Growth of Suzuki-like occlusions in KCl : Pb analysed by means of photoluminescence and absorption spectra

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Growth of Suzuki-like occlusions in KCl : Pb analysed by means of photoluminescence and absorption spectra

P. Bertoldi, R. Capelletti, F. Fermi, M. Manfredi, V. Graveris

To cite this version:

P. Bertoldi, R. Capelletti, F. Fermi, M. Manfredi, V. Graveris. Growth of Suzuki-like occlusions in KCl : Pb analysed by means of photoluminescence and absorption spectra. Journal de Physique Colloques, 1980, 41 (C6), pp.C6-367-C6-370. �10.1051/jphyscol:1980694�. �jpa-00220132�

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JOURNAL DE PHYSIQUE Cbl/oque C6, slrpy1~;nzent uu no 7, Tome 41, Juillet 1980, page C6-367

Growth of Suzuki-like occlusions in KC1 : Pb analysed by means of photoluminescence and absorption spectra

P. G . Bcrtoldi, R. Capelletti, F. Fermi, M. Manfredi and V. J. Graveris (*)

I s t ~ t u t o di Fisica dell'Universita, Parma, Italy

Ciruppo N a z ~ o r ~ a l e di Srru[tu[a dellci klaler~a dcl C N K

Resume. - Une analyse detaillee de la conversion des dip6les impurete-lacune + phase de Suzuki est faite a partir des mesures parallkles d'I.T.C., absorption optique et photoluminescence (des spectres d'emission, excita- t ~ o n et temps de decroissance) dans KC1 : Pb (avec une concentration entre 16 et 100 ppm) recuit a 21 5 O C pendant de longues periodes. Les bandes d'absorption et emission, relatives aux agglomerts de la phase de Suzuki, sont identifikes. On discute de la cinetique du processus et de la possibilitt du transfert d'energie a l'inttrieur des agglo- meres.

Abstract. - A detailed analysis of the impurity-vacancy (I.V.) dipoles ^-+Suzuki-like-phase (S.L.P.) conversion is performed by parallel I.T.C., optical absorption, photoluminescence (emission and excitation spectra and time decays) in KC1 : Pb (concentrations ranging from 16 to 100 ppm) annealed at 215 ()C for long times. The absorption and emission bands related to S.L.P. occlusions are identified. The kinetics of the process and possible energy transfer within the occlusions are discussed.

1. Introduction. ^-The Suzuki phase, namely a n ordered arrangement o f impurity-vacancy (I. V.) dipoles in n.n.n. coordination, was put in evidence since 1961 by means o f X-ray diffraction in heavily Cadmium doped NaCl [I]. In the last years it was investigated extensively by means of electron microscope in different systems [2] a n d in the late stage of the impurity precipitation process, where the new phase occlusions size allows lhcir detection.

More recently the nucleation of Suzuki-like phase (S.L.P.), at the expenses of I.V. dipoles, was followed from the beginning and along the whole process by means of dielectric method, such as Ionic Thermocurrents (I.T.C.), in KC1 doped with P b concentrations higher than 6 ppm a n d annealed for long times in the temperature range 150-260 "C [3, 41.

Moreover, d u c to the localized levels induced by Pb" within the encrgy gap of the alkali halides [5], it is possible to monitor the growth of S.L.P.

also by means o f optical methods such as optical absorption and photoluminescence spectra [3, 61. In this work the nucleation of S.L.P. occlusions and/or its precursors a t thc expenses o f I . V . dipoles is followed by correlating continuously : 1) I.T.C.

measurements, 2) optical absorption, 3) photoluminescence (excitation and emission) spectra and 4) lifetime measurements, for Pb' ⁺ concentrations ranging from 13 up t o 70 ppm.

(*) Semiconductor Physics Laboratory, Latvian State University, Riga, Latvian SSR

2. Experimental details. - F o r the sake of brevity the experimental details are here omitted and the reader is sent t o the quoted references. For I.T.C.

measurements, see (3, 41, for the absorption and photoluminescence spectra and lifetime measurements, sec [7].

For the preparation of KC1 : P b single crystals, see [3] for the detcrmination of the lead contents, see [7].

3. Experimental results. ^-3.1 I.T.C. MEASURE- MENTS. - Figure 1 shows how the nucleation process taking place at 214 " C affects the I.T.C. plot of KC1 :Pb (35 ppm). O n annealing a t 214 "C, the I.V.

peak (open circles, scale on the left) d u e t o I.V.

dipoles, previously dissolved to give a supersatu- rated solid solution by quenching from 350 " C , decreases, while a huge band (1.T.C.-B band) grows on the right (full circlcs, scale o n the right). The hugeness of the 1.T.C.-B band is consistent with a n ionic interfacial polarization [3], built u p at the boundary between KC1 matrix and the embedded S. L.P. occlusions, whenever the sample is polarized by a n electric field. The position shift of the band position up to a saturation value is meaningful o f a shape change of the growing S.L.P. occlusions to reach a n elongate shape (overall in the heavily doped samples).

3 . 2 AESORPTION MEASUREMENTS. - T h e time evolution of the absorption spectrum in the region of the A band (51 along the nucleation process a t 215 "C, is summarized in figure 2, for the sake of

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980694

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C6-368 P. G . BERTOLDI, R. CAPELLETTI, F. FERMI, M. MANFRED1 AND V. J . GRAVERIS

TEMPERATURE \K)

Fig. 1. - I.T.C. plot of KC1 : Pb (35 ppm) annealed for different annealing times 1, at 214 ^OCafter quenching from 350 "C.

Open circles : I.V. dipole peak (scale on the left) ; full circles : S.L.P. band (scale on the right), curve 1 : I, = 0 ; curve 2 : I, = 32h ; curve 3 : I, = 78h ; curve 4 : I, = 160h ; curve 5 : I, = 240".

*.

3

Fig. 2. - Time evolution of the optical absorption spectrum taken a t LNT for a KC1 : P b (16 ppm) annealed for different annealing times I, at 215 OC after quenching from 350 "C. The curves are related t o the following I, : 0, 40", 95h, 14@, 210".

316h, 440h and 654h. The curves for I, < 40" are omitted for the sake of clarity.

example for a 16 ppm Pb doped sample. In the quenched sample only the A band (2712

A)

attributed to the simplest Pb defects, such as I.V. dipoles 171 mainly in n.n.n. coordination [8], is present.

By annealing the sample A band decreases, while new side bands grow, peaked at 2 670, 2 800 and 2 870 A : see for instance their resolution in the front curve. The position, half maximum width of the gaussian components is fairly constant for long annealing times. In the early stages additional small gaussians grow and decrease in short times.

By increasing lead concentration the picture does not change on the whole except : i) for the over- lapping of light scattering which follows the v4 law and increases by increasing the annealing time, see for instance figure 2, front curve and ii) for the

increasing spectrum complexity with additional side bands, furtherly displaced from the original A band.

3.3 EMISSION AND EXCITATION SPECTRA. - AS the absorption spectra the emission ones, induced by exciting in the A band region (either in the original A band and in the additional ones), are followed as well along the nucleation process at 215 " C . The scheme of the absorption, photoluminescence excitation and emission spectra is given, for the sake of the simplicity, in figure 3 for a 16 ppm sample annealed at 215 "C for 654h. In the quenched sample on exciting in the A band (2 712 A ) one finds only the well known emission peaked at 3 360

A

and a much smaller one at 4 100 A , yet reported [9, 101. The 3 360

A

emission can be resolved into three gaussians in agreement with the previous observations and lifetime scheme (71 : this feature does not change along the nucleation process, in spite of the 3 360 emission intensity decrease (see Fig. 36). It is worthwhile noticing that the excitation spectra of 3 400 and 4 100 A emission bands coincide with the absorption A band along the whole nucleation process if the proper corrections are used ; only a small shift (2-3 A ) and widening is observed for the excitation of the 4 100 band.

Along the annealing both 3 360 and 4 100 emissions decrease (even if at different extent), while new bands increase at lower energies on exciting in the new absorption side-bands (see Fig. 3 4 . Here the Stokes shift is remarkable.

For the 16 ppm concentration the main new emission is at 5 800 A and can be resolved into two gaussians, peaked at 5 300

A

and at 6 000

A

(see Fig. 3c).

WAVELENGTH [A)

7000 6000 5000 4000 3000 2700 $

i " I W

2 3 4 5

PHOTON ENERGY (ev)

Fig. 3. - Photoluminescence spectra scheme taken at LNT for KC1 : P b (16 ppm) annealed at 215 "C for 654h after quenching.

a) 0 0 0 absorption spectrum (coincides with the front curve of figure 2) ; A A A excitation spectrum of the 5 800

4

^emis-

sion (S.L.P.) ; excitation spectrum of the 3 360 A (1.V.

dipoles) and 4 100 A emissions. b ) emission spectrum (I.V. dipoles) excited in the A band (the dotted lines indicate the component gaussians). The smaller (by a factor 100) 4 100 emission is omitted for the sake of clarity. c) A A A emlsslon spectrum (S.L.P.), magnified 50 times, excited in one of the additional absorption bands (i.e. 2 800 A ) (the dotted lines are the component gaussians).

Their excitation spectrum, shown in figure 3a, is composed by two gaussians peaked at 2 642 A and 2 790 A (compare them with the absorption resolution, figure 2, front curve).

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GROWTH O F SUZUKI-LIKE OCCLUSIONS IN KC1 : Pb ANALYSED BY MEANS C6-369

Either very long anneaiings and/or higher Pb concentrations are responsible for additional new emissions which appear at 4 750 A and 6 800

A,

which are excited in the absorption band peaked at 2 870 A .

In figure 4a the behaviour vs. annealing time at 215 "C is compared for the 3 360

A

emission (triangles), the I.V. dipole I.T.C. peak (open circles) and the A absorption band (full circles) ; the same is done in figure 46 for one of the optical bands grdwing along the annealing : namely the emission at 5 800

A

(triangles) and the absorption at 2 800 A (full circles) which are compared with the 1.T.C.-B band, attributed to the S.L.P. growth (opcn circles). The values plotted for the optical bands were obtained by using the proper resolved gaussians.

3.4 RADIATIVE LIFETIMES. - On exciting in the new absorption side-bands, as a rule two radiative lifetimes are observed for the emission at

-

5 800

A.

On average, 1) they are longer than those observed for isolated lead defects [7], (i.e. on exciting at 2 712 A and detecting the 3 360 emis- sion) and 2) they are temperature dependent, decreasing by increasing the temperature. For the sake of the example by excitating at

-

2 590 A and detecting at 5 700 A , some figures are :

at 80

K,

T I

-

⁷^{500 ns,}⁷²

-

^{2 700 ns}

and at 160 K 7 ,

-

¹800 ns, 72

-

²⁷⁰^ns.

4. Discussion and conclusions. - 1) Figure 4 shows how the 1.V. dipole concentration, as monitored by the I.T.C. peak is closely related t o the A absorption band, as deconvolved from the complex spectrum, along the whole nucleation process (except for the very early stages). This is remarkable result, as far as the previous studies of clustering at T

<

100 "C in KC1 : Pb by parallel optical absorption and dielectric measurements had shown a much faster decay of the latter if compared with the for-

0 200 400 600 0 200 400 600

ANNEALING TIME (burs) ANNEALING TIME (hours\ -

₃

OPTICAL DENSITY DECREASE

Fig. 4. - Kinetics of optical and I.T.C. bands along the annea- cles) ; 2 800 A absorption (full circles) and 800 A emission ling at 215 "C of KC1 : Pb (16 ppm). A) Behav~our vs. annealing (triangles) ; C) Plot vs. the decrease of 2 712 A absorption band time of : 1.T.C.-I.V. dipole band (oqen circles) ; 2 712 A optical density (induced by annealing) of : 2 670 A (squares) ; absorption band (full circles) and 3 360 A emission (triangles) ; 2 800 A (full circles) and 2 870 (triangles) absorption B) Behaviour vs. annealing time of : 1.T.C.-H band (open cir- bands ; 5 800 A emission (open circles).

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C6-370 P. G . BERTOLDI, R. CAPELI.ETT1, F. FERMI, M. MANFRED1 AND V. J . GRAVERIS

mcr [ I 11. This is due to the fact that the small clus- ters absorb as well in the A band and apparently slow down its decay. The 3 360 A emission decay turns out to be faster than those observed for 1.T.C.-I.V. band and A absorption band (Fig. 4a).

This can be explained by taking into account that the emission, related to the relaxed electronic excited state, can be quenched more efficiently by the surrounding changes than the dipole reorienta- tion ant1 absorption which are related mainly to the electronic ground state [12].

2) The growtl~ of S.L.P. occlusions monitored by the increase of 1.T.C.-B band (see Fig. ¹⁾ has the optical counterpart in the growth of the additional absorption and $mission bands : see big. 46, where only the 2 800 A absorption and 5 800 emission for the sake of the simplicity are compared to the 1.T.C.-B band. The different trend exhibited by the last is consistent with the change of the shape factor (see 3.1. and refs. [3] and [4]) which affects the amplitude of 1.T.C.-B band and not the optical band amplitudes.

3) The I.V. dipole

-

S.L.P. conversion yet proved by I.T.C. measurements [3, 41 is here furtherly supported by figure 4c, where the additional absorption bands amplitude (2 670 A , squares ; 2 800

A ,

full circles and 2 870 A , triangles) and the 5 800

A

emission (open circles) are plotted vs. the decrease of the A band (which is proportional to the I.V. dipole loss as shown by figure 4a). A rather

good linear relationship is found (except for very long annealing times).

4) The kinetics, which rule the I.V.

-

^S.L.P.

conversion for the 16 ppm sample, seem simpler if compared with those found for higher Pb concentrations (3, 41, being here the I.V. dipole loss an exponential decay to a saturation value. For higher concentrations the formation of S.L.P. cylinders was suggested [3] according to the Zener approach [13] : for lower ones probably there are not 1;V.

dipoles enough to allow the full development of cylinders, hence nuclei of not well defined shape are formed.

5) From the point of view of the radiative lifetimes the presence of two lifetimes for the S.L.P.

emissions seems consistent with the two gaussians in which the main emission at 5 800 A is decomposed (see Fig. 3c). Their'lengthening, with respect to sim- ple I.V. dipole emission could suggest a transfer mechanism [14] and could account the ionic rcorien- tation phenomena observed also at 78 K [I 51.

6) Up to our knowledge, it is the first time that photoluminescence is exploited as a very sensitive probe to monitor, yet from the early stage, the growth o f segregate phase occlusions.

Acknowledgments. - The authors wish to thank prof. R . Fieschi for the helpful discussion, Mr.

G . Lenzi and C. Mora for the skilful and valuable technical assistance.

DISCUSSION Question. - N. BONANOS.

What is a Suzuki-like phase ? Is this phase a super- structure of KC1 ? Is it vacancy rich ? Can you arrive at any electrical properties of this phase from your Maxwell-Wagner polarization observation ?

Reply. ^-R. CAPELLETTI.

It is a vacancy rich metastable phase. The X-ray diffraction measurements show an additional line a t the half way of the Brillouin zone suggesting a unit cell with lattice spacing twice that of KC1 as for Suzuki phase. The presence of cation vacancies is supported (1) by the presence of I.V. dipoles in n.n.n.

coordination, which allows an easy formation of the

Suzuki phase as shown by Dr. Corish, (2) by the fact that the activation energy for the 1.T.C.-B band relaxa- tion is 0.69-0.7 eV in agreement with the activation energy for the vacancy motion in KCl. Under the electric field applied at temperatures at which vacancies are mobile, they move to the positive electrode leaving behind the immobile divalent impurities, and accumulate at the boundary between the occlusion and the surrounding KCl. The situation can be frozen in by cooling to low temperature. If the field is turned off and the sample heated up at a constant rate, a depolarization current is detected. This could be seen also in principle by means of dielectric loss and iso- thermal polarization or depolarization current.

References

[I] SUZIJKI, K., J. Phys. Soc. Japan 10 (1955) 794 and ;hill. 16 (1961) 67.

[2] HOBIS, L. W.. J. Phys. 37 (1976) C7-3 and quoted rcfcrcnces.

[3] CAPEL.LETTI, R., GAINOTTI, A., PARFTI, L., Proc. Symposium on Thermal and Photostimulated Currents in Insulators, Ed. Donald M. Srnyth, Lehigh University, Retlehem, PA (USA) (1976) p. 66.

[4] C A P E L L ~ I , R., GAINOTTI, A,, J . Physique Colloq. 37 (1976) C7-316.

[5] FUKUDA, A,, Sci. Light 13 (1964) 64.

[6] BENCI, S. et al., J . Lumm. 18/19 (1979) 341.

[7] BENCI, S. P I al., Phys. Status Solidi H 90 (1978) 657.

18) COLLINS, W C., CRAWFORD, J. H., Phys. Rev. B 5 (1972) 633.

[9] MUGENSKI, E., DAMM, J. Z., Phys. Status solid^ B 80 (1977) K 79.

[lo] PASCIJAI., J. L., AKIZMENDI, L., JAQUE, F., AGULLO-LOY~Z, F., J . Lumin. 17 (1978) 325.

[ I 11 DRYDEN, J S., HARVEY, G. G., J. Phys. C 2 (1969) 603.

[I21 COLLINS, W. C., CRAWFOKD, J . H., Phys. Rev. 8 4 (1971) 3745.

[I 31 ZENER, C., J . Appl. Phys. 20 (1 949) 950.

[14] DEXTER, D. L., J. Chem. Phys. 21 (1953) 836.

[I51 RENCI, S. et al., J. Phys~que Colloq. 37 (1976) C7-138.