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Migration of anion vacancies and formation of complex colour centres in KC1
J. Phys. Chem. Solids
Pergamon Press 1969. Vol. 30, pp. 2 17-223.
Printed in Great Britain.
MIGRATION OF ANION VACANCIES AND FORMATION OF COMPLEX COLOUR CENTRES IN KC1 G. GIULIANI Istituto di Fisica dell’Universita di Pavia, Italy, Gruppo Nazionale di Struttura della Materia de1 C.N.R. (Received 3 May 1968) Abstract-A new technique is presented, which allows to send anion vacancies and electrons separately through an alkali halide crystal. This technique can, in principle, be applied to the production of any complex colour centre involving anion vacancies and electrons. In this paper we report the results obtained by applying this new technique to the production of FA- and Z,-centres. 1. INTRODUCTION
IT IS WELL known that F-light absorbed by an alkali halide crystal containing F-centres and kept near room temperature (RT) leads to the formation of F-aggregate centres (M, Rand N-centres). However, if the sample contains divalent impurities such as Ca, Sr or Ba or suitable monovalent impurities (for instance Li or Na in KCl), the main product of the F-light illumination is given by Z1centres and F,-centres respectively. The process of formation of F-aggregate centres as well as that of FA-and Z,-centres has recently received renewed attention by several authors [ l-61. All these studies have been done by carefully investigating the processes set up by the simultaneous migration of ionic defects and electrons. In this paper results are reported which show how it is possible to produce complex colour centres in two steps by sending anion vacancies and electrons separately through the crystal. The efficacy of this new procedure has been checked by producing F,-centres (whose structure is known and consists of an F-centre with a monovalent impurity ion as a nearest neighbour [7]) in lithium doped KC1 samples; on the other hand the use of this procedure in Z,-centre production shows how it can give useful information about both the intermediate produc.ts of the process
of formation of complex colour centres and the structure of the colour centres themselves*. 2. EXPERIMENTAL PROCEDURE
colour centres’ transformations The produced in KC1 samples by various treatments have been studied by observing the corresponding changes in the absorption spectrum of the samples. A Hilger type E-791 monochromator and a Beckmann DU spectrophotometer have been used in the 170-200 w and in the visible range respec-z tively. The crystals have been grown in our laboratory in nitrogen atmosphere by the Kyropoulos method. The reported amounts of impurities refer always to the quantities added to the melt. All the results reported in this paper have been obtained with X-rayed samples. 3. RESULTS AND DISCUSSION
3.1 Production and migration of vacancies. Formation of FA-centres
anion
To send anion vacancies through the crystal requires: (a) to produce isolated anion vacancies (cu-centres) and (b) to allow the anion vacancies to migrate by, for instance, *Preliminary results on the Z,-centre production have been presented to the ‘Colloque sur les centres color&‘, Saclay, March 1967. The text of the communication has been published as from reference [8]. 217
218
G. GIULIANI
warming up the crystal to an appropriate temperature. Condition (a) can be easily met by X-raying a KC1 sample at a low enough temperature or by shining F-light on a sample containing F-centres (produced by additive colouration or by X-rays) and kept at a suitable temperature. However, when a KC1 sample X-rayed at low temperatures is warmed up to higher temperatures, the disappearence of cw-centres is mainly due to their recombination with mobile interstitial negative ions [9]. Also the warming up of an additively coloured KC1 sample in which a fraction of the Fcentres has been transformed into an equal number of cr- and F’-centres, does not produce a detectable migration of anion vacancies. In fact, in this case, the main effects of the warming are the thermal ionization of F’centres and the capture of the thermally released electrons by empty anion vacancies [lo]. Therefore, it is clear that, in order to observe the migration of anion vacancies, one should warm up a sample containing oxentres but no F’-centres or interstitials which could become mobile during the warming. This may be done as follows. A KC1 sample is X-rayed at liquid nitrogen temperature (LNT), warmed up to RT (~290°K) and kept at this temperature for about 12 hrs*. The sample is then illuminated with F-light and subsequently with F’light at 150°K. The net result of the (F + F’ )illumination consists in a decrease of the F band and in the growth of the a band?. After the (F + F’)-illumination no F’ band is present. In this way one obtains a sample containing isolated anion vacancies but no F’centres. However, it is to be stressed that the electrons released by the (F + F’)illumination may change the structure of the *The sample could be X-rayed directly at RT. However, the RT irradiation of strontium doped KC1 samples results in a large increase of the absorption in the tail of the fundamental edge, making it difficult to observe further changes of the absorption spectrum in this region. tThe (Y band is due to exciton transitions localized near isolated anion vacancies and peaks, in KCl, at 177mp[ll, 121.
hole centres giving rise to some kind of interstitial not present before the illumination. Therefore, the decay of the (Y band which occurs when the sample is warmed up, may be due to the migration of these interstitials. Though we can not definitely rule out this possibility, we shall see below that the decay of the (Yband produced as described above is due, at least partially, to the migration of anion vacancies. The thermal decay of the (Yband has been studied by a pulse annealing method: the sample was warmed up to a temperature higher than 150”K, kept at this temperature for 15 min and then cooled down to LNT for the optical measurement. The results obtained are reported in Fig. 1 and show that anion
3
Fig. 1. Thermal decay of wzentres produced by optical bleaching of F-centres in X-rayed KC1 samples.
vacancies produced by ionization of F-centres in X-rayed samples are stable up to 200% and are totally destroyed if the sample is warmed up to 250°K. From Fig. 1 it appears also that the presence of monovalent (LP) or divalent (Sr++) impurities does not affect the thermal decay of cw-centres. The proof that the thermal decay of a-centres produced by ionization of F-centres in X-rayed samples is due, at least partially,
COMPLEX COLOUR CENTRES
to their own migration, is given by the following experiment. a-centres are produced as described above in a lithium doped KC1 sample. In Fig. 2(a) we have reported the
I
*i5
E
(*VI
2
1 0 -1 -.2
500
550
X (mp)
6”
650
Fig. 2. (a) change in the visible region of the absorption spectrum produced by an (F + F’)-illumination carried out at 15WK on a lithium dooed (lO-p mol) KC1 samnle X-rayed at LNT and warmed up‘to RT. (b) changes in the visible region of the absorption spectrum due to the warming to 250°K of the sample of Fig. 2(a). (c) changes in the visible region of the absorption spectrum produced by an (F + F’)-illumination carried out at 150°K on the sample of Fig. 2(b).
changes in the visible region of the absorption spectrum due to the (F + F’)-illumination at 15CPK: the only observed change is a decrease of the F band. The changes due to the warming up which follows the (F+ F’)illumination at 150°K are shown in Fig. 2(b): here again the only observed change is due to a decrease of the F band. At this point the sample is illuminated again at 150°K with
IN KCI
219
F-light and subsequently with F’-light*: the corresponding changes of the absorption spectrum are shown in Fig. 2(c). From this figure it appears that this illumination, unlike the first one (cf. Fig. 2(a)), not only decreases the F band but causes also the FA bands to grow. In order to explain the above results, we shall begin by emphasizing that the primary effect of the two illuminations is the same and consists in the production of free electrons. However, the final products of each illumination depend on the distribution of the free electrons among the available electron traps. The fact that the FA bands grow only under the second illumination (cf. Figs. 2(a) and 2(c)) shows that during the second illumination electron traps not present during the first one transform into F,-centres by capturing an electron. These electron traps must have been formed during the warming up which follows the first illumination and must be a,-centres (i.e. cu-centres with a lithium ion as a nearest neighbour). The formation of cr,-centres is possible only if anion vacancies migrate and join lithium ions. Therefore, we can conclude that the decay of the cr band which takes place during the warming up of the sample to 250°K (cf. Fig. 1) is due, at least partially, to the migration of anion vacancies. Before leaving this subject, it is worth stressing that: (a) the optical ionization of F-centres at 150°K is not accompanied by the migration of anion vacancies; (b) the migration of anion vacancies is not accompanied, in the experimental conditions described above, by a detectable production of free electrons. In fact, if points (a) and (b) were not true, the FA bands should have grown both during the first (F+ F’)-illumination and during the subsequent warming up of the *In this case the F’-illumination is not essential. However, since the F’ band produced by the F-illumination decreases under the action of the light used for the optical measurement, it is convenient to destroy all the F’-centres before the measurement.
220
G. GIULIANI
sample. However, no detectable growth of bands is observed in either case (cf. Figs. 2(a) and 2(b)).
F,
3.2 Formation of Z,-centres In this section we shall see how Z,-centres can be formed in a strontium doped KC1 sample by sending anion vacancies and electrons separately through the crystal (cf. the end of Section 3.1). The procedure is the same as the one described in Section 3.1 for the production of FA-centres. Nevertheless, in order to make the exposition and the explanation of the results easier, the main steps of the procedure will be recalled here. A strontium doped KC1 sample is: (I) X-rayed at LNT and warmed up to RT; (II) illuminated with F-light and subsequently with F’-light at 150°K: as reported in Section 3.1 this illumination causes the F band to decrease and the (Yband to grow. Furthermore, as in the case of the lithium doped sample
E (e’U
2.5
500
(cf. Fig. 2(a)), the decrease of the F band is the only change produced in the visible region of the spectrum by the (F + F’)-illumination (cf. Fig. 3(a)); (III) warmed up to 250°K; as shown in Section 3.1, the warming causes the (Y band produced in step II to disappear. Furthermore, the disappearance of the cy band is due, at least partially, to the migration of anion vacancies (cf. Section 3.1); (IV) illuminated with F-light and subsequently with F’-light at 150°K. This illumination causes the F band to decrease and the Z1 band to grow (cf. Fig. 3(b)). As in the case of FA-centre formation (cf. Section 3.1), we must draw our attention to the (F+ F’)-illuminations carried out in steps II and IV. The illumination performed in step II does not give rise to the Z1 band (cf. Fig. 3(a)), while the one carried out in step IV does (cf. Fig. 3(b)). This implies that during the illumination carried out in step IV, electron traps not present during the illumination
550
2
600
650
A (my)
Fig. 3. (a) changes in the visible region of the absorption spectrum produced by an (F + F’)-illumination carried out at 150°K on a strontium doped ( 1e3 mol) KC1 sample X-rayed at LNT and warmed up to RT. (b) changes in the visible region of the absorption spectrum produced by an (F + F’)-illumination carried out at 15(PK on the sample of Fig. 3(a) after its warming up to 250°K.
COMPLEX
COLOUR CENTRES
performed in step II, transform into Z,centres by capturing an electron. These electron traps can be referred to as ionized Z,-centres or Z1+-centres and must have been formed during the migration of anion vacancies which occurs in step III. Therefore, since Z1+-centres form during the migration of anion vacancies (just in the same way as cYA-centres do; cf. Section 3.1) it is quite reasonable to conclude that Z1+-centres contain an anion vacancy as a component. As far as the entire structure of the Z,+centres is concerned, we can observe that two configurations are possible: (i) a vacancy pair and, (ii) a divalent impurity with an associated vacancy pair*. Accordingly, the Z,-centre will consist of a vacancy pair with a trapped electron or of a divalent impurity-cation vacancy complex with an associated F-centre. Possibility (i) seems unlikely since the capture of an electron by a vacancy pair is expected to end up in the formation of an F-centre and an isolated cation vacancy. On the other hand possibility (ii) is supported by the Z,-centre ENDOR spectrum obtained by Bushnell[l3]. In any case, it is worth emphasizing that the results reported in this section clearly show that the Z1-centre contains an F-centre as a componentt. 3.3 The Z,+-centre According to what has been said in Section 3.2, the Z1+-centre is very likely made up by a divalent impurity-cation vacancy complex with an associated anion vacancy. Therefore, it is expected to give rise to a localized exciton band. In this connection, let us consider the followi;tg results: (i) the thermal decay of the *That these are indeed the only two possible configurations for the Z1+-centre can easily be accounted for on the basis that Z,+-centres contain an anion vacancy as a component and form only in samples doped with divalent impurities. tVery recently, further evidence for a model of the Z,-centre including an F-centre has been obtained from Faraday rotation (14) and circular dichroismIl51 measurements.
IN KC1
221
CYband which occurs in step III of section 3.2 is accompanied by the growth of a band peaking at about 173 w (cf. Fig. 4); ii) the F-illumE (eV)
6.75
7
7.25
I
oc band
1 150 OK
I 214 OK , 230°K L 246 OK L 260°K
I
+ I_
185
‘
180
175
170
X (my)
Fig. 4. Thermal decay of the a band produced by optical bleaching of the F band at 150°K in a strontium doped (lo-* mol) KC1 sample. All these curves have been obtained by subtracting the absorption curve measured before the F + a conversion to the absorption curve measured after the warming up of the sample to the reported temperature. As evident from this figure, the decay of the a band is accompanied by the growth of a band peaking at about 173 nl&.
ination carried out in step IV of Section 3.2 causes not only the Z, band to grow (cf. Fig. 3(b)) but also the 173 w band to decrease (cf. Fig. 5); (iii) Z,-centres can be bleached by Z,-light at 200°K; the product of the bleaching is given by Z1+-centres as it is proved by the fact that Z,-centres can be reformed by a subsequent F-light illumination carried out at 150°K. Figure 6 shows that the z1 -4 z,+ conversion is accompanied by the growth of the 173 rnp band; (iv) the Z, band does not appear in step IVof Section 3.2 if the sample, in step III, is warmed up to 300°K
G. GTULIANI
222
for 30 min in order to destroy the 173 rnp band. It is clear that the above data, though only qualitative in nature, allows one to reasonably assign the 173 ~E.Lband to Z1+-centres.
E (eV) 6.75
7 f
7.25 r ,
4.
X (my) Fig. 5. (a) change of the absorption spectrum near the fundamental edge produced by an (F + F’)-illumination carried out at 150°K on a strontium doped (lo-smol) KC1 sample containing Z,+-centres; (b) change that would have been observed if the growth of the (Yband were the only effect of the illumination. This curve has been drawn by making use of the known shape of the (Yband and of the fact that the contribution to the absorntion of the 173 w band above 180 rnp is negligible (cf. Fig. 4); (c) difference between (a) and (b) showina that the 173 nq band decreases under the’ (Ft F’F illumination. E
(eV)
7.25
7 173 my
band
I
1
1
180
175 x
170
I
(m/i)
Fig. 6. Change of the absorption spectrum near the fundamental edge produced by a Z,-light illumination carried out at 200°K on a strontium doped (10-s mol) KC1 sample containing Z,-centres (light source: white light from a 500 W lamp filtered through a Kodak 89B wratten filter). The narrow band on the short wavelength side of the 173 m band is of unknown origin. However, it is worth reporting that it has been observed also in some lithium or strontium doped KC1 sample X-rayed at LNT, warmed up to RT and subjected to an (F + F’)illumination (KCI:LiCl) or to a white light illumination (KCI: SrC1.Jat 150°K.
FINAL COMMENTS
The results reported in this paper prove that the processes of formation of FA- and Z,centres take place through the migration of anion vacancies and the formation of cu,- and &+-centres respectively. There is experimental evidence that anion vacancies play the same role in the M-centre formation[l-41. Therefore, it should be possible to produce M-centres by the same procedure used here for the FA- and the Z,-centre production. However, it must be stressed that a necessary condition for this method to be applied to the production of M-centres (or other F-aggregate centres) is that the intermediate product of the process (i. e. the F,+-centre in the case of the M-centre formation) must be stable at least at temperatures just higher than the temperature (200°K; cf. Fig. 1) at which the anion vacancies begin to migrate. As reported in Section 3.3, Z1+-centres are unstable near RT*. In this connection it is worth emphasizing that, since Z,-centres seem to be the product of the thermal decomposition of Z,-centres [ 161, ionized Z,-centres may be one of the products of the thermal destruction of Zr+-centres. If this hypothesis is correct, it should be possible to produce &-centres in two steps by producing ionized Zz-centres which will then transform into Z,centres through electron capture. Measurements to check this possibility (which may give useful information about Z,-centre production and structure) are now in progress. author would like to thank Mr. G. Guala for having performed some of the measurements.
Acknowledgements-The
*Preliminary results on the thermal stability of Zr+centres have already been obtained[ll. More detailed measurements are now in progress in connection with the study of the Z,-centre production (see below).
COMPLEX
COLOUR
REFERENCES M. and SMOLUCHOW1. FARGE Y., LAMBERT SKI R., SolidState Commun. 4,333 (1966). P~VS. Sot. Jaoan 21. 2. ASAI K. and OKUDA A.. J. __ 2197(1966). 3. NAHUM J.,Phys. Rev. 158,814(1967). 4. HIRAI M. and SCOTT A. B., .I. them. Phys. 46, 2896 (1967). 5. HIiRtEL i-l. and LijTY F.,Z.Phys. 177,369(1964). 6. H;iRTEL H. and LtiTY F..Z. Phvs. 182.111(1964). I. MIEHER R. L., Phys. Rkv. t&t. 8, 362 (1962). 8. GIULIANI G., J. Phys. Radium, Paris 28, Suppl., C4-175 (1967). 9. GIULIANI G. and REGUZZONI E., Phys.
CENTRES
IN KCI
223
Status solidi25,431(1968). 10. ONAKA
R., FUJITA
1. and FUKUDA
A., J.
phys. Sot. Japan 18, Suppl. II, 263 (1963). 11. ONAKA R. and FUJITA J., Phys. Rev. 119, 1597 (1960). G.. GIULIANI G. and LYNCH 12. CHIAROTTI D. W., I1 Nuouo hm. 17,989 (1960). 13. BUSHNELL J. C.,.Thesis. Urbana(1964). ii: PAUS H. and LUTY F., Phys. Rev. Lett. 20, 57 (1968). G. and LANGER H., Phys. Lett. 26A, 15. GEHRER 232 (1968). M., J. Phys. Chem. Solids 24, 16. KLEEFSTRA 1567 (1963).
Pergamon Press 1969. Vol. 30, pp. 2 17-223.
Printed in Great Britain.
MIGRATION OF ANION VACANCIES AND FORMATION OF COMPLEX COLOUR CENTRES IN KC1 G. GIULIANI Istituto di Fisica dell’Universita di Pavia, Italy, Gruppo Nazionale di Struttura della Materia de1 C.N.R. (Received 3 May 1968) Abstract-A new technique is presented, which allows to send anion vacancies and electrons separately through an alkali halide crystal. This technique can, in principle, be applied to the production of any complex colour centre involving anion vacancies and electrons. In this paper we report the results obtained by applying this new technique to the production of FA- and Z,-centres. 1. INTRODUCTION
IT IS WELL known that F-light absorbed by an alkali halide crystal containing F-centres and kept near room temperature (RT) leads to the formation of F-aggregate centres (M, Rand N-centres). However, if the sample contains divalent impurities such as Ca, Sr or Ba or suitable monovalent impurities (for instance Li or Na in KCl), the main product of the F-light illumination is given by Z1centres and F,-centres respectively. The process of formation of F-aggregate centres as well as that of FA-and Z,-centres has recently received renewed attention by several authors [ l-61. All these studies have been done by carefully investigating the processes set up by the simultaneous migration of ionic defects and electrons. In this paper results are reported which show how it is possible to produce complex colour centres in two steps by sending anion vacancies and electrons separately through the crystal. The efficacy of this new procedure has been checked by producing F,-centres (whose structure is known and consists of an F-centre with a monovalent impurity ion as a nearest neighbour [7]) in lithium doped KC1 samples; on the other hand the use of this procedure in Z,-centre production shows how it can give useful information about both the intermediate produc.ts of the process
of formation of complex colour centres and the structure of the colour centres themselves*. 2. EXPERIMENTAL PROCEDURE
colour centres’ transformations The produced in KC1 samples by various treatments have been studied by observing the corresponding changes in the absorption spectrum of the samples. A Hilger type E-791 monochromator and a Beckmann DU spectrophotometer have been used in the 170-200 w and in the visible range respec-z tively. The crystals have been grown in our laboratory in nitrogen atmosphere by the Kyropoulos method. The reported amounts of impurities refer always to the quantities added to the melt. All the results reported in this paper have been obtained with X-rayed samples. 3. RESULTS AND DISCUSSION
3.1 Production and migration of vacancies. Formation of FA-centres
anion
To send anion vacancies through the crystal requires: (a) to produce isolated anion vacancies (cu-centres) and (b) to allow the anion vacancies to migrate by, for instance, *Preliminary results on the Z,-centre production have been presented to the ‘Colloque sur les centres color&‘, Saclay, March 1967. The text of the communication has been published as from reference [8]. 217
218
G. GIULIANI
warming up the crystal to an appropriate temperature. Condition (a) can be easily met by X-raying a KC1 sample at a low enough temperature or by shining F-light on a sample containing F-centres (produced by additive colouration or by X-rays) and kept at a suitable temperature. However, when a KC1 sample X-rayed at low temperatures is warmed up to higher temperatures, the disappearence of cw-centres is mainly due to their recombination with mobile interstitial negative ions [9]. Also the warming up of an additively coloured KC1 sample in which a fraction of the Fcentres has been transformed into an equal number of cr- and F’-centres, does not produce a detectable migration of anion vacancies. In fact, in this case, the main effects of the warming are the thermal ionization of F’centres and the capture of the thermally released electrons by empty anion vacancies [lo]. Therefore, it is clear that, in order to observe the migration of anion vacancies, one should warm up a sample containing oxentres but no F’-centres or interstitials which could become mobile during the warming. This may be done as follows. A KC1 sample is X-rayed at liquid nitrogen temperature (LNT), warmed up to RT (~290°K) and kept at this temperature for about 12 hrs*. The sample is then illuminated with F-light and subsequently with F’light at 150°K. The net result of the (F + F’ )illumination consists in a decrease of the F band and in the growth of the a band?. After the (F + F’)-illumination no F’ band is present. In this way one obtains a sample containing isolated anion vacancies but no F’centres. However, it is to be stressed that the electrons released by the (F + F’)illumination may change the structure of the *The sample could be X-rayed directly at RT. However, the RT irradiation of strontium doped KC1 samples results in a large increase of the absorption in the tail of the fundamental edge, making it difficult to observe further changes of the absorption spectrum in this region. tThe (Y band is due to exciton transitions localized near isolated anion vacancies and peaks, in KCl, at 177mp[ll, 121.
hole centres giving rise to some kind of interstitial not present before the illumination. Therefore, the decay of the (Y band which occurs when the sample is warmed up, may be due to the migration of these interstitials. Though we can not definitely rule out this possibility, we shall see below that the decay of the (Yband produced as described above is due, at least partially, to the migration of anion vacancies. The thermal decay of the (Yband has been studied by a pulse annealing method: the sample was warmed up to a temperature higher than 150”K, kept at this temperature for 15 min and then cooled down to LNT for the optical measurement. The results obtained are reported in Fig. 1 and show that anion
3
Fig. 1. Thermal decay of wzentres produced by optical bleaching of F-centres in X-rayed KC1 samples.
vacancies produced by ionization of F-centres in X-rayed samples are stable up to 200% and are totally destroyed if the sample is warmed up to 250°K. From Fig. 1 it appears also that the presence of monovalent (LP) or divalent (Sr++) impurities does not affect the thermal decay of cw-centres. The proof that the thermal decay of a-centres produced by ionization of F-centres in X-rayed samples is due, at least partially,
COMPLEX COLOUR CENTRES
to their own migration, is given by the following experiment. a-centres are produced as described above in a lithium doped KC1 sample. In Fig. 2(a) we have reported the
I
*i5
E
(*VI
2
1 0 -1 -.2
500
550
X (mp)
6”
650
Fig. 2. (a) change in the visible region of the absorption spectrum produced by an (F + F’)-illumination carried out at 15WK on a lithium dooed (lO-p mol) KC1 samnle X-rayed at LNT and warmed up‘to RT. (b) changes in the visible region of the absorption spectrum due to the warming to 250°K of the sample of Fig. 2(a). (c) changes in the visible region of the absorption spectrum produced by an (F + F’)-illumination carried out at 150°K on the sample of Fig. 2(b).
changes in the visible region of the absorption spectrum due to the (F + F’)-illumination at 15CPK: the only observed change is a decrease of the F band. The changes due to the warming up which follows the (F+ F’)illumination at 150°K are shown in Fig. 2(b): here again the only observed change is due to a decrease of the F band. At this point the sample is illuminated again at 150°K with
IN KCI
219
F-light and subsequently with F’-light*: the corresponding changes of the absorption spectrum are shown in Fig. 2(c). From this figure it appears that this illumination, unlike the first one (cf. Fig. 2(a)), not only decreases the F band but causes also the FA bands to grow. In order to explain the above results, we shall begin by emphasizing that the primary effect of the two illuminations is the same and consists in the production of free electrons. However, the final products of each illumination depend on the distribution of the free electrons among the available electron traps. The fact that the FA bands grow only under the second illumination (cf. Figs. 2(a) and 2(c)) shows that during the second illumination electron traps not present during the first one transform into F,-centres by capturing an electron. These electron traps must have been formed during the warming up which follows the first illumination and must be a,-centres (i.e. cu-centres with a lithium ion as a nearest neighbour). The formation of cr,-centres is possible only if anion vacancies migrate and join lithium ions. Therefore, we can conclude that the decay of the cr band which takes place during the warming up of the sample to 250°K (cf. Fig. 1) is due, at least partially, to the migration of anion vacancies. Before leaving this subject, it is worth stressing that: (a) the optical ionization of F-centres at 150°K is not accompanied by the migration of anion vacancies; (b) the migration of anion vacancies is not accompanied, in the experimental conditions described above, by a detectable production of free electrons. In fact, if points (a) and (b) were not true, the FA bands should have grown both during the first (F+ F’)-illumination and during the subsequent warming up of the *In this case the F’-illumination is not essential. However, since the F’ band produced by the F-illumination decreases under the action of the light used for the optical measurement, it is convenient to destroy all the F’-centres before the measurement.
220
G. GIULIANI
sample. However, no detectable growth of bands is observed in either case (cf. Figs. 2(a) and 2(b)).
F,
3.2 Formation of Z,-centres In this section we shall see how Z,-centres can be formed in a strontium doped KC1 sample by sending anion vacancies and electrons separately through the crystal (cf. the end of Section 3.1). The procedure is the same as the one described in Section 3.1 for the production of FA-centres. Nevertheless, in order to make the exposition and the explanation of the results easier, the main steps of the procedure will be recalled here. A strontium doped KC1 sample is: (I) X-rayed at LNT and warmed up to RT; (II) illuminated with F-light and subsequently with F’-light at 150°K: as reported in Section 3.1 this illumination causes the F band to decrease and the (Yband to grow. Furthermore, as in the case of the lithium doped sample
E (e’U
2.5
500
(cf. Fig. 2(a)), the decrease of the F band is the only change produced in the visible region of the spectrum by the (F + F’)-illumination (cf. Fig. 3(a)); (III) warmed up to 250°K; as shown in Section 3.1, the warming causes the (Y band produced in step II to disappear. Furthermore, the disappearance of the cy band is due, at least partially, to the migration of anion vacancies (cf. Section 3.1); (IV) illuminated with F-light and subsequently with F’-light at 150°K. This illumination causes the F band to decrease and the Z1 band to grow (cf. Fig. 3(b)). As in the case of FA-centre formation (cf. Section 3.1), we must draw our attention to the (F+ F’)-illuminations carried out in steps II and IV. The illumination performed in step II does not give rise to the Z1 band (cf. Fig. 3(a)), while the one carried out in step IV does (cf. Fig. 3(b)). This implies that during the illumination carried out in step IV, electron traps not present during the illumination
550
2
600
650
A (my)
Fig. 3. (a) changes in the visible region of the absorption spectrum produced by an (F + F’)-illumination carried out at 150°K on a strontium doped ( 1e3 mol) KC1 sample X-rayed at LNT and warmed up to RT. (b) changes in the visible region of the absorption spectrum produced by an (F + F’)-illumination carried out at 15(PK on the sample of Fig. 3(a) after its warming up to 250°K.
COMPLEX
COLOUR CENTRES
performed in step II, transform into Z,centres by capturing an electron. These electron traps can be referred to as ionized Z,-centres or Z1+-centres and must have been formed during the migration of anion vacancies which occurs in step III. Therefore, since Z1+-centres form during the migration of anion vacancies (just in the same way as cYA-centres do; cf. Section 3.1) it is quite reasonable to conclude that Z1+-centres contain an anion vacancy as a component. As far as the entire structure of the Z,+centres is concerned, we can observe that two configurations are possible: (i) a vacancy pair and, (ii) a divalent impurity with an associated vacancy pair*. Accordingly, the Z,-centre will consist of a vacancy pair with a trapped electron or of a divalent impurity-cation vacancy complex with an associated F-centre. Possibility (i) seems unlikely since the capture of an electron by a vacancy pair is expected to end up in the formation of an F-centre and an isolated cation vacancy. On the other hand possibility (ii) is supported by the Z,-centre ENDOR spectrum obtained by Bushnell[l3]. In any case, it is worth emphasizing that the results reported in this section clearly show that the Z1-centre contains an F-centre as a componentt. 3.3 The Z,+-centre According to what has been said in Section 3.2, the Z1+-centre is very likely made up by a divalent impurity-cation vacancy complex with an associated anion vacancy. Therefore, it is expected to give rise to a localized exciton band. In this connection, let us consider the followi;tg results: (i) the thermal decay of the *That these are indeed the only two possible configurations for the Z1+-centre can easily be accounted for on the basis that Z,+-centres contain an anion vacancy as a component and form only in samples doped with divalent impurities. tVery recently, further evidence for a model of the Z,-centre including an F-centre has been obtained from Faraday rotation (14) and circular dichroismIl51 measurements.
IN KC1
221
CYband which occurs in step III of section 3.2 is accompanied by the growth of a band peaking at about 173 w (cf. Fig. 4); ii) the F-illumE (eV)
6.75
7
7.25
I
oc band
1 150 OK
I 214 OK , 230°K L 246 OK L 260°K
I
+ I_
185
‘
180
175
170
X (my)
Fig. 4. Thermal decay of the a band produced by optical bleaching of the F band at 150°K in a strontium doped (lo-* mol) KC1 sample. All these curves have been obtained by subtracting the absorption curve measured before the F + a conversion to the absorption curve measured after the warming up of the sample to the reported temperature. As evident from this figure, the decay of the a band is accompanied by the growth of a band peaking at about 173 nl&.
ination carried out in step IV of Section 3.2 causes not only the Z, band to grow (cf. Fig. 3(b)) but also the 173 w band to decrease (cf. Fig. 5); (iii) Z,-centres can be bleached by Z,-light at 200°K; the product of the bleaching is given by Z1+-centres as it is proved by the fact that Z,-centres can be reformed by a subsequent F-light illumination carried out at 150°K. Figure 6 shows that the z1 -4 z,+ conversion is accompanied by the growth of the 173 rnp band; (iv) the Z, band does not appear in step IVof Section 3.2 if the sample, in step III, is warmed up to 300°K
G. GTULIANI
222
for 30 min in order to destroy the 173 rnp band. It is clear that the above data, though only qualitative in nature, allows one to reasonably assign the 173 ~E.Lband to Z1+-centres.
E (eV) 6.75
7 f
7.25 r ,
4.
X (my) Fig. 5. (a) change of the absorption spectrum near the fundamental edge produced by an (F + F’)-illumination carried out at 150°K on a strontium doped (lo-smol) KC1 sample containing Z,+-centres; (b) change that would have been observed if the growth of the (Yband were the only effect of the illumination. This curve has been drawn by making use of the known shape of the (Yband and of the fact that the contribution to the absorntion of the 173 w band above 180 rnp is negligible (cf. Fig. 4); (c) difference between (a) and (b) showina that the 173 nq band decreases under the’ (Ft F’F illumination. E
(eV)
7.25
7 173 my
band
I
1
1
180
175 x
170
I
(m/i)
Fig. 6. Change of the absorption spectrum near the fundamental edge produced by a Z,-light illumination carried out at 200°K on a strontium doped (10-s mol) KC1 sample containing Z,-centres (light source: white light from a 500 W lamp filtered through a Kodak 89B wratten filter). The narrow band on the short wavelength side of the 173 m band is of unknown origin. However, it is worth reporting that it has been observed also in some lithium or strontium doped KC1 sample X-rayed at LNT, warmed up to RT and subjected to an (F + F’)illumination (KCI:LiCl) or to a white light illumination (KCI: SrC1.Jat 150°K.
FINAL COMMENTS
The results reported in this paper prove that the processes of formation of FA- and Z,centres take place through the migration of anion vacancies and the formation of cu,- and &+-centres respectively. There is experimental evidence that anion vacancies play the same role in the M-centre formation[l-41. Therefore, it should be possible to produce M-centres by the same procedure used here for the FA- and the Z,-centre production. However, it must be stressed that a necessary condition for this method to be applied to the production of M-centres (or other F-aggregate centres) is that the intermediate product of the process (i. e. the F,+-centre in the case of the M-centre formation) must be stable at least at temperatures just higher than the temperature (200°K; cf. Fig. 1) at which the anion vacancies begin to migrate. As reported in Section 3.3, Z1+-centres are unstable near RT*. In this connection it is worth emphasizing that, since Z,-centres seem to be the product of the thermal decomposition of Z,-centres [ 161, ionized Z,-centres may be one of the products of the thermal destruction of Zr+-centres. If this hypothesis is correct, it should be possible to produce &-centres in two steps by producing ionized Zz-centres which will then transform into Z,centres through electron capture. Measurements to check this possibility (which may give useful information about Z,-centre production and structure) are now in progress. author would like to thank Mr. G. Guala for having performed some of the measurements.
Acknowledgements-The
*Preliminary results on the thermal stability of Zr+centres have already been obtained[ll. More detailed measurements are now in progress in connection with the study of the Z,-centre production (see below).
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REFERENCES M. and SMOLUCHOW1. FARGE Y., LAMBERT SKI R., SolidState Commun. 4,333 (1966). P~VS. Sot. Jaoan 21. 2. ASAI K. and OKUDA A.. J. __ 2197(1966). 3. NAHUM J.,Phys. Rev. 158,814(1967). 4. HIRAI M. and SCOTT A. B., .I. them. Phys. 46, 2896 (1967). 5. HIiRtEL i-l. and LijTY F.,Z.Phys. 177,369(1964). 6. H;iRTEL H. and LtiTY F..Z. Phvs. 182.111(1964). I. MIEHER R. L., Phys. Rkv. t&t. 8, 362 (1962). 8. GIULIANI G., J. Phys. Radium, Paris 28, Suppl., C4-175 (1967). 9. GIULIANI G. and REGUZZONI E., Phys.
CENTRES
IN KCI
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Status solidi25,431(1968). 10. ONAKA
R., FUJITA
1. and FUKUDA
A., J.
phys. Sot. Japan 18, Suppl. II, 263 (1963). 11. ONAKA R. and FUJITA J., Phys. Rev. 119, 1597 (1960). G.. GIULIANI G. and LYNCH 12. CHIAROTTI D. W., I1 Nuouo hm. 17,989 (1960). 13. BUSHNELL J. C.,.Thesis. Urbana(1964). ii: PAUS H. and LUTY F., Phys. Rev. Lett. 20, 57 (1968). G. and LANGER H., Phys. Lett. 26A, 15. GEHRER 232 (1968). M., J. Phys. Chem. Solids 24, 16. KLEEFSTRA 1567 (1963).