Optical properties of divalent rare earth ions in SrAlF5

Journal of Luminescence 8 (1974) 415—427. © North-Holland Publishing Company

OPTICAL PROPERTIES OF DIVALENT RARE EARTH IONS IN SrA1F5 E.W. HENDERSON and J.P. MEEHAN

*

Lighting Research and Technical Services Operation, General Electric Company, Nela Park, Cleveland, Ohio 44112, USA. Received 17 August 1973 Revised manuscript received 26 October 1973

A survey was made of the possibility of reducing trivalent rare earth ions in SrA1F5 to the divalent state by fusion of the trivalent doped material with strontium or aluminum metal. Divalent europium, samarium and ytterbium could be produced in SrA1F5 by this technique and were stable. Single crystals of the divalent ion doped materials were grown and optical properties studied. Absorption, emission and excitation spectra of divalent samarium and ytterbium doped materials are presented and discussed. Materials containing trivalent rare earth ions not reduced by fusion with metal show broad band visible luminescence under 254 nm excitation. This luminescence is in addition to any typical absorption or luminescence of the trivalent rare earth ion.

1. Introduction 2~have been The properties of powde’ and single crystal and SrA1F5Sr2~ Eu sites. This has investigated previously [1—3].The material hasSrA1F5 four inequivalent been confirmed by both X-ray crystal structure analysis and a study of the optical properties of SrA1F 2~.The interesting optical properties of divalent europium 5 : Eu in SrA1F 5 led to an investigation of other divalent rare earth ions in SrA1F5. Methods used by other investigators for the preparation of divalent rare earth ions include gamma or X-ray irradiation [4, 5] chemical reduction with calcium metal vapor [5], electrolysis [6] and heating in a reducing atmosphere [I]. Since we desired to maximize the proportion of the divalent lanthanide ion, molten strontium or aluminum metal was used as the reducing agent. Europium, samarium and ytterbium could be reduced in this way. These have generally been the lanthanides most easily obtained in the divalent state [4]. 2. Experimental method 2.1. Preparation ofmaterials All powder materials were prepared by dissolution of SrCO3, Al (NO3)3 : 9H2O and the appropriate rare earth oxide in nitric acid followed by precipitation with * Present address: Hahnemann Medical College, Philadelphia, Pennsylvania 19102, U.S.A. 415

416

E. 14/. Henderson, J.P. Meehan, Optical properties of RE ions

aqueous HF and evaporation to dryness. The rare earth oxides were four to five nines grade (American Potash and Chemical Company) and all other chemicals Baker reagent grade. The precipitated fluorides were fired at 700 °Cin a flowing HF--N2 atmosphere in a platinum tube furnace for 4 h, fused about 900 °Cand then cooled in an HF—N2 atmosphere. In this work, formulated compositions are cited. For reduction of the rare earths, the doped fluoride powders were melted with strontium (Electronic Space Products, Inc., 2N) or aluminum (ESPI, 5N) metal in a tantalum container in a flowing argon atmosphere. The materials were cooled to room temperature and washed in hot dilute hydrochloric acid to remove any residual free metal. All reduction was done in an NRC model 2805 crystal furnace with induction heating supplied by a Tocco model 2EG 103 450 kc radio frequency oscillator. The tantalum containers were cleaned in a concentrated solution of aluminum nitrate and nitric acid. In the early work, the strontium metal was sealed in a polyethylene packet under nitrogen to minimize oxidation of the surface of the metal. The polyethylene wrapper was vaporized in the flowing argon atmosphere of the furnace and flushed from the system prior to melting of the metal and fluoride mixture. The use of the wrapper was later discontinued because the optical properties of materials prepared with and without it were identical. Crystals used for optical studies were grown by Bridgman technique [3]. 2.2. Optical measurements Reflectance spectra of powder materials were measured between 250 and 750 nm using a Cary 14 with a 1411 diffuse reflectance attachment. Single crystal transmission between 200 and 2500 nm was measured with a Cary 14 and between 2500 and 10000 nm with a Beckman IR-20. The measurement of the ultraviolet transmission spectrum of divalent ytterbium-doped2+ crystals required Failure the use bf filter luminescence. to aremove between the crystal and detector to absorb yb this luminescence from the transmitted light resulted in low optical densities in regions of Yb2+ absorption. No filter was required for transmission measurements of materials doped with divalent samarium as the emission occurred in a region of very low detector sensitivity. Emission, decay, and excitation measurements were made as described elsewhere [7]. 3. Results and discussion The optical properties of divalent europium in SrA1F 5 have been reported elsewhere and will not be discussed here [1—3].

E. W. Henderson, J.P. Meehan, Optical properties of RE ions

417

3.1. Samarium After the reduction process, SrA1F5 doped with 1% Sm has a deep orange body color. The optical properties of the material show only the presence of divalent samarium. However, the strong divalent samarium absorption and emission overlap regions where trivalent samarium absorption and emission would be expected to occur [8]. A more sensitive method for the detection of small amounts of trivalent samarium in the presence of divalent samarium might be through the use of electron 6H paramagnetic resonance3*atcould liquidbehelium temperature. With this technique, observed without interference from the the 7F 512 ground statetechnique of 5m has been alluded to by O’Connor and Bostick [9]. 0 state of 2t This Sm 3.1.1. Optical absorption Between 5000 nm (the IR cutoff of SrA1F 5) and 500 nm, the absorption spectrum of Sr0 999Sm0 001A1F5 shows only reflection and scattering losses due to cracks and inclusions in the crystal. Fig. 1A shows the intense divalent samarium absorption in the crystal between 200 and 500 nm.

200

250

300

350

400

WAVELENGTH

Fig. 1A. Room temperature optical absorption 0.035 in. thick.

450

500

550

600

(nm)

spectrum of single crystal Sr0999Sm~001A1F5,

418

E. W. Henderson, f.P. Meehan, Optical properties of RE ions

~?~IIIIIIII

380

400

420 WAVELENGTH

440

460

mm)

Fig. lB. Absorption spectrum of single crystal Sr

0~999Sm~001AlF5, 0.035 in. thick at 77 °K.

2~,is quite Thissince absorption, attributed primarily 4f-+ Sd with transitions of Sm broad the Sd configuration interactstostrohgly the crystal field of the lattice. The numerous peaks and shoulders observed on this broad absorption band are reasonable because the Sm2+ ion should occupy low symmetry Sr2+ sites. At 77 °K the spectrum shows some minor sharpening of both the narrow and broad absorption lines as compared to the room temperature spectrum. Narrow absorption lines superimposed on this broad f—d absorption are observed 2+ near 465 and 375 nm, see fig. lB. These lines are attributed to Sm f—f absorption. The f-level structure of divalent samarium would be expected to be patterned after that of the isoelectronic ion Eu3~.However, the energy of the levels would be reduced by a constant proportion primarily because of the smaller nuclear charge on samariuni [10, 11]. The amount of reduction, about 15% in this case, was determined by comparison of the divalent samarium emission spectrum with the level structure of trivalent Eu [12]. Determination of the amount of reduction will be discussed in more detail later. The 465 and 375 nm absorption lines are assigned to the 5L~and 5H~levels of the 4f6 divalent samarium configuration, respectively. In trivalent europium these 4f6 levels are numerous and show a number of intense lines [12]. Assuming this line assignment, one might also expect to observe absorption into the 5DJ and 7F~levels beyond about 600 and 2000 nm respectively. However no absorption is observed in these regions which might be attributed to these transitions. The observations of absorption into some f-levels and not others can be explained by consideration of the presence or absence off d absorption bands in the same region as these f-levels. —~

E, W. Henderson, J.P. Meehan, Optical properties of RE ions

5

419

7~

D

0

0

0 0 >-

I— C,,

z L1J Iz 2

5

2

7

D—F 0

Cl,

680

I

700

EMISSION

2~

Fig. 2. Emission spectrum of Sr0~999Sm Emission spectral bandwidth 1.0 nm.

720

740

WAVELENGTH

)rim)

760

780

0~001AlF5 with 254 nm excitation at toom temperature.

5H~and 5LJ levels of the 4f6 conThe absorption is suggested be absorption. into the The coincidence of these abfiguration occurs inwhich a region of strong to f—d sorption bands probably allows mixture of some d-character into the f levels. It is suggested that this admixing increases the f—f transition probability so as to allow observation of the transitions [13]. However, transitions to the 5D~and 7F~levels are not in a region of f—d absorption and therefore are not enhanced so as to allow observation. 3.1.2. Emission Emission spectra of SrlxSmx2+AIF 5 at room temperature and 77 °Kare shown in figs. 2—4 with spectral bandwidths as indicated. The same emission lines are observed with both 254 and 365 nm excitation. spectrum canThe be level attributed entire2~4f6 electronicThe configuration. structure of ly to transitions within the Sm this configuration would be expected to be analogous to that of Eu3+. Trivalent europium has been studied in a variety of materials and its level structure is known [12, 14, 15].

420

E. W. Henderson, J.P. Meehan, Optical properties of RE ions

5D

TF

~

680

684

688

EMISSION

Fig. 3. Emission spectrum of Sr

692 WAVELENGTH

696

700

mm.)

2~ 0~99Sm

0~01AlF5 at 77 °K.Emission spectral bandwidth 0.1 nm.

2~transitions shown in fig. 2 were based on Eu3~levels The assignments of the reduced in energy [12]. A Sm good fit of observed emission lines was achieved with a 15% reduction of Eu3~4f6 levels. With the level structure determined, forbidden f—f absorption lines could be searched for as discussed previously.

680 681 682 683 EMISSION WAVELENGTH(nm)

Fig. 4. Emission spectrum of Sr

2~ 0.99Sm

nm.

0.0jAlF5at 77 °K.Emission spectral bandwidth 0.025

E. W. Henderson, i.P. Meehan, Optical properties of RE ions

421

Transitions from 5D to 7F~levels other than those shown in fig. 2 would be found beyond the limit0of detection of the photomultiplier used. Emission lines for transitions from

5D

7F 1 to 5I~’ 0 1 2level. levelsEmission are also lines observed, considerably due toalthough transitions from otherweaker

than transitions thebe 5D~levels, J> 1,from would

expected to be weak. However emission from these levels might contribute to some of the structure observed in the emission spectrum. The most intense emission “line” in fig. 2, attributed primarily to the 5D 7F 0 0 transition, is shown with higher resolution in fig. 4. Four 2+ lines would be expected sites occupied by Sm2+. to beobservation observed, one from strong each oflines the and four ainequivalent Sr The of three number of weaker lines does not deny the presence of four Sr2~sites. The Sm2~transition probabilities from each of the four possible lattice sites are not necessarily equal. Thus, even with random site occupation four lines of similar intensity might not be observed for the 5D 7F 0 which 0 5D~levels transition. The extra lines are attributed to transitions from other occur in the same region. The positions of the resolved components are: 6803.1 (shoulder), 6804.0, 6806.6, 6811.5, 6816.7 and 6821.1 A. Although the 5D 7F 0 0 transition is forbidden on the basis of the J quantum number, the low symmetry around divalent samarium ions, occupying strontium sites, causes the selection to breaksite, down. has been observed elsewhere 2~occupied a lowrules symmetry as inThis BaClF:Sm2~[16]. when Sm —~

—~

10 0

0

8•

I— C,)

2 ~a I-

6-

z 2

0 C’) C’,

4.

L~J 2

200

300

400

EXCITATION

Fig. 5. Excitation spectrum of Sr

WAVELENGTH

500

600

(nm)

2’ 0.999Sm 0.001A1F5 monitoring emission at 680 nm at room

temperature.

422

E. W. Henderson, J.P. Meehan, Optical properties of RE ions

3.4

-

30

2.6

0

0

-

2,2

Cli C.)

z

‘2 ID

0

.8

C’)

10

.4

1.0

.6

.2 o

I

I

I

200

220

240

260

I

I

280

300

320

WAVELENGTH (nm)

2~

Fig. 6. Absorption spectrum of single crystal Sr

099Yb

001AlF5,0.029 in. thick at room tem-

perature.

The emission spectrum does not show the exact number of lines which might be expected for the assigned transitions. The number of lines expected can be determined from the J values of the transition states, the symmetry of the ion site, and the number of different sites occupied. From observation of the emission spectrum in fIg. 3 it is apparent that numerous 5Dpeaks7Fcould be hidden in the spectrum. The excitation spectrum of the 0 0 emission of Sr0999 Sm~t~01 A1F5 is shown in fig. 5. The excitation spectrum follows the absorption spectrum clseIy (compare figs. IA and 5), indicating direct excitation of the samarium fluorescence. -~

3.2. Ytterbium 3.2.1. Optical absorption 1Q The absorption spectrum of single crystal Sr0 99~ 01A1F5 below 330 nm is shown in figs. 6 and 7. No absorption was obser’.ed from ~beyond330 nm to about

E. W. Henderson, J.P. Meehan, Optical properties of RE ions

423

3.0

2.6

2.2

-

1.8

-

0 -

w U z ID 0

.4

cxl

1.0

.6

.2 I

I

I

I

I

I

220

240

260

280

300

320

WAVELENGTH (nm)

2~

Fig. 7. Absorption spectrum of single crystal Sr

099Yb

001A1F5, 0.029 in thick at 77 °K.

5000 nm, the IR cutoff. Since no absorption is observed around 1000 nm, complete reduction of ytterbium to the divalent state is assumed. 14 ground state. The associated Divalent spectrum ytterbiumwould wouldbebethat expected to have 4f135d a 4f absorption of the 4f14 transitions. The level structure of the 4f135d configuration can be considered to be the combination of Sd1 levels and the 2F 2F 13 levels [17]. In combining these levels the symmeand be 512 4f into account. As divalent ytterbium in SrA1F try of the ion site712 must taken 5 can occupy more than one ion site and the sites would be of low symmetry, the considerable structure observed in the absorption spectrum is not surprising. No analysis of the energy separation of the peaks was made, however 1. This corresponds approximately to the 2Fseveral2Fare separated by about 10000 cm— 712, 512 separation in No f—f absorption would be expected and none is observed. -~

3.2.2. Emission An emission spectrum of Sr0 99Yb2+0 01A1F5 with 254 nm excitation is shown in fig. 8. The lifetime of the emission, monitoring the peak at 405 nm, is 71 ±I ps. The

424

E. W. Henderson, J.P. Meehan, Optical properties of RE ions

8

7

~‘ 0

6

0 0 >.

5

C’)

z

uJ I-

4.

z

0 Cl, Cl,

3.

2

I

360

I

400

440

480

520

560

EMISSION WAVELENGTH mm)

Fig. 8.

Emission spectrum of Sr0~99Yb2~001AlF5 at room temperature.

long lifetime observed for thelevel emission suggests that it arises from a forbidden transi135d [18, 19]. tion, perhaps from a 4f The excitation spectrum of divalent ytterbium-doped SrA1F 5, fig. 9, is very similar to the absorption spectrum (fig. 6). This indicates direct excitation of the divalent ytterbium luminescence. 3.3. Intrinsic emission Aside from europium, samarium and ytterbium, the rare earths and yttrium are not reduced to divalent ions when fused with strontium or aluminum metal. After fusion, the rare earth doped or undoped SrA1F5 assumes a gray body color and the diffuse reflectance spectrum shows approximately uniform absorption over the region 250—750 nm. This absorption might be due to the presence of excess metal in the melt which precipitates out of sblution upon solidification.

425

E. W. Henderson, J.P. Meehan, Optical properties of RE ions

160

95 200

160

210

220

230

240

250

EXCITATION

260

270

280

290

300

310

320

WAVELENGTH (nm)

Fig. 9. Excitation spectrum of Sr

2’ 0~99Yb 0~01A1F5 monitoring emission peak at room tempera-

ture.

In addition to showing absorption throughout the visible these materials were luminescent under 254 nm excitation. The emission is independent of the rare earth doping and occurs in undoped as well as doped samples. The emission consists of two broad bands centered at 330 and 520 nm, see fig. 10. Analysis of the band shapes assuming a Gaussian model shows the peaks are unsymmetric and skewed to lower energy. Each envelope might consist of more than one component indicating more than one emission process. Due to the low intensity of these emission bands no decay or excitation measurements were made. The absorption and luminescence described above are not observed in SrA1F5 doped with europium, samarium or ytterbium after fusion. Each of these ions absorbs strongly at 254 nm as well as in the region of one or both emission bands induced by fusion with strontium or aluminum metal. Thus the presence of the divalent ion could quench the “intrinsic” emission by absorbing the major portion of the 254 nm radiation or by absorbing energy radiated by one of the emission bands. If the two emission bands originate from the same state, efficient transfer of energy to the divalent ion from one band could quench both bands. No further attempt has been made to determine the nature of the center causing this emission. It is noted however, that melting of SrA1F5 under the conditions for ion reduction without strontium or aluminum metal will not produce a material displaying this characteristic emission.

426

E. W. Henderson, J.P. Meehan, Optical properties of RE ions

0 0) C

0 0 0 II,

z 2

2 0 C’, C’) L~J

I

I

320

400 EMISSION

I

480

560

WAVELENGTH mm)

Fig. 10. Characteristic emission for undoped SrA1F

5 (following fusion with strontium or aluminum metal) with 254 nm excitation at room temperature.

4. Conclusions The capability of producing divalent europium, samarium and ytterbium ions in the SrA1F5 lattice in bulk has been shown. The divalent europium ion had been pro. duced and studied in this lattice previously. Both divalent samarium and ytterbium show intense visible emission with ultraviolet excitation. These ions are stable in the

strontium aluminum fluoride lattice when melted in an inert or reducing atmosphere, and single crystals were grown. Incorporation of oxygen in the materials during reduction and/or crystal growth produced crystals with inclusions as observed previously [3]. Such inclusions scattered the radiation used for optical absorption measurements resulting in an increase in measured absorption throughout the spectrum. However, the presence of oxygen in the lattice along with the divalent ions appeared to have no effect on the optical properties of the divalent ions. In cases where the trivalent rare earths could not be reduced to the divalent state or when there was no rare earth ion present, a defect center was apparently produced

by the fusion of the lattice with stron~tiumor aluminum metal. These centers may also have been produced in the europium-, samarium- and ytterbium-doped lattices, but were masked by the intense absorption and emission of the divalent ions.

E. W. Henderson, J.P. Meehan, Optical properties of RE ions

427

Acknowledgements The authors are especially grateful to Dr. R.A. Hewes for help and suggestions on some-of the optical measurements. We also wish to thank Mr. G.W. Armstrong for crystal polishing, Dr. R.L. Bateman for decay measurements and Mr. L.J. Haydu for some materials preparation.

References [1] [2] [3] [4] [5] [6]

M.V. Hoffman, J. Electrochem. Soc. 118 (1971) 933. R.A. Hewes and M.V. Hoffman, J. Luminescence 3(1971) 261. J.P. Meehan and E.J. Wilson, J. Cryst. Growth 15 (1972) 141. D.S. McClure and Z. Kiss, J. Chem. Phys. 39 (1963) 3251. F.K. Fong and P.N. Yocom, J. Chem. Phys. 41(1964)1383. F.K. Fong, J. Chem. Phys. 41(1964) 2291. [71T.S. Davis et al. J. Luminescence 4 (1971) 48. [8] G.H. Dieke, Spectra and Energy Levels of Rare Earth Ions in Crystals (Interscience, New York, 1968) pp. 233—242. [9] J.R. O’Connor and H.A. Bostick, J. Appi. Phys. 33 (1962) 1868. [10] G.H. Dieke and R. Sarup, J. Chem. Phys. 36 (1962) 371. [11] A. Dupont, J. Opt. Soc. Amer. 57 (1967) 867. [12] N.C. Chang and J.B. Gruber, J. Chem. Phys. 41(1964) 3227. [13] W. Kaiser, C.G.B. Garrett and D.L. Wood, Phys. Rev. 123 (1961) 766. [14] Shyama P. Sinha, Europium (Springer-Verlag, New York, 1967) p. 118. [15] G.H. Dieke, Spectra and Energy Levels of Rare Earth Ions in Crystals (Interscience, New York, 1968) pp. 242—249. [16] Z.J. Kiss and H.A. Weakliem, Phys. Rev. Let. 15 (1965) 457. [17] Eugene Loh, Phys. Rev. B. 7 (1973) 1846. [18] G.H. Kieke, H.M. Crosswhite-and B. Dunn, J. Opt. Soc. Amer. 51(1961) 820. [19] H. Witzke, D.S. McClure and B. Mitchell, in: Luminescence of Crystals, Molecules and Solutions, Proc. mt. Conf. Luminescence, ed. Ferd Williams (Plenum, New York, 1973) pp. 598—605.