Preparation and electrical properties of bismuth strontium dimanganese hexoxide [BiSrMn2O6]

Mat. R e s . B u l l . , Vol. 28, p p . 1329-1336, 1993. P r i n t e d in t h e USA. 0025-5408/93 $6.00 + .00 C o p y r i g h t (c) 1993 P e r g a m o n P r e s s L t d .

P R E P A R A T I O N AND E L E C T R I C A L PROPERTIES OF BISMUTH STRONTIUM DIMANGANESE H E X O X I D E [BiSrMn206 ] Zeng Zuotao,

Laboratory of Applied P.R.China

Ren Y u f a n g ,

and

Meng J i a n

of Rare Earth Chemistry and Physics, Changchun Institute Chemistry, Chinese Academy of Sciences, Changchun 130022

( R e c e i v e d J u n e 22, 1993; R e f e r e e d )

ABSTRACT BiSrMn206 i s p r e p a r e d by solid state reaction a t 8500C. I t is tetragonal w i t h a = 0 . 7 8 2 1 n m c = 0 . 3 7 9 0 nm. I t i s a b l a c k n - t y p e semiconductor below 820K. Its resistivity is 3~-CM at room temperature. A semiconductor -metal transition is observed around 8 2 0 K . Bil+×Srl_×Mn206_y i s a s o l i d s o l u t i o n for -0.2~ x~0.2. Its unit cell dimensions increase but resistivity decreases w h e n t h e Bi contents increase. MATERIALS INDEX: semiconductors

bismuth,

strontium,

manganese,

oxides,

INTRODUCTION Some m i x e d v a l e n c e compounds h a v e low r e s i s t i v i t y . The c h a r g e fluctuation make the electrons move e a s i l y in crystal lattices. Some these compounds are metallic or superconductive. Their electrical properties change dramatically when a element in these compounds is partly substituted by another element with different valence. For example, La2CuO ~ i s m e t a l l i c but Lal.85Sr0.sCuO A i s superconductive(l'2); Y2Cu/05 i s a insulator but YBa2Cu307 i s a superconductor (3'4) It is interested to study the electrical 1329

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Z. ZENG et al.

of mixed valent

Vol. 28, No. 12

compounds.

The v a l e n c e o f Mn c a n a l s o c h a n g e f r o m +2 t o +7. B o t h Bi2MnO 4 a n d BiMnO 3 are insulators. BiMnO 3 i s a d i s t o r t e d perovskite structure with triclinc pseudo-unit c e l l (5) Its electrical properties may be c h a n g e d i f Bi i s p a r t l y s u b s t i t u t e d by low v a l e n t element. Recently, T a r a s c o n (6) synthesizes BisSrsMn4025 by substituting for Cu by Mn i n the superconducting Bi2Sr2CuO 6 BisSrsMntO25 i s a l s o p e r o v s k i t e structure. We a l s o i n v e s t i g a t e d the s y s t e m of B i - S r - M n - O and found a compound BiSrMn~6.

EXPERIMENTAL

Analytical reagent SrCO 3, Bi203, MnO2 w e r e u s e d a s s t a r t i n g materials. The s t a r t i n g materials instoichiometric proportion were mixed, ground and sintered a t 8000C f o r 24 h. The r e s u l t i n g powders were reground, pressed and sintered a t 8500C f o r a week a n d t h e n c o o l e d t o room t e m p e r a t u r e in the furnace. Powder X-ray diffraction was c a r r i e d on a R i g a k u D / m a x - I I B X-ray diffractometer. A standard silicon p o w d e r s a m p l e was u s e d t o calibrate the experimental data. The method.

resistivities

were

measured

by

a

standard

The m i c r o s t r u c t u r e observation was c a r r i e d JXA-840 s c a n n i n g e l e c t r o n microscopy.

out

four-probe

on a JEOL

Oxygen contents and oxidation state o f Mn w e r e d e t e r m i n a t e d by a chemical analysis method. Samples are dissolved in hydrochloric acid and oxalic acid mixed solution a t 900C. As t h e solid solution can not dissolve in sulphuric a c i d , HCI h a s t o be used to dissolve the samples. Extra oxalic acid is titrated by KMnO4 s o l u t i o n a r o u n d 600C. CI: c a n be o x i d a t e d by KMnO4 b u t t h e process is slow. Therefore, the outcome is scarcely affected. The oxidation state o f Mn a n d o x y g e n c o n t e n t s c a n be c a l c u l a t e d from the titer.

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SEMICONDUCTORS

1331

RESULTS AND DISCUSSION

The c o m p o u n d BiSrMn206 h a s a t e t r a g o n a l unit cell with a = 0 . 7 8 2 1 nm a n d c = 0 . 3 7 9 0 nm. I t s o b s e r v a b l e density i s 7 g / c m 3. The c a l c u l a t i n g density i s 7 . 2 g / c m 3. The p o w d e r X - r a y d i f f r a c t i o n d a t a o f BiSrMn~O 6 a r e s h o w n i n T a b l e 1.

X-ray

TABLE 1 Pattern of

Powder

hkl

dobs (nm)

dca 1 (nm)

I/I 0

200 001 Iii

0.3914 0.3795 0.3118

0.3910 0.3790 0.3126

220 201

0.2764 0.2722

221 400 002 420 401 202

BiSrMn206

hkl

do~s (nm)

dca /(nm)

I/I O

18 9 25

241 222 440

0.1588 0.1562 0.1383

0.1588 0.1563 0.1383

23 14 4

0.2765 0.2722

49 I00

402 242

0.1360 0.1284

0.1361 0.1285

I0 1

0.2235 0.1955 0.1895 0.1749

0.2234 0.1955 0.1895 0.1749

26 26 12 2

061 260 261 223

0.1234

5

0.1176 0.1148

0.1233 0.1236 0.i176 0.1149

2 1

0.1737 0.1704

0.1738 0.1705

3 3

442 361

0.1117 0.1114

0.1117 0.1114

3 2

BiSrMn206 i s a n n - t y p e s e m i c o n d u c t o r b e l o w 820 K. I t i s b l a c k . Its resistivity is 3~-CM at room temperature. Its oxygen contents, which are determinated by chemical analysis method, are 6.02.The oxidation state o f Mn i s + 3 . 5 2 . In contrast with BiMnO 3, t h e resistivity o f BiSrMn206 i s v e r y l o w . T h e o x i d a t i o n state o f Mn i n BiMnO 3 i s + 3 . T h i s may m e a n t h a t t h e r e s i s t i v i t y of magnates which the oxidation state o f Mn i s n o t a n i n t e g e r is lower than that its oxidation state o f ~n i s e x a c t i n t e g e r . There is the same regular pattern for other compounds with those elements whose valence are changeable. For example, NiO i s n e a r l y an insulator, but its resistivity decreases dramatically (/} when L i r e p l a c e d partly Ni o r there are extra oxygen in its unit cells, thus making the oxidation state o f Mn d e v i a t e f r o m + 2 . Y2BaCuOs, w h i c h t h e o x i d a t i o n state of Cu i s + 2 , i s a i n s u l a t o r b u t YBa2Cu307, w h i c h t h e o x i d a t i o n state o f Cu i s + 2 . 3 , is a superconductor. T h i s may b e c a u s e d b y c h a r g e fluctuation. The charge fluctuation occurs easily when the

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oxidation in lattice

Vol. 28, No. 12

state is not an integer. The valence distribution of BiSrMn~6 can be shown as following:

MnB+

0 2-

Mn4÷

0 2-

0 2-

Mn3+

0 2-

Mnt+

Mn4÷

0 2-

Mn3÷

0 2-

O2-

MnI+

0 2-

Mn3+

o f Mn

Electrons move through Mn-O n e t s easily as the charge fluctuation o f Mn4÷-O2--Mn 3÷ o c c u r s in the lattices o f BiSrMn206. Therefore the resistivity o f BiSrMn206 i s l o w . T h e o x i d a t i o n state o f Mn i s e x a c t l y +3 f o r BiMnO 3. T h e r e are scarcely different valence Mn i o n s in its lattice. The occurrence of charge fluctuation is difficult, so its resistivity is very high. When t e m p e r a t u r e is lower than 820K, the resistivity BiSrMn~6 decreases with increasing temperature. The ln~ vs relation is shown in fig. 1.

of 1/T

0

~.-2 v I:I

'

-4 I

I

I

1

2

3

4

~o3/~(K -I ) Fig.

1.

Temperature

dependence

of

resistivity

of

BiSrMn~6

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SEMICONDUCTORS

1333

The a c t i v a t i o n energies o f t h e s a m p l e a r e 0 . 2 5 e v b e l o w 490K a n d 0 . 1 5 e v a b o v e 490K r e s p e c t i v e l y . There is a resistive minimum in lnp vs 1/T curve around 820K. Its resistivity increases with increasing temperature when t e m p e r a t u r e i s h i g h e r t h a n 8 2 0 K . The semiconductor-metal transition w i l l be s t u d i e d further. Bil+×Srl_×MnlO6_y i s a s o l i d solution for -0.2 ~ x 5 0.2. The variation of oxygen contents and oxidation state o f Mn w i t h x i s s h o w n i n t a b l e 2 . The o x y g e n c o n t e n t s increase but the oxidation state o f Mn d e c r e a s e s w h e n Bi c o n t e n t s increase. As Bi v a l e n c e ( + 3 ) is higher than St(+2), the substitution f o r S r By Bi m a k e s t h e oxygen contents increase or oxidation s t a t e o f Mn d e c r e a s e s for the balance in unit cells.

TABLE 2 Oxygen Contents

and Oxidation

Oxygen Contents -0.2

0 0.2

State

o f Mn D e p e n d e n c e o f Bi c o n t e n t s

Oxidation

State

5.95

3.55

6.02 6.10

3.52 3.49

o f Mn

The v a r i a t i o n of unit cell dimensions with x is shown in Fig.2. Both a and c increase w h e n Bi c o n t e n t s increase. As t h e i o n i c radium of Bi3+(O.O96nm) is smaller than Sr2+(O.113nm), Bi substituting SP s h o u l d make u n i t c e l l d i m e n s i o n s d e c r e a s e if oxygen contents did not change. However, the unit cell dimensions increase w h e n Bi s u b s t i t u t e s f o r S t . T h i s c a u s e d by t h e i n c r e m e n t o f o x y g e n content in unit cells w h e n Bi s u b s t i t u t e St. Fig.3. displays the SEM m i c r o g r a p h s of Bil.2SP0.sMn~O6 a n d BiSPMn206. B o t h t h e i r s h a p e s a n d s i z e s o f g r a i n s a r e s i m i l a r . Their grain sizes a r e a b o u t 3 um. T h e r e a r e s i m i l a r grain shapes and sizes in other s a m p l e o f t h e s y s t e m Bil+×Srl_.Mn~O6_y.% The p a c k s o f g r a i n s a r e more c o m p a c t , so t h e d e n s i t y ( 7 g / c ~ ) of sintered pellet is close to the theoretical density(7.2g/cm 3) o f BiSPMn206.

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"" eq E

Vol. 28, No. 12

O. 235 0.230 O. 225

0.785 <%780 v

0.775 0.385 o. 380 0.375 0

I

-0.2

Fig.

2.

Variation

of

-0.i

lattice

Fig.3a. SEM micrograph of BiSrMn~6

I

I

0

0.i

parameters

O. 2

o f Bil÷xSrFxMn206_ ~ w i t h

Fig.3b.

SEM m i c r o g r a p h

x

of

BilASr08Mn~6

Fig. 4. shows the v a r i a t i o n of r e s i s t i v i t y with x. R e s i s t i v i t y of Bi1÷xSr1_xMn206_y decreases when Bi contents increase. As there are not great v a r i a t i o n in the grain shapes and s i z e s among the samples of Bil+×SrF×Mn206_y, the e f f e c t of grain on e l e c t r i c a l p r o p e r t i e s can be neglected. In mixed valence compounds, valence f l u c t u a t i o n makes

Vol. 28, No. 12

SEMICONDUCTORS

1335

electrons move e a s i l y h e n c e e l e c t r o n s move m a i n l y t h r o u g h t h e a t o m s with changeable valence. I n Bil+×Srl_xMn206_y, t h e v a l e n c e o f B i , Mn and O are all changeable. From c h e m i c a l a n a l y s i s , We know t h a t t h e valence o f Mn i s b e t w e e n +3 a n d + 4 . As t h e c h a n g e f r o m Mn+3 t o Mn+a is easier than that f r o m Bi +3 t o Bi ÷5, e l e c t r o n s may move m a i n l y through Mn-O n e t s . The e l e c t r i c a l properties o f Bil÷×Srl_×Mn206_ s h o u l d be a f f e c t e d by t h e v a l e n c e f l u c t u a t i o n b e t w e e n Mn+3 a n d Mn+~ The o x i d a t i o n state o f Mn s h o u l d also affect the electrical properties of the solid solution. When Bi s u b s t i t u t e St, the oxidation state o f Mn d e c r e a s e . This means that the electron density on Mn-O n e t s increases and the density of carriers may increase as the solid solution is an N-type semiconductor. This makes the resistivity decreases with increasing Bi c o n t e n t s .

12

~8 4

J

0 -

.2

I

I

-0oi

0

I 9.2

0,I X

Fig.

4,

Resistivity

of

Bil+×Sri._×MnlO6_y a t

room

temperature

CONCLUSION

A c o m p o u n d BiSrMn206 is found. It is tetragonal w i t h a=O.7821nm. c = 0 . 3 7 9 0 nm. It is an N-type s e m i c o n d u c t o r below 820 K, In contrast with B i M n O 3, its r e s i s t i v i t y is low. A model is p r o p o s e d to e x p l a i n that. The o x i d a t i o n state of Mn is not an integer(+3.5) in B i S r M n ~ 6 . Charge f l u c t u a t i o n of Mn4+-O2--Mn 3÷ occurs in Mn-O net. That makes the e l e c t r o n i c transport easy. and so B i S r M n ~ 6 has a

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low r e s i s t i v i t y . An u n e x p e c t e d semiconductor-metal 820K. The r e a s o n n e e d be i n v e s t i g a t e d

transition further.

occurs

around

Bi]+xSr1_xMn206_y i s a s o l i d solution for -0.2 ~ x ~ 0.2. The i n c r e m e n t o f Bi c o n t e n t s makes t h e o x i d a t i o n state decrease. Resistivity a l s o d e c r e a s e s w i t h t h e i n c r e m e n t o f Bi c o n t e n t s . Since the variation of the oxidation state o f Mn c a n a f f e c t the electronic s t a t e and t h e d e n s i t y o f c a r r i e r s on mn-O n e t , i t may a l s o make r e s i s t i v i t y c h a n g e . From t h e s t u d y , we c a n s e e t h e relationship b e t w e e n t h e o x i d a t i o n s t a t e and e l e c t r i c a l properties, w h i c h i s i m p o r t a n t t o t h e s t u d y on t h e e l e c t r i c a l properties of mixed v a l e n c e compound.

A c k n o w l e d g e m e n t s - - T h i s work i s s u p p o r t e d by N a t i o n a l Fund o f C h i n a for Natural Scientific Research and L a b o r a t o r y of Rare Earth C h e m i s t r y and P h y s i c s .

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