Solid State Precursors: A Low-Temperature Route to Complex Oxides

7 Solid State Precursors: A Low-Temperature Route to Complex Oxides

Downloaded by MONASH UNIV on June 7, 2013 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1970-0186.ch007

J.

M.

LONGO,

H.

S. H O R O W I T Z , and L. R.

CLAVENNA

Corporate Research Laboratories, E x x o n Research and Engineering Company, L i n d e n , N J 07036

The

atomic

cursors

scale

having

synthesize

mixing

fully

reacted

lower temperatures for

structure

solid

temperatures

in solid

mixed-metal state

oxides at

pre-

have

required

techniques. yielded

These

oxides

with same

surface

areas that are 10 to 100 times higher

than the

oxides

prepared

This

technique

by

portion

tures below 1000°C subsolidus

perature Mn O , 3

8

methods.

synthesis

also has been used to study phase relations

manganese-rich new

conventional

6

Ca-Mn-O

and has resulted

phase diagram

phases CaMn O , 3

of the

having the CaMn O , 4

8

system at

in the tempera-

in the assemblage

containing

several

of a

low-tem-

following compositions: and CaMn O . 7

to

significantly

times than are

synthesis

of reaction

solution

results in the ability

and in shorter

conventional

lower

of cations

the calcite

Ca2-

12

l i y l T e t a l oxides c o n t a i n i n g m o r e t h a n one t y p e of c a t i o n are of interest f r o m b o t h a p r a c t i c a l a n d f u n d a m e n t a l p o i n t of v i e w . oxides

Complex

are able to s t a b i l i z e u n u s u a l o x i d a t i o n states, h a v e significant

nonstoichiometry, a n d contain u n i q u e structural arrangements.

However,

t h e y are l i m i t e d i n several a p p l i c a t i o n s , e s p e c i a l l y c a t a l y t i c a p p l i c a t i o n s , because of the l o w surface area that results f r o m the h i g h t e m p e r a t u r e s u s u a l l y n e e d e d to o b t a i n c o m p l e t e r e a c t i o n . T h e t r a d i t i o n a l c e r a m i c approaches to these c o m p l e x m e t a l oxides i n v o l v e r e p e a t e d h i g h - t e m p e r a t u r e firing of t h e c o m p o n e n t oxides w i t h f r e q u e n t r e g r i n d i n g s . T h e s e h a r s h c o n d i t i o n s are r e q u i r e d to o v e r c o m e the s l o w

reaction kinetics that occur

when

two

solids are

brought

together, as is i l l u s t r a t e d i n F i g u r e 1. T h e r e a c t a n t p a r t i c l e s are s c h e m a t i c 0-8412-0472-l/80/33-186-139$05.00/l ©

1980

American Chemical

Society

In Solid State Chemistry: A Contemporary Overview; Holt, S., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1980.

140

SOLID S T A T E

CHEMISTRY: A

CONTEMPORARY OVERVIEW

100,000 8 TO 2008

Figure 1. Conventional solid state reaction techniques give slow reaction kinetics.


a l l y i l l u s t r a t e d to e m p h a s i z e t h e f a c t t h a t n o m a t t e r w h a t the p a r t i c l e size, e a c h r e a c t a n t p a r t i c l e contains o n l y one t y p e of c a t i o n .

Severe

r e a c t i o n c o n d i t i o n s are necessary to o b t a i n a single-phase p r o d u c t b e c a u s e of t h e d i f f u s i o n a l l i m i t a t i o n s of s o l i d state reactions.

I n i t i a l r e a c t i o n is

r a p i d , b u t f u r t h e r r e a c t i o n goes s l o w e r a n d s l o w e r as the p r o d u c t l a y e r b u i l d s u p a n d diffusion paths b e c o m e longer. A fine p o w d e r of a p p r o x i m a t e l y 10-jum p a r t i c l e size (100,000 A ) s t i l l represents d i f f u s i o n distances o n t h e o r d e r of 10,000 u n i t c e l l d i m e n s i o n s . T h e use of t e c h n i q u e s s u c h as f r e e z e - d r y i n g the c o m p o n e n t

(1,2)

o r c o p r e c i p i t a t i o n (3,4)

i m p r o v e s r e a c t i v i t y of

oxides o r salts because these m e t h o d s

can give

initial

crystallites o n t h e o r d e r of o n l y s e v e r a l h u n d r e d angstroms i n d i a m e t e r . B u t this s t i l l means d i f f u s i o n m u s t o c c u r across 10 to 50 u n i t cells. The

severity of r e a c t i o n c o n d i t i o n s

necessary

diffusional

l i m i t a t i o n s n a t u r a l l y leads

to

materials.

A d d i t i o n a l l y , the h i g h temperatures

to o v e r c o m e

crystalline,

these

low-surface-area

that must be

utilized

l i m i t t h e a b i l i t y to s t a b i l i z e the h i g h e r v a l e n c e states of t r a n s i t i o n m e t a l elements. I d e a l l y , i n o r d e r to a c h i e v e c o m p l e t e r e a c t i o n i n t h e shortest a m o u n t of t i m e a n d at t h e l o w e s t p o s s i b l e t e m p e r a t u r e , one w o u l d l i k e to m i x i n g of the c o m p o n e n t cursors (5,6)

cations o n a n a t o m i c scale.

Compound

see pre

w i l l a c h i e v e this g o a l , b u t t h e s t o i c h i o m e t r y of the p r e

cursor, u n f o r t u n a t e l y , o f t e n does not c o i n c i d e w i t h the s t o i c h i o m e t r y of the desired product.

T h e use of s o l i d s o l u t i o n p r e c u r s o r s ( 7 )

provides

a l l the advantages of c o m p o u n d precursors b u t a v o i d s the s t o i c h i o m e t r y limitations. A extent)

s o l i d s o l u t i o n m a y b e o b t a i n e d b y s u b s t i t u t i o n (to

a variable

of one e l e m e n t i n a host l a t t i c e b y another element, s u c h t h a t

the c r y s t a l l o g r a p h i c s y m m e t r y of the host l a t t i c e is n o t a l t e r e d . F o r t h e purposes of this d i s c u s s i o n , a s o l i d s o l u t i o n m a y be c o n s i d e r e d to b e the interpenetration on

an atomic

scale of

two

c h e m i c a l l y different

but

s t r u c t u r a l l y s i m i l a r lattices. C o n c e p t u a l l y , t h e n , i f one w a n t s to p r e p a r e a c o m p l e x o x i d e at l o w t e m p e r a t u r e s , one c a n f o r m a s o l i d s o l u t i o n b e t w e e n


7.

Solid State

LONGO E T A L .

141

Precursors

t w o ( o r m o r e ) cations i n the p r o p e r r a t i o , h a v i n g a n a n i o n l a t t i c e t h a t c a n b e m o d i f i e d . T h u s i f a s o l i d s o l u t i o n c a r b o n a t e is s y n t h e s i z e d , i t c a n r a p i d l y b e d e c o m p o s e d to a n o x i d e ; l i k e w i s e , i f a s o l i d s o l u t i o n o x i d e p r e c u r s o r is p r e p a r e d , it c a n either b e r e d u c e d o r o x i d i z e d to t h e d e s i r e d p h a s e d e p e n d i n g o n the v a l e n c e states of t h e cations.

I n e a c h of these

cases the cations are a l r e a d y m i x e d o n a n a t o m i c scale i n the single-phase p r e c u r s o r so t h a t d e c o m p o s i t i o n ,

o x i d a t i o n , or r e d u c t i o n is r a p i d a n d

c o m p l e t e at s i g n i f i c a n t l y l o w e r temperatures a n d shorter t i m e s . F i g u r e 2 s c h e m a t i c a l l y illustrates the s o l i d s o l u t i o n p r e c u r s o r c o n c e p t .

Whereas

the r e a c t a n t cations m a y be 100,000 A a p a r t i n c o n v e n t i o n a l s o l i d state reactions ( F i g u r e 1 ) , i n s o l i d s o l u t i o n precursors t h e y are o n t h e o r d e r


of 10 A apart, regardless of p a r t i c l e size. F u r t h e r m o r e , the n a t u r e of t h e s o l i d s o l u t i o n is s u c h t h a t it is p o s s i b l e to c o n t i n u o u s l y v a r y the c a t i o n c o m p o s i t i o n i n the s t r u c t u r e , a n d one is not l i m i t e d to discrete

com

p o u n d precursors. T h e s o l i d s o l u t i o n p r e c u r s o r t e c h n i q u e has, to date, b e e n e x p l o i t e d i n this l a b o r a t o r y p r i m a r i l y for the p r e p a r a t i o n of m i x e d - m e t a l oxides b y d e c o m p o s i t i o n

high-surface-area,

of s o l i d solutions of

carbonates

h a v i n g the c a l c i t e structure. T h i s t e c h n i q u e has w i d e a p p l i c a b i l i t y since the carbonates of C a , M g , Z n , M n , F e , C o , N i , a n d C d a l l f o r m the c a l c i t e structure a n d w i l l , i n g e n e r a l , f o r m s o l i d solutions w i t h e a c h other.

In

this p a p e r w e w i l l d e s c r i b e o u r experience w i t h the s o l i d s o l u t i o n p r e c u r s o r m e t h o d w h e n a p p l i e d to several of these m i x e d - m e t a l systems. T h e most o b v i o u s benefit of this synthesis t e c h n i q u e is that i t gives a r o u t e to h i g h e r - s u r f a c e - a r e a c o m p l e x oxides.

Just as i m p o r t a n t , h o w

ever, is the a p p r o a c h p r o v i d e d b y the s o l i d s o l u t i o n p r e c u r s o r t e c h n i q u e to the d i s c o v e r y of t o t a l l y n e w m a t e r i a l s that are not stable at the h i g h e r temperatures u s u a l l y necessary f o r c o n v e n t i o n a l s o l i d state r e a c t i o n b e tween s m a l l particles. T h e C a - M n - O system w a s chosen as a s t a r t i n g p o i n t i n o r d e r to explore the p o t e n t i a l of the s o l i d s o l u t i o n p r e c u r s o r m e t h o d f o r s y n t h e -

Figure 2. Solid solution precursor techniques give fast reaction kinetics


142

SOLID S T A T E

CHEMISTRY: A


s i z i n g n e w c o m p l e x oxides. T h i s system fulfills the p r i m a r y r e q u i r e m e n t f o r s u c h a s t u d y i n that C a C 0

3

and M n C 0

3

are i s o s t r u c t u r a l , e a c h

d i s p l a y i n g the c a l c i t e c r y s t a l structure. T h u s i t is p o s s i b l e to p r e p a r e C a - M n carbonate C a - M n oxides.

s o l i d s o l u t i o n precursors for s u b s e q u e n t

r e a c t i o n to

T h e r e are a d d i t i o n a l characteristics t h a t m a k e the C a -

M n - O system a p a r t i c u l a r l y versatile one for e x p l o r i n g the p r o p e r t i e s of new

m i x e d - m e t a l oxides.

F i r s t of a l l , t h e M n i o n is a v e r y flexible

t r a n s i t i o n m e t a l o c c u r r i n g i n the s o l i d state as the 2 + cations.

through the

W h e n i n c o r p o r a t e d a l o n g w i t h the electropositive

c r y s t a l s t r u c t u r e , the h i g h e r v a l e n c e

states of

7+

C a into a

M n can be stabilized,

e s p e c i a l l y at l o w t e m p e r a t u r e .


I n this p a p e r w e d e s c r i b e p r e v i o u s l y u n a t t a i n a b l e l o w - t e m p e r a t u r e phase relations i n the m a n g a n e s e - r i c h p o r t i o n of the C a - M n - O system. T h i s n e w low-temperature phase d i a g r a m ( F i g u r e 3)

shows t h e

com

p l e x i t y p o s s i b l e w h e n a c a t i o n c a n h a v e m u l t i p l e valences a n d w h e n t h e r e is sufficient r e a c t i v i t y to o b t a i n e q u i l i b r i u m at l o w t e m p e r a t u r e s .

The

use of the e x t r e m e l y r e a c t i v e s o l i d s o l u t i o n p r e c u r s o r s , C a i ^ M n ^ C O s , also has a l l o w e d us to m o n i t o r synthesis p a r a m e t e r s t h a t are u s u a l l y o b s c u r e d b y conventional high-temperature reaction conditions.

A c c o r d i n g l y , the

influence of h e a t i n g rate, o x y g e n p a r t i a l pressure, p a r t i c l e m o r p h o l o g y , t e m p e r a t u r e , a n d r e s i d u a l surface species o n the final p r o d u c t w i l l

be

discussed. Experimental M o s t m a t e r i a l s syntheses r e f e r r e d to i n this r e p o r t w e r e c a r r i e d out b y u s i n g the s o l i d s o l u t i o n p r e c u r s o r m e t h o d ( 7 ) . P r e c u r s o r s w e r e p r e p a r e d b y p r e c i p i t a t i n g the carbonates f r o m a w e a k l y a c i d i c s o l u t i o n of the a p p r o p r i a t e cations i n the d e s i r e d s t o i c h i o m e t r y . A m m o n i u m c a r b o n a t e w a s t h e p r e c i p i t a t i n g agent. T h e s o l i d s o l u t i o n p r e c u r s o r s w e r e r e a c t e d at t e m p e r a t u r e s r a n g i n g f r o m 800° to 1 0 0 0 ° C for t i m e s r a n g i n g f r o m 0.5 to 150 h r . T h e r e a c t i o n atmosphere w a s p u r e flowing 0 , unless o t h e r w i s e specified. T h e c o n v e n t i o n a l s o l i d state r e a c t i o n syntheses t h a t w e r e c a r r i e d o u t w e r e a c c o m p l i s h e d b y h a n d - g r i n d i n g , w i t h a n agate m o r t a r a n d pestle, the c a l c i u m a n d m a n g a n e s e reagentg r a d e carbonates a n d t h e n firing i n o x y g e n at the specified t e m p e r a t u r e s . T h e s e firings w e r e u s u a l l y i n t e r r u p t e d at f r e q u e n t i n t e r v a l s for a d d i t i o n a l g r i n d i n g i n o r d e r to f a c i l i t a t e the r e a c t i o n . A l l r e a c t i o n p r o d u c t s a n d s o l i d s o l u t i o n precursors w e r e e x a m i n e d o n a P h i l l i p s X - r a y diffractometer to d e t e r m i n e w h i c h phases w e r e present. O x y g e n content of a l l C a - M n o x i d e phases w a s e s t a b l i s h e d b y u s i n g a Fisher Thermogravimetric A n a l y z e r containing a C a h n electrobalance. S a m p l e s w e r e r e d u c e d i n H a n d w e i g h t loss w a s a t t r i b u t e d to m a n g a n e s e w i t h o x i d a t i o n states h i g h e r t h a n 2 + . T h e average m a n g a n e s e v a l e n c e w a s also d e t e r m i n e d b y w e t c h e m i c a l means ( 8 ) . T h e m e t h o d f o r e x p e r i m e n t a l l y m e a s u r i n g the c a t i o n s t o i c h i o m e t r y , as w e l l as t h e p r o c e d u r e f o r d e t e r m i n i n g t h e d e c o m p o s i t i o n t e m p e r a t u r e of t h e l o w - t e m p e r a t u r e C a - M n o x i d e phases, is d e t a i l e d i n H o r o w i t z et a l . ( 9 ) . 2

2


7.

Solid State

LONGO E T A L .

1

1

i

143

Precursors

1

I

1

1

T(°C) 1000

-

CaMn 04 + Tet. Mn 0 2

CaMn0 + CaMnoOd

3

4

3


950

-

CaMn 0 + Mn 03 2

4

2

-

CaMn 04 +CaMn 0 2

7

12

CaMr^Og CaMn 0 7

12

-

900

CaMn03

850

S

Ca2Mri30g CaMn 0 + + CaMn 04 CaMn 0g 2

Ca2Mn303

00 o

2

4

OP

.oc

3

Z u

CaMn 0g + CaMn 0 4

+

7

12

CaMn 0 + Mn 0 7

2

12

3

CO

i

1

CaMn0 3.

1

Mol. % Mn0

3

Figure

•

Mn0

x

x

Isobaric (P = 1.0 atm) subsolidus phase relations manganese-rich portion of the Ca-Mn-O system 02

in the

Specific surface areas w e r e d e t e r m i n e d b y the B r u n a u e r - E m m e t t Teller ( B E T ) method, using nitrogen adsorption. Single-point determina tions w e r e u s e d ; these w e r e f o u n d to agree w i t h i n 5 % of t r i p l e - p o i n t determinations. Results F o l l o w i n g the experimental procedures carbonate

precipitates was prepared.

the p r e c i p i t a t e s

were

single-phase

o u t l i n e d a b o v e , a series of

X - r a y diffraction confirmed

solid

solutions

h a v i n g the

that

calcite

c r y s t a l structure. T h u s the h o m o g e n e o u s m i x i n g of m e t a l cations o n a n a t o m i c scale ( a b o u t 10 A ) w a s a c h i e v e d .

T a b l e I lists t h e i n t e r p l a n a r

spacings of the m a j o r X - r a y d i f f r a c t i o n peaks for some of t h e m i x e d - m e t a l


144

SOLID S T A T E

CHEMISTRY: A


Table I. fhkU Compound CaC0 [Cao67Mno33]C0 [Cao.6oMn .4o]C0 [Cao.5oMn .5o]C0 [Ca 29Mn 7i]CO [ C a 25Mn 75]CO [Cao.2oMn . o]C0 [Ca .i 5Mn .875]CO [Ca .o8Mn .92]C0 MnC0 [Cd .iiMn 9]CO [Cd .i25Mn .875]CO [Cd . Mno. 7]C0 [Cd . oMn .5o]C0 CdC0 CoC0 [Co .67Mn . ]CO [Co .4oMn .6o]C0 MnC0

(104)

(012)

3.03 2.99 2.97 2.94 2.91 2.90 2.90 2.88 2.87 2.85 2.86 2.87 2.88 2.91 2.94 2.74 2.80 2.83 2.85

3.85 3.80 3.77 3.75 3.74 3.71 3.71 3.70 3.70 3.67 3.67 3.70 3.71 3.74 3.80 3.55 3.61 3.65 3.67

3

3

0

3

0

3

0

0

0

3

0

3

0

0

2

8

3

0

0

0

3

3


3

0

0

0

8

3

0

0

3 3

0

5

6

0

3

3

3

3

3

0

0

0

3 3

0

3

3

3

carbonate

precursors p r e p a r e d .

T h e corresponding

data for the

end-

m e m b e r carbonates are also i n c l u d e d f o r c o m p a r i s o n . A s T a b l e I shows, t h e systems d e s c r i b e d h e r e ( C a - M n , C d - M n , C o M n ) a d h e r e f a i r l y closely t o V e g a r d ' s l a w . T h e r e f o r e , the m o n i t o r i n g of l a t t i c e p a r a m e t e r s , o r of a specific i n t e r p l a n a r s p a c i n g , p r o v i d e s a c o n v e n i e n t m e t h o d f o r c h e c k i n g , to a first a p p r o x i m a t i o n , t h e

composition

of a single-phase p r e c i p i t a t e . F o r c o m p o s i t i o n a l i n c r e m e n t s of t h e sizes that are seen i n T a b l e I , the changes i n i n t e r p l a n a r s p a c i n g are g e n e r a l l y r e s o l v a b l e , a n d the p r e c i p i t a t e s of d i f f e r i n g c o m p o s i t i o n s c a n b e r o u g h l y identified

and

estimates

of

distinguished from

composition

one

another.

When

are d e s i r e d , t h i s a p p r o a c h

is

more

precise

unacceptable

because the h i g h - s u r f a c e - a r e a p r e c i p i t a t e s h a v e X - r a y d i f f r a c t i o n patterns w h o s e p e a k s are too b r o a d to m a k e p r e c i s e l a t t i c e p a r a m e t e r d e t e r m i n a t i o n possible. precursor

B e c a u s e of this u n c e r t a i n t y i n t h e c o m p o s i t i o n

(the

metal cation stoichiometry

of

of

the

the p r e c u r s o r w a s

not

a l w a y s i n exact agreement w i t h the s t o i c h i o m e t r y of the i n i t i a l a q u e o u s solution

of

metal cations),

compositional

analysis for

metal

cation

s t o i c h i o m e t r y w a s p e r f o r m e d o n the r e a c t e d m i x e d - m e t a l oxides. T h e i m p o r t a n c e of p r e c u r s o r p a r t i c l e size w a s m i n i m i z e d i n o u r i n t r o d u c t i o n to the s o l i d s o l u t i o n p r e c u r s o r c o n c e p t ; h o w e v e r ,

it can

s t i l l h a v e a n i m p o r t a n t effect o n r e a c t i o n k i n e t i c s . A l t h o u g h cations are m i x e d o n a n a t o m i c scale w i t h i n e a c h reactant p a r t i c l e , the size of t h e


7.

LONGO E T A L .

Interplanar

145

Solid State Precursors

Spacings fhkU


(IIS) 2.27 2.25 2.24 2.23 2.21 2.21 2.20 2.19 2.19 2.17 2.19 2.19 2.19 2.21 2.25 2.11 2.15 2.16 2.17

(202)

(HO)

(116)

(024)

2.09 2.07 2.05 2.05 2.03 2.03 2.02 2.01 2.01 2.00 2.01 2.01 2.02 2.04 2.07 1.95 1.99 1.99 2.00

2.49 2.46 2.45 2.44 2.43 2.42 2.42 2.41 2.40 2.40 2.40 2.40 2.42 2.43 2.46 2.33 2.37 2.38 2.40

1.87 1.84 1.84 1.83 1.81 1.80 1.79 1.78 1.78 1.77 1.77 1.78 1.79 1.81 1.84 1.70 1.75 1.75 1.77

1.90 1.89 1.86 1.86 1.86 1.86 1.86 1.85 1.84 1.83 1.84 1.84 1.85 1.87 1.89 1.78 1.81 1.82 1.83

reactant p a r t i c l e c a n s t i l l l i m i t t h e extent of g a s - s o l i d c o n t a c t a n d heat transfer t h r o u g h the p a r t i c l e . T a b l e I I shows h o w precursor

c a n be m o d i f i e d

procedure.

the B E T surface

a r e a of t h e s o l i d s o l u t i o n

b y m i n o r adjustments

to t h e

precipitation

R e f e r r i n g to T a b l e I I , one c a n see that a 1:1 C a : M n r a t i o

solid solution precursor powder, w h i c h was recovered and dried i m m e d i a t e l y after p r e c i p i t a t i o n , has a surface

a r e a of 9 m

2

• g" . 1

With

the

t h o u g h t t h a t the p r e s e n c e of r e s i d u a l electrolyte, i n the f o r m of a m m o n i u m carbonate,

might

be

causing

agglomeration

of

particles

and

thereby

l o w e r i n g the a p p a r e n t surface area, another b a t c h w a s p r e p a r e d i n w h i c h the n o r m a l p r e c i p i t a t i o n w a s f o l l o w e d cedure.

b y a thorough water rinse p r o

A f t e r i t w a s d r i e d , t h e p o w d e r was m u c h finer i n

appearance

a n d m o r e free f l o w i n g t h a n the u n t r e a t e d p o w d e r , a n d its s u r f a c e

Table II.

Surface A r e a of Precursors C a n Be Modified Surface Area No Treatment

MnC0 CaMn(C0 ) CaC0 3

3

3

area

2

66 9 1

l w Rinse

(m

2

• g' ) 1

H0

+

2

IP A

Rinse

110 32 11


150 74 10

146

SOLID S T A T E

s h o w e d a n increase to 32 m

2

CHEMISTRY: A


• g" . I n order to f u r t h e r decrease a g g l o m 1

e r a t i o n , w e m a d e a n a t t e m p t to d i s p l a c e r e s i d u a l w a t e r w i t h the l o w e r surface-tension i s o p r o p y l a l c o h o l ( I P A ) b y f o l l o w i n g p r e c i p i t a t i o n w i t h c o n s e c u t i v e w a t e r a n d I P A rinses. T h e p o w d e r , w h i c h w a s a g a i n i n appearance

finer

a n d m o r e free f l o w i n g t h a n t h e u n t r e a t e d p r e c i p i t a t e ,

s h o w e d a n o t h e r increase i n surface area to 74 m

2

• g" . 1

T a b l e I I also i n d i c a t e s t h a t a d d i t i o n a l batches of p r e c i p i t a t e s w e r e p r e p a r e d as controls.

The M n C 0

3

p r e c i p i t a t e shows the same t r e n d of

i n c r e a s i n g surface area w i t h the w a t e r rinse a n d a n o t h e r increase w i t h the w a t e r p l u s I P A r i n s e . T h e C a C 0

3

shows a l a r g e increase i n surface

a r e a for t h e w a t e r rinse a n d the w a t e r p l u s I P A rinse w h e n c o m p a r e d t o


t h e u n t r e a t e d p r e c i p i t a t e , b u t does n o t s h o w a significant

difference

between the water a n d water plus I P A - r i n s e d powders. T h e s e results are of p r e l i m i n a r y n a t u r e a n d t h e i r i m p l i c a t i o n s are not f u l l y u n d e r s t o o d . improvements

H o w e v e r , it does a p p e a r that r e a l a n d significant

i n surface

area

can

be

effected

by

relatively

minor

m o d i f i c a t i o n s i n the p r e c i p i t a t i o n p r o c e d u r e . A s m e n t i o n e d before, m a g n e s i u m , c a l c i u m , a n d most of the

first-row

d i v a l e n t t r a n s i t i o n elements f o r m the c a l c i t e structure. T h e o r e t i c a l c r y s t a l c h e m i s t r y considerations w o u l d l e a d one to e x p e c t t h a t i t is p o s s i b l e t o p r e p a r e , b y the process d e s c r i b e d ,

single-phase

solid solution

calcite

p r e c i p i t a t e s of almost a l l c o m b i n a t i o n s of these elements. E x p e r i m e n t a l results w i t h one s u c h c o m b i n a t i o n

(nickel and manganese),

however,

r e v e a l e d that the p r e c i p i t a t e p r e p a r e d i n this case w a s not a single-phase c a l c i t e s t r u c t u r e m a t e r i a l b u t a p r e c i p i t a t e c o n s i s t i n g of M n C 0 c a l c i t e s t r u c t u r e a n d a n a m o r p h o u s N i - c o n t a i n i n g phase. f r o m t h e results of

with a

3

I t is e v i d e n t

this e x p e r i m e n t t h a t t h e s o l i d s o l u t i o n

precursor

c o n c e p t is not as u n i v e r s a l as it first a p p e a r e d , a n d that t h e a p p l i c a b i l i t y of this m e t h o d for a n y c o n t e m p l a t e d c o m b i n a t i o n of elements m u s t b e experimentally determined. A f t e r h a v i n g p r e p a r e d a n d c h a r a c t e r i z e d the s o l i d s o l u t i o n c a l c i t e p r e c u r s o r , o n e m a y fire i t at a p r e s e l e c t e d t e m p e r a t u r e

(generally

less

t h a n 1 0 0 0 ° C ) a n d f o r a t i m e sufficient to y i e l d the d e s i r e d h i g h - s u r f a c e a r e a m i x e d - m e t a l oxide.

S i n c e the cations are a l r e a d y h o m o g e n e o u s l y

m i x e d o n a n a t o m i c scale i n the p r e c u r s o r , the d e c o m p o s i t i o n of the c a l c i t e to the f u l l y r e a c t e d , m i x e d - m e t a l o x i d e takes p l a c e at s i g n i f i c a n t l y l o w e r t e m p e r a t u r e s a n d i n shorter times r e l a t i v e to synthesis b y

conventional

s o l i d state t e c h n i q u e s . In

the

C a - M n - O system,

for

example,

the

preparation

of

the

p e r o v s k i t e , C a M n 0 , b y s t a n d a r d s o l i d state r e a c t i o n r e q u i r e s h e a t i n g of 3

the c o m p o n e n t oxides or carbonates at 1 3 0 0 ° C i n a n o x y g e n - c o n t a i n i n g atmosphere

f o r several days w i t h f r e q u e n t r e g r i n d i n g .

The resulting

p r o d u c t is v e r y c r y s t a l l i n e a n d has a surface area of o n l y 0.2 m


2

• g" . 1

7.

Solid State

LONGO E T A L .

T h e same p u r e c o m p o u n d

147

Precursors

c a n be prepared b y decomposing,

o x i d i z i n g c o n d i t i o n s , a 1:1 C a : M n

under

solid solution calcite precursor at

9 0 0 ° C f o r 3 0 m i n . T h e surface area o f this f u l l y r e a c t e d p e r o v s k i t e , C a M n 0 , is 11 m 3

2

• g" . 1

T h e fully reacted perovskite-related oxide C a M n 0 2

p a r e d w i t h a surface area of 17 m

2

has b e e n p r e

4

• g" b y reacting a 2:1 C a : M n solid 1

s o l u t i o n c a l c i t e p r e c u r s o r at 8 0 0 ° C i n a i r for 15 m i n . S t a n d a r d s o l i d state r e a c t i o n of t h e same c o m p o u n d f r o m t h e c o m p o n e n t carbonates

(1300°C

f o r several days w i t h f r e q u e n t r e g r i n d i n g s ) gives a surface area of o n l y 0.8 m

• g" .

2

1

I n the C d - M n - O system the c o m p o u n d C d M n 0 8 has b e e n p r e p a r e d 2

3


at 5 0 0 ° C f o r 1 h r , i n o x y g e n f r o m a s o l i d s o l u t i o n c a l c i t e p r e c u r s o r h a v i n g a 2 : 3 C d : M n r a t i o . T h i s c o m p o u n d has a surface area o f 9 8 m

• g " ; t h e same

2

1

compound

p r e p a r e d b y c o n v e n t i o n a l s o l i d state

r e a c t i o n h a d a surface area o f 3 m • g " . 2

1

I n g e n e r a l , the s o l i d s o l u t i o n p r e c u r s o r m e t h o d gives i m p r o v e m e n t s i n surface area r e l a t i v e t o c o n v e n t i o n a l s o l i d state t e c h n i q u e s t h a t r a n g e f r o m a f a c t o r of 10 to 100. A n a d d i t i o n a l benefit t h a t c a n b e d e r i v e d f r o m t h e r e l a t i v e l y l o w synthesis t e m p e r a t u r e s afforded b y the s o l i d s o l u t i o n p r e c u r s o r t e c h n i q u e is the a b i l i t y t o synthesize t o t a l l y n e w m a t e r i a l s . F i g u r e 3 shows t h a t the l o w reaction temperatures made possible b y the solid solution pre cursors reveals f a i r l y

complex

subsolidus relations i n t h e C a - M n - O

system, i n c l u d i n g f o u r c o m p o u n d s

t h a t a r e n o t stable a t 1 a t m 0

2

a b o v e 1000°C. T h e first c o m p o u n d t o a p p e a r i s C a M n O i , w h i c h is also the most 7

2

stable m i x e d - v a l e n c e c o m p o u n d o f t h e system. I t c o n t a i n s six M n one M n

and a C a

and

3 +

a n d has a s t r u c t u r e r e l a t e d t o p e r o v s k i t e ( 1 0 ) , w i t h t h r e e M n

4 +

2 +

3 +

o n the A site. T h i s c o m p o u n d w a s first r e p o r t e d b y B o c h u

et a l . (10), w h o e m p l o y e d pressures of 80 k b a r a n d r e a c t i o n t e m p e r a t u r e s of 1 0 0 0 ° C f o r its synthesis. J o u b e r t , i n a p r i v a t e c o m m u n i c a t i o n , reports t h a t t h e y also h a v e b e e n a b l e t o p r e p a r e this p h a s e w i t h o u t h i g h p r e s sure ( I I ) . A b o v e 9 6 0 ° C , C a M n 0 i 7

2

breaks d o w n i n t o C a M n 0 a n d 2

4

Mn 0 . 2

3

A t l o w e r t e m p e r a t u r e s ( T < 940° C ) a n o t h e r m i x e d - v a l e n c e a p p e a r s w i t h a C a / M n r a t i o of 1 / 3 .

shows that o n e - t h i r d o f the m a n g a n e s e is present as M n i n d i c a t e s a f o r m u l a of C a M n 0 . 3

6

phase

T h e r m o g r a v i m e t r i c analysis i n H 4 +

a n d therefore

T o u s s a i n t (12) reports a C a M n 0 i n 3

7

h i s s t u d y , b u t his p u b l i s h e d X - r a y p a t t e r n shows o n l y t h e s t r o n g l i n e s o f CaMn 0 . 4

8

H e states t h a t there w e r e s m a l l a m o u n t s o f t h e c a l c i u m - r i c h

phase C a M n 0

3

present i n h i s p r e p a r a t i o n o f C a M n 0 . 3

7

Below 910°C a

c o m p o u n d w i t h a C a / M n r a t i o o f 1 / 4 appears i n the p h a s e d i a g r a m . I n this case t h e r m o g r a v i m e t r i c analysis shows t h a t o n e - h a l f of t h e m a n g a n e s e

American Chemical Society Library 1155 16th St.. N.W.

In Solid State Chemistry: A ContemporaryO.C. Overview; Holt, S., et al.; Washington. 20036 Advances in Chemistry; American Chemical Society: Washington, DC, 1980.

2

148

SOLID S T A T E C H E M I S T R Y :

is present as M n

4 +

A CONTEMPORARY

, l e a d i n g to a f o r m u l a of C a M r ^ O g .

OVERVIEW

Toussaint

(12)

reports a p h a s e C a M n 0 , b u t h i s X - r a y d a t a s h o w i t to b e a m i x t u r e 4

7

containing predominantly C a M n O i . 7

X - r a y patterns f o r C a M n O 3

e

A t h o r o u g h analysis of o u r

2

own

a n d C a M n O g is n o t c o m p l e t e , b u t i t does 4

a p p e a r that these t w o phases are r e l a t e d s t r u c t u r a l l y . T h e y c a n b o t h b e w r i t t e n as C a . M n 0 , w h e r e x — a

1/3

2

for C a M n 0 3

6

and x —

C a M n O , s u g g e s t i n g t h a t t h e y are r e l a t e d to t h e A a . M n 0 4

s

s c r i b e d b y F o u a s s i e r et a l .

2

1/4

for

phases

de

(13).

Below 890°C a compound

h a v i n g a C a / M n r a t i o of 2 / 3

becomes

stable. T h e r m o g r a v i m e t r i c analysis gives a f o r m u l a of C a M n 0 2

w h i c h i n d i c a t e s that a l l m a n g a n e s e

are 4 + .

Ca Mn 0 2

3

8

3

(14),

8

is a l a y e r e d


s t r u c t u r e c o n s i s t i n g o f i n f i n i t e m a n g a n e s e o x i d e sheets h e l d together Ca

2 +

i n trigonal prismatic coordination w i t h oxygen

by

(15).

T h e s o l i d s o l u t i o n p r e c u r s o r t e c h n i q u e w a s t h e o n l y synthesis m e t h o d f o u n d to y i e l d t h e l o w - t e m p e r a t u r e C a - M n - O c o m p o u n d s i n p u r e f o r m . C o n v e n t i o n a l s o l i d state r e a c t i o n m e t h o d s w o u l d y i e l d these

compounds

o n l y as constituents of m u l t i p h a s e m i x t u r e s , e v e n after p r o l o n g e d hundred hours) regrindings. CaMn0

3

firings

(several

at 8 0 0 ° - 9 0 0 ° C w i t h n u m e r o u s i n t e r r u p t i o n s f o r

T h e l o w - t e m p e r a t u r e phases d i s c u s s e d a b o v e , a l o n g w i t h

a n d C a M n 0 , w e r e the o n l y phases e n c o u n t e r e d i n the M n - r i c h 2

4

p o r t i o n o f the p h a s e d i a g r a m , a l t h o u g h syntheses f r o m precursors

of

intermediate compositions were tried. A s m i g h t be expected

f r o m t h e presence

of

a n u m b e r of

mixed-

v a l e n c e phases, the r e a c t i o n k i n e t i c s of the l o w e r - (less t h a n 1 0 0 0 ° C ) t e m p e r a t u r e p h a s e d i a g r a m are v e r y sensitive to several e x p e r i m e n t a l v a r i a b l e s . S u b t l e changes i n the o x y g e n p a r t i a l pressure d u r i n g r e a c t i o n h a v e r a t h e r d r a m a t i c effects o n the r e a c t i o n k i n e t i c s . S u c h changes i n the o x y g e n ' p a r t i a l pressure m a y b e b r o u g h t a b o u t b y the presence r e s i d u a l surface species.

of

F o r example, washing the solid solution pre

cipitate precursor w i t h a hydrocarbon

c a n i m p e d e the a t t a i n m e n t o f

single-phase p r o d u c t s . P r e s u m a b l y d u r i n g d e c o m p o s i t i o n i n t h e presence of h y d r o c a r b o n s , a C O / C 0

2

a t m o s p h e r e is l o c a l l y g e n e r a t e d a t r e a c t i o n

interfaces. T h e i n i t i a l r e d u c i n g a t m o s p h e r e w i l l f a v o r f o r m a t i o n of phases with M n

3 +

, w h i c h m u s t t h e n react to f o r m the e q u i l i b r i u m p h a s e .

In a

similar manner, g r i n d i n g the starting materials or intermediate products u n d e r a c e t o n e has a m a r k e d effect o n t h e a b i l i t y t o a t t a i n e q u i l i b r i u m . I n fact, g r i n d i n g u n d e r acetone a n d t h e n r e d u c i n g atmosphere that C a M n O 4

s

firing

causes

a sufficiently

( w h i c h has a h i g h M n

r e l a t i v e to its d e c o m p o s i t i o n p r o d u c t s , C a M n O 3

e

and C a M n O 7

content

4 +

i 2

) cannot

b e f o r m e d . T h i s i n d i c a t e s that e v e n t h o u g h t h e r e a c t i o n is c a r r i e d out at t h e r i g h t t e m p e r a t u r e a n d o x y g e n p a r t i a l pressure, t h e r e a c t i o n k i n e t i c s for

the

formation

of

CaMn^Og from

CaMn 0 3

6

and

CaMn O 7

i 2

are

very slow. We

have observed

that the decomposition

of

the solid solution


7.

Solid State

LONGO E T A L .

149

Precursors

p r e c u r s o r does not a l w a y s d i r e c t l y l e a d to t h e d e s i r e d p r o d u c t .

For

e x a m p l e , a p p r o p r i a t e c a l c i t e precursors f o r C a M n O i o a n d for C a M n 0 7 , 4

upon decomposition

i n oxygen,

consist p r e d o m i n a n t l y of C a M n 0

3

l e a d to m i x e d - p h a s e and C a M n 0 .

3

2

3

2

products,

which

I t is a p p a r e n t t h a t t h e

4

u n u s u a l l y h i g h s t a b i l i t y of t h e p e r o v s k i t e (16) structures of C a M n 0

3

a n d perovskite-related

a n d C a M n 0 , r e s p e c t i v e l y , create l a r g e d r i v i n g 2

4

forces, w h i c h i n t u r n create u n f a v o r a b l e k i n e t i c s f o r t h e f o r m a t i o n of C a M n O i o and C a M n 0 . 4

3

3

2

T h e p u r i t y of r e a c t i o n p r o d u c t s c a n also b e

7

affected b y t h e h e a t i n g rate e m p l o y e d .

Precursors should be introduced

i n t o a f u r n a c e t h a t has a l r e a d y b e e n p r e h e a t e d to t h e r e a c t i o n t e m p e r a ture. E x c e s s i v e l y s l o w h e a t i n g rates are to b e a v o i d e d , since t h e y p e r m i t


the f o r m a t i o n of n o n e q u i l i b r i u m phases.

F o r example, Cai-a-Mn^COs

p r e c u r s o r s w i l l s l o w l y d e c o m p o s e at l o w temperatures

(about 500°C)

to y i e l d M n oxides a n d C a - r i c h c a r b o n a t e . T h e d e c o m p o s i t i o n t e m p e r a t u r e s f o r t h e n e w l o w - t e m p e r a t u r e phases a r e v e r y sensitive to t h e e q u i l i b r i u m o x y g e n pressure. O n c e t h e phases h a v e b e e n f o r m e d b y c o n t r o l of t e m p e r a t u r e t r e a t m e n t at 1 a t m 0 , t h e y 2

c a n b e d e c o m p o s e d b y s w i t c h i n g to f l o w i n g a i r — o r m o r e d r a m a t i c a l l y b y firing i n stagnant a i r . F o r e x a m p l e , C a M n 0 , w h o s e 4

t e m p e r a t u r e i n flowing 0 stagnant a i r .

8

decomposition

is 9 1 0 ° C , w i l l d e c o m p o s e i f fired at 8 1 0 ° C i n

2

T h e effect o n t h e s t r u c t u r a l l y r e l a t e d C a M n 0 3

6

is less

p r o n o u n c e d , p r e s u m a b l y b e c a u s e i t contains a l o w e r p e r c e n t a g e of M n . 4 +

A

firing

a t m o s p h e r e of p u r e C 0

2

destabilizes a l l t h e

low-temperature

phases ( C a M n 0 , C a M n O , C a M n 0 , a n d C a M n O i ) to t e m p e r a t u r e s 2

3

8

3

e

4

8

7

2

at least as l o w as 7 0 0 ° C . For

the c o m p o u n d s

i n the m a n g a n e s e - r i c h p o r t i o n of t h e

d i a g r a m there is a clear c o r r e l a t i o n b e t w e e n the p e r c e n t a g e of M n the

decomposition

temperature

i n 1-atm

0 . 2

The

one

phase 4 +

exception

and is

C a M n 0 , w h i c h is a n e x t r e m e l y r e f r a e t o r y p h a s e d e s p i t e t h e f a c t t h a t a l l 3

its m a n g a n e s e is M n . T h i s i n c r e a s e d t h e r m a l s t a b i l i t y c a n b e u n d e r s t o o d 4 +

i n terms of the u n i q u e s t a b i l i t y offered b y t h e p e r o v s k i t e s t r u c t u r e as w e l l as its r e l a t i v e l y h i g h c a l c i u m content.

Conclusions T h e s o l i d s o l u t i o n p r e c u r s o r t e c h n i q u e has b e e n i n t r o d u c e d as a n effective m e t h o d f o r m i x e d - m e t a l - o x i d e synthesis. W e h a v e f o u n d t h a t t h e a t o m i c scale m i x i n g of cations i n s o l i d s o l u t i o n precursors h a v i n g t h e oalcite s t r u c t u r e results i n the a b i l i t y to synthesize f u l l y r e a c t e d m i x e d m e t a l oxides at s i g n i f i c a n t l y l o w e r t e m p e r a t u r e s a n d i n shorter t i m e s t h e n are r e q u i r e d for c o n v e n t i o n a l s o l i d state synthesis t e c h n i q u e s .

These

l o w e r t e m p e r a t u r e s of r e a c t i o n h a v e y i e l d e d oxides w i t h surface areas t h a t are 10 to 100 times h i g h e r t h a n the same oxides p r e p a r e d b y c o n v e n t i o n a l methods.


150

SOLID S T A T E C H E M I S T R Y :

Not only

A CONTEMPORARY

does t h e s o l i d s o l u t i o n p r e c u r s o r

OVERVIEW

synthesis r o u t e

give

h i g h e r - s u r f a c e - a r e a c o m p l e x oxides, b u t i t also p r o v i d e s a n a p p r o a c h t o t h e d i s c o v e r y o f t o t a l l y n e w m a t e r i a l s t h a t are n o t stable at t h e h i g h e r t e m p e r a t u r e s t h a t u s u a l l y c h a r a c t e r i z e c o n v e n t i o n a l s o l i d state reactions. F o r e x a m p l e , t h e s o l i d s o l u t i o n p r e c u r s o r t e c h n i q u e has a l l o w e d u s t o assemble a n e w s u b s o l i d u s C a - M n - O p h a s e d i a g r a m c o n t a i n i n g s e v e r a l n e w l o w - t e m p e r a t u r e phases. T h e s o l i d s o l u t i o n p r e c u r s o r synthesis r o u t e is t h o u g h t t o h a v e w i d e a p p l i c a b i l i t y since M g , C a , C d , M n , F e , C o , N i , a n d Z n carbonates a l l f o r m t h e c a l c i t e structure. A l l t h e m o n o x i d e s o f these same

elements,

p l u s those o f S r a n d B a , f o r m t h e r o c k salt s t r u c t u r e , t h e r e b y p r o v i d i n g


a n alternate l o w - t e m p e r a t u r e synthesis r o u t e u t i l i z i n g r o c k salt s t r u c t u r e , s o l i d s o l u t i o n precursors. T h e s e r o c k salt precursors m a y b e o b t a i n e d b y decomposition,

i n i n e r t o r r e d u c i n g atmospheres,

calcite precursor.

of t h e a p p r o p r i a t e

T h e a r a g o n i t e g r o u p represents a n o t h e r c o l l e c t i o n of

m e t a l carbonates that c a n b e effectively e m p l o y e d as s o l i d s o l u t i o n p r e cursors.

A r a g o n i t e is a p o l y m o r p h of c a l c i t e , a n d t h e cations t h a t w i l l

c r y s t a l l i z e w i t h its s t r u c t u r e i n c l u d e c a l c i u m , s t r o n t i u m , l e a d , a n d b a r i u m . Acknowledgments We

gratefully acknowledge

t h e assistance of H . J . B r a d y , J . T .

L e w a n d o w s k i , a n d G . Springston i n the preparation a n d characterization of t h e m a t e r i a l s m e n t i o n e d i n this s t u d y .

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

Schnettler, F. J.; Monforte, F. R.; Rhodes, W. W. Sci. Ceram. 1968, 4, 79. Kim, Y. S.; Monforte, F. R. Am. Ceram. Soc. Bull. 1971, 50, 532. Stuijts, A. L. Sci. Ceram. 1970, 5, 335. Sato, T.; Kuroda, C.; Saito, M. Ferrites: Proc. Int. Conf. Jpn. 1970, 72. Clabaugh, W. S.; Swiggard, E. M.; Gilchrist, J. J. Res. Natl. Bur. Stand. 1956, 56, 289. Gallagher, P. K.; Johnson, D. W. Thermochim. Acta 1972, 4, 283. Clavenna, L. R.; Longo, J. M.; Horowitz, H . S. U.S. Patent 4 060 500, 1977. Pantony,D.A.;Siddiqi. Talanta 1962, 9, 811. Horowitz, H. S.; Longo, J. M . Mater. Res. Bull. 1978, 13, 1359. Bochu, B.; Chenevas, J.; Joubert, J. C.; Marezio, M. J. Solid State Chem. 1974, 11, 88. Joubert, J. C., private communication. Toussaint, H . Rev. Chim. Miner. 1964, 1, 141. Fouassier, C.; Delmas, C.; Hagenmuller, P. Mater. Res. Bull. 1975, 10, 443. Horowitz, H. S.; Longo, J. M. U.S. Patent 4 049 790, 1977. Ansell, G. B.; Horowitz, H . S.; Longo, J. M. "International Conference of Crystallography, 11th," Poland, 1978, 157. Yoshimura, M.; Nakamura, T.; Sata, T. Bull. Tokyo Inst. Technol. 1974, 120, 13.

RECEIVED September 15, 1978.


Solid State Precursors: A Low-Temperature Route to Complex Oxides

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