Structure and ionic conductivity of mixed lead halides PbCl2xBr2(1−x). I.

Solid State Ionics 18 & 19 (1986) 1179-1183 North-Holland, Amsterdam

1179

STRUCTURE AND PbCl2xBr2(l_x).

IONIC

CONDUCTIVITY

OF

MIXED

LEAD

HALIDES

I.

LUMBRERAS C . L . O . E . S . , L a b o r a t o i r e de G ~ n i e 5 7 0 4 5 M e t z C e d e x i, F r a n c e .

M.

Physique,

Universit~

de Metz,

J. P R O T A S , S. J E B B A R I L a b o r a t o i r e de C r i s t a l l o g r a p h i e , U n i v e r s i t ~ de N a n c y P.O. Box 239, 5 4 5 0 6 V a n d o e u v r e - l e s - N a n c y , France.

I,

DIRKSEN Solid State Department, Utrecht University, 3 5 0 8 T.A. U t r e c h t , The N e t h e r l a n d s .

80.000.

G.J.

J. S C H O O N M A N Laboratory of Inorganic and of T e c h n o l o g y , P.O. Box 5045,

P.O.

Box

Physical Chemistry, Delft University 2600 G A Delft, The N e t h e r l a n d s .

PbCl^ and P h B r 9 s h o w c o m p l e t e m u t u a l s o l i d s o l u b i l i t y . The s o l i d solutions P b C I ~ Br . . . . exhibit the PhCln-type orthorhombic ~x ~ i-x . ~ . symmetry. XRD m e a s u r e m e n t s s h o w p r e f e r e n t l a l s l t e o c c u p a n c y . The ionic c o n d u c t i v i t y has b e e n m e a s u r e d as a f u n c t i o n of t e m p e r a t u r e and composition. The ionic c o n d u c t i v i t y of P b C I B r in w h i c h the anion array is completely ordered, is slightly anisotropic. Homovalent anion d o p i n g c a u s e s the c o n d u c t i v i t y to d e c r e a s e . The role of preferential s i t e o c c u p a n c y , and d o p a n t i n d u c e d l a t t i c e distortions on the n u m b e r and m o b i l i t y of the c o n d u c t i n g d e f e c t s is d i s c u s s e d .

I.

INTRODUCTION

Phase

relationships

PbCI2-PbBr 2 time

ago

(!,~).

More

recently,

the

systems

have

PbCl2-type over

in

in and

All

P b B r 2. occur due

solid

systems In all

which to

the

size

anions.

All

binary

anion

anion

and

by

the

end

ordering

in

the

(5).Homovalent substantial

cation

Frenkel

type

silver

ion

solid

doping

solid

induces

increases

in

Schottky

or

solutions

(6),

the

PbCl2-PbBr 2

conducting

halides halides

anion

of

system

conductivity

several

that

and

decrease

conductivity

solutions

alkali

authors

occupancy

effects

electrical

and PbCl 2-

based

on

respectively,

(Z,~).

compositions effects

between

members via

recently

present

site

the

In

thls

paper

solutions

in

quantitative

conductivity

mechanism,

shown

preferential

exists

ordering

disparity

but

ranges.

solubility

the s y s t e m s

show

the

exhibit

symmetry,

BaCl2-BaBr2,

different the

phase

in

systems

composition

mutual

some

BaCl 2-

described the

orthorhombic

different

the

reported

P b X 2 - P b I 2 (X = C I , B r )

been

literature.

Complete

system

been

(3),

(4)

the

have

relationships BaBr 2

in

a it

exhibit vacancy has

been

0 167-2738/86/$ 03.50 © Elsevier Science Publishers B.V. (North-HoUand Physics Publishing Division)

and

relation

to and

the

for

system

electrical the

the

solid

PbCI2-PhBr 2

information

structural

doping,

we p r e s e n t

regarding

properties

homovalent

in

anion

the anio,1 d i s t r i b u t i o n .

M. Lumbreras et al.

1180

2.

EXPERIMENTAL

ASPECTS

The

preparation

of t h e

has

been

XRD

measurements

published

single

were

taken

automatic

using

or

absorption,

following

proposed and

all

program)

giving

anisotropic

The

and

dependence

ionic

solutions

impedance

and a S o l a r t r o n Analyser. electrode

contact

parameters will

atomic

factor,

be

reported

The

of

interatomic are

using

an

given

of

P b B r 2.

In this

occupy

the

4(c)

entire structure

space

lie on a m i r r o r the

short

3 2.5

are

two

plane,

non-equivalent

the

PbCl2-type

can

accommodate

site

1 (Figure

The v a r i a t i o n and

cell

compositional reported

(5).

z = + 1/4. anion

structure. larger

ions

than

lattice

volume

to

in

site

2

anion

parameter

parameters

versus x

has

x

0

' 35_3

3 1 Figure

2:

x

0

Variation

of

interatomic composition.

I). of the

0

d(A}

3.5

There

sites

Anion

x

atoms

perpendicular

at

0

d(A)

PbCI 2-

special positions 16 g r o u p D 2 h - P n a m : they

c-axis

composition

b

3.5

fourfold

of the

with

as an

is o b s e r v e d

the

II

shortest

3.5,

3 ~

all

part

2.

Et

Response

system

in

d (~)

material.

structure

the

solutions

the

distances

in F i g u r e

25

PbCl2-type

detail

site thermal

solid

in

variations

by

used

isotropic

these

x

throughout

the

(10).

ratio.

(GRI680A),

d a g was

coordinates,

and

for

RESULTS

The

of

projection

structure.

studied

Frequency

Graphite

matrix,

conductivity

bridge

1172

c-axis

fractional

occupancy

composition

spectroscopy

The

(Shelx

vibration

were

capacitance

1.

PbC12

refined

occupancy and

of the

solid

automatic

3.

were

complete

site

Figure

Stuart

procedure

temperature

the

and

thermal

parameters,

from

polarization

data

the

oPb OC[ 1 OCl 2

a CAD 4

empirical

Walker

a least-squares

31&

After

the

Lorentz

corrections, using

by

on

radiation

monochromator.

(9)

1/

(5).The

diffractometer

AgK~

graphite

method

crystals

elsewhere

ENRAF-NONIUS CuK~

/ Mixed lead halides PbCl2xBr2(l_x). I

a.

Pb-CI

the

b.

1 Pb-CI 2

been

c.

CII-CI 1

d.

C12-CI 2

the

shortest

distances

with

M. Lumbreras et al. / M i x e d lead halides PbCl2xBr2(1.x) . I

The

temperature

dependence

of the

and ionic

several

PbCl2xBr2(l_x~

presented

in Figure

composition

conductivity crystals

enthalpy

1181

is given

of

of the data

is

before

in Figure

analysis

4. Details

have been

reported

(5).

3. PbE[2xBr2(l_x}

a 2

b

TO

I

1

1

0

I

I

t

-3

~

,

,

,

3 0

250

200

1 0

105

x

.

_1

Figure

4:

Variation

of the formation

enthalpy 2

b

L3Hf of S c h o t t k y

+

'3

defects,

and m i g r a t i o n

enthalpy

AH m of anion

vacancies

in

PbC12xBr2(l_x)

with

composition. -1

The

ionic

conductivity

conductivity

-2

compound 3

3'0

2'0

2"0 10 5

of

data

the

to be s l i g h t l y

this

ordered

sites

1 and Dr

reveal

the

equimolecular anisotropic.

compound

CI-

ions sites

ions

In

occupy

2.(Figure

5)

Jo _1

DISCUSSION Figure

3: The

ionic c o n d u c t i v i t y

PbC12xBr2 ( l _ x ) measured along b-axis.

crystals the

a.

=

of

All

solid

orthorhombic two

anion

solutions PbC12

b.

=

0(1),

x

0.3(2),

x

=

0.5(3)

x

=

0.75(1),

x

=

0.95(2),

x

=

0.64(3),

x

=

0.5(4).

occurs.

For

completely anion Br-

The

composition

formation

dependence

enthalpy

and

of

the

migration

0.5.

ions.

anion x

>

by

in which discerned.

site

occupancy

0.5

sites

by CI-

reversed

solid

the

be

anion

CI-

and

while

by CI- and

situation

solutions

the

1 remain

ions,

2 are occupied The

in

Here

occupied

can

occupied

sites

observed

structure

sites

Preferential x

adopt

with

sites Br-

ions,

1

is x < are

while

M. Lumbreras et al. / Mixed lead halides PbCl2xBr2(l_x). I

1182

increasing

solute

question,

2

i

therefore, enhanced

defect

modified

defect

mobilities

into

mobility

i0

account. in

reveal

In

the

need

The

anion

undoped

to

region

be

vacancy

end

members

. ~ 10 . from ~VBr ~VcI up to a b o u t 630 K (Ii,

intrinsic

to or

room

12).

~ (PbBr 2)

> 0

(PbCIBr)

2

>

0

(PbCI 2)

(PbCI2).

-3

105

I

T(K 5:

Temperature

dependence

in

conductivity the

U

are

regard

the

extrinsic

to

conductivity

we must

assume

reduced

occur

in

defect

of P b C l B r mobilities

along

i0

defect

end m e m b e r s

With

the

~ the

of the isotherm,

ionic

that

the p u r e

equal.

minimum

u ( P b B r 2)

implies

in

roughly

)<

with

This

densities

Figure

as

densities,

roughly

temperature /

the

The

arises

whether

taken

b

content.

a-axis(1)

to

the

solid

and solutions,

and h e n c e

slightly

enhanced

b - a x i s (2) . thermal sites

2 remain

In

the

occupied

ordered

equimolecular occupied ions. a

by

CI-,

and

results

are

structure

in

accommodate

isotherms

conductivity

in

decrease

the

A

minimum

content,

while

conductivity minimum. positions composition formation

PbCIBr, enthalpy

not

be

taken

Together occupancy,

in the

the

enthalpy solute extrinsic

and

+ Cl~r,2

need

preferential

a com-

sites

can

be

ions 1.

for

exchange,

one

the

a-axis. 0

pathways

will

solute

sites

inspection

ignoring

with

that

compound

By

the

with

site

imply

content.

illustrated

occupy

conduction

defect

preferential may

increasing

equimolecular Br-

the

conduction

with

This

account.

this

equimolecular

decreases

size local

represented

+ BrcI,2

. ~ VBr,I

into

with

develop

reveal

V B•r , 2

+

The

several

the

to the

jumps

+ V~I, 2 ~ V~l,l

the

solutions

from

around

B r cxl , l

around

isotherms

Apart

distortions

PbCIBr.

the

and

to

solute

increasing

effects,

due

by

CI x Br,l

ionic

region

migration

with

In a d d i t i o n ,

that

1.

e P h C I 2.

solid

vacancy

Br-

can

site

decreases

present

lattice

densities.

likely

with

the

occurs

In

increases

than

composition

conductivity

the

2

increasing

PbBr 2 e PbClBr

anion

site

defect

seems

disparity

1 are

2 by

reveal

sequence the

sites

extrinsic

with

equimolecular intrinsic

of

concordant

anions

Conductivity

ions.

sites

which

larger

Br-

array

composition

These

content.

with

it

can

in

2,

C1-

and

of

Figure

(// a - a x i s )

> ~

ions

1,

and

ion

a continuous

for Brwould

which

CI--Br-

envisage

This

the

PbC1Br

simplicity

pathway

for

be

ions

along

concordant

(// b - a x i s )

as

M. Lumbreras et al. / Mixed lead halides PbCl2xBr2(l_x). I found

experimentally.

Therefore,

increasing

solute

ions

be p r e s e n t

will

pairs,

and

domains. (7)

It

that

domains which

strain

may well

effects.

A

this

anomalous

(tCl),

(viz.

and

well

behaviour data

bromide

ions

of

function

E.

6).

7). of

to a w a i t

for

chloride

8).

Such being

C.

Carabatos,

critical

of Metz,

discussions.

authors

(M.L.)

professor

would

G.

University,

One like

of to

Blasse,

for k i n d

for the

thank Utrecht

hospitality.

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ACKNOWLEDGEMENTS

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the

composition. are

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In press.

has

(tBr)

J.

ion

4).

explanation

number

experiments

be

dependence

Jebbari,

A.G.T.G.

on ~ { f

Figure

quantitative

transference

and

solute

Lumbreras,

S.

(Eds.

region

< x < N 0.7.

detailed

isolated

M.

Inorganic

before

effects

very

as

PbCiBr-type

of p a i r s

anomalous

on x in the

and more

suggested

produce

may

the

more

fields

from

This

of

N 0.3

been

5).

the s o l u t e

in o r d e r e d

has

differ

cause ~Hf

also

content

with

l 183

12).

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Hoshino,

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