DIELECTRIC AND OPTICAL PROPERTIES IN ALKALI HALIDES DOPED WITH COBALT

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DIELECTRIC AND OPTICAL PROPERTIES IN

ALKALI HALIDES DOPED WITH COBALT

R. Cappelletti, R. Fieschi, G. Martegani, L. Pirola

To cite this version:

J O m N A L DE PHYSIQUE Colloqrie C 4, Suppliment au d 8-9, Totne 28, Aorir-Seplembre 1967, page C 4-1 30

DIELECTRIC

AND

OPTICAL

PROPER'rIES

IN ALKALI HnLIDES

DOPED

WITH

COBALT

(*)

R. CAPPELLETT~ ( * *),

R.

FIESCHI

(*

*), C . MARTEGAWL and L. PIROLA (**)

Tstituto di Fisicn dell'UniversitA, Parma (l taly)

Rksurnt.

-

On ctudic l'influence de Saddition de CO dans ks cristaux de NaCI au moyen de

mesurcs optiques et diklectriques.

L'existence de courants de dCpoIarisation causes par la presence dcs dipdles (( impurcte de Co-

lacune d'un cation M est dtmontrk, et on etudie I'effet d u recuit isotherme sur la concentration

des dipoles.

On remarque une cinttique de prkipitation du troisitrne ordre, comme dans tc cas de la prki-

pitat ion des impuretes alcalino-terreuses.

Des bandes d'absorption optique engendrkes par I'addition de CO sont revC1ccs lorsquc l'echan-

tillon est irradik aux rayons X.

h presence de CO catalyse la formation des centres F, cornme le font Ics impuretb alcalino- tcrreuses. On n'a trouvi aucunc bandc sembtable i la bande Z.

Abstract.

-

The influence of CO addition in NaCl crystals is studied by mcans of dielectric

and optical measurements. Dcpolari7ation currents (ionic thermo-conductivity) due to the presence

of dipoles (( cobali impuritysation vacancy are detected, and the effect of isothermal annealing

on thc dipole concentration is studied. The precipitation process which is observed has a third

order kinetics, according to the results on the alkaline earth impurities.

Optical absorption bands duc to CO additions are detected.

The presence of CO impurities cntalyzes the formation of F centers, just as the presence of

a1 kaline earth impurities, when the sampIe is X-irradiated. Nothing analogous to t h e Z bands is found.

1. Introduction.

-

The study o f the influence of

polivalent metal addition on the physical properties

of alkali halide single crystals has pIayed an important

role, since the early work of Pick 11, 21. The main emphasis was devoted to the study of the optical properties {color centers, luminescence, influence of the ionizing radiation

p

J and to thc electrical proper-

ties (ionic conductivity, dielectric losscs [4, 51). An

appreciable amount of work was carried out also on the E. P.

R.,

mainly on M n ions [6], bul also on V,

and Z centers due to alkali earth ions [7, 8, 91. One of the problems of interest was thc association

process between the impurity and thc positive ion

vacancy to build dipoles and the precipitation of

these dipoles into aggregates of various sizes [10, 11,

121.

LJnfortunateIy the impurities which give the most

interesting color centers, namely alkali earth, are not (*) This research has bccn sponsored in part by AFORS undcr Grant AE' EOAR 65-7 with thc European Ofice of Aerospae Research.

(**) Gruppo Italiano di Struttura della Materia (G. N. S. M,)

of the Consiglio Nazionalc dellc Riccrchc.

suitable for E.

P.

R. measurements, unless they capture

an efectron or a hole, bccause the divalent ions have

closed shell configurations. Converse1 y M n impurities

do not give 2-type centers suitable of a convincing

interpretariw.

Cobalt impurities are, according to us, suitable for

a n extensive study of the properties of polivalent ~ n e t a l

impurities in alkali halides. I n facts : a) there are

optical absorption bands due to the presence of CO in untreated samples ; b) the irradiation with ionizing radiation affects thc absorption spectrum ; c) divalent

cobalt has unfilled siiell, therefore gives E. P. R. [I31 ;

d) the presence of' positive ion vacancies in excess

over the thermodynamical equilibrium valuc, as

well as association and precipitation processes can

be studied hy mcans of ionic conductivity and dielec- tric losses ; c) moreover "CO is the most advantageous

isotope for the study of' the Mossbauer effect : this

provides on indirect and powerful wily of studying

the symmetry of the fcltI a t the

"CO

nucleus (actually

at the ' ' ~ e resulting from the

0

decay of "CO),

through the quadrupole splitting [14, 15, 161 ; isomer

shift measurements should provide additional infor-

DIELECTRIC AN D OP'I-ICAL PROPERTIES C 4 - 131

mations on the change of charge at the nuclei, due

to additive coloration, opticaI conversion and to X ray

irradiation of the sample.

We present here some preliminary results on the

electrical and qptical properties of Co doped NaCl

singIe crystals. Our results will be compared with the

onac obtained by means of Mossbauer measurements.

KC1 doped samples were also investigated, but they

are not suitable for our purpose, because the solubi-

lity is too low, as detected by means of ionic thcrmo-

conductivity measurements ; the optical absorption

bands due to CO are also weak [17j.

2. Experimental procedure. - a) Cryslal growth :

Thc samples used were cleaved from crystals obtaincd

from Harsaw Chemical Company or grown in our

Iaboratory by means of Kyrapulos method in dry

nitrogen atmosphere. In the last case a small fraction

(10-3 in weight) of CoC1, .6H,O was added to the

molten NaCl. The bottom of thc grown monocrystal

showed blue opalescence, due to anhydrous CoCl,

microscopic occlusions. This coloration was partially

removed by heating the crystal at 400 O C then quen-

ching at R .

T.

b) Ionic tlrermooonductiviiy (I.

T.

C.) measure- ments : The samples (15 X 15 X 0,6mm3 about)

were polarized 11 81 in a static field of 104 V/cm for

3 minutes at 232 OK and cooled down to 100 OK. The

field was switched off and the crystals warmed up at a

constant ratc of 0,1 OKlsec. The depolarization

current a t linearly increasing temperature was defected

by a vibrating reed electrometer (Vibron 233 C) and

recorded by a Speedomax. The apparatus can mcasure

currents down to 1 0 - ' v . Neither colloidal graphite

nor silver paint succeeded i n improving the contacts

between crystals and silver electrodes ; on the contrary

they gave rise to very intense pcaks, perhaps duc to

some kind on interfacial polarization.

c) Oplicul measuremmts : Optical absorption spec-

tra were obscrved at liquid nitrogen temperature by

means of a Cary 15 spectrophotorneter. X-irradiation

was performed with a MachletC OEG 50 T tube

operating at 40

kV

and 40 mA. For optical bleaching

thc tungsten lamp and monochrornator of Cary

spcctrophotometcr were employed.

d) Thermal trealnlents : For I. T. C. mcasurcments

the samples were heated in vacuum in thc cryostat,

thcn the vessel[ was filled with dry nitrogen and cooled

down to 273 OK in 4 minutes,. The crysPals used for

optical ~ncasurements were healed in a Vycor e~~velopc

put in an oven at the required temperature.

3. Ionic thermuconductivity results. - The

depolarisation currents, a t linearly increasing ternpe-

rature

0.

T. C. curves) are plotled in figure 1 for a NaCI : CoCI, sample, not submitted to any thermal

treatment (curve (a)) and quenched from 423 OK to

room temperature (curve (b)). Two bands, peaked at

205 OK and 222 OK are present ; their intexity is

linearly dependent on the intensity of the polarizing

electric field.

T E M P E R A T U R E (in OK)

FIG. 1.

-

Ionic thcrrnoc~nductivity curves of NaCl [loped

with CoCIz (10 -3) :

curve a : the sample has nor been previousIy heated : curve b : the saniple has been heaied at 150 O C thcn

quenched at R. T.

The other curves o f figure 2 show the e r e c t of

isothermal annealing a t RT ; the band at 222 "K does

not change appreciably during ageing, while the band

at 205 OK decays, much in the sanle way as the 1.V

bands to alkali earth or to Cd ions [10, 121. This

remarkable analogy suggests that it is due to orienta-

tion of dipoles CO"-sodium vacancy. The tempera-

ture of the peaks is in the same range as for NaCI.

crystals doped wit11 different divalent ions (Cd, Sr),

showing that the activation energy for dipolar orien-

tation is of the same order ('2: 0.5 eV) ; no carcful

determination was possible, up to now, because of

the overlapping of the two bands. Thc precipitations

becomes morc and Inore appreciable at higher tempe-

ratures : in figure 3 the dipole peak decay curves are

compared for ageing at R T and 61 OC.

C 4 - 1 3 2 R. CAPPELLETTI, R. FIESCHI, G. MARTEGANI AND L. PIROLA

T E M P E R A T U R E ("K)

FIG. 2. -Ionic thermoconductivity curves of NaCl : CoClz (10-3) :

curve a : the sample has been heated at 150 OC then quen- ched at R. T. ;

curve b, c and d : after increasing times of ageing at R. T.

.l6

n.z-5.10 dipoles crn'l

I

0 I I I l I I I I l I

0 10 20 30 40 50 60 70 80 90 100 TIME OF PERMANENCE t AT CONSTANT TEMPERATURE t i n hours) FIG. 3.

-

NaCl : CoC12 (10 -3) : the normalised concentration

of dipoles n/no is plotted versus time of ageing :

curve a : the ageing has been performed at R. T. ;

curve b : the ageing has been performed at 6 1.5 OC.

solution with monomolecular kinetics, one can

attempt to fit the experimental results of I. T. C.

with a chemical rate equation in the form :

Hence, under boundary conditions : t = 0, n = no

one gets :

Here n and no are the concentrations of dipoles at

time t and at the time t = 0 respectively, a a coefficient

which determines the order of kinetics, E the activa-

tion energy for the process and v, the frequency

factor. From (l') the kinetics order a can easily be

fitted, since a plot of (n,/n)"-l versus time has to be a

straight line.

For (( cobalt-cation vacancy dipoles D, in the range

of temperature of our isothermal annealings, a is three,

see figure 4, and E is 0.61 eV.

Cobalt dipoles excess anneals with the same law that Ca, Sr, Ba, Mn, Cd follow in KC1 and NaCl [lO, 11, 121.

TIME O F PERMANENCE AT 2 I 0 C I N H O U R S ( c u r v e @)

0 100 200 300 LOO

I I I I

TIME OF P E R M A N E N C E AT 61.S°C IN H O U R S l c u r v e @ I FIG. 4. - NaCl : CoC12(10-3) : n$/n is plotted versus time

of ageing at RT, curve (a ; and at 61.5 OC, curve (b.

The experimental observation that cobalt has a quite similar behaviour of some extensively studied divalent impurities in alkali halides seems to us a good starting point to check by means of Mossbauer effect measurements, the microscopic symmetry of the precipitation products.

Mullen [l41 has obtained two kinds of Mossbauer

spectra, in NaCl : 57CoC1, : a doublet (A-type)

DIELECTRIC AND OPTICAL PROPERTLES

ling ; a broad peak, with high isomeric shift (B-type),

-

absent in samples slowly coolcd.

It i s known from conductivity measurements that

in NaCl crystaIs the equilibrium between divalent

cation impurities and positivc ion vacancies is reached

.

in a short time, even at ternpcratures as low as 0 OC [l91

and that at room temperature the concentration of

2

_-

the dissociated impurity is very IOW

( 5

5

X).

Accor-

_3-

ding to us, this rules out thc possibility that B and A

_$

_.

Mossbauer spcctra are due to isolated and to associa- h

ted iron, respectively. 5

-

-

Our rcsults show that quenching and low tempera- 9

ture annealing affect the dipole concentration and ₁

_-

the fraction of the precipitated impurities (probably

as trimers) ; the rate of decrease of the concentration 6 5

W D T O S CXERLY IN CV

4

dipoles, as measured by I. T. C., is qualitatively of the

FIG. 5 .

-

Absorption spectra at liquid nitrogen tempera-

same order as the rate of annealing of l? to A Mass- _{turc of NaCi : CoC712 (10 -3) :}

bauer spectra. curve A : tllc samplc has not becn previously hcated ;

A similar behaviour has also been detected for CO' + curve B : after 45' minutes at 400 "C, then quenched at R. T.

cation vacancy dipoles in Lil: and NaF, by means

of E. P. R. measurements [13]. The concentrations

employed by Mullen (- 30 p. p. m.) are also relatively

high with rcspect to the solubility of CoCl,, so that

precipi~tion processes irito large complexes necessa-

rily occur (see also the interpretation of De Coster

and Amclinckx

II

S]). According to us, the interpreta-

tion of the Mossbauer spectra should be reexamioed,

keeping into account the results of more direct measu-

rements on dipole concentration. I t would also be

desirable to employ samples where the 57Co concen-

tration is k n o w n and uniform, hence samples where

the impurities are added to the mclt ; at lower CO

concentration, the precipitation of dipoles could be

cornpletcly avoided.

Optical absorption results.

A) A s s o i r ~ ~ r o ~ SPECTRA OF NaCl : CoCI, A N D TI~ERMAI, TREATMENTS.

-

A thin slice, cleaved from the clear side of the ingot, far from tbe blue opalescent

zone, shows in the ultraviolet range of tile absorption

spectru~n three main peaks at 6.40 eV, 5.82 eV and

5.16 eV ; other weak b a ~ l d s appair in the spectral

range from 4 to 4.8 eV. No appreciable absorption is

shown in the visible range (see Fig. 5, curve A). Quen-

ching from high temperature ( E 400 OC) to R. T. shifts

towards low energy by 0.1 eV the position of the

absorption bands (Fig. 5, curve

B).

This effect is not

understood at present.

If thc sample i s cleaved near the blue opalescent

part, the absorption spectrum shows, in addition to

the previous bands, other weak bands, in the region

between 1.8 and 2.3 eV, and a monotone increase of

absorption coefficjent versus photon energy induced

by the scattering of the light by the microscopic

occlusions of anhydrous CoCl,. Quenching from

high temperatures weakens to a certain extent the

effects due to scattering ; conversely the U--v. bands

are enhanced and resolved

from

the background.

This results allows us to ascribe the U,-v. bands to

optical transitions at afornically dispersed cobalt.

Moreover, since the relative, heights of the three

main absorption bands is approximateIy the same,

indepcndenrly of thc sample and of the thermal

treatments, wc think that the transitions occur at

the sanic ccntcr.

B) X-I RRADIATION EFFECT ON THE ADSORPTION SPECTRA. -. The absorption spcctra of two samples

of a pure and a CO doped NaCl, X-irradiated with

the same dosc at R .

T.

are compared in figure 6 ; the

samples werc quenched froin 400 OC to R. T. The

presence of C O impurities catalizes the formation of

F-centers.

No anaIogous to the Z bands, due to electrons

bound to a1 kali earth impurities, was found neither

i n X-rayed crystals, nor after opticaI bleaching of the F

band. Optical bleaching of the

F

band in NaCI :

CoCI, sainples causes the formation of R and M

centers ; the same occurs, as it is known, in pure

NaCl samples.

The enhancement of F center formation rate under

X-irradiation, as is known, is found in alkali halides

containing alkali earth additions 120, 211 and i s due

to the presence in the sampIes of X. V. complexes [22,

23, 241 ; ihis is consistent with our results o f I. T. C.,

which show that I. V. dipoles arc present in NaCl :

C 4 - 134 R. CAPPELLETTI, R. FIESCRI, G. MARTEGANI AND L. PIROI,A

FIG. G.

-

Absorplion spectra at liquid nitrogcn tempera- ture of sarnples heated at 400 O C then qucncherl) at R. T., arter X-irradiated a t R. T. for 10 minutes :

curve A : purc NaCl (Hxrshaw) ; curve B : NaCl : CoCl2 (10 -3).

111 PICK (H.), r l t ~ t z . P / ~ y s i k , 1939, 35, 73.

[2] SE~TL (F.), P I I ~ s . Rev., 1951, 83, 134.

[ 3 ] SCHULMANN (J. H.), D A L E ~ M P T O N (W.), C0l0r

Ccnters i n Solids. Pergamon Press, 1967.

[41 DREYFUS (R. W.), PAYS. RPV., 1961, 121, 1675. [S] HAVLN (Y.), J. Chem. Physics, 1953. 21, 171.

[6] WATKINS {G. P.), Phys. Rev., 1858, 113, 91.

171 HAYE (W.), NICHOLS ( G . M.), Phys. Rev., 1960, 117, 993

[g] CONKLIN (G. E.), FUUF (R. J.), Phys. Rev., 1963,

132, 189.

191 BUSHNELL (J. C.), Internatio~~al Sy~nposium on

Color Centers in Alkali Halides. Urbana (IlIi-

nois), October 1965 (unpublished).

1101 COOK (J. S.), DRYDEN (J. S.), Proc. Phys. Soc., 1961,

80, 479.

[Ill ALZEYCA (G.), C K ~ P P A (1'. R.), SANTUCCI (S.), Nttovo Cimet~ro, 1966, 42, 100.

[l21 CAPPELLF~TI (R.), DE BENEDLTTI (E.), Aggregation

of divalent impurities in sodium chloridc doped

with cadmium in press on Ph-vs. Rev.

1131 HAYES (W.), 3. App'pl. Physics, 1962, 33, 329.

[l41 M ~ L L E N (1. G.). Phys. Rev., 1963. 131. 1410 ; 1963.

. . . . .

131, i415.

[l51 DE COSTER (M.), AMEL~NCKX (S.), Plzj*~. Left., 1962

1, 245.

[l61 WERTI-IEIM (G. K.), GUGFENHBM (H. J.), J. Chem.

Physics, 19.55, 42, 3873.

1171 WASH~UIYA (S.). J . P ~ Y s . SOC. J a n . 1963. 18. 1719.

i18] Buccr (C.), F~E:SCH~

(d.),

Gur~r

( ~ . j ,

P ~ ~ s . ~ k v . , 1966, 148, 816.

[l91 DREYFUS (R. W.), NOWICK (A. S.), 1. Appl. P h y ~ i c ~

1962, 33, 473.

[201 ETZEL (H. W.), Phys. Rev., 1952, 87, 906. [21] R A B ~ N (H.), Phys. Rev., 1959, 116, 1381.

[22] CRAWFORD (J. H.), NEI,SOS (C. M,), P~J's. Rev. L c I ~ .

1960, 5 , 314.

[23] BELTRAMI (M.), CAPPELLETTI (R.), FIESCHK (R.), Phys. I,crlcrs 1961, 10, 279.

E241 C I ~ A W F O K D (J. H.), Lectures on c( Radiolysis of Alkali

Halides )) presented at the international Summer

Course on Solid State Physics, Gherlt, September