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Temperature dependence of F-centre production in KCl
Solid State Communications Vol. 4, pp. 537-540, 1966. Pergamon Press Ltd. Printed in Great Britain.
TEMPERATURE DEPENDENCE OF F-CENTRE PRODUCTION IN KC1 J. D. Comins and P. T. Wedepohl Department of Physics, University of the Witwatersrand Johannesburg, South Africa (Received 25 August 1966 by F.R. L. Schöntng)
The rate of production of F-centre by X-irradiation of KC1 has been measured as a function of temperature. The rate increases sharply at about 175 °K. Re-irradiation of bleached KC1 samples shows that vacancies are destroyed during bleaching when the initial irradiation is performed at 77°K,but not when it is performed at 298°K.
TEMPERATURE is an important variable in investigations of defect production by ionizing radiation in the ailcali halides. The rate at which defects are produced may be strongly temperature-dependent (see, for example, work on the temperature dependence of F-centre different types of defect production in Kr1),at and may be produced different temperatures 2~
-
-
:z This paper reports: (a) the temperature dependence of F-centre production in KC1;
-
(b) the effect of optical bleaching with F-band light at room temperature on the defects formed in KC1 by irradiation at room and lower temperatures.
iC
-
-
(a) Freshly cleaved KC1 crystais obtained from the Harshaw Chemical Company were irradiated with X-rays at various temperatures between 77 and 300°K. The X-ray tube (tungsten target) was operated at 41 kVp and 2OmA, the X-rays being filtered through 0. 13mm Al and 1mm Si0 3 to promote uniform colouration ~ crystals which were 8cm from the target. The F-centre concentration was plotted as a function of irradiation time. No attempt was made to correct for F ‘-centre concentration. However, this correction will not alter the broad features of the results, which will be suitably corrected In a future publication. The rate of F-centre formation (dF/dt) was determined from the growth curves at an 537
C
-
-
2
-
~o
100
140
1~O
220 250 3C!X) TEMPERATURE (K)
FIG. 1. The variation of F-centre production rate with temperature; measured at an F-centre concentration of 10 ~‘ cm
538
TEMPERATURE DEPENDENCE OF F-CENTRE PRODUCTION IN KC1
each F-centre 10’~in cmthe3in case, thisconcentration concentrationofbeing intrinsic (“slow-stage”) region of colouration. The results are shown In Fig. 1. It can be seen that (dF/dt) increases rapidly near 175°K,rises to a maximum at about 215°K,and decreases at higher temperatures. The temperature dependent F-centre growth between 165 and 220°Kis not predicted by the defect production mechanism of Pooley ~, which successfully predicts a similar effect in Xi’ It should be emphasized, however, that the effect in Kr is very much larger. (dF/dt) increases by a factor of about 1000 in Xi, compared with our factor of about 3 for KC1 Thus refinements in Pooley’s theory may be necessary to account for the KC1 results. A feature of the Kr experiments is that the temperatures region over which (dF/dt) increases rapidly is the same as that at which disorientation of the Vk (trapped-hole) centre occurs. ‘ ~It seems significant that this is also found here in KC1, the Vk disorientation temperature being 173°K. Thus hole mobility may play a significant role in F-centre production at higher temperatures. -
The decrease in (dF/dt) at temperatures above 220°Kcould be due to recombination between defects which become mobile above this tem~rature. For example, if one of the primary products of the thmage event were a negative ion vacancy. ~d this were mobile above 220°K, an (im’-~obi1e)F-centre would only be formed if ~& vacancy captured an electron before moving to an annihilation site. Alternatively, the decrease in (dF/dt) may be due to a mechanism of the type suggested for Kr by Hall et aL 1 (b) From e. s. r. work ~it is known that halogen interstitial defects are formed at 4°K. There is also evidence of their being formed at higher temperatures. The following experiments were performed with the object of studying the reaction of these “high temperature” interstitials with vacancy centres. Irradiations were performed as in (a) and growth curves of F-centres were plotted. Each crystal was warmed from the temperature of irradiation to 298°K, and then irradiated with light having a wavelength within the F-band. F-centre concentration was measured as a function of time during the bleaching in each case. On cessation of bleaching, each crystal
Vol.4, No.10
was re-cooled to Its original temperature of irradiation and then re-Irradiated. Again growth curves of F- centres were plotted. The results are shown In Figs. 2 and 3.
2.oxld
-
18 —
~ 12
i.c
-
C’6
-
u
\
\B
-
C
0~2-
A -
0
10
•
20
30 BLEACHING
40 TIME
50 (MINUTES)
FIG. 2. The decay of F-centre concentration during optical bleaching for crystals irradiated at 77°K(A),214°K(B)and 298°K C The results shown in Fig. 2 are similar to those of Sibley and Sonder ~, and show a much faster rate of bleaching for the crystal irradiated at 77 K (Si) than for that irradiated at 298°K(S2). Sonder and Sibley’s suggestion that the mechanism of bleaching is different in the two cases is further supported by observations on aggregate centre formation in our experiments. The total number of M and R centres formed, per F-centre destroyed was much higher for (S2) than for (Sl). The re-irradiation experiments (Fig 3) were performed to obtain more Information about the bleaching processes. In the case of (S2), it can be seen that the F-centre concentration recovered to its pre-bleach value very quickly and then followed an extrapolation of the original growth curve, the recovery being accompanied by the destruction of the M and R bands. Thus the predominant process during the bleach is F-centre aggregation, the aggregated centres being equal in number to the F-centres destroyed. For (Si),
Vol.4, No.10
TEMPERATURE DEPENDENCE OF F-CENTRE PRODUCTIONtN KC1
2Oxld’ ~
539
—
1•~ —
•I~ I~
A
IRRADIATION
TIME
(HOURS)
FIG. 3. The growth of F-centre concentration with irradiation dose, in crystals irradiated at different temperatures. The effect of bleaching at room temperature and subsequent re-irradiation at the original temperature is shown in each case. Irradiation temperatures were 77°K (A), 214°K(B) and 298°K(C).
however, the F-centre growth curve after bleaching was very similar to that during the first irradiation. Thus the bleachi~~ urocess here is quite different, vacancies being destroyed in the process. Crystals irradiated at temperatures between 77 and 298°K showed evidence for both mechanisms, the (S2) mechanism becoming more predominant as the temperature of irradiation was raised.
A plausible explanation of the results is that the (Si) mechanism is due to the mutual destruction of interstitials and vacancies. A preliminary study of the (Si) bleaching kinetics indicates that these are more nearly second-order than firstorder. If interstitials are formed at room temperature, as the radiation-induced hardening experiments ~ ~ suggest, then they might be in a form which cannot easily recombine with F-centres.
Acknowledgement- One of the authors, J. D. Comins, would like to thank the Council for Scientific and Industrial Research, Pretoria, for their support in this work.
540
TEMPERATURE DEPENDENCE OF F-CENTRE PRODUCTIONtN KC1
Vol.4, No.10
References 1.
HALL T. P. P.,
POOLEY D. and WEDEPOHL P. T. Proc. Phys. Soc. 83, 635 (1964).
2.
DORENDORFH., Z. Phys. 128,
3.
POOLEY D., Proc. Phys. Soc. 87, 245 (1966).
4.
DELBECQ C.J., HAYES W. and YUSTER P. H., Phys. Rev.
5.
KA.NZIG W. and WOODRUFF T. 0., J. Phys. Chem. Solids 9, 70 (1958)
6.
NADEAU J. S., J. Appi. Phys. 35, 1248 (1964); 33, 3840 (1962).
7.
SIBLEY W.A. and SONDER E., J. Appi. Phys. 34
8.
SIBLEYW.A. andSONDERE., Phys. Rev. Letters 14 900 (1965)
166 (1950).
121, 1043 (1961).
Die Geschwindigkeit der Bildung von F-Zentren die durch R6ntgenbestrahlung von KC1 erzeugt werden, ist ais Funktion der Temperatur gemessen worden. Diese Geschwindigkeit nimmt bei einer Temperatur von etwa I 75°Kpl~iz1ichzu. Wiederbestrahiung von gebleightem KC1 zeigt, dass Gitterleerstellen wãhrend des Bleichungsprozesses vernichtet werden wenn die erste Bestrahlung bet einer Temperatur von 77°KausgefiIhrt wurde. Das ist nicht der Fall wenn die erste Bestrahiungs-temperatur 298°Kwar.
TEMPERATURE DEPENDENCE OF F-CENTRE PRODUCTION IN KC1 J. D. Comins and P. T. Wedepohl Department of Physics, University of the Witwatersrand Johannesburg, South Africa (Received 25 August 1966 by F.R. L. Schöntng)
The rate of production of F-centre by X-irradiation of KC1 has been measured as a function of temperature. The rate increases sharply at about 175 °K. Re-irradiation of bleached KC1 samples shows that vacancies are destroyed during bleaching when the initial irradiation is performed at 77°K,but not when it is performed at 298°K.
TEMPERATURE is an important variable in investigations of defect production by ionizing radiation in the ailcali halides. The rate at which defects are produced may be strongly temperature-dependent (see, for example, work on the temperature dependence of F-centre different types of defect production in Kr1),at and may be produced different temperatures 2~
-
-
:z This paper reports: (a) the temperature dependence of F-centre production in KC1;
-
(b) the effect of optical bleaching with F-band light at room temperature on the defects formed in KC1 by irradiation at room and lower temperatures.
iC
-
-
(a) Freshly cleaved KC1 crystais obtained from the Harshaw Chemical Company were irradiated with X-rays at various temperatures between 77 and 300°K. The X-ray tube (tungsten target) was operated at 41 kVp and 2OmA, the X-rays being filtered through 0. 13mm Al and 1mm Si0 3 to promote uniform colouration ~ crystals which were 8cm from the target. The F-centre concentration was plotted as a function of irradiation time. No attempt was made to correct for F ‘-centre concentration. However, this correction will not alter the broad features of the results, which will be suitably corrected In a future publication. The rate of F-centre formation (dF/dt) was determined from the growth curves at an 537
C
-
-
2
-
~o
100
140
1~O
220 250 3C!X) TEMPERATURE (K)
FIG. 1. The variation of F-centre production rate with temperature; measured at an F-centre concentration of 10 ~‘ cm
538
TEMPERATURE DEPENDENCE OF F-CENTRE PRODUCTION IN KC1
each F-centre 10’~in cmthe3in case, thisconcentration concentrationofbeing intrinsic (“slow-stage”) region of colouration. The results are shown In Fig. 1. It can be seen that (dF/dt) increases rapidly near 175°K,rises to a maximum at about 215°K,and decreases at higher temperatures. The temperature dependent F-centre growth between 165 and 220°Kis not predicted by the defect production mechanism of Pooley ~, which successfully predicts a similar effect in Xi’ It should be emphasized, however, that the effect in Kr is very much larger. (dF/dt) increases by a factor of about 1000 in Xi, compared with our factor of about 3 for KC1 Thus refinements in Pooley’s theory may be necessary to account for the KC1 results. A feature of the Kr experiments is that the temperatures region over which (dF/dt) increases rapidly is the same as that at which disorientation of the Vk (trapped-hole) centre occurs. ‘ ~It seems significant that this is also found here in KC1, the Vk disorientation temperature being 173°K. Thus hole mobility may play a significant role in F-centre production at higher temperatures. -
The decrease in (dF/dt) at temperatures above 220°Kcould be due to recombination between defects which become mobile above this tem~rature. For example, if one of the primary products of the thmage event were a negative ion vacancy. ~d this were mobile above 220°K, an (im’-~obi1e)F-centre would only be formed if ~& vacancy captured an electron before moving to an annihilation site. Alternatively, the decrease in (dF/dt) may be due to a mechanism of the type suggested for Kr by Hall et aL 1 (b) From e. s. r. work ~it is known that halogen interstitial defects are formed at 4°K. There is also evidence of their being formed at higher temperatures. The following experiments were performed with the object of studying the reaction of these “high temperature” interstitials with vacancy centres. Irradiations were performed as in (a) and growth curves of F-centres were plotted. Each crystal was warmed from the temperature of irradiation to 298°K, and then irradiated with light having a wavelength within the F-band. F-centre concentration was measured as a function of time during the bleaching in each case. On cessation of bleaching, each crystal
Vol.4, No.10
was re-cooled to Its original temperature of irradiation and then re-Irradiated. Again growth curves of F- centres were plotted. The results are shown In Figs. 2 and 3.
2.oxld
-
18 —
~ 12
i.c
-
C’6
-
u
\
\B
-
C
0~2-
A -
0
10
•
20
30 BLEACHING
40 TIME
50 (MINUTES)
FIG. 2. The decay of F-centre concentration during optical bleaching for crystals irradiated at 77°K(A),214°K(B)and 298°K C The results shown in Fig. 2 are similar to those of Sibley and Sonder ~, and show a much faster rate of bleaching for the crystal irradiated at 77 K (Si) than for that irradiated at 298°K(S2). Sonder and Sibley’s suggestion that the mechanism of bleaching is different in the two cases is further supported by observations on aggregate centre formation in our experiments. The total number of M and R centres formed, per F-centre destroyed was much higher for (S2) than for (Sl). The re-irradiation experiments (Fig 3) were performed to obtain more Information about the bleaching processes. In the case of (S2), it can be seen that the F-centre concentration recovered to its pre-bleach value very quickly and then followed an extrapolation of the original growth curve, the recovery being accompanied by the destruction of the M and R bands. Thus the predominant process during the bleach is F-centre aggregation, the aggregated centres being equal in number to the F-centres destroyed. For (Si),
Vol.4, No.10
TEMPERATURE DEPENDENCE OF F-CENTRE PRODUCTIONtN KC1
2Oxld’ ~
539
—
1•~ —
•I~ I~
A
IRRADIATION
TIME
(HOURS)
FIG. 3. The growth of F-centre concentration with irradiation dose, in crystals irradiated at different temperatures. The effect of bleaching at room temperature and subsequent re-irradiation at the original temperature is shown in each case. Irradiation temperatures were 77°K (A), 214°K(B) and 298°K(C).
however, the F-centre growth curve after bleaching was very similar to that during the first irradiation. Thus the bleachi~~ urocess here is quite different, vacancies being destroyed in the process. Crystals irradiated at temperatures between 77 and 298°K showed evidence for both mechanisms, the (S2) mechanism becoming more predominant as the temperature of irradiation was raised.
A plausible explanation of the results is that the (Si) mechanism is due to the mutual destruction of interstitials and vacancies. A preliminary study of the (Si) bleaching kinetics indicates that these are more nearly second-order than firstorder. If interstitials are formed at room temperature, as the radiation-induced hardening experiments ~ ~ suggest, then they might be in a form which cannot easily recombine with F-centres.
Acknowledgement- One of the authors, J. D. Comins, would like to thank the Council for Scientific and Industrial Research, Pretoria, for their support in this work.
540
TEMPERATURE DEPENDENCE OF F-CENTRE PRODUCTIONtN KC1
Vol.4, No.10
References 1.
HALL T. P. P.,
POOLEY D. and WEDEPOHL P. T. Proc. Phys. Soc. 83, 635 (1964).
2.
DORENDORFH., Z. Phys. 128,
3.
POOLEY D., Proc. Phys. Soc. 87, 245 (1966).
4.
DELBECQ C.J., HAYES W. and YUSTER P. H., Phys. Rev.
5.
KA.NZIG W. and WOODRUFF T. 0., J. Phys. Chem. Solids 9, 70 (1958)
6.
NADEAU J. S., J. Appi. Phys. 35, 1248 (1964); 33, 3840 (1962).
7.
SIBLEY W.A. and SONDER E., J. Appi. Phys. 34
8.
SIBLEYW.A. andSONDERE., Phys. Rev. Letters 14 900 (1965)
166 (1950).
121, 1043 (1961).
Die Geschwindigkeit der Bildung von F-Zentren die durch R6ntgenbestrahlung von KC1 erzeugt werden, ist ais Funktion der Temperatur gemessen worden. Diese Geschwindigkeit nimmt bei einer Temperatur von etwa I 75°Kpl~iz1ichzu. Wiederbestrahiung von gebleightem KC1 zeigt, dass Gitterleerstellen wãhrend des Bleichungsprozesses vernichtet werden wenn die erste Bestrahlung bet einer Temperatur von 77°KausgefiIhrt wurde. Das ist nicht der Fall wenn die erste Bestrahiungs-temperatur 298°Kwar.