0'0

TR-1350

INFLUENCE OF CHARGE COMPENSATION ONUV-EXCITATION OF RARE-EARTH FLUORESCENSE

byW, Viehmann

May 1967

RECEIVED

AUG23 1967 r - v.

G ", ' I U G 2 '19 '

U.S. ARMY MATERIEL COMMAND

HARRY DIAMOND LABORATORIESWASHINGTON. D.C. 20438

DISTRIBUTI(ON Or THIS DO)CUM!NT IS UNL!MIIED

Best Available Copy

gd

Ilie findings in thj. report are nut to boconstrued as an off icial Department of the Armypv3.ttlon, unless so designated by other author-ized documents.

Destroy this report when it is no longorneeded. Do not return it to the originator.

EFUZ

TR-1350

!.NFLUENCE OF CHARGE COMPENSATION ONUV-EXCITATION OF RARE-EARTH FLUORESCENCE

byW. Viehmann

May 1967

US ARMY MATERIEL COMMAND

~~HARRY DIAMOND LABORATORIES______WASHINGTON DC 20438

CONTENTSPage no.

ABSTRACr . . ............... 5

1. INTRODUCTION . 5

2. CRYSTAL GROTM ............... 7

3. MEASUREo.. . 7

4* RESULTS a 8

5. DISCUSSION . .. . . . . . 20

6. REFERENCES ........... 22-23

ACKNOWLEDGMENT .......................... 23

ILLUSTRATIONS

Figure Number:

1. UV-absorption spectra of 7h3+-doped CaWOY CaF2, and SrF2with various charge compensating defects, measured at 77°K. . . . 9

2. Excitation spectra for the 5D4-7F5 fluorescence transi-

tion of Tb3+ in CaF2 and SrF2 at room temperature, .. ..... 11

3. Excitation spectra for the 5D4-7F5 and

5D3-7F4 fluores-

cence transitions of Tb in CaWO4 at room temperature. Thebase line for the latter has been shifted for clarity. Spectrafor Ta-compensated crystals are i entical to those with Nb-com-pensation. Known 4f-levels of Th are indicated in the figure. .. 12

4. Room-temperature excitation spectra for the fluores-cence-transitions of Tb3+ in SrWO4 with various charge compen-sations. Base lines have been shifted for clarity. . . o .. . . . 13

5. Room temperature excitation spectra for the flaorescence-transitions of T in PbWO4 and PbMoO4. Base lines have beenshifted for clarity. ..... ... ........................ 15

6. Room-temperature excitation spectra for the fluorescencetransitions of Tb3+ in CaMoO4 and SrMoO4 with various charge com-pensations. Base lines have been shifted for clarity. o......... 16

3

ILIBSTRATIONS (Cont 'd) Page no.

7. Excitation spectra for the 5 DO-TF2 fluorescence transi-tion of Eu3 at room temperature in various Scheelites as indi-cated in the legend. Known 4f-levels of Eu 3 + are shown ......... 18

8. Excitation spectra for the 4 F 2- 6 HI 3/ fluorescence3+9/ti,,nsition of Dy in various Scheelites as indicated in thelegeid. Known 4f-levels of Dy3 + are marked. . . . . . . . . . . . 19

4

ABSTRACT

Excitation spectra for the fluorescence transitic:ns of Tb3 +, Eu3 +,

and Dy3+ were obtained at room temperature on single crystals of the

tungstates and molydates of Ca, Sr, Pb, and Cu, having the Scheelitestructure, and on terbium-doped crystals of CaF2 and SrF2 , Rare-earthconcentrations were nominally one atomic percent and charge compensa.-

tion was provided by vacancy formation or interstitials, by substitu-tion in cation-sites or by anionic substitutions. Strong 'iV-excita-tion bands were found for Th- and Eu-doped crystals and weaker bands

for Dy-doped matrices. The b-,nds generally occur near the host-latticeabsorption-edge but their intensities and energy positions are strongly

influenced by the charge-compensating defects near the active ion.

The long wavelength peaks of the bands occur at energies coincidingwith 4f-level positions of the rare-earth ions, suggesting a strong

perturbation of 4f-levels by charge transfer states. The short-wave-

length regions of the excitation bands are ascribed to exchange trans-

fer from UV-absorbing groups.

1. INTRODUCTION

Following the work of Botden and Kroeger on energy transfer andenergy transport in samarium-activated calcium tungstate (ref 1, 2),host sensitization of rare-earth fluorescence has been studied ex-teasively in a number of oxide matrices (ref 3-13).* Many of theseinvestigations have been stimulated by the importance of these pro-cesses for solid-state laser materials. In Y203 : Eu

3+. for example,the broad UV-excitation band for Eu3+ fluorescence just below thelattice absorption edge of Y203 enabled laser action to be induced in

this material (ref 4). However, from a theoretical point of view,investigations of luminescence sensitization by lattice processes arealso of considerable interest, since, at least in principle, such ex-periments yield information on excited states of the lattice. Thepresent investigation was, undertaken to elucidate some of the mecha-nisms and processes rsponsible for energy transfer ,nd energy trans-port in rare-earth doped crystals. Specifically, the influence ofthe defect structure of the host lattice on these processes is of

interest here.

The incorporation of trivalent rare-earth ions into host lattices

having divalent cations, such as alkaline earth fluorides, tungstatesand molybdates, creates specific electronic defects in the lattice,depending on the mechanism of charge compensation employed (ref 14).

*For a review of recent work on luminescence in rare-earth doped ma-

trices see: L. G. Van Uitert-"Luminescence of Insulating Solids for

Optical Masers" in "Luminescence of Inorganic Slids." Ed. by PaulGoldberg, Acqtd. 2ress. 1966.

5

If no charge-compensating ions are intentionally incorporated,charge compensation is accomaplished by the creation of cation vacan-cies, as in the case of the tungstates and mol-bdates, according totLe scheme: 2RE3+ + (Ca) vacancy replace 3 CaZ+.

In the fluorides, the extra positive charge of rare-earth ions is*thought to be compensated by the incorporation of interstitial fluo-

rine ions (ref 15, 16). Another mecharism of charge compensation influorides as well as tungstates and molybdates is provided by thesubstitutional incorporation of monovalert cations, such as Na, ac-cording to: R 3 + + Na+ replace 2 Ca2+ (ref 14, 17).

Proper substitutions in the anionic site can also be employed.

Fcr instance, RE3+ + Nb5+ replace Ca2+ + W6 + in the case of tung-states and RE3+ + S2 - replace Ca2+ + F- for tne fluorides.

The latter type of substitutional charge compensation has beenemployed in Eu3+-doped CdF2 by Kingsley and Prener (ref 18) who foundan intense excitation band for the Eu3+ fluorescence in this materialbelow the absorption edge of pure CdF2 and ascribed it to excitonprocesses in europium-sulfide ion pairs and subsequent transfer of

excitation to Eu3+. In the present work, single crystals of CaF2 ,.SrF2 , CAW04 , SrWO4 , PbW04 , CaMoO4 , SrMoO4 , PbMoO4 , and CdMoO4 acti-vated by Tb, Eu, and Dy and with the various charge compensation ions

as outlined above were prepared and their excitation spectra measured.

Table I. Materials Investigated

Active Ion Charge Comp Matrix+ Na CaF 2, SrF2

Fi- CaF2 , SrF2

Vac(=8) CaWO4, SrW04 , SrMoO4 , PbMoO4Na CaW0 4 , SrWO4 , SrMo04, CaMoO4 , CdMoO4Nb CaWO4 ,SrWO 4 ,SrMoO 4 ,CaMoO 4 ,PbWO 4 ,PbMoO 4 ,CdMOO 4Ta CaWO4

Dy3+ Vac CaWO4 , SrWO 4 , CeMo04, SrMoO4Na CaWO4, SrWOj, CaMoO4 , SrMoO4Nb CaWO4, SrW04 , CaMoO4, SrMoO4 , CdMoO4 ,PbMoO4

Eu3+ Na CaWO4Nb C&'O4, CdMOO4, PbWO4, CaMoO4Ta CaW04 , SrW04, CaMoO4

6

2. CRYSTAL GROWTH

Rare-earth doped crystals of the tungstates and molybdates listed

in table I were grown by the Czochralski method in an apparatus similar

to that described by Nassau and Broyer (ref 19). Optical-grade com-

mercial powders were used as starting materials except for PbmoO4 and

PbW04, which were prepared from reagent-grade constituent oxides.

The rare earths were added to the melt in the form of their trivalent

oxides of 99.9 percent purity, sodium was added in the form of reagent

grade Na2 (W04 )3 and Na2 (Mo04 )3 , and niobium and tantalum as optical-

grade Nb2O5 and Ta2 05 . For uncompensated and sodium-compensated crys-

tals., melt compositions were computed on the basis of the various dis-

tribution coefficients as published by Nassau and Loiacono (ref 14)

to yield crystals having rare-earth concentrations of one atomic per-

cent. Rare-earths and niobium were added to the melts in molar ratios

of 1:1 and distribution coefficients as given by Brixner (ref 20, 21)

were used. Since no exact distribution coefficients for the cadmium

arid lead compounds are known, the crystal compositions given in these

cases represent only approximate values.

Vacancy-compensated molybdates showed pronounced formation of

color centers, which could generally be annealed out by heating the

crystals in oxygen for about 24 hr at 13000 C. Europium, which enters

the Scheelite structure in the divalent form, could be converted toEu3 + by heating in oxygen in CaW04 : Eu; Nb, CaW04 : Eu; Ta, CaMoO4:

Eu; Nb, CaMoO4 : Eu; Ta, PbWO4 : Eu; Nb and CdMoO 4 : Eu; Nb. In CaWO4 :

Eu; Na and SrWOA: Eu; Ta the conversion was only partial, even after

60 hr in 02 at 13000 C.

The doped fluorides of calcium and strontium were grown in a

graphite crucJbie by the Stockbarger method. The starting materials

were optical-grade commercial powders to which lead fluoride was added

as a scavenger. Sodium was added as reagent grade NaF and the rare

earth as trifluoride of 99.9 percent purity. Growth proceeded in anatmosphere of dry argon. Tb concentrationr in the melt were one atomic

percent and the sodium-to-terbium ratio was 5:1 to insure complete

compensation.

3. MEASUREMENTS

Absorption spectra were measured on optically ground and polished

crystals of 6 mm thickness using a Carey 14 spectrophotometer. Exci-

tation and emission spectra were obtained on the same rimples at room

temperature with the aid of a commercial instrument ha ing two mono-

chromators, which could be programmed independently to yie'ld either

excitation or emission spectra. The excitation source was a xenon

arc lamp and the detection system consisted of a 1P21 phototube and an

Aminco photomultilier microphotometer. The resolution of the instru-ment is about 25 A. The excitation spectra were not corrected forconstant excitation energy nor for the wavelengths dependence of thedetector sensitivityo since we were interested in relative resultsonly.

4. RESULTS

4.1 Absorption Spectra Near the Banc Edge

Figure 1 depicts the UV-absorption spectra of Tb-doped CaF2,SrF2 , and CaWO4 . measured at 77

0K, with the various charge-compensat-ing ions as indicated in the legend. The doping level is nominallyone atomic percent in all samples.

The line spectra associated with 4f-4f transitions withinthe Tb3+ ion are weak compared with those for other rare-earth ions,even in CaWO4 ., where the forbiddenness is lifted by interconfigura-tional mixing in the crystal field of S4 symmetry. They are consid-erably weaker still in the fluorides, since interconfigurationalmixing is absent and 4f-4f transitions are forbidden in crystal fieldsof cubic symmetry. In the fluoride crystals containing no intentionalcharge-compensating ion and in which charge compensation is believedto take place by the incorporation of interstitial fluorine, 4f-absorption transitions of Tb3+ are barely discernible and they arepractically absent in the sodium-compensated samples.

0 A surprising result is the appearance of a rather broad(100 A) band with a superimposed line spectrum in the CaF2 : Tb; Nasamples about 350 below the absorption edge.

In SrF2 : Tb; Na, the band is somewhat weaker and is shiftedto higher energy. Absorption measurements at room temperature showedthat the bands broaden but do not shift with temperature. No suchbands are found in the crystals without sodium added for charge com-pensation.

In CaWO4, in which charge compensation takes place in sub-stitutional sites only, the bands appearing below the intrinsic ab-sorption edge of the pure base lattice are much stronger and are notresolved as such within the absorption range obtainable with ourequipment.

8

I.,

0

0

It t-

IL))

CIS

044 1-

ci C,

c;c-r 91 A.ISM IV)Id

4.2 lb3 Excitation

The strong interaction between these absorption bands and4f-levels of 7b3 * becomes apparent in the excitation spectra. Figuze2 shows the intensity of the 5D4-7F5 transitlon of 7bU as a functionof the exciting wavelength in CaF2 : Th; Sa and CaF2 : Th. In the so-dic-compensated crystal, a rather pronounced excitation band centeredat 2600 A coinciding with tre absorption band of figure 1 Is found, incontrast to the interstitially compensated sample, where only directexcitation of Th1 in its 4f-levels is observed. It should be notedhere that in this sample pumping Into the lower-lying 4f-levels ofTb3 + leads to stronger excitation than In the Xa-conpensated crystalin accordance with the stronger absorption lines observed in CaFP: Tl.

C In SrF2: Th; Na, the strong excitation band is centered at2550 A. It is only about 1/3 as strong in the interstitially compen-sated sample in excitation as well as absorption (latter not shown infigure 2). Direct excitation of 4 transitions, however, leads tostronger fluorescence of lb3 + in this material, an observation in ac-cordance with the effect previously observed in CaF2 : Th and CaF2 :Th; Na. In both mtric s, rather weak host sensitization bands cen-tered at 2200 and 2300 can be discerned, which correspond to bandsalso seen in absorption.

Tb3 + when present in low concentrations shows fluorescencefrom a second, higher metastable state, 51)3, to the ground-state mani-fold in various matrices.

With increasing concentration, however, fluorescence fromthis level quenches strongly due to a Varsanyi-Dieke type interactionbetween coupled lb-ion pairs, which is made possible by the circum-stance that the width of the 7 F !ultiplet is almost exactly equal tothe separation between the 5D3 and 5 D4 manifolds (ref 22, 23, 24).In the fluorides studied here, 5 D3 -

7 F4 fluorescence is only veryweakly excited by pumping into the exciton band and practically ab-sent under direct absorption into the 4f-levels of lb 3 + .

Similir results are obtained on CaWO4 and SrWO4 . As shownin figures 3 and 4, strong excitation bands for the D4 fluorescenceare observed below the intrinsic absorption edge of the base lattice.

Weaker excitation bands are present for the 5D3 -7F fluores-

cence transitions. The do not, however, generally coincide in energywith those for the 5D4- F transitions and they are weaker in SrWO4than in CaW04 . Below these bands, the usual line Epectra for

5D3- 7F

as well as 5D4-7F transitions are found.

10

I0

* 0

a 00

0

I- IZ* U%

i C .) to

IN 0

-- T - I 1 -

77 0 X

0 it

s.L.afl 3A[IV3ki AJLISN3JLNI r

I

! : II I I II I1: I0- ELK J(I) ,oW 4

(2) C! 9 8 .IC60OjW42 (3) Ce% ,Nb w,,0

/(' I II, II SOLID LINES:

I 5D4 -v TF EMISSION

I I .I1li i, I

IIII 2.3! BROKEN LINES:

II/# I1jl II , aI l

i5 -0 .- EMISSIONif 4f

-U F'- 1,of IIS W I a I

II ,I',',

4'

I- 4

3- 3

2000 3000 4000 5000

AVELENGTH IN A

Figure 3. Excitation specti,4,for tne 5D 4-"IP nd 5D -7F4 fluorescencetrasi'Lnsof'i-'in Ga; 01.4 at j ,teiplratum . 'I -e b as e

line for the latter nas been snifted for clarity. Spectrafor Ta-compemsated crystals ame Iderntjcal t(, those witn Nb-comp~ensation. Known 4f-levels of 7b JN re indicated in thefrir'e.

10

13 (1) Sr Tb 0 WO49 .905 '.0054

(2) SrseTh0 .0 1 W O

(3) Sr9Tb Nb W90&S .'*.01 8904

SOLID LINES:S7 5D-6 7 EMISSION

4 F5

/ROKEN LINES:

I- 5 b 7 EMISSION

z2 3 Ill,

4pI

I-1 I! II Iz

I 1/3

,22000 3000 4000 5000

0

WAVELENGTH IN A

Figure 4. Roam-temperature citation spectra for the fluorescencetransitions of Tb- in SrWO with various charge copensations.Base lines have been shifted for clarity.

13

ISuperimposed on figure 3 are the energy levels of Tb3 + and

their labels (where known) " published by Dieke (ref 25). It is ap-parent from figures 3 and 4 that the excitation bands for both tran-sition groups peak at energies coinciding with 4f-levels of the Tb

3 +

ion. This evidence strongly suggests that interactions between ex-citon bands and 4f-levels lead to these pronounced excitation bands.Which 4f-levels are involved depends on the base lattice as well ason the charge-compensating ion or local defect structure, An an ap-parently complicated manner. These findings are supported by the re-sults obtained on lb-Nb-dcped PbWO 4 , shown in figure 5. Only a linespectrum is found for the % 3 -

7 F4 fluorescence excitation; i.e., this*transition is only excited by direct pumping of the Tb3-4f levels,

whereas in addition to a line spectrum, a pronounced band is found forthe 5D4-

7F5 fluorescence excitation, which peaks ovcr Th-4f levelsThis band is centered at about the absorption edge of PbWO4 (3400

In the zolybdates of Ca and Sr, the energy position of thebroad excitation bands is essentially independent of the charge-com-pensating ion, as shown in figure 6. In both hosts the bands peak ata.pprcximately 3100 %, a wavelength that falls between two 4f-multi-plets of Tb3 +, (J, K).

It may be noted here that the intensity of the excitationdepends quite strongly on the charge compensator, with Nb giving thehighest, vacancy compensation the lowest, and Na an intermediate in-tensity in SrMoO4 . In CaMoO4 , this order is reversed, with sodiumgiving noticeably higher intensity than Nb. (A satisfactory vacancy-compensated CaMoO4 : Tb crystal coula not be grown due to excessivecolor center formation.)

In PbMoO4 the excitation bands are very weak (fig.5), withniobium compensation giving the stronger band, and in CdMoO4 no Tb

3 +

fluorescence could be observed, regardless of charge compensationemployed.

In SrMoO4 t.he 5D3 -

7F4 fluorescence transition could be ex-cited by direct pumping into Tb3+ -4f levels below the absorptionedge; whereas in CaMoO4 and PbMoOA this transition was abo-it 2 to 3orders of magnitude weaker than the 5D4-

7F5 fluorescence transition.

4.3 Eu3+ Excitation

Our data on the excitation of Eu3+ fluorescence are limitedto a few host crystal-charge compensator systems, in wbich Eu couldbe obtained in trivalent form (sect 2). Trivalent Eu in Scheelitestructures shows fluorescence transitions from four metstable levels,5D3 ,

5 )2, 5D1, and 5D0, to various levels of the 7F ground-state

manifold, with those originating from the 5D0 level being the more

intense (ref 26).

14

1 0 1 6 " T 0 0 ! OW,

(3) Pb Th MO

a SCUID LINES:70~ EMISSION I

54 FS if

7 BROKEN LINES:-4z 5 1) T EMISSION I III

3 4

I4h

200

2000 000 ~QQ 000WAVELENGTH INA

Figure 5. Room-temperat um, q*ci tat or, spbctra f'or the fluorescence-.transitions of ATh' In ?bWO0 ano PbmoO4. Base lines havebeen sriifted for clarity,.4

I0-

(9) Sr,Tb 0 MoO095 1x.005 4

I ' (2) SrT.oNb, Mo1 0

j (3) SrsTbol1 No 01 OO4/ (4) Cqo9 Tbo! Nb01 !04

(5) Co 96 Tb0 iN!0 iMoO4

zI SOLID AND -- LINES:6 5 D4 * 7 F, EMISSION

4u -'I .

3 3i \ (i

-I I I

22

2000 3000 4000 50,00

WAVELENGTH IN A

Figure 6. Room-temperature Spiltation spectra for the fluorescencetraniLIons of ab- in Ca £) and BrMoO 4 with variouscharge compensations. i~se ines have been shifted forclarity.

16

Excitation spectra of the 5 DO- 7 F2 transition of Eu3 + areshown in figure 7 for Eu3+ concentration of nominally one atomicpercent. Incomplete conversion of Eu2 + to Eu 3 + in CaWO4 : Eu; Na andSrWO4 : Eu; Ta causes a tail on the absorption edge of these crystals,extending to about 4500 %. This band accounts for the observed reduc-tion of excitation below this wavelength. Whereas Tb3 + did not fluo-resce in CdMoO4 , a very intense excitation band is observed for Eu

3 +

in this material with a peak at 3600

In CaWO4 and CaMoO4 , Nb compensation and Ta compensationyield identical excitation spectra. The excitation band for CaMoO4 :Eu; Nb and CaWO4 : Eu; Nb peaks at the same wavelength, 3200 % but 1s

quite weak in CaWO4 , as it is in SrWO4 : Eu; Ta, where the peak posi-tion is at approximately 2850 %. if Na is used as charge compensa-tion in CaWO 4 , a rather intense band is observed at 2750 % with ashoulder at 2500 A.

No excitation band could be found in PbWO4 : Eu; Nb. 5 D0 -

F2 fluorescence in this host could be excited only by pumping intothe 5D2 and

5DI levels directly. Fluorescence from the higher meta-stable levels of Eu3 + could be excited only by direct excitation of4f-levels; i.e., no excitation bands were observed for these transi-tions in any of the hosts examined here.

Some of the lower-lying 4f-levels of Eu3+ are identifiedand indicated in figure 7, using the assignments by Dieke andCrosswhite (ref 25). Above the 5 D3 levels, experimentally verifiedassignments are not available. Calculations by Ofelt (ref 27) on the

basis of free-ion wave functions indicate a very dense group of levelsconsisting of thq 5G and 5L multiplets, as well as the 5D4 level tooccur above the 5D3 level. I'ollowing this group, there is the

5 H mul-tiplet, followed by a complex grouping of the

5F, 51, and 5K multi-

plets and the 3p levels.

It is apparent from figure 7, that as previously observedin Tb-doped crystals, the excitation bands for the 5Do-7F2 fluores-

cnc .transition of E .u3 + .k at energieo coinciding with 4f-levelsof the Evu3 + ion. The intensity and position of thse peaks deperon the base lattice, as well as the charge-compensating ion, i.e.,

the local defect structure associated with the incorporation of the

rare-earth ions.

4.4 Dy3 + Excitation

Excitation spectra for the 4F9 /2 -6H1 3/2 fluorescence transi--

tion of Dy3 + in various Scheelite matrices are presented in figure 8.Apart from direct excitation of Dy-4f levels, only comparatively weakexcitation bands are observed for this ion in the tungstates as well

17

10. (I) Co96 Eu0 Io , W04

(2) CoEu Nb, Mo 0I9II I

3p, 1 S 54 5u5L (3) Cc.9 9EU0 NbW.99048 2,35 (4) SrEuo TaoI W904

5 I "01 .01 .99 4

\ 2 I (6) Pb EuiNb WO0(N /.99 .01 .01 .994

I / 15 F - -Eu' "-EMSSON

% 2

> 1 50

-

zh,.. /sIiI"/ // I ,

/ / I I',i 12 2,,

44

2000 3000 4000 5000

WAVELENGTH IN

Figore 7. ExcitLon spectra for tne 5D -7F fluorescence transitionof gur at room teperature iR vakoIS ScheelitS asindicated in the legend. Known 4f-levels of E-u arcshown.

18

10-

(I) CC.m111% O0 W04

9- ~(2) Ca " Dy~N 01".0%

(3) Sr"DY01W.0*O 4

8- (4) DyA0 0 Nb0 M 4 4(5) Cdgg DPoj Nb0 (H 9J

7-1.2,3,4 (16) Pb",D01 t6 MoJ* 4z

II 4p H*6 EMISSION6-f 9/2 13/2

J 5-W

z.

-

'00

66

Figure 8. Excitation spectif for tne 4 - 6H fluorescencetransition of Dy in Various 96heel s as indicatedIn the legend. Known 4f-levels of ID' are marked.

19

as in the molybdates. In CaMoO4 and SrMoO4, the charge-compensatingion has no influence on the position and very little effect on theintensity of the excitation bands. In all cases, however, the bandmaxima occur at energies coinciding with those of various Dy 3 + -4flevels. A particularly interesting behavior is presented by 'd~oO 4 ;Dy; Nb. In this case the band peaks at Dy 3 + levels centered at3400 % (PQ), a wavelength well inside the host absorption of CdMoO4 .The Dy-levels that lie directly below in the band edge of the base

lattice and yield strong fluorescence in the other materials, are

strongly quenched (L,N). The Dy-level at 3400 W on the other hand

yields only weak fluorescence in the other matrices.

5. DISCUSSION

Sensitization of rare-earth fluorescence in hosts containing UV-absorbing groups, such as W04

2 -, Mo 42 -, NbO4

3 -, and V043 -, is thought

to be due to absorption in these groups and subsequent transfer ofenergy to the rare-earth ions via other absorbing groups. The condi-

tions necessary for efficient sensitization have recently been dis-

cussed by Blasse (ref 12), on the basis of a model in which transport

from absorbing group to absorbing group as well as transfer to therare-earth ion are regulated by exchange. The probability of energytransfer by exchange is equal to the product of wave function over-

lap and energy overlap (ref 28). Blasse and Bril (ref 11) conclude

from studies of the system Y2Wl-xMOx06 that transfer from tungstate to

tungstate or molybdate group takes place, -whereas there is no trans-

fer from molybdate to molybdate or tungstat. group due to lack of

energy overlap. Similar results had been obtained earlier by Kroeger

(ref 2) for the system CaWl.xMoxO 4 .

Exchange-regulated energy transfer from the absorbing groups tothe rare-earth ions is also determined by the wave function overlap

of the respective orbitals, and the energy overlap of the group emis-

sion and the rare-earth absorption levels.

Scheelites emit broad bands centered around 23,000 cm-1 for the

tungstates and around 18,000 cM- 1 for the molybdates (r.f 2). Therequirement of energy overlap with emitting levels of Tb, Eu and Dy

is therefore satisfied and energy transfer to these ions is deter-mined mainly by orbital overlap (ref 12). For the tungstates of Caand Sr, the energy overlap with the Tb- 5D3 level is actually better

than with the 5D4 -level. Judging from figures 3 and 4, the excita-

tion of the former appears to be derived mainly from the W04 group,since the excitation bands for the 5D3 -

7F4 emission of terbium coin-

cide with those for the host-lattice emission (not shown) and are

influenced in intensity but not in position by the various charge com-

pensations.

20

In the molybdates, enerfy overlap with the Tb-5D3 level is quitepoor, but very good for the D 4-level. Excitation spectra for thislevel are little influenced in their position by the various chargecompensators as previously ngticed in the tungstates for the excita-tion of the D3-level. NbO4 - substitution shows opposite effects ontransfer probability to the OD4 -level in CaMoO4 and SrMoO4 and ontransfer to the 5% 3-level in the calcium and strontium tungstates.Niobium substitution reduces transfer in the Ca compounds and enhancestransfer in the Sr compounds. A possible explanation would be thatgood energy overlap between W042- or MO4

2 - emission with the NbO4 3-emission is present in SrWO4 and SrMoO4 , and poor ovgrlap exists inthe Ca compounds. Due to lack of this o erlap, NbO4 groups willinhibit energy transport from remote W04 groups to the groups nextto the rare-earth ions.

The excitation bands for the 5D4-7F5 fluorescence transition ofTo3 + in CaWO4 and SrWO4 consist of two or more overlapping bands. Ofthese, the high-energy portion coincides with the excitation band forthe host-lattice emission and is therefore ascribed to energy trans-fer from WO4

2 - groups.

A aifferent excitation mechanism must be responsible for thelow-energy portions of the bands. Broad excitation bands below theintrinsic absorption edge of rare-earth-doped oxide matrices havebeen investigated by Ropp (ref 5) and by Blasse and Bril (ref 13).Ropp ascribed these bands to allowed transitions in highly perturbed4f-levels and Blasse and Bril have presented evidence that a charge-transfer process from 02- to Eu3+. resulting in orbital mixing betweenthe two ions, is responsible for these bands. It appears that ourfindings on Tb-doped fluorides lend support to these arguments. Fromthe absorption measurements presented in figuge 1, it is evident thatTb3 is dir ,ly excited in the 2500 to 2600 A region. The fluoridehosts do not absorb in this region. The band appearing in the sodium-compensated crystal below the natrix absorption edge is thought to bedue to a charge-transfer process from F- to Tb3 + ions, the energy forsuch a transfer being lowered by the replacement of divalent cationsby monovalent ions.

Similarly, a shift of the charge transfer bands to lower energyis observed in Tb-doped SrWO4 upon substitution of monovalent sodiumfor Sr2+ and to still lower energy in the vacancy-compensated crystals.

These charge transfer states apparently couple quite stronglyinto certain 4f-states of the rare-earth ions, inducing strong transi-tions within this configuration. In Eu-Nb or Eu-Ta-doped CaWO4 andSrWO4, the excitation bands clearly occur below the host-absorptionedge with their peak positions closely coinciding with 4f-level posi-tions of Eu3+. In the molybdates, the situation is not so clear.

21

Bands due to transfer from Uco042- groups and those due to charge trans-fer apparently overlap 1"te clorely. In the Dy-doped matrices, trans-

fer from 042- and Ho04 groups as well as charge transfer leads toweak excit'tion. The quenching of the host matrix emission of CaVO4aod SrVP4 under short wave UV-excitatlon is smaller in Dy-doped crys-tals than in Tb- or FM'-doped crystals, suggesting that the low effi-

ciency of Dy exctation is not due to greater losses to the phonon

spectrum or radlationless transitions but to weak transfer efficiency.

Since the energy overlap of the Dy-4 F9/2 level and 1042- emission isonly slightly loas favorable than for the Tb -kD4 level, the low trans-fer probub-1ity must be due to lack of wave function overlap.

6. RFRENCKS

(1) T.P.J. Botden, Philips Res. Rpt. 6, 426. (1951), ibid. 7,

197, (1952).

(2) F. Kroeger, "!ome Aspects of the AmInescence of Sclids,"

Elsevier Pubi. Co., Inc., New York, '948.

(3) L. G. Van Fitert, R. R. Soden and R. C. Linares, J. Chem.

Phys., 36, 1793, (1952).

(4) 1. C. Chang, J. Appl. Phys., 34, 3500, (1963).

(5) R. C. Popp, J. Electrochem. Soc., 111, 311, (1964), ibid.

112, 181, (1965).

(6) W. L. Wannaker and A. Bril, Philips Res..tpt., 19, 479,(1964).

(7) A. K. Levine and F. C. Palilla, Appl. Phys. Letters, 5, 118,

(1964).

(8) F. C. Palilla, A.K. Levine and M. Rinkewics, J. Electrochem.

Soc., 112, 776, (1965).

(9) W. L. Waniizker et al. Philips Res, Rpt. 21, 270, (19$56).

(10) F. J. Avel'a, J. Electrocher. Soc. 113, 855, (1966).

(11) G. Blasse and A. Brill J. Chem. Phys., 45, 2350, (1966).

(12) G. Blasse, J. Chem. rhys., 5, 2356, (1966).

(13) G. Blasse, and A. Bril, J, Chem. lMys. 45, 3327, (1966).

(14) K. Nn;asau and G. M. Iolac.eno, J. Phys. Chem. Sol.ds, 24,1503, () q63).

(15) E. FrIedunm and W. Low, J. Cher. Phys., 33, 1275, (1960).

(16) C. Baron, J. 1Lys. Chem. Solids, 25, 1206, (!964).

(17) E. XJWLmb ot al., Pbys. Nev., 131, 920, (1963).

(18) J. D. Kingsley and J. S. Prover, Phys. 3bv., 126, 45,(1962).

(19) K. Nassau and M. Broyer, J. Appl. PRys., 33, 3064, (1962).

(2C) L. H. Brizner, J. Electrochm. Soc., 111, 690, (1964).

(21) L. H. Brixner, J. Electrochem. Soc., 113. 621, (1966).

(22) F. Varsanyl and G. H. Dieke, Phys. Rev. Letters, 7, 442,(1961).

(23) L. G. Van Uitert., J. Electrochem. Soc., 107, 803, (1960).

(24) G. E. Peterson and P. V. Bridenbaugh, J. Opt. Soc. An. 53,1129, (1963).

(25) G. H. Dieke and H. M. Crosswhite, Applied Optics, 2, 675,(1963).

(26) L. G. Van Uitert and R. R. Soden, J. Cben.Phys., 32, 1687,(1960).

(27) G. S. Ofelt, J. Chew. Phys., 38, 2171, (1963).

(28) D. L. Dexter, J. Chem. Phys. 21, 836, (1962).

A(XNOWLEDGEN

rhe author would like to thank R. T. Farrar, C. A. Morrison,and J. Nemarich for umny helpful discussions.

23

DOCI3NIT COhITL ErATA -R 9 D

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Excitation spectra for the fluorescence transitions of 7b3+, Eu+, and D3

were obtained at roon temperature an single crfstals of the tuagstates androlybdates of Ca, Sr, Pb, and Cd, having the Scheelite structure, and on

terbium-doped crystals of CaF2 and SrF2 . Rare-ear*." :oncent rations werenou.naally or., atooic percent and charge compensation was provided by vacancyformation or Interstittals, by substitution In cation-s'tes or by anionic sub-

stitutions, Strong LJ-excitation bands were found for i- and Eu-doped crys-

tals and weaker bands for Dy-doped mnat rices. The bands generally occur nezr

the host-lattice absorption-edge but their intensities and energy positionsare strongly influenced by the charge-compensating defects near tne active ion.

The long wavelength peaks of the bands occur at energies coinciding with 4f-

level positions of the rare-earth ions, suggesting a strong perturbation ofif-levels by charge transfer states. The short-wavelength rcgionb of the

excitation bands are ascribed to exchange transfer from LN-absorbing groups.

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