Optical properties of Sm2+ and Eu2+ in natural fluorite crystals

Nucl. Tracks Radiat. Meas., Vol. 17, No. 4, pp. 557-561, 1990 Int. J. Radiat. Appl. Instrum., Part D

0735-245X/90 $3.00 + .00

Pergamon Press pie

Printed in Great Britain

O P T I C A L P R O P E R T I E S O F Sm 2+ A N D Eu 2+ I N N A T U R A L FLUORITE CRYSTALS T. CALD~RON,*A. MILLAN,*F. J^ot~? and J. GAgcx^ SOL~? *Departamento de Quimica Agricola-Geologia-C~oquimica, Universidad Aut6noma de Madrid, Cantoblanco 28049 Madrid, Spain and ?Departamento de Fisica Aplicada, Universidad Aut6noma de Madrid, Cantoblanco 28049 Madrid, Spain (Received 17 March 1989)

Abstract--The absorption, photoluminescence and luminescence decay time of Eu2+ and Sm2+ ions in natural CaF 2 crystals are studied and compared with results reported for synthetic crystals. The results obtained suggest Eu2+ to Sm2+ energy transfer in the natural samples.

INTRODUCTION

EXPERIMENTAL

OPTICAL properties (absorption and photoluminescence) of minerals represent a very useful role for their rapid characterization and detection of some trace elements. Moreover, these properties need to be studied to explain the nature of the observed colour. It is well known that natural fluorite (CaF 2) occurs in a wide range of colours and hues (Przibram, 1956; Bill et aL, 1967; Bill and Galas, 1978; Mackenzie and Green, 1971). Rare-earth elements are known to be capable of existing in a divalent state substituting to host cation ions. Some of them, like Sm 2+, have been related to the specific light green coloration observed in CaF2 natural crystals (Kaiser et aL, 1961; Bill et al., 1967; Recker et aL, 1968; Bill and Galas, 1978). Optical spectra of divalent europium and samarium ions in CaF2 have been largely investigated (Kaiser et aL, 1961; Lob, 1968, 1969; Vagin et al., 1969; Kfihner et aL, 1972; Hurrel et aL, 1972; Ignatev and Ovsyankin, 1980; Murrieta et aL, 1983) and related with 4f~--,4f~5d ~ ion transitions. In fact, CaF2:Sm 2+ synthetic crystals have been reported to operate as a solid state laser at the strongest zero phonon line at 708.5nm (Sorokin and Stevenson, 1961) as well as in a number of lines of the vibronic emission region (Vagin et al., 1969). On the other hand, CaF2:Eu 2+ synthetic crystals show a broad emission band peaking at 420 nm and at low temperature a very sharp peak at 413 nm which was attributed to a zero-phonon line (Kobayasi et al., 1980). Although the optical absorption spectra of rareearth ions and the origin of coloration in natural fluorite have been well studied, no details of photoluminescence have been reported for the green fluorite up to now. Therefore a systematic study of photoluminescence in these natural crystals appears to be necessary for complete understanding of the optical behaviour of this mineral.

Natural green-coloured fluorite samples were cut from a specimen from Badajoz (Spain) parallel to the (100) natural faces. The impurity content was analyzed by atomic absorption spectrometry and emission spectrometry (ICP) for a number of ions (Table 1). The cell parameter, a0--5.4 ~, was obtained with the aid of a Paran-X-Ray computer program. Optical absorption measurements were performed with a Cary 17 Spectrophotometer, and luminescence measurements were made with a Jobin Ybon JY-3CS Spectrofluorimeter coupled with a device to chill the sample. Decay time measurements were made with a N2pulsed day laser EG and G model 21, capable of exciting at 337 nm (N2-1ine), in the wavelength range of 350-700 nm, using different dyes. For detection a Bausch and Lomb single monochromator and a Philips digital storage oscilloscope (model PM3311) were used. Spectral resolution was 1 nm.

557

RESULTS AND DISCUSSION Absorption and photoluminescenee (continuous and decay time) measurements have been performed Table 1. Impurity content analyzed by atomic absorption and emission spectrometry (*) Element Si Al Mg Na Fe Sm (*) Eu (*)

Impurity content (ppm) 6125 485 60 69 10 0.08

558

T. CALDERON et al.

~r.~

E

E

~.~

~

,'C

=G C - ~ ..~ • - - ~:

J

I

I

I

t

I

0

i

z~

•

~

o

o"

o"

~

o

o

r~1+

(-s;jun

-qJD )

"~

('s;!un

'ClJo)

I

I

.-t

C)

E E E

0

z 6 (.s4!un

"ClJO) "0 "0

c;

Sm 2+ A N D Eu 2+ OPTICAL PROPERTIES in our natural green fluorite single crystals and correlated with results formerly reported for synthetic crystals. The liquid nitrogen temperature (LNT) optical absorption spectrum in the 200-800 nm wavelength range is displayed in Fig. 1. It shows a number of lines which have been previously assigned to different transitions between the 4f~ and 4f~5d ~ electronic configuration of Sin 2+ (Kaiser et al., 1961; Loh, 1968, 1969; Vagin et al., 1969). In fact the spectrum can be considered as consisting of two broad and structured absorption bands corresponding to transitions up to the et (~14.000-30.000cm -~) and t~ ( ~ 30.000-45.000 cm- ~) sub-levels of the d ~ electron configuration in cubic symmetry. The structure in each band is due to electronic interaction between the d' electron and the 4f 5 core (Loh, 1969). This structure will be later analyzed taking advantage of the excitation spectrum. No signals of Eu-absorption were detected, according to the low concentration of this ion obtained by chemical analysis. At this point it may be said that the absorption spectrum is also very similar to that reported for other natural green fluorite (Bill and Galas, 1978) showing that samarium is a very important coiouring ion in this kind of fluorite. The photoluminescence of natural green CaF2 crystals have been studied under excitation in the optical absorption region corresponding to Fig. 1. Two kinds of emission spectra have been obtained. Figure 2(a) shows the LNT emission spectrum under excitation of wavelength light at 427 nm. This spectrum is quite similar to that previously reported for CaF2:Sm 2+ synthetic crystals showing a maximum at

559

708.5 nm superimposed to a broad and structured emission band. This spectrum corresponds to the reversal 5d -, 4f transition. On the other hand, excitation at the high energy side of the absorption spectrum (Am = 341 nm) produces an emission band peaking at 421 nm and a weak i.r. emission band with maxima at 708 and 770nm [Fig. 2(b)]. The 421 nm emission band is the same observed for Eu2+-doped CaF2 synthetic crystals, and the peak at 708 nm has been previously commented on as being due to Sm 2+ ions. The maximum at 770 nm should indicate the possibility of perturbed Sm 2÷ (perhaps due to aggregation), giving a slightly different emission spectrum to that in Fig. 2(a). In order to obtain additional information on the nature of the centre responsible for the luminescence spectra, the excitation spectra corresponding to the above-commented emission bands were taken and are displayed on Fig. 3. The excitation spectrum of the 708.5 nm emission band can be easily identified, and the peaks corresponding to the e~ component can be labelled in terms of the electronic levels of the 4f 5 configuration following the work of Loh (1968) in synthetic CaF2:Sm 2+ crystals. The peaks reported for synthetic crystals have been marked with vertical arrows on the figure for the sake of comparison with the peaks appearing in the natural crystals. The similitude between both synthetic and natural crystals becomes evident. However, a similar identification in the t~ component (high energy region) was not carried out because of strong defect absorption (Loh, 1968, 1969; Bill and Galas, 1978).

! (o r b. units

0,4

0,2

200

T

t

"t"

°e

:500

400

500

600

(ms}

FIG. 3. LNT excitation spectra of the emission bands. Dashed line: ~.m = 421 nm; full line: )-m = 708.5 nm.

700

560

T. C A L D E R O N et al.

[

I

I

100

200 T(K)

I

I

I

300

Fv~. 4. Temperature dependence of the Sm2+ emission lifetime. In fact, an inspection of Fig. 3 reveals that the high energy component of the Sin 2+ absorption does not practically contribute to the emission spectrum of our natural crystals. Figure 3 also includes the excitation spectrum corresponding to the Eu 2+ emission, and a broad and structured band at 340 nm is shown. This band corresponds to one of the two absorption bands reported for synthetic Eu-doped CaF2 crystals (Kobayasi et al., 1980). In fact, the absorption spectrum of Eu 2+ consists of two broad structured bands assigned to transitions from the lowest stark

component of the 4f~ configuration to the two excited states (t:s) and (es) of the 4f65d configuration, which resulted as a consequence of crystal field splitting of the 5d electronic configuration. It should be noted that in our crystals the high energy absorption band (et component) is not present in the excitation spectrum of europium emission. It means that excitation into this band decays non-radiatively (or emits) through other levels. The decay time of Sm ~+ and Eu e+ luminescences have also been studied in the temperature range from LNT to room temperature (RT). The luminescence of Sm 2+ shows a single exponential decay with a lifetime of 2.1/as (LNT) according to the electric dipole character of the d -, f transition and is in good agreement with the lifetime value observed for synthetic crystals. The temperature dependence of this lifetime (Fig. 4) can be explained by thermal quenching of emitted light according to the well-known equation: 1 T

-

1 TO

1

f- - -

e - ~z/rr

(I)

Tnr

which gives the de-population probability of the excited state, 1/To and l/%, e- ~/rT being the radiative and non-radiative transition rates, respectively.

i O II

m e, =)

o w )Ik~ Z kd I.Z

i

2., h , I0 !

Z 0 ~n (n W

I

2 TIME

3

4

()kbS)

FIG. 5. LNT decay time plots of the emissions observed under excitation in the Eu absorption band; ,;.~,= 337 nm [see also Fig. 2(b)].

Sm 2+ A N D Eu 2+ O P T I C A L P R O P E R T I E S F r o m the fitting of the experimental results, we obtained: zo = 2.1 #s "Cnr~ 4 X 10-10S A E ----0.16eV. On the other hand, excitation with light lying into the Eu-absorption band (N2 laser line, 337 rim) produces (see Fig. 5): (i) a luminescence of Sm 2+ [see also Fig. 2(b)] with the same lifetime o f 2.1 #s; (ii) a two-exponential decay for the europium emission with lifetimes 0.15/zs and 0 . 7 # s (LNT). The component o f 0 . 7 # s is the same reported for the 4f65d - , 4f 7 transition in C a F 2 : E u 2+ synthetic crystals (Kobayasi et al., 1980). The shorter component, 0.15#s, can be explained as a result of energy transfer from Eu 2+ to Sm 2+ ions, this mechanism reducing the luminescence decay time of the europium ions which are making the transfer. In fact, the luminescence of the Sm 2÷ ions is observed under excitation with light lying in the europium absorption bands [Fig. 2(b)]. However the Eu 2÷ spectrum is not well resolved in the excitation spectrum of samarium luminescence, due to the low Eu/Sm concentration ratio. It is reasonable to assume this energy transfer takes place, considering the great overlapping between the europium emission and the region from 400 to 460 nm of the Sm 2+ absorption spectrum. This fact indicates that the Eu 2+ ions should be very close to Sm 2+ ions, although this suggestion needs to be confirmed using more powerful techniques, such as electron paramagnetic resonance. In summary, the absorption and luminescence of our natural green fluorites are due to the presence of Eu 2+ and Sm 2+ ions. The optical measurements show similar results to those observed for synthetic crystals. Transfer of excitation light from Eu 2+ to Sm 2+ appears to take place.

561

REFERENCES Bill H. and Galas G. (1978) Color centers, associated rare-earth ions and the origin of coloration in natural fluorites. Phys. Chem. Miner. 3, 117-131. Bill H., Sierro 5. and Lacroix R. (1967) Origin of coloration in some fluorites. Am. Miner. 52, 1003-1008. Hurrell J. P., Kam Z. and Cohen E. (1972) Theoretical survey of the sidebands of Sm +2 fluorescence in BaF~. Phys. Rev. 116, 1992-2012. Ignatev I. V. and Ovsyankin V. V. (1980) Spectrum and symmetry of vibrations forming the vibranic wing of the 4f ~- ~ 4f k- ) 5d - 4f k luminescence of MeF 2 - p~,2 crystals JAHN TELLER pseudoeffect in CeF 2 - Sm +2 and SrF 2 - S m 2+ crystals. Optics Spectr. 49, 45-50. Kaiser W., Barnet C. E. B. and Wood D. L. (1961) Fluorescence and optical maser effects in CaF2: Sm + +. Phys. Rev. 123, 766-776. Kobayasi T., Mzoczkowki S. and Owen J. F. (1980) Fluorescence lifetime and quantum efficiency for 5d-4f transitions in Eu +2 doped chloride and fluoride crystals. J. Luminiscence 21, 247-257. Kfihner D. H., Lauer H. V. and Bron W. E. (1972) Lattice dynamics and electron-phonon-coupling field and vibranic spectra. Phys. Rev. B5, 4112-4126. Loh E. (1968) 4f-5d Spectra of rare-earth ions in crystals. Phys. Rev. 175, 533-536, Loh E. (1969) Ultraviolet-absorption spectra of europium and erbium in alkaline earth fluorites. Phys. Rev. 184, 348-352. Mackenzie K. J. D. and Green J. M. (1971) The cause of coloration in Derbyshire Blue John bauded fluorite and other blue bauded fluorites. Miner. Mag. 38, 459-470. Murrieta H., Hern~ndez J. and Rubio J. (1983) About the optical properties of the Eu ions as an impurity in non-metallic crystals. Kinam 5, 75-121. Przibram K. (1956) Irradiation Colours and Luminescence. Pergamon Press, London. Recker K., Neuhaus A. and Leckebusch R. (1968) Vergleichende Untersuchungen der Farb- und Lumineszenzeigen schaften natiirlicher and geziichterer definiert dotierter Fluorite. Proc. I.M.A. Cambridge, pp. 145-152. Sorokin P. P. and Stevenson M. J. (1961) Solid state optical maser using divalent samarium in calcium fluorite. IBM. J. Res. Dev. 5, 56-59. Vagin Y. S., Marchenko V. M. and Prokhorov A, M. (1969) Spectrum of laser base on electron-vibrational transitions in a CaF2:Sm 2+ crystal. Soviet Phys. Jetp. 28, 904-909.