Site-selective laser spectroscopy of BaF2:Eu3+

Journal of Luminescence 37(1987) 159—165 North-Holland, Amsterdam

159

SITE-SELECHVE LASER SPECL’ROSCOPY OF BaF2: Eu34J.P. JOUART, C. BISSIEUX and G. MARY Laboratoire de Recherches Optiques, Faculté des Sciences, Université tie Reims, BF 347, 51062 Reims Cidex France Received 24 November 1986 Revised 2 April 1987 Accepted 3 April 1987

The selective laser excitation of the fluorescence of Eui+ ions is used to investigate the defect sites in BaF 3±for Eu342 Eu concentrations ranging from 0.03 to 2.43 mol%. We identified the fluorescence lines arising from the (Eu34-, ~T) dipole of C 3~ 3±ion. (Eu3±,Fr)ion dipole dominates in BaF symmetry and established its energy level diagram. TheThis compensating is an interstitial F~ ion located in the next-nearest-neighbour site, with respect to the Eu 2. Fluorescence lines 3~,Fr) dipoles have been found above 0.1 mol% dopant concentration. assignable to next-nearest-neighbour pairs of (En

1. Introduction In the alkaline earth fluorides (MF2 with M3~) = Ca, Sr, Ba), the trivalent rare-earth ions (RE substitute for the M24- ions, thus requiring a charge compensation. The symmetry and the strength of 3 + ion depend on the field acting onitsa position RE the crystal compensator and on relatively to the RE34- ion. The crystal field splits the energy levels of the RE3 + free-ion, into several components; each distinct surroundings of the RE34- ion gives rise to its own splitting, therefore its own absorption or fluorescence spectrum. A large number of RE3 + sites with different symmetries and different field strengths has been observed [1—4]in the alkaline earth fluorides by using selective laser excitation34of ions the fluorescence. occupy a trigonal site In BaF2, the RE [5—11].The C3~symmetry is due to a compensating fluorine iondominance located in the ,~)interstitial position. The of (~, the ~C 3~ site is of in agreement with the theoretical calculations Corish et al. [12] on the stability of this site in BaF 2. The3+cubic sitesometimes (without compensator ion) has been observedclose but to the RE its presence depends on the conditions of crystal growth [5,8,13] A RE3 + ion having another RE34ion in its neighbourhood has also been observed

[11] in BaF 2 but to a minor extent than in CaF2. Up to now, so far as we lchowñ, no study of site-selective has been reported for BaF 3laser Inspectroscopy this paper, we present an anal2 Eu ysis of the fluorescence spectra of the Eu3~ion in BaF for Eu frombetween 0.03 to 2.432,mol% andconcentrations we discuss theranging interaction the Eu3 + ions. ~.

2. Exusenmental The oxygen-free BaF 2 crystals of fluorite structare investigated in this work were kindly supplied by Dr. H.W. den Hartog from the University of Groningen. crystals activated 0.15, 0.32, Five 1.15 BaF2 and 2.43 mol% EuF with 0.03, 3 (nominal concentration) have been studied. All along these 34- cubic site is missing. An appreciaseries, the Eu 24- ions has ble of divalent Eu[14]. beennumber revealed(10—15%) by EPR measurements The techniques used to record the fluorescence spectra already been [15].The sep. aration have of different Eu3described + sites was achieved through site-selective excitation A Cary-16 spectrophotometer was used to record the absorption spectrum.

0022-2313/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

160

J.P. Jouari et aL

/

3±

Site-selective laser spectroscopy of BaF

Table 1 Positions of the fluorescence lines for Eul± (C

2 : Eu

Table 1 (continued) 3~centre) in

BaF2

Transitions

Transitions

5D

Observed lines at 77 K

Irreducible

Wavelength (air) (A)

representation of the levels Upper state,

Energy (vacuum) 1) (cm

7F 2 —~ 0 7F 2 —~ 1

5D

5D

7F 2 —+

5D

2

7F 2—’ 3

4634.5 4655 4706 4707 4720.5 4721.5 4727 4741.5 4826.5 4828.5 4832.5 4834 4849 4855 4929.5 4930.5 4952 5068 5070 5076.5 5078 5090.5 5092.5 5095 5097

21571 21476 21244 21238 21179 21173 21149 21084 20713 20705 20688 20681 20617 20592 20281 20275 20187 19726 19719 19694 19687 19640 19631 19622 19615

5120 5242.5 5246 5334 5337.5 5352.5 5356 5340 5342 5345 5 53475

19526 19070 19057 18742 18730 18677 18665 18722 18715 18703 18696

5D

—

7F 3

lower state A1 orE —‘A1 E—~A1 A1 or E —+ E A1orE—~E A1 orE—’ A2 A1 orE—~A2 E —‘ B E—+A2 A1 or E --‘ E A1 or E —‘ E A1 orE—’ A1 A1 orE—’ A1 E—’E E—~A1 A1 or E —* E A1orE—~E B—B

5D

Observed lines at 77 K Wavelength Energy (air) (A) (vacuum) (cm’)

7F~

5804 5808 5815 5819 5830.5 5835 5839.5 5844 5889.5

*

17224 17212 17192 17180 17146 17133 17120 17107 16975

5912* 6089 * 6243.5 * 6168 6171 6175.5 6468 * 6481.5 * 6501* 6543 6548 6917.5 * 6926.5 *

16910 16418 16012 16207 16200 16188 15457 15425 15378 15280 15268 14452 14433

0 —‘ 5D 0 —‘ ~F2 5D 7F 1—’~ 4 5D

7F 0—’ 3

5D

7F 1—’ 5 5D 7F 0 —~ 4

*

Irreducible representation of the levels Upper state, lower state

A 1 —‘ E A1 —* A2 A1 —~ A1 A1 —~ E

Observed under selective excitation.

3. Results 5D

7F 1—’

5D

0 7F

1 —‘

1

5D

7F 2 —‘ 4

5D

A2—’A1 E—~A1 A2 ‘ E E -~ E A2 —‘ A2 E -~ A2

7F 1—+ 2

5D 7F 5D 2 —~ 5 0 —‘ ~

54895 54935 5497 5501 5627

18211 18198 18186 18173

5652 5778

17688 17302

*

A2-+E E -~ E A2 A1 E—’ A1 E-+E

A1 —~A1

3.1. Trigonal centres (C3~) A system of 70 fluorescence lines (table 1) prevails at all concentrations. This spectrum which has been analyzed and compared3~ions with the located results in of field a the group of tngonal theory, issymmetry due to Eu The energy-level diagram (table 2) of the 7F~and 5D~states has been constructed for J = 0, 1, 2 The position of the 5D 7F 0 level is fixed at 17302 cm’, the 1 and 5D 17Flevels are 5D split mto two components A2 and E, 2 and 2 m three components 2E and A1 The strength of the crystal 3~sitefield is descnbed mduced by by the compensator at the Eu sphtting of the 7F~(or 5D 1) level As the magmtude of this sphttmg is smaller than those of C4~, and C3~,centres in SrF2 (fig. 1), we conclude that

J.P. Jouàrt Ct al.

/ Site-selective laser spectroscopy of BaF2

3+

161

Eu

i::~:

~-

0

dj~Fo~ceEu3F~A)

Fig. 1. Indication of the effect of the interstitial Fr on the magnitude of the splitting of 5D

7F 1 and

the compensator is an interstitial F— ion located in the second nearest-neighbour site. The dependence of the relative intensity of the C3~centre (corresponding 5D to the 7F magnetic dipole3~total transitions 5D con0 ‘F~and 1 0) on the Eu centration shows that the relative maximum concentration of C3~centres is obtained for Eu concentrations of about 1 ismol%. Interaction between 34- F~)dipoles observed from 1 mol%, as the (Eu by the broadening (fig. 2) of the “C testified 3~” lines and the decrease in the concentration of the isolated C3~centres. —~

—~

Positions of the Stark

~

~

3.2. Additional centres We5Dhave 7F~ studied the5D~ magnetic (fig. 2) ±s~F dipole transitions 0dipole transition 5J)~ 07F(fig. 3) and the electric 0 7F (fig. 4)5D(no absorption was recorded in the region 0 0). Beside the “C3~”lines, several additional lines (labeled “a”Eu3t and “b”) were belonging observed (fig. 2) from 0.15 mol% The lines to each site —*

—~

—*

are isolated from the general spectrum by the site-selective spectroscopy technique (figs. 5, 6 and table 3).

2

BaF2

2

~\ 2850 1950 1924 1877 1845

1 multiplets.

43~oI%Eu

115~oI/

~—--------——--——:0.15rr~oI

~

7F 2 7F 1 7F 0

E A1 B E A2

1290 884 859 327 392

Ai

0

______________________________________________________1 2 Effect onen-~the 5D - Fig 16900 of the Eu3~concentration 16950 17000 3± at 77 K under 457.9 nm0 —~ laser fluorescence of BaF2 : Euexcitation.

3

162

J.P. Jouart et a!.

/ Site-selective laser spectroscopy of BaF2

~2243~

Eu ~

__

17307 3~ ~

~:1.15

~oI.%

17395

17309

E~

17354

—:0-32rr,oI.%E~3~

-

Ba F 2 0.03mol% Eu

34Fig 4 —+ 0 fluorescence spectra of EuB+ m BaF2 Eu at 77 K excited with the 4579 nm laser line The arrows show where selective excitations (figs. 5 and 6) are performed. 17250

~j

0 15,,~oI/

0

3

7F

17300

cm~

small shift of this level. Hence, the 5D

C3

19000 19050 19100 i~0.03~1%~Su 3~concentration on the 5D, —+ ~F Fig. 3. Effect of the Eu 0 fluorescence excitation The ofright BaF2hand Eu~ msert at shows 77 K details under of 4579 the 5D urn laser 7F 3~The left 1 -~ short0 7F of5DBaF2 032 mol% Eu fluorescence spectrum insert shows the 0 —~ 1 absorption spectrum at 300 K of BaF ~243 mol% B •

U

We can propose two structures for the “a” and

0 levels of far-away compensated centres should be situated between those of C3.~,(17 302 cm’) and °h (17 304 cm —1) centres (the latter has been detected in a BaF2 crystal doubly doped with 0.07 mol% EuF3 and 0.17 mol% NaF). (2) A pair of nearest-neighbour 34- ions(NN) or next nearest neighbour (NNN) Eu been Pairsobserved of nearest m neighbouring CaF Eui+ ions have 2 [17,18] and in CdF2 5

[15,19,20]. The D1 fluorescence is strongly 34- ions by Thecross quenching rate between (defined by = quenched relaxation the Wtwo Eu l/T l/r 0 where is the radiative lifetime of the isolated centres and the lifetime of pairs) is 6 X iO~s~in CaF 2 [18] 5D A much weaker quenching of the 1 fluorescence is observed for the “a” and “b” centres in BaF2 At 032 mol% the additional lines5D represent 7F about 15% of the total mtensity of the 0 1 transition (fig 2) The radiative decay probabth5D ties of the 0 level of “C3~.,”“a” and’ b 3~ions centres bemg almost equal, thus 15% of lines the Eu would be paired As the additional constitute 5D only 4% of the total mtensity of the 1 transition (fig. 3), the relative quantum efficiency —

‘~

‘b’ centres 3~ion and one (1) Centres consisting of one Eu compensating fluorine ion located in third or fourth nearest neighbour [16]. For such centres, the perturbation of the cubic crystal field becomes smaller the F~ion moves 3 ion Weasindeed observe that away7Ffrom the Eu the 1 splittings of “a” and “b” centres are smaller that of theif we C3,,, take centre this pro posal canthan be discarded intoBut account the 5D 0 level positions (17 301 and 17299 cm_i) Through J mixing, the crystal field induces a +

T

—~

—~

J P Jouart et al

/

3+

Site selective laser spectroscopy of BaF

163

3 Eu

= ‘r/z~of the 5D~level fluorescence is about 0.25. Assuming a lifetime r~between 3 and 10 ms, we obtam a quenching rate for the “a” and “b” lines between 1000 and 300 51, which is 60 to 200 times smaller than the quenching rate for the pairs of nearest-neighbour Eu3 ions in CaF 2. This result reasonably agrees with the theoretical predictjons if we suppose the additional lines to arise from a NNN pair: the ratio of the quenching rates

for the NN and NNN pairs is given by

w

=

\ NN where R~ and RN~ are the Eu—Eu separations NNN

+

~

LI L

~J

and n = 6, 8 or 10 according to the type of multipolar interaction [21]. With R~ = 3.8 A and = 6.2 A, W~/WNNN amounts to 18 (for n = 6), 50 (for n = 8) and 133 (for n = 10). This

c3v

b

-

-

c3~~

~c:vb

cm 1

173j~

16900

16950

5D

Fig. 5

17000

~_i

16900

16950

17000

Fig. 6

17050

17100

7F 3D Figs 5 6 0 —÷~l7~fluorescence 1) is mdocated on spectra the left of BaF2 of eachBui+ spectrum (0 15 mol%) Below this at 77 the K fluorescence tinder dye-laser spectrum excitation excited Thewith0 the —‘ 457 0 excitation 9 nm line energy (air) (ingives cm the overall picture of the spectral diversity. Notice that the third “b” line is masked by a “C 3,,,” line.

164

J.P. Jouartet aL

/ Site-selective laser spectroscopy of BaF2 4. Conclusion

Table 3 7F 5D 5D 7F 0 —‘ 0 excitation lines and 0 —‘ 1 fluorescence lines for th C “ “ d “b” B F -E ~ e 3v’ a an centres m a 2 U Centres Excitation lines Fluorescence lines (m vacuum) 7F 5D (cm~)) (m vacuum 5D 7F (cm’) 0 -+ 0 0 -~ 1 C3~

17302

“a” “b”

17301 17299

3± Eu

3 ions in BaF The selective laser excitation of the fluorescence of Eu 2 has been carned out We identified 3~,F~)dipole the fluorescence of C lines ansmg from the (Eu 3,, symmetry and established its energy level diagram. The compensating ion is an mterstitial F— ion located m the next 3 nearest-neighbour This (Eu3, F~)dipole site, dominates as it is usualininBaF BaF2 : RE 2. 3 ~, F~)dipoles have been Fluorescence lines assignable to next-nearestneighbour pairs of (Eu found above 0.1 mol% dopant concentration. +

16975 16910 16959 16922 16972

~.

19 952 16927

ratio should be still larger in the case of a superexchange interaction mechanism [22,23]. We tentatively assign the “b” spectrum to a pair of Eu3~ ions located in next nearest neighbours (fig. 7). The stability of this Eu—Eu

Acknowledgment

-

We wish to thank Dr. H.W. den Hartog for proVidmg the BaF 2 Eu crystals used m this .

pair has been found by Consh et al [12] This pair has the C2~,symmetry with an Eu—Eu separation of 6 5D 2 A The three “b” lines (fig 5 and table 3) of the 0 ~F1 transition and their shifts with respect to the “C3,,” lines are consistent with the C2~, symmetry and the strength of the crystal field 3 ,, 5D only7Ftwo a lines are around the Eu ions. As observed m the region 0 1, the ‘a” spectrum cannot be assigned to the NNN pair Of C2,, symmetry. Thus, further work is needed to reveal the structure of “a” centres.

References

—‘

~

+

-

—~

IC2 ~.

~—‘

I

I

3’

¶

~ —

EU I

— -

• hI - -

~-

-~-

-

¶

Eu3~

-

-

-

t

‘

I Fig. 7. A pair of next-nearest-neighbours (NNN) Eu3~ions.

[1] D.R. Tallant and J.C. Wright, J. Chem. Phys. 63 (1975) 2074. [2] J. Khava, P. Evesque and J. Duran J. Phys. C: So!. St. Phys. 11 (1978) 3357. [3] J P Jouart C Bissieux G Mary and M Egee J Phys C Sol. SI. Phys. 18 (1985) 1539. [4] J.C. Wright, Cryst. Latt. Def. and Amorph. Mat. 12 (1985) 505. J. Sierro, Phys. Lett. 4 (1963) 178. U Ranon and A Yamv Phys Left. 9 (1964) 17 J. Makovsky, J. Chem. Phys. 46 (1967) 390. F Z Gilfanov L D Livanova A.L Stolov and Yu P Khodyrev, Opt. Spectrosc. 23 (1967) 231. [9] LB. Aizenberg, LD. Livanova, I.G. Saitkulov and A.L. Stolov, Soy. Phys. Sol. St. 10 (1969) 1595. [10] St. MV12 Eremm,RK (1971) 2820. LuksandAL Stolov Soy Phys So! [11] M.P. Miller and J.C. Wright, J. Chem. Phys. 68 (1978) [5] [6] [7] [8]

[12] J Consh CR.A Catlow PWM Jacobs and SH Ong Phys. Rev. B25 (1982) 6425. [13] LA Boatner R W Reynolds and MM Abraham J Chem Phys 52 (1970) 1248 [14] P Dorenbos and H W den Hartog pnvate communica tion (1986). [15] J.P. Jouart, C. Bissieux, M Egée, G Mary and M. de Marcia J Phys C Sol St Phys 14 (1981) 4923 [16] RH Heist and F K Fong Phys Rev Bi (1970) 2970 [17] S Kh Batygov Yu K Voronko L S Gaigerova and V S FedOrov, Opt. Spectrosc. 35 (1973) 505.

J.P. Jouartet aL

/ Site-selective laser spectroscopy of BaF2

[18] R.J. Hamers, J.R. Wietfeldt and J.C. Wright, J. Chein: Phys. 77 (1982) 683. [19] S. MJio and J.C. Wright, J. Chem. Phys. 77 (1982) 1183. [20] J.P. Jouart, C. Bissieux, M. Egée and G. Mary, J. Phys. C: Sol. St. Phys. 16 (1983) 3359.

3+

165

Eu

[21] D.L. Dexter, J. Chem. Phys. 21 (1953) 836. [22] RJ. Birgeneau, AppL Phys. Lett. 13 (1968) 193. [23] J.C. Vial and R. Buisson, J. de Phys. Lett. 43 (1982) 745.