the regions below Fermi level EF are quite sim-ilar to that of V2þ

O-: PWO crystal and prefect PWO crystal. The most

obvious differences are the band gap of the former narrows about0.5 eV than that of V2þ

O-: PWO crystal and the small peak of O2p

state occurred in the forbidden band of V2þO : PWO crystal, but it dis-

-2 0 -1 5 -1 0 -5 0 5 1 00

50

100

150

200

250

300

0 1 2 30

2

4

6

8

Pb6s

O2p

O2p

W5d

Fermi level TDO

S

Energy [eV]

O2p

Fig. 3. The total density of states for PWO crystal containing oxygen vacancy, theinset shows the total densities of state in the forbidden band on an expanded scale.

-2 0 -1 5 -1 0 -5 0 5 1 00

20

40

60

80

Pb5d

Fermi level

Pb6sW5dNb4d

O2p

TDO

S

Energy [eV]

Fig. 4. The total density of states for PWO crystal containing defect cluster[Nb�W + V2þ

O +Nb�W].

354 T. Chen et al. / Current Applied Physics 10 (2010) 351–354

appears in the Nb5+: PWO crystal. Since Nb5+ substituting W6+

exhibits univalent negative electricity, the oxygen vacancies willno longer trap electrons to maintain charge balance, but compen-sated by Nb�w. Meanwhile, the F and F+ color center which has aclose relation with the 350 nm absorption band do not exist inthe Nb5+: PWO crystal, and then we can predict that the dopingof Nb5+ can suppress the 350 nm absorption band. The calculationresults are in good concordance with the experimental results thatdoping with Nb5+ can enhance the transmittance near UV regionsand suppress the 350 nm absorption band [4].

4. Conclusion

The formation energies of point defects and defect clusters havebeen simulated through computer technique. The calculation re-sults indicate that Nb5+ will only occupy the position of W6+ form-ing defect cluster NbO3 + VO in Nb5+: PWO crystals. By furtheranalyzing simulation results, it reveals that Nb5+ ions will not onlyoccupy the W6+ position in the (WO3 + VO), the W6+ ions nearest tothe (WO3 + VO) but also can be replaced by Nb5+. It will form thecharge compensating mechanism [Nb�w+V2þ

O +Nb�w]. To make furtherconsideration of this calculation results, the densities of states for

perfect PWO crystal, V2þO : PWO crystal and Nb5+: PWO crystal were

calculated. The results show that doping Nb5+ can effectively sup-press the 350 nm absorption band.

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