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IL NUOVO CIMENTO VOL. 19 D, N. 1 Gennaio 1997

Investigating crystal defects in BaF

and SrF

by thermoluminescence experiments

M. SALIS

INFM and GNSM - Cagliari, Italia

Istituto di Fisica Superiore dell’Università - Cagliari, Italia

(ricevuto l’8 Ottobre 1996; approvato il 19 Novembre 1996)

Summary. — The thermoluminescent emission of Ba and Sr fluorides was recorded

simultaneously vs. temperature and wavelength. Data obtained indicate that Ba and Sr ions move to interstitial positions, thus originating electron traps active in the thermoluminescent process.

PACS 78.60.Kn – Thermoluminescence.

PACS 61.72.Ji – Point defects (vacancies, interstitials, color centers, etc.) and defect clusters.

Lattice defects in alkali fluorides have been widely investigated in the past three decades [1-4]. While many studies have been devoted to the exploitation of luminescent properties of suitably doped materials, less attention has been reserved to defects in undoped materials. This is true, in particular, for intrinsic defects in alkali earth fluorides. For this reason, experiments were performed by spectrally resolved

thermoluminescence (TL) on lattice defects in SrF2 and BaF2. Indeed, the TL

emission, taking place when X-irradiated crystals are warmed up, is originated by recombination of opposite-sign carriers trapped at point defects of the crystal lattice. TL experiments, therefore, represent a valid tool for gaining information on these

defects. Results obtained are compared with those of a previous study on CaF2

examined with the same technique [5].

Samples were supplied by J. Matthey: SrF2 with purity 99.995% and BaF2 with

purity 99.99%. Samples were analyzed in standardized conditions, that is, irradiated for one hour with soft X-rays (Cu Ka) and then warmed from 300 to 690 K with the

constant speed of 0.9 Ks21_{. The ensuing TL emissions were detected by a}

vacuum-operating apparatus capable of spectral resolution from 375 to 730 nm. The TL

emissions of SrF2 and BaF2 are shown in fig. 1 and 2, respectively, as recorded by

contour plots. The relative data are reported in table I, which also contains relevant

data of the CaF2TL emission. For both samples, it appears from fig. 1 and 2 that two

emission peaks are present on the temperature axis with the same wavelengths, that is,

653 nm and 670 nm for SrF2and BaF2, respectively. The CaF2TL emission is somewhat

M.SALIS

96

Fig. 1. – Contour plot of the SrF2thermoluminescent emission.

more complex. It consists of six peaks at three temperatures and two wavelengths. Table I also shows the TL yields, that is, the TL intensities integrated over the entire

spectral and temperature ranges. The CaF2TL yield exceeds that of the other

fluorides by one order of magnitude.

To single out the nature of lattice defects originating the observed TL emission, table II shows the simplest defects which can be present in the alkali earth fluorides.

Among these, the Frenkel defects, which originate when F(2) ions move to interstitial

positions, are to be considered. Owing to the presence of Frenkel defects, when carriers are injected in the crystal by X-ray activation, F and H centres are formed. There is experimental evidence showing that these centres lie so close in the lattice that the prevailing process is tunnel recombination of opposite-sign carriers [6]. This gives rise to a radioluminescent emission in the spectral range 300–350 nm. For this reason, these centres are unable to trap steadily the carriers in such a way as to contribute significantly to the TL emission. At most, no more than a faint contribution to TL patterns in expected for them. It is worth noting, in this connection, that the T3

peak of Ca F2 (see table I), which was attributed to F centres [5], is not detectable in

the Ba F2 and Sr F2 samples examined.

Some indirect arguments can be presented to single out interstitial cations as the best candidates for explaining the electron traps responsible for T2peaks (see table I).

Indeed, in the fluorite-type cell (S. G. Fm3 m) eight equivalent positions exist in which cations can be held. Since the cell contains only four cations, room is available for interstitials. Indeed, a divalent cation can move to an unoccupied eight-fold site without

INVESTIGATING CRYSTAL DEFECTS INBaF2ANDSrF2ETC. 97

Fig. 2. – Contour plot of the BaF2thermoluminescent emission.

originating a structural deformation of the lattice. Thus, the only significant energy involved in the excess electrostatic energy of the divalent ion electric field. Therefore a large density of interstitial cations is expected to be present in these fluorides. It is to be noted that, owing to their double positive charge, two electrons can be trapped by these defects.

It is of interest to devote some attention to the actual depths of such traps. It has been shown that, by assuming for the trapping centre a hydrogen-like model with the effective electron mass equal to the real one, thermal activation energy is given by [7] W(Z) 4 RHhc 162 e2 0

g

h

TABLE I. – Data on emission wavelengths, glow-peak temperatures, and thermoluminescence

yields of CaF2, SrF2and BaF2.

l1 l2 T1 T2 T3 Yield

Ca 459 607 378 480 600 518

Sr — 653 393 523 — 40

M.SALIS

98

TABLE II. – Ion or ion cluster originating interstitials (round brackets) or vacancies (square

brackets) in alkali earth fluorides ( M1 1_{4 Ca , Sr , Ba).}

Ion or ion cluster

Electron centre

Charge Ion or ion cluster

Hole centre Charge

[ F2_{] F 11 ( F}2_{) H 21} ( M1 1_F2_{) — 11 [ M}1 1_F2_{] — 21} ( M1 1_{) — 12 [ M}1 1_{] VF 22} [ F2_F2_{] M 12 ( F}2_F2_{) — 22} ( M1 1_{)[ F}2_{] — 13 [ M}1 1_{]( F}2_{) — 23} [ F2_F2_F2_{] R 13 ( F}2_F2_F2_{) — 23}

where RH is the Rydberg constant, Z the effective nuclear charge, e0 the static

dielectric constant and eQthe high-frequency dielectric constant. Equation (1) actually

accounts for lattice relaxation following release of the trapped electron. According to eq. (1), the thermal activation energy for removing the first electron is given by

E( 1 )

4 2 W(Z( 2 )) 2W(Z( 1 )_{) ,}

(2)

where, taking into account the Slater screening constants [8], we have Z( 2 )

4 2 2 0.3 and Z( 1 )

4 2. The dielectric constants can be evaluated utilizing optical and dielectric data [9]. We obtain e04 6.81 and eQ4 2.06 for Ca F2, e04 6.33 and eQ4 2.13 for Sr F2,

e04 7.33 and eQ4 2.33 for Ba F2. It follows, from eq. (2), that E( 1 )4 1.22, 1.24 and

1.07 eV for Ca, Sr and Ba fluorides, respectively. These figures appear consistent with the temperatures of the recorded glow peak. It is to be expected that glow temperatures will show the same increasing behaviour as the thermal activation energies. Indeed, it can be shown, from data in table I, that both glow temperatures T2

and energies E( 1 )_{increase with the sequence Ba, Ca, and Sr. As to the actual values of}

activation energies, the found figure, E( 1 )_{4 1.22 eV, can be considered in reasonably}

good agreement with energy 1.07 eV previously found for CaF2[5].

R E F E R E N C E S

[1] HAYES W. and STOTT J. P., Proc. R. Soc. London, Ser. A, 301 (1967) 313.

[2] BEAUMONTJ. H., HAYESW., KIRKD. L. and SUMMERG. P., Proc. R. Soc. London, Ser. A, 315 (1970) 69.

[3] CATLOW C. R. A., J. Phys. C, 12 (1979) 969.

[4] CHAKRABARTIK., MATHURW. K., ABBUNDIR. J., KRISTIANPOLLERN. and HORNYAKW. F., J.

Lumin., 48-49 (1991) 828.

[5] BROVETTOP., DELUNASA., FLORISA., MAXIAV., MURGIAM. and SPANOG., Nuovo Cimento D,

12 (1990) 1651.

[6] SHIC., KLOIBER T. and ZIMMERER G., J. Lumin., 48-49 (1991) 597. [7] LEHOVEC K., Phys. Rev., 92 (1953) 253.

[8] SLATER J. C., Phys. Rev., 36 (1930) 57.