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ELECTRICAL EFFECTS IN ADDITIVELY COLORED KCl CRYSTALS. INFLUENCE OF COLLOIDS

Denys Durand, G. Chassagne, J. Serughetti

To cite this version:

Denys Durand, G. Chassagne, J. Serughetti. ELECTRICAL EFFECTS IN ADDITIVELY COLORED

KCl CRYSTALS. INFLUENCE OF COLLOIDS. Journal de Physique Colloques, 1973, 34 (C9), pp.C9-

465-C9-469. �10.1051/jphyscol:1973976�. �jpa-00215452�

(2)

JOURNAL DE PHYSIQUE Colloque C9, s u p p l k m ~ t ~ ! au I I O 11-12, Tome 34, Novenzbre-Dhcenzbre 1973, page C9-465

ELECTRICAL EFFECTS IN ADDITIVELY COLORED KC1 CRYSTALS.

INFLUENCE OF COLLOIDS

D.

D U R A N D ,

G.

C H A S S A G N E and

J.

S E R U G H E T T I

Dtpartement de Physique des Materiaux ("), Universitt Claude-BernardILyon

I,

43, boulevard du

I

1-Novembre-1918, 69-Villeurbanne, France

R6surn6. - De 470 a 873 K, la conductivite de cristaux de KC1 de haute purete, colorks addi- tivement, est plus elevee que celle des cristaux non colores. A partir de la theorie des semi-conduc- teurs, on peut expliquer cette conductivite a I'aide de la relation :

-

-

ZITI* kT)'l4 EF

5 = j l e JII~.,

(T

- ~- exp - -

11 2 2 k T

oh p est la mobilite de I'electron, III., le ~iombre de centres F par cn1'. On trouve une energie d'ioni- sation du centre F, El., de 2,08 eV. Le facteur pre-exponentiel est en bon accord avec les resultats experimentaux. La relation (1) indique que la concentration en lacunes anioniques est determinee par le nombre de centres F ionises et non par un Cquifibre de Schottky. On pourrait expliquer les ecarts a la relation precedente par la presence d'un exces de lacunes anioniques.

La representation de log rr en fonction de IOjlT fait apparaitre un phknoniene de precipitation quand la concentration en centres F est superieure a IOl'/cn~s. Ceci est lie a I'apparition de collol- des observks par spectrophotonietrie. Dans le doniaine 470-630 K, il y a apparition de colloi'des et une diminution relative de la conductivite. Dans la gamme 630-750 K, les collo'ides disparaissent et la conductivite augniente a nouveau. Neannioins, la conductivite obeit:toujours a la relation (1).

Ceci semble indiquer q~l'il n'y a pas d'kmission tliernio-ionique due aux colloi'des dans nos expe- riences.

Abstract. - From 470 K to 873 K the conductivity of additively colored, high purity KC1 crystals is found to be greater than that of uncolored crystals. From semiconductor theory it is possible to explain this conductivity using the relation :

n ~ l ~ * k ~ ) 314 El.

rr = p e J I I ~ (l-- - exp -

11 2 k T

where j~ is the electron mobility, 111: is the F center concentration. The F center ionization energy EF is found to be 2.08 eV. The preexponential factor agrees well with the measured values. It follows from (I) that the anion \.acancy concentration is determined by the number of ionized F centers and not by a Schottky equilibri~~ni. The departures to the above relation could beexplained by an excess of anion vacancies.

Plots of log rr vs 1 000/T show a precipitation phenonienon when tlie F center concentration exceeds 1017/cm3. This is related to the appearance of colloids observed by optical measurement.

Over the range from 470 K to 630 K there is an appearance of colloids and a relative decrease of conductivity. Over tlie range from 630 K to 750 K there is a desappearance of colloids and a relative increase of conductivity. Nevertheless, tlie conductivity follows always the relation (1) during this process. This seems indicative of no therniionic enlission due to colloids in these experiments.

1.

Introduction. - The electrical conductivity of additively colored alkali halides and the mobility of

F

centers have been the object of many studies [I]-

[I I].

Recent works o n electrical conductivity [6]-[9]

in additively colored KC1 crystals containing ( o r not) a colloidal band shown that there exist divergent interpretations of the origin of the carriers and of the activation energy of the electrical conduction process.

Jain and Sootha [6], [7],

[8]

assume that the conduc- tivity increase of add~tively colored KC1 cryst:~ls

(*) Associe au CNRS

observed between 30O0-450 OC is due only t o a therrn- ionic emission of electrons from

K

colloid particles into the conduction band of the crystal. Contribution to the electrical conductivity by the ionized

F

centers is found to be nezligeable

in

this temperature range.

They deduce from their measurements a n electron aflinity of

0.32,

eV for KCI crystals. The! assurnr t h ~ t the \vork function for tliermionic c.niission t'rc.rn .I

colloid has the hulk metal valuc ( 2 . 2 6 c'\'!.

On the other Iii~nd Servers x n ~ l Sc<)rt [ " j irl us.ih!>

additi\cl> colc?!.ed KCl ci-jstals. \ ~ i l t ~ ~ > ~ r t ~*ol!,~i~8.~U banci. t.or~nd a n incrcaxe ot' t ' > t ' cnraiiu~.ti\ir? b ~ . r i \ i . ~ n ~

150 "C-550

"C.

I'l-orn the

F

ccnrz: tran>pora marrrik-i.:-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1973976

(3)

C9-466 D. DURAND, G. CHASSAGNE AND J. SERUGHETTI

measurement (0.96 at 400

O C )

they conclude that the

conductivity increase is due to ionized F center.

The thermal ionization energy of the F center is found to be 2.04 eV.

In this work, we have studied the electrical conduc- tivity of additively colored ultra pure crystal over the regions of both extrinsic and intrinsic ionic conduc- tivity. The F center concentration is varied for studing the conductivity without and with colloids.

2.

Experimental procedure. -

The KC1 crystals used in these experiments are zone melting refined using Voszka's method [I21

(*).

The samples (20 x 20 x 8 mm3) are additively colored by a stoichiometric excess of potassium between 500

OC-

700

OC

employing Van Doorn's technique [13]. In order to ensure uniformity of the F center concen- tration, the samples are maintained at the coloration temperature for 24 h. Then, the crystals are rapidly quenched in an argon jet. The absence of colloids after quenching has been verified by electron-micro- scopy observation of replicas [I41 and by spectropho- tometry. The spectra are recorded at room temperature using a Beckman DK 2 A recording type spectro- photometer. The F center concentration is determined using the Smakula equation in the form

:

where

cr,

is the absorbance per cm at the maximum of the F band. Our samples contained between

1016-1.2 x 10" F centers per cm3.

The conductivity is measured between 100- 100 000 Hz using a General Radio Type 1621 impe- dance bridge, over the temperature range from 470 K to 873 K. Specimens are cleaved (10 x 10 x 1.5 mm3) from the coloration bloc removing initial crystal faces. Then they are polished on silk cloth with pure ethanol as lubricant. The samples are held between two platinium foils, in order to ensure the contact a light pressure is maintained between the platinium foils, using two springs situated in the cold part of the furnace. The sample is intro- duced in the furnace when the temperature is controlled to + 0.25

OC.

A purified nitrogen stream is maintained in the enclosed furnace during all the measurement.

The conductivity measurements are done after 30 min in the furnace. The conductivity is calculated for frequencies where the measured capacity is that of the crystal. Optical spectra are recorded at room temperature after each conductivity measurement and thus correspond to annealing of 30 min for successively increasing temperatures between 470 K- 873 K. All the sample manipulations are done under weak red light to avoid the formation of F aggregate centers and colloids under the action of white light.

3.

Experimental results. -

Figure I shows plots of log,, a versus 1 000/T K for an uncolored crystal which has not been subjected to any thermal treatment and for colored ones with varied F center concen- trations. The uncolored crystal exhibits the usual behaviour for the alkali halides. For colored crystals there appears an increase of the conductivity bet- ween 470 K and 873 K.

FIG. 1. - The electrical conductivity of additively colored KC1 crystals Log10 o vs 1 0001T K. Curve 1 : uncolored KC1 crystal.

Cl~rve 2, 3, 4 : colored KC1 crystals with initial F center concen- trations 1.15 :< 1016cm-3 i2.5

x

1017cm-3and 1.2 >: 1018 cm-3

respectively.

Figure 2 gives, for the same colored crystals, the F center evolution as a function of the temperature for annealings of 30 min at successively increasing temperatures. There can be seen a regular decrease of the F center number for F center concentration

<

5 x 1016 emd3. For F center concentration

(*) We would like to thank Prof. J. P. Chapelle and the . , , . I . I 2 -T'k _

5 00 500 600 700 000

(( Laborntoire rle Pl~ysique C ~ i s f o l l i i ~ e )) of Faculte des Sciences

d'Orsay, France, for making and purifying crystals ~ ~ s e d in FIG. 2. - Concentration variations of F centers vs temperature

these experiments. for isochronal annealing of 30 min.

(4)

ELECTRICAL EFFECTS I N ADDITIVELY C O L O R E D KC1 CRYSTALS. I N F L U E N C E O F COLLOIDS C9-467

>

5 x 1016 cm-"gure

2

shows that the

F

center

number passes through a minimum and a maximum which depend on the initial concentration. The maxi- mum

F

center concentration is not the same as the initial concentration.

Optical spectra given in figure 3 explain these phenomena for a crystal containing

2.5 x

I O l 7

F

centers per cm3. These spectra shown the appearance of a n reversible transformation

F

centers 4 colloids.

Thus for

F

center concentration

r 5

x 1016 c m - 3 and for annealing time of 30

min

at 200

(IC

a band begins to appear at

he

long wave-length side of the

F

band. This band is attributed to the

F

center preci- pitation into

K

colloids [15], [Ib]. It appears always at iL = 730

nm

whenever

F

center concentration

> 5 x 1016 cmL3. The peak position of the colloidal

band shifts to the low energies as a function of the thermal treatment. The final position depends on the initial

F

center concentration. When equilibrium between

F

centers and colloids is obtained, one observes a minimum

F

center concentration and a maximum aera under the colloidal band. At higher temperatures.

the colloidal band decreases and consequently the

F

band increases. The thermal bleaching due to

F

center diffusion out of the crystal explains the decrease of the

F

center concentration at still higher temperature.

In fact, this thermal bleaching begins to be operative near

200 OC.

This explains the difference between the maximum

F

center concentration and the initial ones. This thermal bleaching gives an

F

center concen- tration gradient normal to the faces.

FIG. 3.

-

Changes in absorption spectra of a colored KC1 crystal with an initial F center concentration of 2.5 r 1017c1n- 3

as a function of isochronal annealing temperatures of 30 min.

Curve 0 : before annealing. Curves 1-1 1 : annealing ternpel.aturcs of470 K, 502 K, 535 K, 566 K , 597 K , 627 K , 659 K, 679 K ,

722 K, 752 K and 780 K respectively.

In thesc experiments no observable band appear in the UV region of the spectra.

4.

Discussion. -

The

increase of conductivity in additively colored crystals can be attributed to an electronic conductivity.

Mott and Gurney [3] from Smakula's experiments [2]

suppose that in the region of intrinsic ionic conductivity the thermal ionization of

F

centers gives conduction electrons according to the relation :

where 1 1 ~ 1 s the

F

center concentration per unit \olume.

N

the ion pair concentration per unit volume. E, the thermal ~onization energy of an

F

center and E, the enthalpy of formation of

a

Schottky defect. Other symboles have their usual significance. They assume that the thermal ionized

F

center number is small compared to negative ion vacancies produced b>

intrinsic disorder.

Shamovskii. Dunina and Gosteva [4] in conduc- tivity measurements of additively colored

KC1

crjstals found an activation energy of 1.03 eV over the region of extrinsic ionic conductivity.

If

vacancy concen- tration was established by the Schottky equilibrium, the activation energy of conductivity should be about zero, the E , and E, values being not very different.

Thus to explain their observed activation energj these authors proposed that the conduction electron number.

released from the donor levels (i. e.

F

center) at any temperature is given by :

exp - EF

.

( 3 )

2 kT

This relation itnplies that the anionic vacancy concrn- tration is equal to the conduction electron numb'er.

and thus is only produced by thermal ionization of

F

centers.

It must be noticed that the two above interpre- tations d o not pertain to the same coloration method.

Seevers and Scott [9] found that the electrical conductivity of weakly colored ultra pure KC1 cr!-snls.

without colloids, varies as

J<

in the resions of hoala extrinsic and intrinsic ionic conductivit!,. Thus rhe!

verified the hypothesis of Shamovskii c.1 '11. They found an

F

center thermal ionisation energy ot' 2.04 eV in good agreement with that of Sh~tmu\skii et al. They conclude that in additivel! colored KC1 the concentration of anionic vacancies is not ests- blished by the Schottky equilibrium, but ratl?er is only that produced by the ionization of

F

centers.

In our experiments data such as that

of

figure 4 sl1o\vs that the conductivity is still propor1ion;il lo

JIG

for crystals containing only

F

centers o r cant:iining

F

centers 2nd colloids. Our results are in good agrec- ment wit11 tllosc 01' St.c\.crs : ~ n d S c o ~ t for \ve:ih!\.;

1 I

(5)

C9-468 D. DURAND, G. CHASSAGNE AND J . SERUGHETTI

FIG. 4. - Dependence of conductivity upon the F center concen- tration at 666 K. Loglo a vs log1 0 !IF. Ours results.

x

Data

of Seevers and Scott 191.

colored crystals as it is shown on figure 4 on which some of their data have been reported using eq. ( I ) to calculate their F center concentration. On the other hand, the linearity of

o

with 4; is obeyed over the temperature range from 470 K to 720 K. It is difficult to establish the linearity when the crystal bleaching becomes important. The F center diffusion towards crystal faces is accompanied, in our measu- rements, by the increase of the measured capacity, even at high frequency. This increase is related to a decrease of the measured conductance. These pheno- mena could be explained by the appearance of a diffusion impedance. The higher initial concentration, more pronounced are these phenomena.

Our conductivity measurements are well repre- sented by the relation

:

In a plot of Log,,

o/JG

versus 1 000/T K (Fig. 5) using relation (4), we obtain an F center thermal ionization energy

E, =

2.08 eV. We have supposed that the electron mobility is independant of the F center concentration and equal to the value given by Brown and InchauspC [I71

pe =

3.6 exp

-

300 cm2 .V-' .s-' .

T

On the other hand, the preexponential factor of relation (4) is in good agreement with experimental

FIG. 5. - plots of ~ oa/

J-

gI Z F ~vs 1 OOO/T K for KC1 crystal ~ with an initial F center concentration of 1.2

x

101s cm-3

according to eq. (4).

results. Tliis is not the case if we use relation (2).

The introduction, in relation (2), of an entropy factor due to formation of Schottky defects does not give better agreement. For nF

=

5 x 10'' cm-3 at 500 K eq. (4) gives a conductivity

o =

2.4 x

lo-' R - ' .cm-'

which is, to within a factor of 5, our result for crystal (4) figure I .

Thus,

-

in our experiments, the linearity between

o

and

JnF

seems indicative of electronic conductivity due to F center ionization and not to a thermionic emission of electrons from colloidal particles as assumed by Jain and coworkers [6], [7], 181.

A comparison of our results with those of these authors is difficult since their results are presented in tlie form of a ratio

o,/a,

where

o,

is the conductivity of colored crystals and

on

is that of uncolored ones.

o,

and

on

have not the same temperature variation and thus the ratio

@,/on

does not give explicit data.

Jain

et 01.

found a ratio

o,/o,

d 1 below 250 OC.

From this they assume that there is no contribution to the conductivity due to F center ionization. Some conductivity measurements [I81 have been conducted by us on KC1 crystal purchased from Quartz et Silice Co. These crystals show a large region of extrinsic ionic conductivity (estimated to be a posi- tive divalent impurity

:

about I ppm). The conduc- tivity of weakly colored crystals is decreased in this case. This phenomenon has already been observed by Maycock [5]. However we can conclude that Jain's crystals are less pure that those used in these experiments.

Jain

et

al. found a ratio

a,/a, >

1 and increasing above 300 OC. This ratio falls again above 450 OC when colloids have completely disappeared. From this Jain and coworkers assume that the conductivity increase above 300

OC

is due to thermionic emission of electrons from colloids. Tliis has not been observed in our experiments. It could be possible to explain the increase of Jain's ratio

a,/a,,

by tlie increase of F center concentration in this temperature range and by the different variations of

a,

and

on

with temperature. Above 450

O C

tlie decrease of tlie ratio could be explained by the decrease of F center concen- tration and tlie appearance of diffusion phenomena which perturb conductivity measurements.

But these explanations are not completely satisfying.

An alternative explanation could be found in the presence of anionic divalent impurities in the crystal.

In effect in this case the number of conduction elec-

trons would be reduced by the presence of an anionic

vacancy excess. On the other hand, the conductivity

would be proportional to exp

- E J k T

in the tempe-

rature range where tlie charge coliipensating anionic

vacancies are dissociated and larger in number

t h a n

the ionised F centers. This supposes that the Schottky

equilibrium is always inoperative. It should be very

interesting to verify this hypothesis by measurementh

on additively colored crystals wliich have been doped

with heterovalent anionic impurities.

(6)

ELECTRICAL EFFECTS IN ADDITIVELY COLORED KC1 CRYSTALS. INFLUENCE OF COLLOIDS C9-469

References [l] STASIW, O., Giitt. Nachr. (1933) 387.

[2] SMAKULA, A,, Gott. Nachr. (1934) 55.

[3] M o n , N. F. and GURNEY, R. W., Electronic processe~ in ionic crystals (Oxford University Press, London) 1948.

141 SHAMOVSKII, L. M., DUNINA, A. A. and GOSTEVA, M. I., Sov. Phj>s. Sol. Srare 2 (1960) 2252.

[5] MAYCOCK, J. N., J. Appl. Phys. 35 (1964) 1512.

[6] JAIN, S. C. and SOOTHA, G. D., J. Phys. & Cl~etn. SoliclJ. 26 (1965) 267.

[7] JAIN, S. C. and SOOTHA, G. D., Phys. Rev. 171 (1968) 1075.

[8] JAIN, S. C. and JAIN, V. K., Phys. Rev. 181 (1969) 1312.

[9] SEEVERS, R. E. and SCOTT, A. B., J. Phys. & Chern. Solids 31 (1970) 729.

[lo] GRAVIT~, J. C., GROSS, G. E., BENSON, D. K. and SCOTT, A. B., J. Cheni. Pliys. 37 (1962) 2783.

[ l l ] KUCZYNSK~, G. C. and BYUN, J. J., Phys. Stat. Sol. ( h ) 50 (1972) 367.

[I21 VOSZKA, R., TARJAN, I., BERKES, L. and KRAJSOVSZKY, J., Krist. rrrld Tech. 1 (1966) 423.

[13] VAN DOORN, C. Z . , Rev. Sci. Instrum. 32 (1961) 755.

(14) CHASSAGNE, G., DURAND, D., SERUGHETTI, J., Europhysics Topical Meeting (( Lattice defects in ionic crystals >)

July 2-6 (1973) Marseille, Luminy, France. This volume p. C9-465.

[I51 SCOTT, A. B., SMITH, W. A. and THOMPSON, M. A. J., J. Phys. Chen~. 57 (1953) 757.

[I61 DURAND, D., CHASSAGNE, G. and SERUGHETTI, J., Phys.

srat. sol. ( a ) 12 (1972) 389.

[17] BROWN, F. C. and INCHAUSPE, N., Phys. Rev. 121 (1961) 1303.

[18] DURAND, D., CHASSAGNE, G. and S E R U G H E ~ , J., to be published.

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