Ionic thermo currents in doped CsBr and KCI

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Ionic thermo currents in doped CsBr and KCI

S. Radhakrishna, S. Haridoss

To cite this version:

S. Radhakrishna, S. Haridoss. Ionic thermo currents in doped CsBr and KCI. Journal de Physique,

1977, 38 (7), pp.841-844. �10.1051/jphys:01977003807084100�. �jpa-00208646�

Résumé. ²⁰¹⁴ On étudie par la méthode des thermocourants de

dépolarisation

la réorientation des

complexes

lacune-impureté

^dans les monocristaux de CsBr

dopés

par

Ca2+, Pb2+, Ba2+, MnO24-

et KC1

dopés

par

CrO24- .

^Les

pics

^de thermocourant sont

analyses

par différentes méthodes. Dans

chaque cas, l’énergie

d’activation et les facteurs

pré-exponentiels

^sont calcules. Les résultats obtenus pour les

impuretés MnO24-

^et

CrO42-

^{sont très} intéressants.

Abstract. ²⁰¹⁴ The reorientation of

dipolar

complexes ⁱⁿ

Ca2+, Pb2+, Ba2+, MnO24- doped

^CsBr

single crystals

^{and in}

CrO24- doped

^KC1

single

crystals ^are

reported

in this article. The ionic thermo- current

peaks

^were

analysed using

^different methods. The reorientation parameters ²⁰¹⁴ the activation energy and the

frequency

^{factor 2014 are} calculated in each case. The results

pertaining

^{to the molecular}

impurities

^{are found to be}

interesting.

1. Introduction. ^- The

transport properties

^of pure and

impurity-doped

cesium halides have been inves-

tigated previously by

^different researchers

[1-10].

The results of all these

experiments

show that cesium halides behave in a manner very different from the KCI type

crystals

^{and in}

particular,

it has been found that the contribution to

conductivity

^{comes from}

both cation as well as anion vacancies. The intrinsic

region

^{is more}

complicated

^to

interpret

ⁱⁿ

simple

terms. In the case of

heavily doped crystals

^the

charge compensating

vacancies dominate the

conductivity.

With a view of

getting

^more

insight

^{into the}

problem

of defect motion in cesium halides the

study

^{of ionic}

thermo currents in CsBr

doped

^{with different}

impuri-

ties has been undertaken. The

impurities

^introduced

into the lattice are

Ca2+, Ba2+, ^{pb2 +}

^and

MnO’-.

ITC

experiments yield

^the activation

parameters

for the reorientation of

impurity-vacancy (I.V.) complexes

which involves the motion of bound vacancies whereas

conductivity experiments give

^the

activation

parameters

^for the motion of free vacancies.

In the case of KCI type lattices the

migration

energy of anion vacancies is more difficult to determine.

In

conductivity experiments

the contribution due to anion vacancies become ^more

significant

^{in a tem-}

perature

region

^{which is not}

easily

accessible unless certain

precautions

^{are taken. Even} in the few cases

where measurements are available the

solubility

^{of the}

impurity

is low and hence some doubts have been

expressed

about these measurements. However there

are some indirect means to get ^the

migration

energy of anion _vacancy in KCI type ^lattices

[I I]. Along

^these

lines the

investigation

^{of ITC on}

CrO’- doped

^KCI

has been undertaken. It has to be noted that in the

case of

KCI-type lattices,

the activation _energy for reorientation of I.V.

complexes

^is

usually

^smaller

by

0.1-0.2 eV than the activation _energy for motion of free vacancies

[12].

2.

Experimental.

^-

Single crystals

^{of CsBr were}

grown from melt

by

^the

Bridgman technique.

^{In the}

case of calcium and barium

doping

^the

impurity

^was

added in the melt while in the case of lead

doing

the

impurity

^{was diffused into the}

crystal.

The presence of lead was confirmed

by optical absorption

^measu-

rements. The concentration

of Pb"

was found to be 100 _ppm, that of

Ca 2 +

80 _ppm and that of

Ba 21

100 ppm.

Single crystals

^{of CsBr}

doped

^with

Mn 4

^{were ’}

grown

by Kyropoulos

^method.

MnBr2

^and

K2CO3

in the

weight

^ratio

K2C03/MnBr2 >

^{2 were added}

in the melt. The color of the

crystals

^{obtained was}

green.

They

^{did not show} any visible

turbidity.

The _manganese concentration used ^was 100 ppm.

Such a

procedure

^has

already

^been

adopted

^to grow

single crystals

^of

potassium

halides with

Mno2- [13, 14].

The

optical absorption

spectrum ^{of the}

crystal

is shown in

figure

^{1. In table I} the data is

compared

with

optical

^{bands for}

Mno2 -

in solution

[15]

^{and for}

Mno2-

^{in different}

potassium

halide lattices. The agreement confirms the _presence of

Mno2-

^{ions in}

the

crystal.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01977003807084100

842

FIG. 1. - Optical absorption spectrum of CsBr :

MnO-.

TABLE I

Optical Data for MnO’

ⁱⁿ

Different ^Systems

Single crystals

^{of KCI}

doped

^with

CrO’

^were

grown from melt

by

^the

Kyropoulos

method. The

impurity

^{content was found to be 300} ppm. The presence of the

impurity

^{was confirmed}

by

^{the cha-}

racteristic

C.0’ optical

^bands.

The

samples

used for the ITC

experiment

^{were of}

thickness 1-2 mm and square cross-sections 5 x 5

mm2.

The

samples

^were mounted in a

cryostat

and were

polarised

^at

optimum temperatures

^{and with}

appropriate voltages

for about 30 min. The

samples

were then heated from about 100 K to room

tempe-

rature

(300 K)

^{at a rate of 5}

K/min.

^{The currents were}

measured

using

^{a 610C}

Keithley electrometer;

^the

current and

temperature

^{were recorded}

by

^{a two-}

channel servogor recorder.

3. Results. ^-

Figure

^{2 shows ITC curves obtained}

for CsBr :

Ca2+,

^{CsBr :}

Ba2+,

^{CsBr :}

^:Pb2+.

^{In the}

case of CsBr :

Ca2+

the

crystal

^was

polarised

^at

250 K and in the case of CsBr :

Ba2 +,

^{CsBr :}

^{pb2 +} polarization

^{was done at 270 K. In all the cases the}

FIG. 2. ^- I.T.C. in doped ^CsBr.

polarization voltage

^was 250 V. The

curves

were

analysed using

^the

following

^{methods : .}

(i)

Initial rise

method, (ii)

^Area

method, (iii)

^Chen’s

method and

(iv)

^Curve

fitting procedure.

^{All these}

methods have been discussed in detail elsewhere

[16].

Recently

^a nomogram has been constructed and

reported [17]

for the direct evaluation of activation energy

knowing

the half-width and maximum tem-

perature of the ITC

glow

^curve. This has also been used for the activation _energy calculation. For all the

required accuracies,

^the nomogram

really

^serves

better. As a

typical example

the results for CsBr :

Ba2+

is

given

in table II.

TABLE II

Reorientation

Parameters for

^{CsBr : Ba + +}

(*) w, 0, ^{a refer to full} half-width, high temperature ^side half- width and low temperature side half-width of the ITC curve.

A

comparison

of activation _energy obtained from ITC

experiments

with that obtained from other

experiments

^is

given

in table III.

Together

^{with it are}

presented

^some of the results for CsI lattice.

Figure

3 shows the ITC

peak

for CsBr :

MnO’-

and

figure

⁴

gives

^the spectrum obtained for KCl :

Cr04 - .

^{In all the cases the currents were} of the order

10-13

_amps.

FIG. 3. ^- I.T.C. in CsBr :

Mno2; o

represents the values calcu- lated using the best choice of parameters ^{E and} To, continuous line

represents experimental ^curve.

FIG. 4. ^- I.T.C. in KCI :

Cro’-.

Continuous line represents expe- rimental curve. represents calculated value.

interpret peak fully knowledge

^{of the}

position

^{of the}

charge-compensating

vacancy and the model for the relaxation of I.V.

complex

^is necessary.

The dielectric loss results in CsBr :

Cd2 +,

^{CsBr :}

Pb 2+ [6, 7, 9],

EPR studies on CsBr :

Mn 2+ [20],

CsBr :

Pb2+ [21]

and studies on reorientation of

dipoles

ⁱⁿ

CsI : Ca2 +, Sr 2 1 [10]

suggest ^{that the}

impurity

substitutes for the host cation and

charge- compensating

vacancy is situated in a nn

position.

The mechanism for reorientation of the I.V.

complex

thus formed has been

suggested by

^Varotsos

[10], involving

^the

jump

^{of a nn} vacancy ^to

another

nn

position

^{via a nnn}

position.

The fact that

only

^one

ITC

peak

^is observed in the

present

cases, and that

even for the

impurity ^{Mn 2+} only

^{one nn} vacancy is

found, amply justifies

^the

assignment

^{of the}

peak

to nn relaxation.

A

peculiar

feature obtained

by

^Varotsos

[10]

^is

that the activation _energy does not

depend

very much upon the radius of the

impurity

ion in cesium halide lattice. A naive

theory

^was

proposed

^to

explain

^this

feature. Nevertheless the

specific

^{variation seen in the}

present investigations

^{and the}

possibility

^{of the}

displacement

^{of the}

impurity

towards the _vacancy

as deduced

by

ESR results

[20] suggest

^{that the}

theory proposed by

Varotsos is to be modified.

(ii)

^{CsBr :}

Mno2 -. -

^The

analysis

^{of the}

experi-

mental data indicate an activation _{energy -} 1.07 eV and a

frequency

^{factor -}

^{101’ s-1}

^{which is}

unusually

FIG. 5. ^- Variation of the reorientation parameters with various divalent impurities introduced. 0 activation energies [E pre expo-

nential factors.

844

large.

^In

spite

of the fact that ^a

good

^{fit to the ITC}

equation

^is

^obtained,

^a

larger

value of the activation energy and

frequency

factor make the

assignment

^of

the

peak

^difficult.

Assuming

^a substitutional

position

^for

MnOl- (MnO’-

radius - 3

A,

^Br- radius 1.96

A)

^{an I.V.}

complex

^{can be} formed with

charge-compensating

vacancy in a nn or nnn

position.

The value of the activation _energy obtained is too

large

^{to be inter-}

preted

^{in terms of nn or nnn} relaxation.

Noting

^the

radius of the

impurity,

^a face centred

position

^with

one bound _vacancy

nearby

^{can be}

thought

^{of as a}

possible impurity position.

The

expression

^for

To ’

^{involves an} entropy ^term also.

Owing

^{to the}

large

radius and inherent

dipole

moment, the presence of the

MnO’- 4 impurity

^{ion in}

the lattice _may alter the entropy ^{so much that}

To

results in a value such as the one observed. Such an

argument ^{has been}

successfully

put forward ^{for the}

explanation

^{of TL}

peaks

^{in KCI : Cu + +}

[22].

^How-

ever this

argument

^{for the}

present investigations

^is

subject

^to theoretical

justification. ^Further,

^the

jump

mechanism is different in the ^case of cesium

halides,

which

might

^be

responsible

^{for the}

large

values observ- ed and it _may not be correct to correlate the observed value with results obtained from other

experiments

or

assign

^{it to a}

single

vacancy

jump.

(iii)

^{KCI :}

Cro2-. -

The activation _energy and the

frequency

^factor

corresponding

^{to the}

peak

^at

260 K are found to be - 1.06 eV and -

1018 s-’

respectively.

The value of activation _energy is closer to that for the motion of free vacancies. Also it has

already

been shown from IR and Raman stu- dies

[11, 23]

^that

CrO’- occupies

^a substitutional site in KCI with ^a

charge-compensating

vacancy ^as its

first

neighbour

^thus

constituting

^{the I.V.}

dipole

^{in the}

anion sublattice. As the activation _energy obtained is closer to that for motion of free vacancies the

peak

may be attributed to the nn relaxation. The

argument

for

’Co 1

^{is the same as} that for CsBr :

MnO’-.

^The

peak

^{on the low} temperature ^{side of the}

prominent

one is

tentatively assigned

^{to nnn} relaxation.

Though normally

^the

peak

^{due to nnn} relaxation is

expected

in a

high

temperature

region compared

^{to that due to}

nn relaxation the abnormal

frequency

^{factor can}

alter the situation. Hence the

assignment

^is

plausible

but not definite.

It is known that in KCI

crystals

grown in an

ordinary atmosphere, background

divalent cationic

impurity

^is

invariably

present.

Charge compensation

^{can also}

be achieved

by

^the

pairing

up of

CrO’ -

^{and the}

background ^M21 impurity.

However this forms a

rigid dipole

^{and hence} any relaxation mechanism is difficult to conceive for such a

configuration.

^The

peak

^{at 300 K cannot} thus be accounted for

by

^{this. It}

may

quite possibly

^{be due to} vacancy

pairs.

^The

assignment, though

^not

fully justified,

^{is still mea-}

ningful,

because of the _presence of both anion and cation vacancies.

5. Conclusion. ^- The

present investigations

^throw

light

^{on two facts :}

(i)

^the variation of activation parameters with ionic radius of the

impurity

^{in the}

CsBr

lattice, (ii)

^the

complexity

of the results in the

case of molecular

impurities.

^Further

experiments

on some more divalent cationic

impurities

^{in CsBr}

may

fully

^establish

(i).

^Extensive

investigations,

^both

theoretical ^as well as

experimental,

^{are needed to} say

anything

definite about the molecular

impurities

ⁱⁿ

crystals

^{like CsBr or KCI.}

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