Formation of the spinel Cu0.5Fe2.5O4 from CuFe2O4 and αFe2O3

Mat. Res. Bull. Vol. 3, pp. 787-796, 1968. in the United States.

FORMATION

Pergamon P r e s s , Inc.

Printed

O F T H E SPINEL Cu0. 5Fe2. 504 F R O M CuFe204 A N D ~Fe203 A. Bergstein Institute of Solid State Physics Czechoslovak Academy of Sciences Prague, Czechoslovakia

(Received August 14, 1968; Communicated by E. F. Bertaut) ABSTRACT

The reaction and phases occurring during the heating of a mixture of the spinel CuFe204 and of ~Fe203 in the ratio 1 : 1.5 at t e m p e r a t u r e s up to 1350°C, and the homogeneity range of the spinel Cu 0 5Fe2 504+.~ were investigated. This composition forms a hovnogene~)us sp{uel phase, Cu 0 5Fe2. 504+~ between 1240 and 1300°C; at lower t e m p e r a t u r e s ~t compIex s e r i e s of precipitation reactions occur. Introduction

The cubic spinel Cu0. 5Fe2 504 (or CuFe508) was found by Kordes and RSttig (1), who considered it to be'a compound Cu~. 5Fe3+50~4-, because its ferrimagnetic Curie temperature differed from the value expected for a solid solution of the spinel Cu 2+ Fe 23+n2"4 and (magnetite) Fe2+ F~^3+..22 u 4 . This opinion is shared by Jefferson (2), who found a m a x i m u m of the lattice constant and a m i n i m u m of the Curie temperature near the composition Cu0. 5Fe2. 504 in the system CuFe204-Fe304.

However, according to the phase diagram by

Schmahl and M~iller (3), Cu0. 5Fe2. 504 is at 1000°C but one of an infiniteseries of solid solutions on the join CuFe204-Fe304.

No range of homogeneity

was observed at this temperature, except for solutions nearest to CuFe204. Also, Bertaut and Delorme describe Cu0. 5Fe2.504 as a solid solution of Cu 2+ Fe23+O2- and Fe304, because at low temperatures (400°C) no ordering as in

lithium f e r r i t e Li + _~ e 35+ . .u2 -4 was found but, instead, a decomposition into 787

788

THE SPINEL Cu0. 5Fe2. 504

delafossite CuFeO 2 and hematite ~Fe2 0 3 .

Vol. 3, No. 10

This a g r e e s with the phase dia-

g r a m of the s y s t e m C u - F e - O for t e m p e r a t u r e s below 800°C by Yund and KuUerud (5).

Miyahara and Kino (6) found a homogeneous spinel phase for the

composition CuFe508 in a i r between 1200 - 1350°C.

Below 1200°C, spinel

and ~ F e 2 0 3 w e r e the equilibrium phases; above 1350°C, spinel, cuprous oxide Cu20 , and hematite Fe203 w e r e found in the quenched samples.

The val-

ues of the lattice constants, r e p o r t e d for the cubic spinel lattice of CuFesOs, a r e 8 . 3 9 ~ (1, 4) and 8 . 4 1 ~ (2). This paper deals with the reactio n s and the phases observed during the heating of a mixture of CuO and Fe203 in the ratio 1 : 2.5 between 850 - 1350°C. Experimental Mixtures of CuO p.a. and ~Fe203 p r e p a r e d by the decomposition of oxalate w e r e repeatedly homogenized by common c e r a m i c techniques.

They

w e r e p r e f i r e d in a i r at 750°C, so as to a s s u m e an oxidation state c o r r e s p o n d ing to the c h e m i c a l composition 1 CuFe204 - 1.5 Fe203 containing a spinel and a hematite phase a c c or di ng to X - r a y analysis.

This composition was used

as starting m a t e r i a l for the subsequent investigations. The t h e r m o g r a v i m e t r i c m e a s u r e m e n t s w e r e p e r f o r m e d in a i r at a heating rate of 5 - 7 ° C / m i n with powdered and with p r e s s e d samp les of the starting material. The d i l a t o m e t r i c m e a s u r e m e n t s w e r e p e r f o r m e d in a i r at heating r a t e s v a r i e d f r o m 0.2 - 1 0 ° C / m i n .

The s a m p l e s w e r e rods 4 m m in d i a m e t e r and

35 - 40 m m long, pr e he a t e d so as to eliminate the influence of sintering r e a c tions on the shape of the d i l a t o m e t r i c curves. F o r the c h e m i c a l a n a l y s i s of active oxygen and for the X - r a y analysis at room t e m p e r a t u r e p r e s s e d s a m p l e s heated for 20 - 48 hours at 1200°C at partial oxygen p r e s s u r e s 0. 1 - l a t . or in a i r at t e m p e r a t u r e s between 970 and 1300°C, quenched into water, w e r e used. e t e r and F e K

A Debye c a m e r a 114 m m in d i a m -

radiation s e r v e d for the X - r a y analysis at room t e m p e r a t u r e in

a i r and a Unicam high t e m p e r a t u r e c a m e r a for the X - r a y analysis at t e m p e r a t u r e s up to 1360°C at pa r t i a l oxygen p r e s s u r e s 0. 002 - 0. 21 a t m o s p h e r e s . the high t e m p e r a t u r e X - r a y a n a l y s i s e i t h e r the s t a r t i n g m a t e r i a l or samples

For

Vol. 3, No. i0

THE SPINEL

Cu0. 5Fe2. 504

789

e q u i l i b r a t e d in a i r at d i f f e r e n t t e m p e r a t u r e s between 1100 - 1350°C w e r e used. F o r a few s a m p l e s , the phase c o m p o s i t i o n was c h e c k e d with a m e t a l o g r a p h i c microscope. R e s u l t s and D i s c u s s i o n High T e m p e r a t u r e X - r a y A n a l y s i s In a i r , s p i n e l and h e m a t i t e a r e the p h a s e s found up to 1220 - 1240°C. B e t w e e n t h e s e t e m p e r a t u r e s and 1280 - 1300°C only spinel is p r e s e n t . h i g h e r t e m p e r a t u r e s d e l a f o s s i t e a p p e a r s b e s i d e s the spinel phase.

At

A result

r e p o r t e d in (6), n a m e l y that the single spinel phase d e c o m p o s e s into t h r e e solid p h a s e s spinel, h e m a t i t e , and c u p r o u s oxide at the highest t e m p e r a t u r e s , was not c o n f i r m e d .

Such a f o u r p h a s e s y s t e m (three solid and one g a s e o u s

p h a s e s ) with t h r e e c o m p o n e n t s would be n o n - v a r i a n t a c c o r d i n g to Gibbs phase rule.

It is t h e r e f o r e i m p r o b a b l e that t h e s e p a r t i c u l a r r e s u l t s r e p o r t e d in (6)

r e p r e s e n t e q u i l i b r i u m conditions. The i n c r e a s e of the lattice constant with t e m p e r a t u r e in a i r (Fig. 1) is l i n e a r within the t w o - p h a s e r e g i o n s p i n e l - h e m a t i t e .

Its slope d e c r e a s e s at

the t r a n s i t i o n to the s i n g l e phase s p i n e l r e g i o n and c h a n g e s again when the d e l a f o s s i t e phase a p p e a r s on the I

X-ray patterns, When the p a r t i a l oxygen

i

r

asp~

8,50

p r e s s u r e is d e c r e a s e d at !

constant t e m p e r a t u r e in the

I

12i3

t w o - p h a s e r e g i o n (Fig. 1), the FIG. 1 High t e m p e r a t u r e lattice constant a of the s p i n e l p h a s e vs. t e m p e r a t u r e T. 1 - two phase r e g i o n s p i n e l - ~FegO.~ , 2 - single h o m o g e n e o u s sp~ndl phase, 3 - two p h a s e r e g i o n spinel - delafossite.

8.45

860

'

1o'oo

12'oo T°C

790

THE SPINEL Cu0. 5Fe2.504

Vol. 3, No. 10

lattice constant of the spinel lattice i n c r e a s e s and its value depends linearly on the log. of the oxygen p r e s s u r e (Fig. 2).

This proves that the slope of the f i r s t

linear part in Fig. 1 is due not only to the t h e r m a l expansion of the spinel phase, but also to the change of its composition by gradual dissolution of hematite. Thus, the amount of dissolved Fe203 can be determined from the lattice constants of the spinel.

The slope of the second part in Fig. 1 r e p r e s e n t s the

t h e r m a l expansion of the single homogeneous phase Cu0. 5Fe2. 504+~,, influenced by the changes of the oxygen nonstoichiometry :v with temperature.

Judging

from the dependence of the lattice constant of homogeneous quenched spinel samples on y (Table 1), this influence is probably negligible.

a,p

{ 8.56

,

-2.5

,

-1.5

-0.5 log po 2

FIG. 2 High temperature lattice constant a of the spinel phase at 1200°C in the two phase region spinel - e F e 2 0 3 vs.-the log. of the partial oxygen pressure. Also, the lattice constants of the spinel phase in quenched or slowly cooled samples are lower when ~Fe203 and higher when delafossite are p r e s ent together with the spinel than in single phase samples (Table 1), as shown by the high t e m p e r a t u r e X - r a y patterns. The t e m p e r a t u r e dependence of the lattice constant in the two-phase region spinel-hematite (Fig. 1) and the values of the lattice constants of the spinel phase in the quenched samples (Table 1) agree well with the results reported in (2) for single phase spinels CUl_xFe2+xO 4 with O .~ x .~ 0. 5.

How-

ever, the lattice constant of spinels with 0. 5 -~ x which are present in the two

Vol. 3, No. 10

THE S P I N E L Cu0. 5 F e 2 . 5 0 4

791

TABLE 1 Lattice constants a of the spinel lattice and oxygen a t o m s ~ above or below the spinel f o r m u l a Cu0. 5 F e 2 . 5 0 4 in Cu0. 5Fe2.504+~, mixed oxides. s - spinel, ~ - F e 2 0 3 , d - delafossite a.~: ~: phases:

8.44

8.410

8. 412

8.411

8.411

8.400

8. 387

-

0. 017

0. 022

0. 032

0. 041

0. 11

0. 168

s + d

s

s

s

s

s + ot

s + ot

phase region s p i n e l - d e l a f o s s i t e at the highest t e m p e r a t u r e s in Fig. 1, dec r e a s e s in the d i a g r a m of the quenched single phase spinel in (2), w h e r e a s it i n c r e a s e s in our case.

The composition x = 0. 5 a p p e a r s to have s p e c i a l

p r o p e r t i e s in both c a s e s , but it is doubtful whether it is to be c o n s i d e r e d as a compound, as proposed in (2). The rmo.g_ravimet ric A n a l y s i s A m i x t u r e of 1 CuFe204 and 1.5 F e 2 0 3 steadily l o s e s oxygen during heating in a i r from 800 - 1350°C (Fig. 3).

T h r e e s t a g e s of decomposition r e -

actions, analogous to the t h r e e r e g i o n s in Fig. 1, a r e encountered.

In the

f i r s t region, non-homogeneous spinels with oxygen e x c e s s (cation vacancies) and gaseous oxygen a r e f o r m e d by the gradual dissolution of hematite in the spinel phase a c c o r d i n g to the r e a c t i o n CuFe~z+xO..(a+~x+4 A) + (1, 5 - 2 ) F e 2 0 3 = CuFe(2+x')O(4+4x'+v A) + ÷

x'

(I, 5- 5 )Fe203

÷

--

O2(g),.

T h i s r e a c t i o n c o m e s to an end when all the hematite is d i s s o l v e d and a single spinel phase Cu0. 5 F e 2 . 5 0 4 + ~ is left.

However, the t r a n s i t i o n between the

f i r s t two r e g i o n s is continuous in Fig. 3 and not so distinct a s in Fig. 1.

This

homogeneous s p i n e l d e c o m p o s e s (region 2) a c c o r d i n g to Cu0. 5Fe2. 504+7 v Cu0. 5Fe2. 504+7, + ~ O 2 ( g ) until the s t o i c h i o m e t r i c composition Cu0. 5Fe2. 504 is reached.

Then stage 3,

the p r e c i p i t a t i o n of d e l a f o s s i t e , begins: Cu0. 5Fe2. 504 ~ x CuFeO 2 + Cu0. 5 _ x F e 2 . 5 - x ~04 (3_2x)

X

+ 02(g).

792

THE S P I N E L Cu0. 5Fe2. 504

Vol. 3, No. 10

I

It s e e m s that, at least

!

u n d e r the conditions of our °/°

the r m o g r a m s , h o m o g e n e o u s s p i n e l s Cu0. 5Fe2. 5 0 4 + ~ with negative values of ~, that is, with i n t e r s t i t i a l cations o r oxygen v a c a n c i e s , a r e not stable. The l i m i t s of t h e i r h o m o g e n e i t y 1

with positive values of ~ (with cation v a c a n c i e s ) w e r e c h e c k e d by c h e m i c a l and X - r a y a n a l y s i s !

of q u e n c h e d s a m p l e s . i m a l value

2 __ 2 3 800

" cr = 0 , fO00

, 1200

The m a x -

of 7 was found to

be +0. 04.

Thus at h i g h e r t e m -

, 1400

p e r a t u r e s than 1000°C a field of

°C

s p i n e l would extend between the

FIG. 3 T h e r m o g r a v i m e t r i c d i a g r a m of an oxide m i x t u r e 1 C u F e 2 0 4 + 1.5 F e 2 0 3 in a i r . Heating r a t e 5 ~ C ] m i n . ; ~W - weight loss, T - t e m p e r a t u r e , 1 - two p h a s e r e g i o n s p i n e l - ~ F e 2 0 3 , 2 - single phase h o m o g e n e o u s s p i n e l phase, 3 - two phase r e g i o n spinel - d e l a f o s s i t e .

homogeneous nonstoichiometric

s p i n e l - ~ F e 2 0 3 and the spinel - d e l a f o s s i t e fields in the t e r n a r y phase d i a g r a m in (3). Dilatometry The s a m e t h r e e s t a g e s of the d i s s o c i a t i o n p r o c e s s as

in Fig. 1 (and l e s s d i s t i n c t in Fig. 3), a p p e a r on the n o n - i s o t h e r m a l d i l a t o g r a m in Fig. 4.

The d i s s o l u t i o n of the ~ F e 2 0 3 in the spinel is m a r k e d by dilatation

(1}, which b e c o m e s v e r y smooth in the single p h a s e r e g i o n (2) of the d e c o m p o sition of the h o m o g e n e o u s s p i n e l C u 0 . 5 F e 2 . 5 0 4 + .

The p r e c i p i t a t i o n of

d e l a f o s s i t e is a c c o m p a n i e d by a c o n t r a c t i o n of the s a m p l e in r e g i o n 3 [but the lattice constant of the s p i n e l p h a s e i n c r e a s e s (Fig. 1)]. H y s t e r e s i s Effects S e v e r a l kinds of h y s t e r e s i s e f f e c t s of the r e o x i d a t i o n p r o c e s s e s when c o m p a r e d with the d i s s o c i a t i o n p r o c e s s e s a r e o b s e r v e d .

On the n o n - i s o t h e r m a l

Vol. 3, No. 10

THE S P I N E L Cu0. 5Fe2. 504

HEA TING

I f

793

COOL ING 2

4L

980 I

1180

I

I

flSO

I

I

1080

980

I

I

1280

I

I

1080

I

I

880 T "C

a.

R e v e r s a l of t e m p e r a t u r e change b e f o r e the two p h a s e r e g i o n spinel d e l a f o s s i t e is attained.

FIG. 4 Non- i s o t h e r m a l d i l a t o g r a m s in a i r of rods pre-sintered from an oxide m i x t u r e 1 CuFe204 + 1.5 F e 2 0 3 .

HE A T ING

COOL IN(2

2

al

3

2

A~ _ change of length, T- temperature, 1 - two p h a s e r e g i o n spinel- aFe203, 2 - single p h a s e h o m o g e n e o u s spinel, 3 - two phase r e g i o n spinel - delafossite. 1040 ,

,

,

990

i

i

1290 1290 t

i

1190

i

,

,

1340

,

I040 ,

,

,

~

i 9 9 0

i

,

1190

T "C b.

R e v e r s a l of t e m p e r a t u r e change a f t e r the b e g i n n i n g of the p r e c i p i t a t i o n of d e l a f o s s i t e .

794

THE SPINEL Cu0. 5Fe2. 504

Vol. 3, No. 10

t h e r m o g r a m s (Fig. 4) the reoxidation during cooling proceeds in any case m o r e slowly than the dissociation in consequence of the difference in the r e a c tion mechanism. ple.

On heating, oxygen is liberated through the bulk of the s a m -

It has to migrate to the surface, from where it passes into the gas phase.

On cooling, the oxygen must be brought to the surface from the gas phase, migrate through the c r y s t a l and occupy its place in the lattice.

The same is

true for i s o t h e r m a l dissociation-reoxidation p r o c e s s e s , where at constant t e m p e r a t u r e the partial oxygen p r e s s u r e of the atmosphere is abruptly changed (Fig. 5).

dissociation

reoxicl ation

al

I

I

0

I

I0

I

I

20

I

I

30

I

!

I

40

l

I

60

I

FO

I

I

80

I

I

I

I

90 rain

FIG.

5

I s o t h e r m a l d i l a t o g r a m s of rods pre-sintered f r o m an oxide m i x t u r e 1 CuFe204 + 1.5 Fe203. a - dissociation, b - reoxidation; A L - c h a n g e of length, t - time.

As long as the dissociation did n o t p a s s stage 2 in Figs. 1, 3 and 4, the shape of the reoxidation curves in Fig. 4 are analogous to the shapes of the dissociation curves, in spite of the h y s t e r e s i s .

However, once stage 3 has

been reached during the dissociation, i.e. when the precipitation of delafossite has already begun, a second h y s t e r e s i s effect appears on the reoxidation curves.

The precipitation of hematite is no longer a continuous process, be-

Vol. 3, No. 10

THE SPINEL Cu0. 5Fe2. 504

795

ginning at the t e m p e r a t u r e of the respective phase boundary (1220 - 1240°C) on the n o n - i s o t h e r m a l dissociation curves, but is shifted to lower temperatures. In air it sets in abruptly at ~- 1075°C.

This effect is probably due to the nu-

cleation of the hematite phase. Conclusions The mixed oxides Cu0. 5Fe2. 504 form a single homogeneous phase Cu0. 5Fe2. 504+ with cubic spinel structure in a definite t e m p e r a t u r e range. In air, this range is --- 1240 - 1300°C. oxygen non-stoichiometry is + 0. 04. precipitated.

The maximum possible value of the At higher oxygen contents ~Fe203 is

It s e e m s that the composition here investigated does not form a

stable homogeneous spinel phase with negative values of ~.

When the oxidation

state corresponding to the stoichiometric spinel with ~ = O is reached, further decomposition is accompanied by the precipitation of a d e l a f o s s i t e - l i k e phase. A mixture of spinel and hematite 1 CuFe204 + 1.5 Fe203 t r a v e r s e s three different phase regions during its decomposition, when the t e m p e r a t u r e is raised in air from 800 - 1300°C.

In the first region, hematite and spinel

coexist and the hematite is gradually dissolved in the spinel with evolution of gaseous oxygen.

The concentration of the dissolved Fe203 at constant oxygen

p r e s s u r e depends linearly on t e m p e r a t u r e ; at constant t e m p e r a t u r e it depends linearly on the logarithm of the partial oxygen p r e s s u r e .

In the second region,

a single n o n - s t o i c h t o m e t r i c spinel phase Cu0. 5Fe2. 504+Twith positive values of y is decomposed, when the t e m p e r a t u r e is increased at constant oxygen p r e s s u r e or the oxygen p r e s s u r e d e c r e a s e d at constant t e m p e r a t u r e , until the stoichiometric composition Cu0. 5Fe2. 504 is reached.

In the third region, the

loss of oxygen due to an increase of t e m p e r a t u r e or d e c r e a s e of oxygen p r e s sure leads to the precipitation of a delafossite (CuFeO2)-like phase. The reoxidation p r o c e s s e s during the d e c r e a s e of t e m p e r a t u r e or inc r e a s e of oxygen p r e s s u r e are c h a r a c t e r i z e d by pronounced h y s t e r e s i s , due to the nucleation of the hematite phase.

This happens when the preceding decom-

position goes f a r t h e r than to the stoichiometric single phase spinel Cu0. 5Fe2. 504 , that is, when delafossite begins to be precipitated.

As no such

h y s t e r e s i s is observed during the reoxidation of the single phase spinels

796

THE SPINEL Cu0. 5Fe2. 504

Vol. 3, No. 10

Cu0. 5 F e 2 . 5 0 4 + ~ we must assume that nuclei of Fe203 or at least c l u s t e r s of Fe 3+ ions are present in these spinels but not in the spinels CUl_xFe2+xO 4 (x • 0. 5), which exist in the two-phase region spinel - delafossite. The different r e s u l t s as to the lattice constant, magnetic moment and ion distribution found in l i t e r a t u r e for the spinel Cu0. 5Fe2. 504 may be partly due to a neglect of the oxygen non-stoichiometry or of undetected amounts of a second phase and the corresponding differences in the composition of the spinel phase resulting from the preparational conditions.

This seems to be true even

for the c l a s s i c a l paper of Kordes and R~ttig (1), who were the f i r s t to find the spinel Cu0. 5Fe2. 504 . T h e i r samples were heated at 1200°C in a i r and quenched only from l l 0 0 ° C so that, according to our results, they probably contained undissolved or precipitated ~Fe203. •

J V

I thank M. Mtkulas, J. Volf and E. Kitzinger for technical a s s i s t a n c e and P. Holba for discussion of the results. References 1.

E. Kordes and E. RSttig, Ztscldt. anorg. Chem. 264, 34 (1951).

2.

O. F. Jefferson, J. Appl. Phys. 3.~6, 1165 (1965).

3.

N . G . Schmahl and E. MtHler, Archiv f. Eisenhflttenwesen 3..55, 527 (1964).

4.

E. Bertaut and C. Delorme, Compt. Rend. 236, 74 (1953).

5.

R. A. Yund and G. Kullerud, The Am. Mineralogist 4..99, 689 (1964).

6.

S. Miyahara and Y. Kino, Jap. J. Appl. Phys. 4, 310 (1965).