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CENTRES COLORÉS ET
IMPURETÉSRADIATION-INDUCED DYNAMIC MOTION OF INTERSTITIAL HALOGEN IN ALKALI
HALIDES
N. Itoh, M. Saidoh
To cite this version:
N. Itoh, M. Saidoh. CENTRES COLORÉS ET IMPURETÉSRADIATION-INDUCED DYNAMIC
MOTION OF INTERSTITIAL HALOGEN IN ALKALI HALIDES. Journal de Physique Colloques,
1973, 34 (C9), pp.C9-101-C9-105. �10.1051/jphyscol:1973917�. �jpa-00215392�
CENTRES COLORES ET IMPURETES
RADIATION-INDUCED DYNAMIC MOTION OF INTERSTITIAL HALOGEN IN ALKALI HALIDES
N. I T O H and M. S A I D O H
Department of Nuclear Engineering, Nagoya University, Nagoya, Japan
Rbumk. - Les experiences rkcentes sur le mouvement dynamique de I'halogene interstitiel induit par des collisions ionisantes sont decrites dans les halogknures alcalins. A la lumiere des rksultats experirnentaux de I'influence de la temperature sur le volume d'interaction de I'halogene interstitiel avec les defauts, il est suggkre qu'un remplacenient sequentiel de I'halogene interstitiel a un trou dans une orbitale p perpendiculaire a la direction du mouvement. Un nouveau modele de formation du centre F est egalernent suggire.
Abstract. - Recent experiments on tlie dynamic motion of the interstitial halogen induced by ionizing collisions in alkali halides are reviewed. In view of the experimental results on the temperature dependence of the interaction volunle of the interstitial halogen with defects, it is suggested that a replacement sequence of the interstitial halogen has a hole in a p-orbital perpen- dicular to the direction of motion. A new model on the F ccnter forniation is also suggested.
1. Introduction. - Series of experimental know- ledges on the mechanism of tlie F center formation has been accumulated [I] and it is known that a Frenkel pair is formed associated with the de-exci- tation of a n exciton. Recent pulse radiolysis experi- ment by Kondo [2] has indicated tliat a pair of an F center and a n H center is formed primarily. It follows that a halogen atom, not a halogen ion, is ejected into an interstitial position by the de-excitation process of an exciton. Moreover it has been shown that the relaxation time of the F center formation is smaller than the life time of tlie singlet exciton [2]. It follows that the de-excitation process which produces the energetic interstitial atom is not tlie recombination of the relaxed singlet exciton nor of tlie relaxed triplet exciton. One should consider another channel of the exciton recombination which results in tlie ejection of an interstitial halogen.
Theoretical treatment of the motion of the energetic interstitial has been made by several authors [3], [4].
It is believed tliat the repl~tceinent sequence of an interstitial halogen in the lialogen lattice along
<
110>
causes the migration of energetic halogen from the site where it is generated. The experimental information concerning [lie replacement sequence, however, is limited.In order to have further insight into the F center formation process, it is of interest to clarify the nature of the motion of the energetic interstitial atom. In the present paper, we revicw recent studies made at our laboratory on the dynamic motion of rhe inter- stitial halogen. New models of the F center formation are suggested.
2. Interaction of the rcplacemerlt scqucnce with defects. - The motion of tlie replacement sequence could be studied through its interaction \\.it11 defects.
In pure crystal tlie energetic interstitial halogen may become a n H center a t the end of the range of the replacement sequence. If its motion is interrupted by the distortion caused by a Na' impurity, for example, the final products are a n HA center and an F center.
Saidoh et al. [5] have measured the formation of tlie F, H and H A centers in N a f -doped KBr. Figure I
FIG. I. - The ratio of the initial formation yield R I I of the H center divided by the resultant initial formation yield R,, of the H and H \ centers. as a function of Na+ concentration. The curves are calculated curves with the values of the interaction volume of the replacement sequence with a Nav impurity as
shown in the figure.
shows the ratio of the H center concentration to the s u ~ n of the concentrations of the H and H , centers produced in Nn+-doped KBr, as a function of tlie Na' concentration at liquid lieliuni tempcrature. Assuming tli:lt at least one N a f impurity is within a given volume along the range of the replacement sequence.
one obtains the lines shown in the figure. With the best fit, the interaction volume of an energetic inter- stitial halogen with a sodium impurity is obtained to be 150 times tlie volume of an ion pair.
In order to e s t c n ~ l the rnei~s~~rernents of tlic inter-
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1973917
C9-102 N. ITOH AND M. SAIDOH
action volume to higher temperature, one should take the thermal migration of tlie H center into conside- ration. The rate constant (l/r) of the thermal annihila- tion of the H center obtained by several authors is put together in figure 2. The activation energy for
TEMPERATURE (OK)
FIG. 2. - Temperature dependence of the reciprocal of the relaxation time of the H center annihilation in KBr. Data points shown by open circles are obtained by M. Ueta : J.
Phys. Soc. Japotz 23 (1967) 1256, those shown by closed circles by M. Saidoh er 01. (ref. [6]) and those shown by triangles by M. Saidoh and N. ltoh : J. P / i ~ ~ s . C/ir,t?~. Solicls 34 (1 973) 1 165.
the thermal annihilation is 0.09 eV, in accordance with previous observation. The interaction volume between the energetic interstitial atom and the Na' impurity should be obtained within r . Such experiments were made by means of the pulse radiolysis tech- nique [6], using a Febetron 707, operated at 2 MeV, 1 0 0 0 A and 20 ns. The concentration of the H and H A centers 200 ns after the initiation of the electron pulse was measured, between 80 K and 200 K. The interaction volume was obtained as a function of temperature, and the result is shown in figure 3. It is clear that the interaction volun~e decreases as the temperature decreases. with the activation energy of 0.025 eV. In the figure the value obtained at liquid helium temperature is also shown. This result indi- cates that the interstitial atom at the end of the replacement sequence undergoes thermal migration.
The activation energy of tlie thermal migration of
TEMPERATURE ( OK)
FIG. 3. - Temperature dependence of the interaction volunle of the replacement sequence with a Na+ impurity, measured with pulse radiolysis experiments (open circle) and with steady
irradiation experiments (closed circle).
the interstitial atom at the end of the replacement sequence is smaller than the activation energy of the thermal migration of the H center by a factor of 4.
The interaction volume of the replacement sequence with a n H center was measured through the forma- tion of the V, center, the di-H center. It is observed that the ratio of the formation efficiency of tlie V, cen- ter 200 ns after the initiation of the pulse increases with increasing temperature. The activation energy for the increase in the interaction volume in this case agrees with the value obtained from tlie interaction with the N a f impurity.
The interaction volume may be expressed as
where s and 1 are the area of the interaction and tlie range of the replacement sequence, respectively. E is the activation energy obtained as 0.025 eV. lo is the range of the replacement sequence at liquid helium temper;iture and has been considered to be about 7 times the lattice constant [5]. Using the interaction voluliie obtained at liquid helium temperature, .Y is about 20 times the area of the unit square of NaCI lattice. Using this value of s, I, is obtained as
1.4 x
lo3
from figure 3.In view of the present experimental results of the Increase in the interaction volume with temperature.
one can explain the temperature dependence of the
RADIATION-INDUCED DYNAMIC MOTION OF INTERSTITIAL HALOGEN IN ALKALI HALIDES C9-103
F center formation by steady irradiation, comnlonly observed [I] in alkali halides, such as KC1 and KBr.
The formation yield of the F center decreases from highest value a t liquid helium temperature to the lowest value around 30 K. The yield begins to increase again as the temperature increases from 100 K. Recent pulse radiolysis experiment by Hirai [7]
indicates that the decrease is due to the annihilation of the close pairs of an interstitial halogen atom and an F center. The increase of the F center formation rate above 100 K has been ascribed to the increase in the primary defect production efliciency [8]. One can consider that close pairs with a separation smaller than a critical value I.,, :ire annihilated above 30 K i n the case of steady irradiation. Since the range of the replacement sequence increases with tcmpcr:~ture as shown by eq. ( I ) , the probability that the path length of the replacement sequence is larger than I., would increase. Therefore the escape probability of the interstitial halogen atom from the vacancy increases with increasing temperature. The escape probability may be expressed as e-'"". Assuming tllat the forma- tion efficiency of F centers is proportional to the escape probability 11, the tempel-atul-e dependence of the experimental formation eficiency between 120 K and 180 K was fitted to
The best fit was obtained with /,/I., = 0.4, / , / I . , = 130.
Tile ratio I,//, shows a good agreement with the value obta~ned from figure 3. The value of r , is deduced to be 17 times the lattice constant by assuming the value of I, described above. There is uncertainty in the values of I, and I.,, however, since the length can not be determined separately from the area of inter- action [ 5 ] .
3 . The structure of the replacement sequence. - The experimental results discussed in the preceding parngrapli indicate that the interstitial halogen at the end of the replacement sequence migrates with an activation energy which is milch smaller than that of the thermal migration of the H center. It follows that the interstitial halogen :it tllc end of the repla- cement sequence is either a n H center in a n excited state or another interstitial atom center than the H center. A possible conlipration of the interstitial atom center would be that the interstitial atom is situated at the body centcr of the simple cube. This configuration would have ;I liigher lattice energy than the H center configuration. and i t is unprobablc that the
<
I I0 >-oriented replacement scquencc takes tile body cenler configuration at its range end. Therefore it is concluded that the H center at the end of the replacement sequence is at the evcited state. I t is very likely that the Id center i n ttir / I , htate requires an activation energy to jump into the nearest pohition.Since crowdion d o not acquire the energy during its motion, the conclusion derived above indicates that the interstitial atom is not electronically at the ground state and possibly a t the
17,
state during repla- cement sequence. It is of interest to consider the motion of the crowdion of this type, which is referred as I7-crowdion, in detail. The IT-crowdion has interesting features that it has a large effective radius alons the direction of motion and it has a small r~idius perpen- dicular to the direction of nlotiotl. One could approsi- mate the radius parallel to the direction of the motion by the ion radius of BI.- and the radius perpendicular to the direction of motion by the covalent radius of Br". Srnolucho\\~ski rr 01. [3] have c~~lcul:~tcd the energy for the migration of the replacement sequence which has thc same configuration as the H center- ground state. The value 0.2 eV they obtained Ibr KC1 agrees well with the activation energy of the thermal migration of the H center. Thc energy for the mig1.a- tion of the 17-crowdion is considered to be much lower, since the center is more closed packed along the<
110>
direction, and has smaller repulsivc poten- tial to the direction perpendicular to its motion. The energy for the nligration of [he U-crowdion \\,auld be of the same order as the thermal migration enersy of the interstitial halogen center at the end of the replacement sequence.4. A model of the F center formation. - I n this section the process to impart a n kinetic energy to the I7-crowdion is discussed. The description is made referring to KCI, as an example. Figure 4 illustrates a possible process of the formation of the I7-crowdion.
A chlorine atom is excited into a P-state, which is composed of a hole in a p-orbital of chlorine and a n
I-.I(;. 4. -- - A possible formation mechanism of a n F centel- I>y ionizing radiation. r r ) shows an exciton hefore relaxation, 1)) sho\+s a n exciton after relaxation, an electron trapped by a CI1 molec~~le-ion. I n (,) the CI, ~nolecule-ion is deviated \lightly and the electron is m o w populated in thc h e l l from \vliicli t!lc C l i molcculc-ion is 1.cn1ovcd. r l ) is the conlig~~ration ill \\liich
the ClT is p~i5lied into one vacant .;itc.
C9-104 N. ITOH AND M. SAIDOH electron in an s-like orbital in the potential formed
by a negative ion vacancy and a neutral chlorine atom, as described by a). The following step is the relaxation of the hole into a C1; molecule ion either in a C,, l7, o r h', states. This process has been dis- cussed by Kabler [9] and by Wood [lo]. After relaxa- tion, the potential for the electron would have a form of a double well potential formed by two halogen- vacancy and by two C1-'I2 ions lying in each vacancy potential well. This configuration is described by b) in figure 4. The total energy of this configuration is not much lower than the energy of the forbidden gap and would be about 10 eV.
The C1; molecule-ion has a negative charge and if the movement of this negative charge is not restric- tive, the configuration shown by figure 4b can not be stable. A slight deviation of the CI; along the
<
110>
direction would niake one of the double well potential deeper than the other and the electron wave function would be populated more in the poten- tial well from which the C1; is removed, as des- cribed by c). Thus the total energy would be lowered by the movement of CI; in this case. In reality, however, the repulsive potential between C1; and K + would keep the C l i at the configuration b). It is the case when the V, center has an electron configuration of 1, and the luminescence emitted by the recombi- nation from the singlet and triplet states of an elec- tron plus a V, center in tlie I,, state has been observed.It is suggested that if the C1, has a ITz configura- tion the motion of the C12 along
<
1 10>
is lessrestrictive. The repulsive interaction between K f and C1; against the movement along
<
110>
is consi- dered to be much lower, when the CIT has a hole in the p-orbital in the (100) plane, than when it is in theZ,,
state. Simple calculation, based on the radius of the orbitals described before, indicates that the repulsive force between a CI- ion and two K f ions at the configuration when the CI- ion lies between two K + ions is 4.6 eV and the repulsive energy at the same configi~ration is reduced to 0.8 eV if one of the electrons in a R-orbital is removed. One could expect that the resultant potential near the equilibrium position of C1, in a R g state for its niotion along the<
110>
direction is rather flat.Figure 5 describes the configuration coordinates curves for the (V,
+
e) states. After relaxation of an exciton into (V,(I,,)+
e):::, where ::: denotes the excited state, the system is stable against the devia- tion of the V, center along its molecula~- direction. On the other hand, when the exciton relaxes into (V,((7,)+
e)*, the change in the niolecular distance in CI, is smaller and potential curve against the devia- tion of C1, along the molecular axis would have a form shown in the figure. If the deviation of the CI, exceeds the saddle point configuration, the potential curve would be a downhill. The total energy at theFIG. 5. - Configuration coordinates curves for the relaxation of an exciton. Along the coordinates of CI-CI distance, the exciton is relaxed into
'z:,
or Up states. The potential for the deviation of the Cl; molecule-ion as a whole is shown in the other plane.'c:
and 3 ~ : states arestable and recornbi- nation luminescence is emitted. The 17E state would result in a replacenlent sequence with the kinetic energy Ek and final pl.oducts would be a pair of well-separated F and H centers withformation energy El..
configuration of figure 4d may be the summation of the forn~ation energy of an F center and an H centel minus the interaction energy between the two centers.
Since the energy required to form an F center and an H center is 5.0 eV according to Smoluchowsk~
ct crl. [3], the CI; would acquire the kinetic energ) at least 5 eV. This energy is sufficient to separate tht interstitial halogen atom from the F center. In tht other model suggested previously, tlie kinetic energ) imparted to the interstitial halogen is much smaller.
Further theoretical studies of the configuration coordinates curve and of tlie migration of the L7-crow- dion would be of interest. It is interesting to note that the present model describes recent experimental results that the F center is the priniary product and that the H center is ;it ill1 excited state after termina- tion of the replacement sequence. This lnodel is consistent with the conclusion that the F center formation is not followed by the I-ecombination of the singlet nor triplet exciton. Tile present model would not be applicable to alkali halides containing halogen with larger radius, such as KI, for which Pooley's model may be applicable.
RADIATION-INDUCED DYNAMIC MOTION O F INTERSTITIAL HALOGEN IN ALKALI HALIDES C9-105
References
[I] SONDER, E. and SIBLEY, W. A., Point Defects in Solids vol. 1 ed. J. H. Crawford, Jr. and L. M. Slifkin, p. 201 (Plenum Press, New York), 1972.
[2] KONDO, Y., HIRAI, M. and UETA, M., J. Phys. Soc. Japan 33 (1972) 151.
[3] SMOLUCHOWSKI, R., LAZARETH, 0. W., HATCHER, R. D.
and DIENES, G. J., Phys. Rev. Lett. 27 (1971) 1288.
[4] POOLEY, D., Proc. Phys. Soc. 87 (1966) 257.
[ 5 ] SAIDOH, M. and ITOH,N., J. Phys. SOC. Japan. 29 (1970) 156.
[6] SAIDOH, M., HOSHI, J. and ITOH, N., Solid State Cornm~n~., 13 (1 973) 43 1.
[7] KARASAWA, T. and HIRAI, M., J. Phys. Soc. Japan 33 (1972) 1728.
[8] SONDER, E., Phys. Rev. B 5 (1972) 3259.
[9] KABLER, M. N., Phys. Rev. 136 (1964) 1296.
1101 WOOD, R. F., Phys. Rev. 151 (1966) 629.
DISCUSSION
J.
M. SPAETH. - We have observed in experiments explained by the just proposed model by Itoh, that of bleaching U, centres in KC1 with polarised light the p-type hole associated with the C1° is perpendi- the formation of dichroic H centres (see also abstract cular to the migration direction of the CIO.156). The dichroism observed could possibly be
