T. NAGARAJAN et al. : Positron Annihilation in Colonr Centres 75

phys. stat. sol. (b) 88, 75 (1977)

Subject classification: 10.2 22.5.3

Department of Nuclear Physics, University of Madras1)

Posit'ron Annihihtion in Colour Centres') BY

T. NAOAXAJAN, P. RAMASAMY, and 8. RAMASAMY

The angular correlation of positron annihilation in NaCl and KCl is compared for various con- centrations of F-centres produced by y-irradiation as well as additive doping. The intensity and the "full width a t half-maximum (FWHM)" of the narrow component are slightly different for each of the X-irradiated, electron-irradiated, y-irradiated (light and heavy doses), and addi- tively doped crystals. The possible role of additional defect centres in trapping positrons is dis- cussed. Also angular correlation measurements of positrons annihilating in the reversible F + F' conversion centres in equilibrium in additively coloured KCI at ~ 100 "C are given.

Es werden die Winkelkorrelationen der Positronenannihilation in NaCl und KCI fur verschiedene Konzentrationen von F-Zentren verglichen, die sowohl durch y-Bestrahlung als auch durch addi- tive Verfarbung hervorgerufen wnrden. Die Intensitat und die ,,Halbwertsbreite (FWHM)" T1p der schmalen Komponente sind leicht unterschiedlich fur die rontgenbestrahlten, elektronen- bestrahlten, y-bestrahlten (starke und schwache Dosen) und additiv verfairbten Kristalle. Die mogliche Rolle von zusatzlichen Storstellen beim Anhaften des Positrons wird diskutiert. Dar- uber hinaus wird iiber Winkelkorrelationsmessungen von Positronenannihilation an den rever- siblen F + F'-Konversionsspektren im Gleichgewicht in additiv verfarbtem KCI bei -100 "C be- richtet.

1. Introduction

The spectra of lifetimes and the angnlar correlation curve of 2y annihilation are considerably affected by the presence of lattice defects in metals as well as ionic solids. Recent investigations (cf. [ 11) have shown the considerable influence of the defect structure on the physical properties of positroniuni-like states in ionic crystals. In particular, the effects of defect concaentrations as introduced by additive doping on the positron lifetime spectra of ionic crystals were studied by Dupasquier [a]. A comparative study of the effect of y-irradiation, optical bleaching, and thermal annealing on the time spectra for differently treated KC1 crystals was done by Mallard and HSLI [ 3 ] . A siniilar study of the effect of different treatments of KaCI crystals on the lifetime spectra and the narrow component of angular correlation was done by Dannefaer and Sniedskjaer [4]. Herlach and Heinrich [5] found that the angular distributions for additively coloured crystals are narrower than those for 'pure' crystals of KCl. The same effect has been observed by us [Cj] for y-irradiated NaC'1 single crystals, by Vorobev et al. [ 7 ] for electron-irradiated crystals, by Brandt et al. [8] for X-irradiation, and by Williams and Ache [9] for proton irradiation. All these techniques produce apart from F-centres many other centres such as positive ion vacancies, trapped holes, colloids, etc. A comprehensive interpretation of the results becomes often difficult by the lack of a precise knowledge of the defects produced in a sample. _ _ ~

Madras 600025, India. *) TT'0rk supported by the University Grants Commission, India.

76 T. NAGARAJBT, P. RAMASAMY, and S. KAMASAMY

d positron can be easily trapped at either an F-centre or a cation vacancy. The annihilation characteristics of alkali halides with defects should show a lifetime spectrum and an angular distribution associated with at least these two inodes of annihilation. The situation concerning the quantitative explanation of the observed lifetime and the angular distribution is not satisfactory. Any model suited to explain the lifetime disagrees with the angular correlation and vice versa [lo, 111. The situation is very siniilar to the case of 'pure' alkali halide crystals where no particwlar model can qiiantitatively fit both lifetimes and angular distributions. For a coniprehensive understanding of positron processes in colour centres, there is a need for nieasurenients in different alkali halides under different conditions. The present work compares angular distributions of KaC1 and KC1 for various concentrations of F-centres produced by y-irradiation as well as additive doping. In additively coloured KC1, angular distributions were taken a t three different temperatures.

Although the positron annihilation with cotnplex colour centres (e.g. M, R, F, etc.) has not yet been investigated in detail, some results are already available on P'- centres [ 131. The present paper reports precise angular correlation nieasurenients of positrons annihilating with the reversible F + F' conversion centres in eqixilibriuni in additively coloured KCl a t - 100 "C.

2. Experiment

The standard parallel slit geometry used in the present work is described in our earlier papers 113, 141. The techniques of growing single crystals of alkali halides and cutting them parallel to various crystallographic directions are described in [ 141. Colouring is done both additively and by y-irradiation. Additive coloration of KCI was done by excess sodium nietal by a method due to Schulman and Conipton [ 151 and also by Van Doorn's method [16]. The latter method is used for controlled doping. The technical details are given elsewhere [17]. Quenching was done a t liquid nitrogen temperature and the transfer time involved was less than 1 s. All manipulations were done under subdued red light.

For measuring the positron angular correlation at liquid nitrogen temperature and at room temperature, the additively eoloured KCl saniple was mounted on the cold finger of a bath cryostat made of glass. For the investigation of F-centres a t -100 "C a continuous flow cryostat was used. On one side of the speeimen the positron source was disposed almost in contact, but on the other side F-light was incident. The maximum efficiency of photoconversion of F-to F'-centres was found to he a t -100 "C [MI.

The optical absorption measurements were carried out with thin slices cut from crystal samples used in positron angular correlation. The optical absorption was measured by a Cary-14 spectrophotometer fitted with a nietal cryostat. The homo- geneity of coloration was quite good. In the present work, additive coloration of RC1 was done at 600 "C with pure sodiuni nietal. In order to avoid other colour centres or colloids, however, it is necessary to quench the crystal, and this introduces plastic deformation and also freezes in vacancies which subsequently coalesce to produce vacancy clusters. The difficulties accompanying quenching are, of course, avoided by the use of irradiation; but other colour centres such as M-centres are formed. A single-crystal slab of KaCI (1 nini thick) was y-irradiated with a 3800 Ci GoBo source for 30 h at room temperature. The F-centre concentration produced was 1.3 ., lO1'/cn13. This sample was then bleaehed with F-light produced by focussing a Phi l ip lamp (150 W) through an interference filter (Schott & Gen., Mainz, type AL 10019.35,Am,, = 430 nm, T',,, = 44"t, BW = 30 nni) at -100 "C. The F-centre concentration decreased considerably. The appearance of the 3"-band is very weak

77 Positron Annihilation in Colonr Centres

M-band

U n m ) -- a b C

Fig. 1. Optical absorption spectrum of y-irradiated NaCl: a) (1) room temperature, (2) LKT. (3) bleached with F-light for 2 h a t -100 " C ; b) shows the presence of M-band a t 730 nm; c ) optical absorption spectrum of additively colonred KCI: ( I ) room temperature (composite of F- and Z-

band), (2) LiYT (F- and Z-band are resolved), (3) bleached with F-light for 2 h a t -100 "C

(see Fig. la). The creation of F'-centres by F-bleaching is not very snccessfnl in y-ir- radiated crystals, presumably because of thc presence of hole centres which arc efficient traps for electrons. This has been experienced with X-rayed crystals too [19], For 'hcavily irradiated crystals, the appearance of the M-band at 730 nm is evident in Fig. l b . Fig. l c shows the optical absorption of additively colonred KC1 quenched to liquid nitrogen temperature. At HT the F-band peak appears a t 570 m i l .

Its half-width is 0.27 eV. But when the temperature is lowered to T,NT, the peak is resolved into two peaks, onc at 545 nni and the other at 610 nm. The latter is the Z-

T a b l e 1 R,esult.s of the present work

crystal history of the crystal

KBr

NaCl

IL'aCI

NaCl

KCI

K C1

KCI KCI

y-irrad iated, 4.5 x 1015

y-irradiated, 0.8 x 10'7 ~ m - ~ y-irradiated, 1.3 x cm-3 y-irradiated, 1.5 x additively coloured, 1.8 x 1016 ~ r n - ~

same sample as above samc sample as above same sample as above

temp.

RT

RT

RT

RT

RT

LNT -100 "C -100 "C

with F-light)

percentage of positrons

mnihilated, IN

not significant,

(5 $I 0.1)

(10.6 & 0.2)

(15 4: 0.2)

25

20

17

12

I's*) (mrad)

-

-

2.86

3.80

2.65 2.50 151 2.60 L20]

2.93

2.93

1.95

*) Half-width a t half-height of the narrow component.

78 T. NAGARAJAN, P. RAMASAMY, and S. RAMASAMY

Fig. 2. Folded angular distribution of addi- tively coloured KCI : (1) room temperature, (2) LNT, (3) -100 "C, (4) -100 "C with F-light, ( 5 ) untreated KC1 single crystal

a t RT

band due to alkaline earth impurities. The appearanve of a Z-band indicates that the crystal used in the present work is not pure. Then the crystal was warmed to -100 "C and bleached by F-lightl produced by focussing a Philips lamp (150 W) through an interference filter (Schott & Gen., Mainz, type AL 10374.204, A,,, = 543 nni, T,,, = = 65%), BW= 22 nrn). The appearance of the F'-band is clear from Fig. 1c.

The angular correlation measurements on y-irradiated crystals are described in [C;]. But the results relevant t o the present discussion are collected in Table 1. Measure- ments were also taken on additively coloured KC1 with an 3'-centre concentration of 1.8 x 1016/cd a t RT, -100 "C, and LNT. At each temperature, seven to eight runs were taken. At -100 "C four runs were taken with F-light focussed onto the sample. The folded angular distributions are shown in Fig. 2 for various temperatures. The narrow component corresponding to positroniuni formation with P-centres is obtained by a procedure due to Herlach and Heinrich [ 5 ] . For an F-centre concentration of 1.8 x 1016/ciw~ at ltT, the positron annihilation with F-centres is 25O:. This is stir- prisingly large when compared to the values of other authors. For instance, Herlach and Heinrich [5] obtained 25'$/, a t an F-centre concentration of 5 x 101s/cn13 and Hautojarvi I201 got 3 . 7 O / , , at 9 x 1016/cm3. In the present work, quenching has been done very fast from BOO "C to LNT. This introduces plastic deformation and freezes in a large number of vacancy clusters. The effect of quenching 'pure' crystals to 80 K has been investigated by Dannefaer and Smedskjaer [4]. They obtain a narrow component of width rN = 2.1 nirad and I, = l2();. This narrow component in the sample of Dannefaer and Smedskjaer [4] is due to trapping of positrons in the vacan- cies frozen in during the quenching process. It is interesting to note that the width of the narrow component TN is 2.1 mrad for positrons annihilating in vacancies.

Width and intensity of the narrow components are found to be T, = 2.93 mrad, I, = 17"/, a t -1100 "C and rN = 2.93, I, = 20°4, a t LNT. With F-light irradiation at - 100 "C, F $ P' equilibrium is established. Jn this situation the narrow component has the values TK = 1.92 nirad and I, = 1276.

3. Discussion

Table 2 gives information on the parameters of the narrow component of the angular distribution reported by various authors. The quenched-crystal results of Dannefaer and Smedskjaer [4] show that positron trapping in vacancies is quite high (la./,) and the nionientuni distribution of electrons is a little broader (r, = 2.1 mrad) than in usual F-centres [4]. According to Farazdel and Cade [el] the cation vacancy is energetically more favourable than the F-centre for positron annihilation. Convincing

Positron Annihilation in Colour Centres 79

T a b l e 2 Comparison of narrow-component results due to various authors

authors I treatment

Herlach and Heinrich [5] Hautojarvi [20]

Vorobev et al. [7]

Dannefaer and Smedskjaer [4]

Brandt e t al. [S] present work

additively coloured KC1 additively coloured KCI electron irradiation, KCl quenched from

transfer time 0.4 s, KCI X-irradiation. NaCl additively colourcd KC1 at 600 "C and quenched t o LNT, transfer time < 1 s y-irradiated KaCl y-irradiated NaCl

700 "C to -193 "C

F-centre concentration

per cm3

5 X 1018

4 x 1017

0.7 x 1017 2.6 x 1017 4.9 x 1017

9 x 10'6

no F-centres

1.8 x 1016

1.3 x 1017 1.5 x 1017

1 9 *) (mrad)

2.5

2.6

3.3

2.1

2.0 2.65

2.86 3.8

percentage I,

25.0

8.5 3.7 4.8 8.4

14.2 12.0

8.0 25.0

10.6 15.0

3randt and

*) Half-width a t half-height of the narrow component.

evidence for this has been supplied by Mallard and Hsu [3] and also by _ _ _ - - - ~ ,_ . " - - - Waung IXXJ Uur crystal sample contains a composite 01: 4 - and /i-centres (because 01: alkaline earth impurities) and also a large number of vacancy clusters due to quench- ing. Sincc Z-centres are electron-trapped near an impurity and the trapping potential is very shallow, the presence of Z-centres should cause further narrowing of the narrow component. Also the trapping rate for positrons should be smaller than that for F-centres. But the width of the narrow component obtained in the present nieasure- ment (T, = 2.65 nrrad) and the very high intensity (I, = 25%), when compared to the results of Herlach and Heinrich [5] (see Table l), definitely confirms the presence of a high concentration of vacancies in the coloured and quenched crystal used by us. The present work and that of Dannefaer and Smedskjaer [4] suggest that the trapping rate of positrons in vacancies is higher and the angular distribution is also broader than for F-centre annihilation. Positrons trapped in cation vacancies annihilate with the wave function components of crystal electrons near the trap. The magnetic quenching experiments of Rrandt and co-workers [ 2 2 ] have shown that the overlap with an unpaired electron is muoh more pronounced when positrons annihilate a t F-centres than a t cation vacancies.

Kext the characteristics of the narrow component were investigated a t - 100 "C and LNT. The width has increased to TN = 2.93 mrad and remains the same a t LNT. This implies that the binding potential of ef a t the F-centre has increased a t LNT. But the percentage of the narrow component decreases to I, = 170/, and again increases to 207; a t LNT. The therinalization of positrons is temperature-dependent as inelastic phonon scattering is involved, and the trapping rate is dependent on the positron mobility. But the reason for the decreased intensity of the narrow component a t -100 "C and its increase a t LNT is not clear a t present. This behaviour may be perhaps caused by the presence of alkaline earth impurities as evidenced by the presence of %-centres.

80 T. KAGARAJAN, P. RAMASAMY, and S. RAMASAMY

The results of the y-irradiated crystal are given in Table 1 for comparison. The width of the narrow component is larger than in the caseof additively coloured crystals, and width and intensity of the narrow component increase with higher doses. It is evident from Fig. l b that M-centres are formed during heavy irradiation. It can be concluded from this work that the trapping rate of the M-centre should be higher than that of the F-centre and that also the electronic momentum distribution should be wider.

The present work is the first detailed measurement on F + 3” conversion. Danne- faer et al. [23] have measured the Doppler broadening of positron annihilation in F + F’ equilibriunr. But the Doppler broadening measurement was very insensitive to F + F’ conversion. The folded curves due t>o positron annihilation in F- and F’- centres are shown in Fig. 2 and their parameters are given in Table 1 : 1’, = 1.95 mrad, IN = la”/,. The present measurements on F + F’ equilibrium is vitiated by the presence of Z-centres. The general characteristics of %centre annihilation must be similar to those of F-centre annihilation. So i t can be judged from the present ex- periment that F’-centres should have a narrow momentum distribution and a much larger capture rate than F-centres. The higher capture rate of positrons by F-centres is consistent with the conclusion of Bosi et al. [la], but the smaller value of rN will be inconsistent with the binding energy of the F’e+ centre deduced theoretically [Zl]. But the trapping depth of F’-centres deduced from therrnolurninescence experiments [24] is siiialler than that for F-centres and supports our conclusion. Berezin [25] used HulthBn’s wave functions for computing the binding energy of the F’e+ centre and obtained E,(F’e+) = 2.96 el7 for KCl. We used the same wave functions to compute the positron angular distribution; but these give a very broad angular distribution r, = 5.5 mrad. So HulthBn’s wave functions (see [SS]) are unsuitable for explaining the angular distribution. If we assume that the positron trapped in the F’-centre fornis a positronium ion Ps- which is analogous to H-, it niay be possible to explain the observed low niomentum distribution,

4. Conclusion

Precise angular distribution nieasurenients have been made on the annihilation of positrons in additively coloured KC1 at three different temperatures. The intensity and the width of the narrow component due to positronium formation are strongly temperature-dependent. In additively coloured and quenched crystals, the width and the intensity of the narrow component are larger due to vacancy aggregates, which also play a doniinant role in the trapping of positrons. Also in heavily y-irradiated

. crystals, the width and the intensity of the narrow component increase due to fornia- tion of M-centres which also have high trapping rates for positrons. The angular distribution of positron annihilation in F + F’ equilibrium in KC1 is also measured. The present experiment suggests a high trapping rate of positrons in F’-centres, but a narrower angular distribution. HulthBn’s wave functions for F’-electrons give an unsatisfactory account of the angular correlation. There is a need for measurements of lifetime as well as angular correlation of positron annihilation in various types of controlled defects such as cation vacancies, %-centres, M-centres, and F‘-centres in different alkali halide systems.

Acknowledgements

The authors thank Prof. V. Devanathan for his keen interest and Prof. R. Srinivasan of Indian Institute of Technology for providing liquid nit,rogen. I t is a pleasure to thank Dr. Y. V. G. S. Murti for fruitful discussions and Mr. V. R. R’arayanasamy for correcting the manuscript.

Positron Annihilation in Colour Ccntrcs 81

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(Received January 24, 1977)

6 physica (b) SZ/l