India n Jo urn a l o f Pure & A pplied Physics
Vo l. 40, Dece mber 2002, pp. 862-866
Study of ionic conductivity in KBr: (Cr04)2- crystals
G Sai babu
l
,
I De partme nt
N Vasanth Kumar2, A Ramachandra Redd y' & 0 Srikanth 3
of Ph ysics, Ra ilway Juni o r Co ll ege, Kazipet, Waranga l 506 003
?
-Bharath Sanc har Ni ga m Limited, Wa ranga l
-' De partmen t o f Ph ysics, Region al Engi neerin g College, Waranga l 506 OOl)
Received 12 April 2002; re vised 6 Au gust 2002
The io ni c co nducti vity o f pure KBr and KBr c rysta ls doped with chromate ani o n has been meas ured in th e te mperature
range o f 200-600 DC. In pure KB r crystals, large co ntributi o n to condu ctivit y by cati o n vaca nc ies has bee n o bserved at
hi gher te mperatures. Io ni c condu cti vity measure ment s are made o n pure KBr crystals and KBr crystals doped w ith 0 .25. 0.5
and 0. 75 mo le % o f chro mate a ni o n in as-grown state. Furthe r, the e ffects o f que nch ing and annealin g o n conductivit y of
KBr crys ta ls d o ped with 0.75 mo le % chro mate io n are measured. The condu cti vity versus tempe rature pl o ts s how th at. as
co ncen trati o n o f impu ri ty a ni o ns in creases, th e condu cti vi ty decreases in cxtrin sic associated reg ion. As the concentrati o n
inc reases to 0.75 mo le %, the co ndu cti vit y in creased to high va lu e compared to 0.5 mo le %. But, it is less th an lhat o f pu re
c rysta ls. T hc extri nsic un -associated reg io n shows a s li ght vari ati o n in co nducti vit y b ut , te nd s to be sa me th at o f as-grow n
pure KBr c rysta ls. T he va riatio n in co nd ucti vit y of doped KBr c rysta ls are exp lained o n the basis of fo rmati o n of com pl exes
with bac kg round impuriti es, form ati o n o f ne utra l pairs, introducti o n o f fres h di s locati o ns, etc. Th e variati o n in condu c ti vit y
due to que nc hin g and ann eal in g is ex pl ain ed o n the basis o f di ssociati o ns and aggregati o n o f co mpl exes, vanis hin g of newly
introd uced d islocations, e tc. The co rres pondi ng ac ti vati o n ene rgy va lu es a rc eva lu ated.
1 Introduction
Most of the recent work s have been dea ling with
latt ice defec ts in a lka li ha lide c rysta ls doped with
d iva le nt cati o ns 1.2,} and to a muc h lesser e xte nt with
anions45 . The nature of latti ce de fects can be studi ed
by us ing va ri ous tec hniqu es like, ioni c conductivi ty,
the rma ll y stimul ated de-po lari zati o n curre nts, etc .
Among th ese tec hniqu es, io ni c conductivity is o ne
of the conveni e nt meth ods to prov ide va lu abl e
informati o n on the state-o f- po int defects, nature of
impurity prec ipitate and compl exes.
Io ni c co nduc ti vity as a functi on of te mpe rature
has been carri ed out by a numbe r of wo rkers o n
a lka li ha lide c rys ta ls doped with di va le nt catio n
impurities I .} and , to a muc h less ex te nt, o n dival ent
ani o ns)!'. The e ffect of que nc hing-induced c hanges
on conductivity of cati o n-doped c rysta ls has been
in vesti gated by a numbe r of wo rke rs:l 7x . Speci a lly,
the co nduc ti vity o n potass ium bromide c rysta ls
doped with cati o n has been reported by a numbe r of
worke rs' 7.". A very littl e work has been re po rted o n
KBr c rys ta ls do ped w ith ani o ns£'. Parti c ul arl y, KBr
c rys ta ls do ped w ith c hromate ion was studi ed by
va rious wo rke rs on o ptica l and spectroscopi ca l
properti es Ill. B ut, no atte mpts have been made on the
stud y of io ni c conduc tivity of KBr c rysta ls doped
with c hromate ani on. So, in the present paper, an
atte mpt has been made to stud y the e ffect of
c hromate io n 111 KBr c rystal s usmg 10 lll C
conductivity meth od . An atte mpt i. made to
eva luate the acti vati on e nerg ies fo r mi grati o n of an
ani o n vacancy, the formati on o f separate pa ir o f
Sc hottky vacanc ies or I-V dipo les and , d issolu tio n
of impurity io n in host latti ce. A pl ausible
mec hanism is su ggested to ex pl a in the va ri ati on of
conductivity in the presen t syste m.
2 Experimental Details
Single c rystal s of pure K Br and KBr c rysta ls
doped with 0.25 , 0.5 and 0.75 mol e % of po tassium
c hromate we re grown by us ing me lt tec hnique in
air. The starting mate ri a ls used fo r the gro wth of the
c rysta ls we re Sarabha i, Me rc k ana lar g rade KBr sa lt
and potass ium c hromate sa lt. The we ig hted sa lt s
we re g round separate ly to form fin e powde r and
th en mixed to get ho mogeni zed mi xture . T he mi xed
sa lt was take n in a platinum c ru c ibl e and ke pt in the
furn ance in a zone , whe re the tempe rature is
unifo rm .
Slo wl y, the te mperature of the furna ncc was
inc reased abo ve the me lting po int of the host
SAIBABU e/ ai.:IONIC CONDUCTIVITY IN CRYSTALS
material. The melt was kept at a temperature of 100
to 125 DC above the melting point, for four to five
hours, to ensure a steady temperature. The
temperature of the melt brought to below 50 DC of
melting point of host material, at the rate of 5 DC
per hr and further the temperature decreased at a
rate of 50 DC per hr. The samples were ground once
and the growth process was repeated. When the melt
was brought to room temperature, small cracks were
observed and the sample broke into pieces. A fine
shape of the crystals of about 2 x 2 x 0.4 cm
dimensions were achieved. They were annealed at
600 DC for four to five hours and cooled to room
temperature at the rate of 20 DC per hr. After
annealing of the crystals, the presence of potassium
chromate was analysed by chemical reaction
method. The optical absorption spectra showed
similar peaks as reported earlier 'o .
[J-{]
-2
fr--f>
It---j(
0--0
Pure KBr Cryslal
2-
0·25 mole % of ICr04 )
0.5 mole % of I Cr0 )24
0'75 mole % of I Cr04)2-
-3
b
go -5
-6
-7 ~~I--~IL2---1L3 --IL4---IL5---IL
'6---li
' 7---li'B---1~9~
Fig. I - Conductivity versus temperature plots of pure and
(Cr0 4 l-doped KBr crystals of different concentrations
863
The dc conductivity of samples was measured in
the temperature range 200 to 600 DC. For measuring
the conductivity, a two-probe conductivity ce ll was
used. The cell consists of two copper electrod e~ . The
sample mounted in between the electrodes are ke pt
in a furnace . For measuring the current, a potential
difference of 1.5 volts (dry battery) applied in
between the electrodes . The current was measured
by using a cary 40 I e lectromete r. The temperature
of the sample was measured with a chromel- alumel
thermocouple. The samples were annealed at hi gh
temperature of 600 DC for 3 hr and suddenly
quenched to room temperature by pouring the
sample on metal plate. Further, the quenched
samples were once again annealed at 500 DC
temperature for 2 hr and brought to room
temperature slowly by decreasing the te mperature at
the rate of 30 DC per hr. The conductivity of
quenched and annealed samples were measured at
identical conditions.
3 Results and Discussion
The results of the ionic conductivity of pure KBr
and KBr crystals doped with chromate ion, in
different percentages are shown in Fig. I. These are
plotted as log crT versus 10"'/T. The domains of
conductivity plots of intrinsic and extrinsic region s
have been marked as regions J, JT, III and fY
according to Drefus & Novick3.5·7. In the present
study, the plots show only regions II, 11I and fY.
Neither for grown crystals nor for quenched or
annealed samples the region I is shown.
In the present study, when the KBr crystals
doped with 0 .25 , 0.5 mole % of chromate ion , the
conductivity decreased in e xtrinsic associated and
When
the
dopant
un-associated
regions.
concentration increases from 0.5 to 0.75 mole % the
conductivity increases in extrinsic associated and
un-associated regions. Particularly, in un-associated
region, the conductivity tends to be same as that of
the pure KBr crystals.
Fig. 2 depicts the conductivity of KBr: (Cr04 f
crystals of 0.75 mole % of as-grown crystals,
quenched from 600 DC and annealed at 500 DC for 2
or 3 hr. From the plots, it is clear that, as the
samples are quenched from higher temperature, the
conductivity increases compared to as-grown
crystals. The annealed crystals conductivity lies in
intermediate state of pre-quenched (as-grown) and
quenched crystals in extrinsic associated region . The
864
INDIAN J PURE & APPL PHYS , VOL 40, DECEMB ER 2002
conductivity also vari ed in extrinsic un-assoc iated
region. But, as the te mperature of the sample
inc reases, the conduct ivity tends to be same as that
of the pre-que nched sa mpl es. The co rresponding
activati on e nergies, of each regton of diffe re nt
samples are shown in Tab le I .
Tab le I -
Activation e nergies o f pure KBr and
doped KBr crystals
Sample
KBr (Pure)
KBr +(CrO.j)2 (0.25 mole % )
KBr + (CrO.j)2. (0.5 mole % )
KRr +(CrO., f (0.75 mole %)
Quenched
A nnealtx l
Activation energies for
diffe rent regio ns (ev)
II
III
IV
0.1\6
0.9 1
I.lg
I. I I
0.91
1.00
1.03
1.17
1.35
1.23
1.11
1. 111
1.9X
I. X
2. 14
2.47
2.57
exactly in its surroundings". But, in case of divalent
cation-doped crystals, the cation vacancies are
favourably fo rmed in its surroundin gs, 7. '2 Thi s
anion vacancy effective ly be haves as (]j positive o ne.
Similarly, the background impurity li ke Me 2+ (Ca 2+
and others) present in the host mate ria l produces
pos itive io n vacancies for c harge compe nsation .
These positi ve ion vacancies be have effect ive ly as
negative. These two defects fo rm as a ne utral pair.
These neutral pairs do not co ntribute to
conductivity. Further, (CrO.,f ion forms co mpl exes
with background impuntt es of Mc 2+. These
co mpl exes are also not invol ved III the
conductivity5.
-2r-- - - - -- - - - -- - -- ---.
<>--<> KB r:lCr04)2-af 0 IS mal. %
ot
)f-.K
In alkali halide c rystal s, io ni c conducti vity is
mostly due to the mi grati on of cation vacancies at
lower te mpe ratures. At hi g h te mperatures, the
co ndu cti vity is due to mi grati on of cations as we ll as
ani ons. Th e mobi lity of anions are lesser th an the
cati o ns du e to hi g her ioni c radiu s. The mobility of
ani o n is negli g ibl e at lower te mpe ratures .
The io ni c conductivity of pure KBr c ry sta ls is
shown in Fig. I. The co nducti vity of pure crystals
was measured onl y up to 575 °C. The plot ex hibits
onl y the prominent regions as II , IH a nd IV region s.
T he condu ctivity mec hani sm of diffe rent regions
was disc ussed by various investi gators U 5 .7 . The
conducti vity plots of log crT versus IOJ/T o f
KB r:(Cr01f ' a lso de pi cts three regions for pure
c rysta ls. Whe n the crysta ls are doped with graduall y
increasing co ncentrati on of dopant , th e conductivity
first decreases and the n inc reases, but always
re ma lllS lower th an th at of pure crystal. The
va riati o n in conduc ti vity can be ex plained as
fo ll ows.
It is ascerta in ed by vari ous in vest igato rs",lo that,
impuri ty anion (CrO.jf admixture e nters in the KCI
latt ice, substituti o nall y, in th e CI ' site. In th e same
manner, th e c hromate io n e nte rs in the KBr c rysta l
matrix , substituti o na ll y, in the Br- s ite . Due to
a lo iva le nt impurity; the ex tra c harged negati ve
de fec t is introdu ced to keep the c rys ta l e lectrica ll y
ne utral. The negati ve de fect (or) the ani o n vacancy
is favo urabl e in the formation of vacancy not
05
grown Crysiois
6---0 Quenched fr om 6CXJ °c
An neoied 01 500 °c
-3
-
'"
'Eu
-4
'E
.c
o
I-
6
'" - 5
E
-6
- 7
I'~
I --I,~2--I-L
, 3---'
1 , 4--"L5-----L
' .6--..lI,;. - -,.L B
1
--'1'9
-\
Kl/T[KJ
Fig. 2 - The conduct i vity versu s temperature pl ots or K Br
crys tals doped w ith 0.75 mole 0/" or chromate ion or as-grown,
quenched and annealed crys tals
3.1 Effect of
conductivity
impurity
concentration
on
In genera l, when alkali ha lides li ke, KBr c rysta ls
are doped with impurity admixture, K 2 C rO,,, the
impurity admixture appears as a seco nd -o rde r phase
o n the surface of the crystals . Wh en conducti vity
SAIB ABU et a l.:IONIC CONDUCTIVIT Y IN CRYSTALS
was measured at gradu a ll y inc reasin g te mpe rature,
the n, the
second -orde r
impurity
prec ipitate
di ssoc iated as cati o ns and a ni ons and e ntered into
the c rys ta l. Due to the moti on of cati o ns and ani o ns,
a s ma ll c urre nt fl ows in the c rysta l. In the log crT
versus ( 10' /7) pl ot, it a ppeared as reg io n IV. The
regio n IV is ca ll ed a no ma lou s prec ipitate region (or)
ex trin s ic segregated reg io n. In thi s reg ion, it is very
diffi c ult to e xpl a in th e va ri ati on o f co ndu ctivity with
the in c rease in pe rcentage o f d opa nt concentrati on
(or) a ni o ns because, the formati on o f second-o rder
prec ipita te de pe nd s upon th e rate o f coo lin g .
With furth e r inc rease in te mpe ra ture, th e
impurity a ni on e nte rs int o th e c rys tal matrix a nd
substituti o na ll y occupi es in Br s ite. Acco rdin g to
earli e r d iscuss ion, more and mo re ex tra negati ve
defec ts are introdu ced in c rysta l matri x whi c h a re
occup y lll g
not
exactl y
JI1
the
nearest
ne ighbourh ood II .
T hese negati ve de fec ts be have e ffec ti ve ly
pos iti ve . S imil a rl y, du e to the presence o f
backgroun d impurity like, Me~ +(Cal+ a nd othe rs) in
host ma te ri a l, pos iti ve ion vacanc ies a re produced
fo r c harge co mpe nsati o n . These pos iti ve ion
vacanc ies be have as e ffecti ve ly negative. The
effecti ve ly pos iti ve a nd negati ve c ha rge carri e rs
be have as ne utra l pa irs du e to coul ombi c inte racti o n
fo rces. These pairs a re not in vo lved in the
conducti v ity, i.e., the concentrati o n o f Sc hottky
vacanc ies a re suppressed . As
the
(C rO~r'
concentrati on inc reases up to 0 .5 mo le 90 , more and
more ne utra l pairs a re formed . Due to thi s, th e
condu c ti vity dec reases in ex trin s ic assoc iated reg ion
(reg io n 1II) as compa red to pure c rysta ls.
Furthe r inc rease in concentra ti o n o f c hroma te to
0.75 mo le %, the conduc ti vity li es in between th a t
o f pure KBr c rysta ls a nd 0 .5 mo le % c hrom a tedoped c rysta ls. The inc rease in condu cti vity o f 0.75
mo le % of (C rO,,)"' in KBr is ma inly due to
introd ucti on o f fres h di s locati o ns in th e c rysta ls as a
result o f hi gher ioni c radiu s (3A) of (C rO,,) ]' th at
produ ces certa in loca l stress in its surroundin gs.
These fres h di s locati o ns possess a prope rt y to attract
the so lute a t o m sl ' I~. These so lute ato ms can eas il y
move a nd settl e do wn in a pos iti on o f lo w pote ntial
e ne rgy. As the te mpe ra ture in c reases, more and
more so lute a to m s prefe r to move to a pos iti on o f
low pote nti a l e ne rgy, du e to inc re ase in the rma l
e ne rgy a nd attracting di s locati o ns. Due to thi s, th e
865
conducti vity graduall y inc reases , in ex trin s ic
assoc ia ted region o r regio n III . He re, th e poss ibili ty
of oth e r mec ha ni s m is, as the te mpe ra ture inc reases,
the di s locati o ns a lso try to move in the c rys ta l
ma tri x. Due to moti on of these di s locati o ns,
condu cti vity may e nh a nce furth e r.
3.2 Effect of quenching and annealing on
conductivity
The th e rma l treatme nt of c rysta ls pl ays a vita l
ro le on th e ir e lec tri cal a nd mec ha ni ca l prope rti es. [n
the present studi es, (CrO.I )"· o f 0.75 mo le % doped
c rysta ls were que nc hed fro m 600 DC in a ir. T he
co nducti vity of these c rysta ls a re g reater th a n asgro wn c rysta ls . Th e inc rease in cond ucti vity is
ex pl a ined on the bas is of di sso luti o n o f Me 2. (C rO~f compl exes.
Whe n (CrO ~f d oped KBr c rysta ls a re a nnea led
a t hi g he r te mpe ra ture , th e Ca 1+ (C rO,,)"" (o r) M e~+­
(C rO"f compi exes a re d issoc iated as M e 2+ a nd
(C rO"f ions. These io ns a re ra nd o ml y di s tributed in
the sampl e, w he n the c rysta ls were qu e nc hed 10
roo m te mpe rature, the (C rO"r ' is unabl e to j o in w ilh
the impurity io n, due to sho rte r time ava il a bl e to
move to the nearest ne ighbour pos iti o n of M e"+ s ite.
Du e to fo rmati o n o f iso la ted M e~+ a nd (C rO,,) 2., the
concentra ti o n o f cati o n vacanc ies a nd a ni o n
vacan c ies IS in c reased . Due to In c rease in
concentra ti on of cati o n vacanc ies, the conducti vity
inc reases in ex trin s ic assoc ia ted reg io n (region III)' .
In case o f ex trin s ic u n-assoc ia ted reg ion at low
te mpe ra ture ra nge, th e cond uc ti vity a lso va ri es. Thi s
is ma inl y due to the rma l Iy-gene ra ted vacanc ie s,
whi c h a re frozen in the fo rm o f c luste rs. These
c lu ste rs are di ssoc ia ted w ith inc rease in te mperature
a nd destroy the m se lves a nd m a inta in the vacanc ies
conce ntrati on of as-grown c rys ta ls. S o, th e
conducti vity te nds to be same as tha t of as -grow n
sa mpl es in regio n-ll ( Refs. 5,7).
Th e qu e nc hed sa mpl es a re, o nce again , a nn ea led
a t 600 DC a nd th e co nducti v ity is s hown in F ig. 2.
Th e cond uc ti vit y of a nneal ed sampl e li es in
inte rmedi a te sta te of as-grow n a nd que nc hed
c ryslal s states. Thi s IS m a in ly att ributed 10
associati o n o f co mpl e xes with backgro un d impurit y
like Ca 2+ ions, o nce aga in . Du e to fo rmati o n or
co mpl exes, concentrati o n of cati on vacancies
dec reases and co nduc tivity is supp ressed in e xtrins ic
assoc iate regio n (region III ).
INDIAN 1 PU RE & APPL PHYS , VOL 40, DECEMB ER 2002
X66
In
ex trin sic
un -associated
region,
th e
condu ctivit y is mainl y due to thermall y-generated
vacancies and it is independ ent of impurity dopant
co nce ntrati on. The condu cti vity remains as that of
grow n crys tals.
Reddy A. ill di{l// .I Pllre & Appl Phrs , 40 (2002.) 290.
4
Prakash J, PhI's SIa/1lS So lidi (h ) 135 ( 19 ~6) 774 1.
5
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Proceedin gs or Illlem at iOll a l Wo rksholJ 0 11 Solid Slall'
Malerials, Malays ia , 199 1, pp. 49-54.
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Saih ahu G, PhD lhes is, Osmani a U ni Yersn y. Hyderahad.
India , I lJ95.
R
Chadw ick A C. Radial
9
Cook .l S. Dryden J S. Ferguson J & Gwan P B . .I Phvs C.
19 ( 19X6) 4999.
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Jain S C & Wa rTi er A V R. Opl ica l hl'i/(lvio llr.l· (~l
Illalerials , (Ed) Ka sluri L Chopra (Th omson Press. Indi a).
1973 . pp . 2:1 3-253.
II
Delll Y:lIwnkoyu P T. Syaschenko & Eerl an E M.
Tela . 12( 1970)3 124
12
A lybokov A A. Toi chiew N & Akc halnv S H, Phr.l· 51(//11 .1
Solidi (h), 109 ( 19R2) 295.
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COllrell & Bilhy 13 A. P/'()(' Pln's Soc fl . 62 (I ()4lJ ) 4lJ.
14
Solid .I'lale phrsics . (Ed) Ryogo Kuhn , Uni versit y ol'Tokyo
& Tak eo Naga mi ya. Osaka Uni versily. (McGraw-Hi II
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Ack nowledgemen t
The aut hors wish to th an k Prof P 0 Townsend,
Department of Physics and Astronomy, Univers it y
of Sussex, Brighton, UK, Prof M N Chary, Prof U V
Subba Rao and Dr K Nars imha Redd y, Department
of Phys ics , Osman ia University, Hyderabad , for
th eir useful suggesti ons , di scussions and constant
encourageme nt in bringin g out thi s paper. 0, e of
th e autho rs (GS B), wishes to thank , CPO South
Ce ntral Railway, Hyderabad and Sr DPO (SC
Di vision), Secunderabacl , for th eir constant
encou rage ment in brin ging out this paper.
Refere nces
Kess ler A . Phrs Slallls Solidi (a ). 1:1 1 ( 1992) 235 .
:2
Suzynska M & Cappell etli R. AC/(f Phys Sol A. RO ( 199 1)
l:2lJ.
.'
Saihahu G. Vasanlh Kumar N. Sri kan lh 0 & Ramachandra
EfT /)cji'l'I .I· Solids .
121 ( llJ92) 6 19.
Fi ~
TI'(,1'l1