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Journal of Luminescence 126 (2007) 165–170

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Optical and crystal-field analysis of Er3+ ion in Y2O3–P2O5 thin films

Ramzi Maaleja,�, M. Dammaka, S. Kamouna, J.-L. Deschanvresb, Mohamed Kamouna

aLaboratoire de Physique Appliquee, Groupe de Physique Theorique, Departement de Physique, Faculte des Sciences de Sfax, 3018 Sfax, TunisiebLaboratoire des Materiaux et du Genie Physique, ENSPG, B.P. 46, 38402 Saint-Martin-d’Heres, France

Received 7 January 2006; received in revised form 26 April 2006; accepted 26 June 2006

Available online 9 August 2006

Abstract

Luminescence spectra of erbium ions doped in Y2O3–P2O5 thin films, with different P2O5 content (from 3% to 47%), were analysed

with crystal-field Hamiltonian model with D2d symmetry including J-mixing effect. The empirical crystal-field parameters (CFPs)

obtained for the best fit of calculated to experimental energy levels allows us to confirm the well-established YPO4 phase for 47% of

P2O5. The CFPs are compared to those calculated for Ce3+, Nd3+ and Dy3+ in the YPO4 host. This work is a continuation of our

previous results for erbium-doped Y2O3 thin films.

r 2006 Elsevier B.V. All rights reserved.

PACS: 71.70.Ch

Keywords: Erbium; YPO4; Luminescence; Crystal-field

1. Introduction

4fn intraconfigurational transition studies of rare-earthions doped into wide band gap crystals are of importanceto several optical domains of application, such asluminescent materials, optical amplifiers, tunable visibleand UV lasers and scintillators. Concerning the scintillatoraspect, more particularly, several works were recentlydedicated to the study of lanthanide orthophosphates, withRPO4 chemical formula (where R most commonly is Y orLu). Accordingly, they are currently being investigated forapplications as thermo phosphors for remote temperaturemeasurements, as X-ray and gamma-ray scintillators formedical imaging application and laser hosts. Furthermore,it was found recently that neodyme doped YPO4 andLuPO4 crystals can be promising VUV scintillators forgamma detectors [1].

In this work, we focus our interest on erbium Er3+ (4f11)ion doped in Y2O3–P2O5 thin films. Recently, this patternwas experimentally investigated in the aim to develop anew wide band amplifier [2]. The spectroscopic analysis inthe visible range shows that erbium ions exhibit different

e front matter r 2006 Elsevier B.V. All rights reserved.

min.2006.06.006

ing author. Tel.: +216 74 45 26 32; fax: +216 74 24 28 70.

ess: ramzi.maalej@fss.rnu.tn (R. Maalej).

spectroscopic behaviours when the P2O5 molecular contentis varied systematically from 3% to 47%. Indeed, either atlow P2O5 content (3%) or at high P2O5 concentration 47%,the emission spectrum is well resolved. However, from 17%to 37% content, erbium spectra was mainly composed oftwo wide bands superposed on the peaks.Our purpose in this paper is concerned with the

interpretation of the observed erbium doped Y2O3–P2O5

thin films emission spectra. We focus our interest onemission spectrum obtained for P2O5 content about 47% inorder to confirm the new formed YPO4 phase. Thisinterpretation is based on a detailed theoretical study ofthe crystal-field effect on erbium energy levels. This work isa continuation of our previous analysis of erbium iondoped Y2O3 layer [3].

2. Material structure

The lanthanide orthophosphates RPO4 crystallize withtwo different structural types depending on the position ofthe lanthanide in the transition series. The first half ofseries (i.e., LaPO4 to GdPO4) crystallizes in the monoclinic‘‘monazite’’ structural form, and the second half of the

ARTICLE IN PRESSR. Maalej et al. / Journal of Luminescence 126 (2007) 165–170166

series (i.e., TbPO4 to LuPO4) crystallizes in the tetragonalstructure.

Yttrium orthophosphate (xenotime) YPO4 is uniaxialcrystal with the optical axis lying along the fourfoldcrystallographic c-axis. It crystallizes in the I41=amd D19

24

� �tetragonal space group. The yttrium Y3+ (Er3+) haveeight-folds oxygen coordinations with D2d point symmetry[4,5].

3. Experimental results

Erbium-doped yttrium orthophosphates Y2O3–P2O5

thin films were deposed by aerosol assisted MO-CVDprocess. Experimental details were described in theprevious work [2]. Fig. 1(a) presents the room temperatureemission spectrum in the visible range obtained at low P2O5

content (3%). This spectrum is well resolved whichcharacterises the Er3+ ions in the pure Y2O3 layer. Indeed,all detected emission lines have been identified in theerbium doped Y2O3 layer in our previous work [3].

At 14% of P2O5 content, luminescence recordedspectrum presented in Fig. 1(b) corresponds also to yttriamatrix. However, a new peak (marked by an arrow),located at 18420 cm�1, appears and characterises theformation of a new phase corresponding to erbium inYPO4 as it was demonstrated by X-ray measurements [2].The luminescence intensity of this peak increases withincreasing P2O5 content, which confirms the abundance ofthe YPO4 formed phase upon Y2O3. Accordingly, since27% of P2O5, the most intense line located at 17300 cm�1

(marked by an asterisk in Fig. 1) characteristic of erbium inyttria matrix disappears completely.

Furthermore, between 17% and 37% content of P2O5

(Fig. 1(c)–(f)), the emission spectra shows two wide

0

10000

20000

17400 17600 17800 18000 18200

Wavelen

Lu

min

esce

nce

In

ten

sity

(a.

u.)

*

Fig. 1. (a)–(g) Erbium luminescence spectra of thin films in the Y2O3–P2O5 sys

appeared and asterisk shows the characteristic line of Er3+ doped Y2O3 layer

luminescence bands which are superposed on the erbiumemission lines. This is probably due to a disordered Y–P–Ophase caused by yttria substituting.Finally, for 47% of molecular content of P2O5, the

sample has shown a great structural similarity with YPO4

crystals; this result was based on X-ray measurements [2],which confirm the tetragonal structure of our thin films.Furthermore, the emission spectrum is in a good agreementwith the work of Morrison involving YPO4 crystals [6],indicating that the incorporation of rare earth does notperturb structural arrangement of atoms in the layer.Fig. 2(a) and (b) presents the room temperature

luminescence spectrum of Er3+ (1%) doped Y2O3–P2O5

(47%) layers, obtained at room temperature with anexcitation at 488 nm for an incident power of 80mW. Thisspectrum was recorded in visible spectral range between18050 and 19250 cm�1. Two sets of emission lines weredistinguished: the first set is centred at 18300 cm�1

corresponding to transitions from the 4S3/2 (E1,2) excitedstates to the 4I15/2 (Z1,y,8) ground states (Fig. 2(a)). Thesecond set, manifested around 19100 cm�1, is attributed totransitions from 2H11/2 (F1,y,6) excited states to the 4I15/2(Z1,y,8) ground states (Fig. 2(b)).Using the relevant energy scheme established by Morrison

and Leavitt [6], most emission lines have been identified in thespectrum with some reassignments (Table 3). These linesallow us to elucidate crystal-field levels resulting from thesplitting of both fundamental and excited states. We report inTable 1 all observed lines, their relative intensity and theircorrespondent assignments. All these levels are doublydegenerated since erbium Er3+ is a Kramer’s ion. We noticethat some emission lines were not included in the fittingprocedure since they are not well resolved. These emissionsare indicated in Fig. 2(b) by an asterisk.

18400 18600 18800 19000 19200

gth (cm-1)

37%

47%

27%20%

17%

3%

14%ab

c

de

f

g

tem for different P2O5 concentrations. Dashed arrow shows the new peak

.

ARTICLE IN PRESS

18223

18189

1816318129

18421

18388

18370

18338

18317

18278

3000

6000

9000

12000

18050 18100 18150 18200 18250 18300 18350 18400 18450

Wavelength (cm-1)

Lu

min

esce

nce

Inte

nsi

ty (

a.u

.)

19207

18891*18935*

18981*

19044

19096

19116 1915319170 19188

0

2000

4000

18850 18950 19050 19150 19250

Wavenumber (cm-1)

Lu

min

esce

nce

Inte

nsi

ty (

a.u

.)

(a)

(b)

Fig. 2. (a) and (b) Luminescence spectra of Er3+ (1 at%) doped Y2O3–P2O5 thin films for 47% molar concentration of P2O5 corresponding to YPO4

phase.

R. Maalej et al. / Journal of Luminescence 126 (2007) 165–170 167

The agreement of our emission spectrum with the workof Morrison involving YPO4 crystals indicates the properincorporation of the rare-earth ion in the crystal layer.Indeed, the typical emission line width of erbium ionsdoped thin films is approximately 10–20 cm�1 at roomtemperature, which is in the same order as that in YPO4

crystals. This suggests that thin film preparation does notstrain the lattice.

4. Theoretical considerations on the energy levels scheme

The different interactions that govern the energy levelstructure of the lanthanide ions within the fn (n ¼ 1,y,13)configurations are usually those due to the intraioniceffects as the electrostatic repulsion between 4f electrons(except for one electron or one hole system), the spin–orbitcoupling as well as the interaction of the 4fn configurations

with the excited ones. In addition to the intrinsic free-ioninteractions, the external effects that arise from thesurrounding charges on the 4f electrons of lanthanide ions,the crystal-field effect must be accounted for. The effect ofthese interactions on the energy level scheme of thelanthanide ion can be described by Eq. (1), where theHamiltonian H is a sum of the free-ion HFI and crystal-field interactions HCF:

H ¼ HFI þ HCF, (1)

where

HFI ¼ H0 þX

k¼0;2;4;6

Fkf k þ xsoAso þ aLðLþ 1Þ

þ bGðG2Þ þ gGðR7Þ ð2Þ

ARTICLE IN PRESS

Table 1

Observed emission lines of Er3+ in YPO4 thin film with their energy

positions and relative intensities

Multiplets Wavelength Relative intensity Assignment

4S3/2-4I15/2 18370 0.87 E1(G6)-Z1(G7)

18338 0.49 Z2(G6)

18317 0.92 Z3(G7)

— — Z4(G6)

18223 0.61 Z5(G7)

18189 0.78 Z6(G6)

18163 0.68 Z7(G7)

18129 0.72 Z8(G6)

18421 0.55 E2(G7)-Z1(G7)

18338 0.48 Z2(G6)

18370 0.87 Z3(G7)

— — Z4(G6)

18278 1.00 Z5(G7)

— — Z6(G6)

— — Z7(G7)

— — Z8(G6)

2H11/2-4I15/2

19096 0.35 F1(G6)-Z1(G7)

— — F2(G7)-Z1(G7)

19153 0.17 F3(G6)-Z1(G7)

19170 0.15 F4(G6)-Z1(G7)

19188 0.14 F5(G7)-Z1(G7)

19207 0.12 F6(G7)-Z1(G7)

19044 0.21 F3(G6)-Z4(G6)

Table 2

Computed eigenvectors of intermediate coupled states for Er3+ in YPO4

Multiplet Eigenvector

2H11/24G 2H(11) 2H(21) 4I 2I

�0.6438 0.1340 �0.6624 �0.0562 0.35434S3/2

4S

2P

2D(20)

2D(21) 4

D4F

0.8519 �0.4051 �0.2496 �0.0201 0.0410 0.21344I13/2

2I 4I 2K

�0.0335 0.9940 0.10394I15/2

4I

2K

2L

0.9855 �0.1682 �0.0172

R. Maalej et al. / Journal of Luminescence 126 (2007) 165–170168

and

HCF ¼Xk;q

BkqCðkÞ

q . (3)

In Eq. (2) the first term H0 represents the isotropic terms inthe atomic Hamiltonian including only spin-independentspherically symmetric terms. The Fk parameterise thecoulomb interaction between the 4f electrons. The a, band g parameters are associated with two electroncorrelation corrections to the coulomb repulsion.

The HCF operator defined in Eq. (3) represents thecrystal-field contribution, which is described as a sum ofthe tensorial Racah operators Ck

q acting within the 4fn

electronic configuration. The Bkq ’s are the non zero crystal-

field parameters (CFPs) which contain the radiallydependant parts of the one-electron crystal-field interac-tions. We note that the tensorial Racah operators Ck

q arerelated to the spherical harmonics Y k

q by

Ckq ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffi4p

2k þ 1

rY k

q .

4.1. Free-ion wave functions

The free-ion eigenstates of Er3+ in the intermediatecoupling scheme were calculated by diagonalisation of thehole matrix associated to the HFI operator within the

complete SLJMJ basis set of the 4f11 electronic configura-tion that includes 17 LS states and 43 2S+1LJ multiplets.The set of free-ion parameters used here is derived directlyfrom our relevant work [3].The resulting wave functions are expressed in terms of

Russel–Saunders coupled wave functions:

cJM

�� �¼

XaSLJMJ

CðaSLJMJÞ��4f3 aSLJMJ½ �

�.

The symbol a in the above expression represents any otherquantum numbers that are needed when the set SLJMJ isnot sufficient to define states uniquely. The 4I15/2,

4I13/2,4S3/2 and 2H11/2 eigenvectors with their coupling coeffi-cients C(SLJMJ) are obtained and summarised in Table 2.

4.2. Crystal-field splitting calculations

When the f-ion is placed in a crystal, the electrostaticfield due to the surrounding anions removes some ofdegenerescence of the free-ion J levels and makes them tosplit. The erbium Er3+ ion occupy a D2d site symmetry inYPO4 host.For this site symmetry the crystal-field Hamiltonian

comprises five none zero Bkq parameters and is written as

HCF ¼ B20C

2

0 þ B40C

4

0 þ B44 C

4

�4 þ C4

4

� �þ B6

0C6

0 þ B64 C

6

�4 þ C6

4

� �.

The HCF matrix elements are computed numerically with acomputer program originally developed in our laboratory[7]. It has been used also to diagonalise the CFHamiltonian on the basis of states spanning 4I15/2,

4S3/2and 2H11/2 multiplets. The empirical CFPs are obtained bya least-square fitting program that minimises the root meansquare (rms) deviation between calculated and experimen-tal energy levels derived from optical spectra. This rms isgiven by

s ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiXi

Ecali � E

expi

� �2N � P

vuut ,

where N denotes the number of levels and P is the numberof independent free-parameters.

ARTICLE IN PRESS

Table 4

Fitted crystal-field parameters values (in cm�1) for Er3+ doped YPO4

layers in comparison with those of Morrison and Leavitt [6] and Dy3+ ion

in YPO4 crystal [8]

CF parameters B20 B40 B44 B60 B64

3+

R. Maalej et al. / Journal of Luminescence 126 (2007) 165–170 169

In the calculation of the CFPs, the centroıds associatedwith the free-ion multiplets are introduced as parameters.In this way, the centroıds are freely varied so that, togetherwith the full diagonalisation of the crystal field interaction,we obtain a fitting to experimental crystal energies as closeas possible.

YPO4:Er (this work) 356 262 �649 �442 �28

YPO4:Er3+ [6] 370 144 800 �646 88

YPO4:Dy3+ [8] 352 112 �800 �848 151

Table 5

Electric dipole selection rules in D02d site symmetry

D02d G6 G7

G6 s s, pG7 s, p s

Table 6

Comparison of crystal-field strength parameter Nv for different rare earth

ions doped YPO4 host material

Nv (cm�1)

Ce3+ (4f1) [8] 1962

Pr3+ (4f2) [9,10] 17313+ 3

5. Results and discussion

5.1. Energy level calculations

Table 3 presents the calculated and observed CF energylevels arising from the splitting of the 4I15/2,

4I13/2,4S3/2 and

2H11/2 multiplets. Table 4 shows the final set of CFparameters of erbium in YPO4 thin films obtained for thebest fit with quite good rms about 7 cm�1.

A tentative to explain the relative intensity is made: onthe basis of the Table 5, we can easily remark that the ppolarized transition can occur only between two differentrepresentations G6 and G7 in contrast to s polarizedtransition which is always allowed between these represen-tations. We affirm that s transitions are two times moreprobable than those p polarized, which affect theluminescence intensity. Moreover, based on our crystal-

Table 3

Experimental and calculated Stark energy levels (in cm�1) of Er3+ ion

doped YPO4 layers

Stark

level

Irrep. Ecal

(cm�1)

Eexp (cm�1)

(this work)

Eexp (cm�1)

(Morrison and

Leavitt [6])

4I15/2Z1 G7 3 0 0

Z2 G6 28 32 32

Z3 G7 47 52 52

Z4 G6 121 115 107

Z5 G7 149 147 143

Z6 G6 186 185 —

Z7 G7 204 206 —

Z8 G6 241 241 —

4I13/2Y1 G6 6540 — —

Y2 G7 6543 — —

Y3 G7 6591 — —

Y4 G6 6620 — —

Y5 G7 6655 — —

Y6 G6 6658 — —

Y7 G6 6669 — —

4S3/2E1 G6 18373 18370 18367

E2 G7 18417 18421 18416

2H11/2

F1 G6 19111 19096 19087

F2 G7 19131 — 19119

F3 G6 19146 19153 19145

F4 G6 19177 19170 19183

F5 G7 19180 19188 19201

F6 G7 19206 19207 19235

Nd (4f ) [11] 1717

Dy3+ (4f9)[8] 1392

Er3+ (4f11) (this work) 1091

field calculations, we deduce that the most intense linelocated at 18278 cm�1 is s polarized since transition occursbetween G7 and G7 stark levels. Due to the lack of polarizedmeasurements, we cannot compare and appraise ourresults, but we believe that future polarized spectra oferbium in YPO4 will confirm these results.

5.2. Crystal-field strength

Further tests of the accuracy of the obtained CFparameters can be performed through the comparisonwith previous sets of CFPs of other lanthanide ions in thesame host. Indeed, simulations performed for different 4f n

configurations in YPO4 host matrix should provide CFPswith only smooth variations. As reported in Table 4, ourparameters are in good agreement with those of Morrisonand Leavitt [6] and Dy3+ ions doped YPO4 crystals [8].More reasonably, we can also use for this comparison

the crystal-field strength parameter Nv defined by [9]

Nv ¼Xk;q

4p2k þ 1

ðBk

qÞ2

" #1=2.

The calculated Nv parameter obtained for Ce3+, Dy3+ [8],Pr3+ [10,11], Nd3+[12] and Er3+ (this work) ions dopedYPO4 host material are reported in Table 6. We notice adecrease of Nv parameter when the number of 4f electronsis increased. This reflects clearly the ‘‘contraction’’ of the

ARTICLE IN PRESSR. Maalej et al. / Journal of Luminescence 126 (2007) 165–170170

radial distribution of 4fn electrons along the rare-earthseries. This result is in a good agreement with the linearcontraction demonstrated by Auzel and Malta [9].

6. Conclusion

Erbium doped Y2O3–P2O5 thin films were investigatedfor different P2O5 molecular content varying from 3% to47%. Yttria phase was obtained for a very low P2O5

content (3%) but for high content (47%) a new crystallineYPO4 phase was well reached.

Our interest was mainly focused on luminescence spectraof erbium ions doped in YPO4 phase. A detailed crystal-field analysis has been carried out for Er3+ ions substitutedin YPO4 with D2d site symmetry. The fitting of 14 starklevels of the 4I15/2 fundamental and the 4S3/2 and 2H11/2

excited states was performed. The rms deviation of the fit isabout 7 cm�1, which indicated the success of the fitting anddemonstrates a well-established D2d symmetry within theYPO4 phase. The Stark levels have been obtained withtheir correspondent irreducible representations of D2d

double group. Calculated irreducible representations arein a good agreement with theoretical prediction. Further-more, the most intense line located at 18207 cm�1 waspredicted to be s polarized line, since transition occursbetween G7 and G7 irreducible representations, as may beconfirmed by appropriate future measurements.

The CF strength parameter is also calculated andcompared with those calculated for Ce3+, Pr3+ and

Nd3+ and Dy3+ ions in the YPO4 host which demonstratesthe contraction of the radial distribution of 4fn electronsalong the rare-earth series.

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