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Topotactical reactions with ferrimagnetic oxides having hexagonal crystal structures—II
J. Inorg. Nucl. Chem., 1960.Vol. 16, pp.
100
to 108. PergamonPress Ltd. Printed In Northern Ireland
TOPOTACTICAL REACTIONS WITH FERRIMAGNETIC OXIDES HAVING HEXAGONAL CRYSTAL STRUCTURES--II F. K. LOTGERING Philips Research Laboratories, N. V. Philips' Gloeilampenfabrieken Eindhoven, The Netherlands (Received i 7 November 1959; in revisedform 4 February 1960) Abstract--A number of reactions between grains of non-oriented and magnetically oriented oxides of the system BaO-MeO-Fe2Oa are described. Such conversions, for which the term "topotactical reactions" was introduced in a previous paper, can yield polycrystalline final products with oriented crystals. The reaction mechanism and the influence of sintering on the degree of orientation of the final product is studied for some topotactical reactions.
lN the BaO-MeO-Fe2Oa system (Fig. 1), in which Me = Co, Ni, Zn etc., a.o. the ferrimagnetic compounds M : BaFe12019, MezW = BaMe2FelsO~ 7, Me~Z = Ba 2 Me2Fe24041 and Me2Y := BazMe.,Me2FelzOz2, having closely related hexagonal Fez03
BoO
~0
Fit,. I.- Composition diagram of the system BaO-MeO-Fe~Oa (Me 2+ = Fe 2+, Mn s+, Co z+ etc.) BaFel~.O19(M), BasMe:Fe.~404t(Z), Ba2Me2Fex.oO22(Y), BaMe~Fe160~7(W), MeFe~O4(S) and BaFe±O4(B). The compounds Ba2MezFe28Oao and BaaMe2Fe3eO60, which also exist, will not be mentioned m this paper and are therefore omitted in the diagram.
crystal structures, occur. (1} The grains of these magnetically hard oxides can be oriented by means of a magnetic field. This leads to the possibility of studying crystal orientation phenomena during reactions between oriented and non-oriented grains in mixtures of oxide powders. As described in a previous paper (2~(referred to as paper I) many of such reactions lead to polycrystalline materials with oriented grains. An example of such a "topotactical" reaction is: BaFe12019 ~ + 2 C o 0
+ 2 F e 2 0 3 --* BaCo2FeleO27
(t) j. SMIT and H. P. J. WUN, Ferrites. Philips" Technical Library, Eindhoven; John Wiley, New York (1959). G. H. JO>~KER,XVI- Congres international de chimie pure et appliquee, 1957. M~moires pr~sent~es h la Section de Chimie Min~rale, Soci~t~ d'Editions d'Enseignernent Sup~rieur, Paris, p. 117 (1958). ~sj F. K. LOTGEalNG,J. htorg. Nucl. Chem. 9, 113 (1959). 100
Topotactical reactions with ferrimagnetic oxides having hexagonal crystal structures--I I
or
101
M ~ + 2CoO + 2Fe~O3 --* Co~W
The hexagonal M crystals in the initial mixture are oriented in a magnetic field with their c-axes mutually parallel (indicated by the arrow). The reaction takes place by firing the mixture at 1250°C. The final product consists of sintered, polycrystalline Co2W, the hexagonal crystals of which show the same crystal orientation as the M crystals had in the initial mixture. A conservation of crystal orientation during a solid state reaction has, as far as we know, never been described in the literature. However, similar, but not exactly the same phenomena, are known. E.g. WESTRnCand ZWIETERING(8) observed that the reduction of a F%O4 single crystal gives a conglomerate of iron crystals with a preferred orientation. This reaction takes place between a solid and a gas phase and not, as in our case, between solid phases. The Experimental part of this paper gives (1) the degree of orientation for a number of reactions between oriented and nonoriented grains, (2) the analysis of the intermediate reaction products of some of these reactions; (3) the results of some marker experiments and (4) experiments concerning the influence of sintering on the crystal orientation, The experimental results (1) (2) and (3) will be used for the discussion. EXPERIMENTAL 1. Degree of orientation
Table 1 gives the reactions investigated between oriented and non-oriented grains and the degree of crystal orientation of the final products. We refer to paper 1 for details of the preparation techniques and for the determination of the orientation factor f from the X-ray diffraction pattern of a sample ( f = 0 a n d f = 1 correspond to no orientation and complete orientation, respectively). For the sake of brevity the equations are formulated with BaO and CoO instead of BaCO8 and CoCOs, which actually were used as raw materials. Further we introduce the symbols S for MeFe~O, and B for BaFeiO4.
The initial mixtures of the reactions V, VI and X contain Col/~Zs/sZ as a second ferrimagnetic component. The Col/IZns/sZ crystals, which have a low magnetic anisotropy, (2) are not oriented in a magnetic field so that the initial mixture contains only one oriented crystal phase. Most of the reactions investigated are conversions of M, Me~W, Me~Z or Me2Y into each other so that the crystal structure of the final product is closely related to, or identical with, that of the oriented component in the initial mixture. The question arises whether oriented material can be obtained if this is not the case. We therefore tried to prepare BaFe~O,, BaFeOl_~ (~ < 0.5) and TiFe~Os, the crystal structures of which are completely different from those of M, Me2 W etc., from the ferrimagnetic hexagonal compounds by the reactions XXI-XXIII. From each of the initial mixtures two pellets were pressed, one in the presence and one in the absence of a magnetic field. The two pellets were fired in the same way. Reactions XXI and XXII occurred as anticipated by the equations. The reaction product of XXIII consisted of the pseudobrookite phase TiFe205 together with a small amount of unidentified material. A comparison of the X-ray diffraction patterns of the magnetically oriented and the non-oriented pellets did not give any indication for crystal orientation of the BaFe20~, BaFeOs_0 or TiFesO5 phases in the oriented samples. 2. Intermediate reaction products
For some reactions given in Table 1 the intermediate products that lead from the initial mixture to the final, product were investigated in the following way. A sample was heated up slowly (200-300°C/hr) to a certain temperature (900, 1000, 1100 or ~s~R. W~STRIKand P. ZwxE'rgmNo,Proc. Kon. Ned. Akad. v. Wetenschappen B $6, No. 5, 492 (1953).
102
F . K . LOTGERING
TABLE
I . - - R E A C T I O N S BETWEEN ORIENTED A N D N O N - O R I E N T E D G R A I N S , THE DENSITY A N D CRYSTAL ORIENTATION OF THE F I N A L P R O D U C T S *
No. I
II llI
Reaction ~ M ~ + ~ B a O + ÷-~CoO + ~-~'~TiOs~ BaTil. a Co~,x Fes.~ O~ ½Co~Z.t + ]BaO + ZnO + 6F%Oa ~ Cox.oZnl.oZ -~Zn~Z.t + -~ BaO + CoO + 6Fe20 a --~ Cot.oZnx.oZ
Density (g/cma) 4.5 4-2 5.1
Orientation very good ( f = 0.97) good i f = 0.8) good i f = 0.7)
IV
~a B aF%O 4 + ~ C o O ~o-6CO2YI (35 wt ~ ) + y~ + ~-SF%Os ~ Cos Y
4"4
very good
V
g C o ~a Z nai Z ( 3 3 w t % ) + _~r,o ~,~ ~xZn~t a -v~, ( 3 6 w t ~ ) + -~BaO + 4F%Os -~ Co½Zn~Z
5.0
non-oriented
VI
~ C o - ~ Z n ] Y . t ( 3 6 w t % ) + -5~Co~Zn~Z ~ ~ 8 (33 wt %) + -~-~BaO + aZo~CoO + -~o~ZnO + -~R ~ ~FesO~ --~ C o i5 Z n ia Y
5"0
very good ( f = 0.91)
VII
M ~' + CoO + ZnO + 2F%O8--,- Cot.oZnt.oW
4.3
VIII
~ZnzZ ~" + CoO + ~aZ nO + 4F%Oa---~ Col.oZnl.oW
2.8
IX
]Co~Z ~ + -~CoO + ~-ZnO + 4F%O~ -- Coo.sZnl.,W
3-3
X
~M
very good ( f = 0.89) good (f= 0-84) good ( f = 0.86)
i' (15 wt %) + ~Co-~Zn-~Z(42 wt ~o) q i~CoO av + -~Fe2Os --* Cot.etZn0.as W
( f = 0.95)
(f=o)
3.3
good ( f = 0.80) M non-oriented. Co2 W oriented good i f = 0.7) poor ( f = 0.1) poor ( : = O.I)
XI
CozZ.I. + 8F%Os ~ 2M + CozI'V
2.7
XII
Co~W.L + 2BaO + 4F%Oa --* Co2Z
4.6
XIII
2M t + B~O + CoO -k ZnO ---*Cot.0ZnroZ
4.8
XIV
CosY.t + BaO + 6F%Oa ~ Co~Z
4.9
XV
~CosW-L + -~BaO + -~CoO ~ CosY
5.1
XVI
½ZnsZ ~ + ½BaO + ZnO ~ Zn s Y
5" 1
XVII
~Co2Z.L + ½BaO + CoO ~ CozY
4.9
XVIII
2ZnsZ ~ + 8ZnO ~ 3Zn2 Y + 6ZnFesOa
5.2
( f = 0.8) poor ( f = 0.2) good ( f = 0.6) poort
XIX
2 Co2Z-L + 8CoO --~ 3Cosy + 6CoF%O4
4"8
moderate1"
XX
Co2Z1 + BaF%O, + 5Fe.Os ~ CosZ + M
--
M moderatet
XXI
¼M ~' + ~BaO ~ BaF%Oa
XXII
"ax~M 1' + ~x~a2BaO + (~ -- ½6) 02 --* BaFeOs'_,~
xxIII
Fe2IIW ~ + 10TiO2 + ~O2 --*-9TiF%O 5 + BaTiOs
good
* The ferrimagnetic grains in the initial mixture were oriented in a static ( t ) or rotating (2.) magnetic field. All samples were fired 3 hr at 1250°C in oxygen. For meaning of symbols M, M%W etc. see introduction and Fig. 1. f ' f c o u l d not be determined with sufficient accuracy for samples with more than one phase,
Topotactical reactions with ferrimagnetic oxides having hexagonal crystal structures--II
103
1200°C) in oxygen. As soon as the desired temperature was reached, the pellet was quenched and the reaction products were identified by X-ray analysis. The results for four reactions are given in Table 2. From the phases found at each temperature the partial reactions indicated by arrows in Table 2 can be derived. Since the X-ray reflexions of phases present in small amounts may escape observation, it is not possible to reconstruct the partial reactions in great detail. The principal reactions, however, could be recognized clearly. VII, VIII,
T A B L E 2.--INTERMEDIATE P R O D U C T S OF THE REACTIONS
Reaction VII Heated up to M + CoO + ZnO + 2FesOs - *
COl.
M + FesOs + ZnO + CoO I 900 °
I000 °
1100° 1200° 1250° with annealing
I
Z+
[
900 °
Fe~Os
q-
I
I
I
Z(s) -k FesOs(s) + S(s) Z(s) -k FesOs(s) -{- S(s) k__l
M(s) + S(s) M(s) + S(s) [__l
M(s) -k S(s) M(s) + S(s) i I
W(s)
W(s)
Reaction XII Co~WJ. q- 2BaO -k 4FesO8 -* CoeZ
Reaction XVI ½ZnaZ ~' q- ½BaO q- ZnO --* Zn s Y
W + BaO + FesOs
Z q- BaO -k ZnO
W(a) -I- B(s) -{- FesOs(m) -{- S(w) + M(vw) W(s) + B(s) -k FesOs(m q- S(w) q- M(vw) l i. I
,
,J,
i
Z(s) "k B(s) q- ZnO(m) Z(s) -~ B(s) -[- ZnO(m)
I
I
1100 °
w(s) + B(s) + y(m) + S(w) + M(m) I L
Z(s) + B(m) + ZnO(m) + Y(s) L I
1200°
W(w) + Y(s) + M(s) + Z(cw)
e(vw) + Y(s)
1250° with annealing
I*
-{- Z n O
CoO
M(s) + FesOs(w ) + $(s) M(s) -{- FesOs(w) q- S(s)
I 1000°
XII A N D X V I , T A B L E
Reaction VIII ½ZntZ + CoO + ½ZnO + 4FetOa-~ o Znz. o W Col.0Znl.o W
Z(s)
Y(s)
* The X-ray intensities of the phases are indicated by (s) strong; (m) moderate; (w) weak; (vw) very weak. From Table 2 it is seen that e.g. reaction VII proceeds as follows. ZnO, CoO and Fe~O3 react and give a spinel phase S at temperatures below 900°C. From 900 ° to 1200 ° the reaction product, which exists mainly of M and S, remains practically unchanged. The final product is formed from M and S at 1250 ° after heating for some hours at 1250°C.
3. Marker experiments Two sintered pellets of different compounds were brought into contact. The contact surfaces were ground flat carefully. The boundary was marked by platinum wires (diameter 30/~) placed in grooves (depth 30/~) in one of the pellets. The pellets were heated 24 hr at 1250°C and cooled. They could be separated easily and under a stereoscopic microscope it was observed that the position of the wires was unchanged with respect to the boundary. (The error in the estimated position of the wire was much smaller than the diameter of the wire.) X-ray diffraction patterns of the surface layers of the two pellets gave the following results.
104
F.K. LOTGEIUNG
In the cases BaFesOJFeiOs, BaFe~=O~,/CoFetO, and Ba=ZnaFes,O,x/ZnO (i.e. reactions of a Ba compound with a compound free from Ba), the reaction products were present exclusively on the pellets of the Ba compound. For the reactions BaFex=O~,/Ba=ZnsFel=Oasand BaFesOJBaCo=Fex,O s7 (i.e. for which both components contain Ba) the reaction products were found on both pellets. From these experiments we conclude that the large and heavy Ba + ions diffuse much more slowly than the small and light Co +, Zn + and Fe a+ ions under circumstances present during topotactical reactions. 4. The influence of sintering on the degree of orientation S l ~ r r s et al. ~4) observed on oriented polycrystalline M material that the degree of orientation increases with increasing heating time and temperature, which shows that the oriented crystals grow at the expense of the wrongly oriented ones. This was attributed to a lowering of the surface energy. We studied this phenomenon on the following topotactical reactions:
x Z n , Z + ( 1 - x) Co~Z--* Zn~Co~_~T Co,Z, has a preferential plane of magnetization whereas Zn~Z has a preferential direction and a magnetic anisotropy 4--5 times weaker than that of Co~Z. 18~ In a rotating magnetic field cs~the Co2Z crystals arc oriented with their c-axes mutually parallel (the "correct" orientations) whereas the c-axes of the Zn,Z crystals are oriented at random in a plane perpendicular to the c-axes of the Co~Z crystals (the "incorrect" orientations). The "correctly" and "incorrectly" oriented crystals increase and decrease the orientation factor f, respectively. Pellets of mixtures with x = 0; 0.15; 0.30; 0.50 and 0.70 were pressed in the presence of a rotating field andfwas determined from the X-ray diagrams. Then the pellets were heated for 16 hr at 1250°C in oxygen, furnace cooled a n d f w a s determined again. The samples were well sintered and had densities of 5.1-5.2. Thefvs. x curves are given in Fig. 2. For all samplesfis increased by siutering and the increase is considerable for the higher ZnsZ contents. Thus the "correctly" oriented crystals are grown at the expense of the "incorrectly" oriented ones by sintering for a long time. Using the same experimental procedure, the increase of fduring sintering was determined as a function of the sintering time for the mixture x = 0.5. From the result, given in Table 3, it is seen that the improvement of the orientation increases with increasing density. We therefore think that the improvement of orientation by sintering during a topotactical reaction may be important, but only in the case of a well-siutered final product (with a density of at least 4.5). DISCUSSION We divided the reactions I - X X (Table 1) into four kinds, the prototypes o f which are: (a) M ~ + C o O q- Z n O + 2F%Oa
- * Col.0Znl.0 W (reaction VII)
(b) ½Co~Z.I. + I B a O + Z n O + 6F%Os
--* Col.0Znl.o Z (reaction II)
(c) 2o-7-6C%YJ. + ZlB10+ x~CoO + ~ F e s O s --* Cos Y (reaction IV) (d) Co~Z.L q- B + 5F%O s
--* C % Z + M (reaction XX)
In p a p e r I a topotactical reaction was distinguished f r o m an eptitaxial crystal growth by the fact that the oriented c o m p o n e n t in the initial mixture, which determines the orientation of the final product, takes p a r t in the reaction and disappears completely. (a) and CO) are covered by this definition o f topotactical reaction whereas (c) and (d) are not. The difference between (a) and CO) is that the crystal structures o f the oriented c o m p o n e n t in the initial mixture and o f the final p r o d u c t is equal for Co), but different for (a). During (c) oriented Cos Yis f o r m e d in the presence o f oriented Cos Y crystals i.e. a simple case o f crystal growth. In reaction (d) an a m o u n t o f oriented M is ~ A. L. S~-trrs, G. W. RAI"tm~qAUand G. H. Wear.R, Philips Techn. Rev. 16, 141 (1954).
Topotactical reactions with ferrimagneticoxideshaving hexagonalcrystal structures--II
105
formed in the presence of oriented CoBZ crystals, which does not take part in the reaction i.e. an epitaxial growth of M on C%Z takes place.
I. Reaction of the types Co) and (c) (I-VI, Table 1) If the initial mixture contains a well oriented phase with the crystal structure of the final product, we found a highly oriented final product in all cases, investigated. i.o
O.E
f (>4
02 ! 0
c~
02
04
06
0.8
z
Fio. Z--Orientation factors f for CotZ-Zn~, mixtures oriented in a rotating magnetic field.
(a) Beforefiring; (b) after 16 hr firingat 1250°C.
The high degree of orientation of reaction (c) means that the oriented C o s y crystal present in the initial mixture act as a substrate for the growth of the Cos Y produced so that the amount of oriented material increases during the reaction. Apparently practically no new Cos Y nuclei are formed on the BaO, CoO or FesOs crystals (or on their partial reaction products like B or S). This behaviour may be TABLE3.--INcrease OF THE ORIENTATIONYACTORfDURING DIFFERENT ~ 1 " ~ ^ l ~ e w r s ov ^ mrrtn~ oF Co,Z + Zn~Z (l:l), oRn~creD IN AROTATINGFIELD Non-fired ~mple: fo
Heat treatment: heated up slowly to
0"4 0"3 0"3
I100°C, cooled rapidly 1250°C, cooled rapidly 1250°C, i hr 1250°C furnace cooled 1250°C, 3 hr 1250°C furnace cooled 1250°C, 16 hr 1250°C furnace cooled
0.3 0.5
fired sample density I f 3"4
f -- f0
4-6
0"4 0"5
0.0 0.2
4"9
0"5
0.2
5"0
0"6
0.3
5"1
0.7c6~
0.2o~
connected with the fact that BaO, CoO etc. have crystal structures and compositions completely different from Cos Y. Then the question arises what happens ff the reaction mixture contains two components like MesZ and M% Y that have closely related crystal structures. This case was investigated by means of the reactions V and VI. Final product crystals grown on the non-oriented C01/2Zs/2Z cannot be oriented, whereas the formation of an oriented final product takes place ff the oriented C%Zn=_ e Y phase acts as a
106
E K. LOTGERINO
substrate. Reactions V and VI gave non-oriented and very well oriented final products* respectively. This pronounced difference indicates that the component with the crystal structure of the final product acts preferentially as a substrate for the final product, even if a component with a closely related structure is present in the reaction mixture. Reaction VI is essentially similar to reaction XVI, both being the formation of Mes Yfrom Me2Z, BaO etc. From the appearance of an Me~ Y phase during reaction XVI after firing for a short time at relatively low temperatures of 1100-1200°C (see Table 2) it is seen that nucleation of Me2 Y takes
place easily. It seems to us therefore rather surprising that during reaction VI the segregation of Cos/,Zns/4Y takes place mainly on the Cov4Zns/4Y crystals present in the initial mixture. The formation of well-oriented final products by topotactical reactions of the type (b) can be easily understood from the above mentioned preference for crystal growth by the assumption that the reaction (b) (or II, Table I) starts with an epitaxial growth of ZnzZ on the oriented Co2Z crystals. Such an eptaxial process differs, however, hardly from crystal growth because the substrate (CozZ) and the segregate (Zn2Z) are practically the same materials (the difference being the kind of Me ~+ ions, which constitute only 1/13 part of the small cations) A mixed Cov0Znv0Z crystal is then formed by a diffusion of the Co s+ and Zn ~+ ions through a composite grain consisting of Co2Z and ZnzZ.
2. Reactions of the type (a) (VII-XIX, Table 1) In contrast to the (b)-type, not all (a)-type reactions give well-oriented final products (Table 1). In particular, the reactions XIII, XIV and XVI gave poorly oriented final products. (These reactions we carried out several times. It was verified by X-ray analysis of the unfired pellets that the ferrimagnetic phase in the initial mixtures were indeed well oriented). During reaction (a) (or VII, Table 1) nucleation of a new phase CopoZnx. 0 W takes place. The production of highly oriented C01.oZnx.oW indicates that the oriented M phase acts mainly as a substrate for the COl.0Znl.o W crystals. For the formation of CopoZnl.0W nuclei on M grains a transport of MeO and FelOn from the other reaction components toward the M grains is necessary whereas a transport of BaO in the opposite direction must take place for nucleation on the other reaction components, which do not contain Ba. The observed preference for nucleation on the M grains may, therefore, be explained by assuming that the Ba 2+ ions diffuse much more slowly than the Me 2+ and Fe z+ ions. This assumption was indeed confirmed by the marker experiments (see Experimental Part 3). Not all the experimental data can, however, be explained in this way. The pressence of BaO in the initial mixture should promote the formation of a non-oriented final product. Thus the reactions VII-IX are expected to give better oriented final products than the reactions XII-XVII. Such a tendency is indeed observed. The formation of well-oriented final products by the reactions XII and XV and of a poorly oriented Zn 2 Y by reaction XVIII are exceptions, however, for which we are not able to give any explanation. KEDESDY and DRUKALSKY~s~ proposed an atomic mechanism of the formation of a mixed crystal (having NiO-structure) from a NiO and a ZnO crystal. Oxygen atoms diffuse from the ZnO to the * The high degree of orientation of the final product VI is not caused by sintering (c.f. Experimental Part 4). This was checked by heating the mixture up to 1250°C and cooling the pellet rapidly. The sample was poorly sintered (d = 4-3) and still very well oriented (f = 0.92). ts~ H. KEDESDYand A. DRUKALSKY,J. Amer. Chem. Soc. 76, 5941 (1954).
Topotactieal reactions with ferrimagnetic oxides having hexagonal crystal structures--II
107
NiO crystal and build up a new oxygen layer on the NiO surface. Then a diffusion of Zn atoms from the ZnO into the NiO crystal follows. According to this picture the oxygen "frame" of the NiO lattice remains intact and increases layer for layer during the reaction. This process would lead to a topotactically oriented mixed crystal with the orientation of the original NiO crystal. A similar mechanism for e.g. the formation of a MezW nucleus on a M crystal (reaction VIl) would explain nicely the orientation of the MesW crystal. Such a mechanism, which seems to us quite possible in the ease of the mixed NiO-ZnO crystal, is, however, very unlikely for our much more complicated reaction. Although the crystal structures of Mez W and M are closely related, the oxygen frame and the positions of the Ba+ ions in this frame are different for both structures. For a transition of the M into the Me1 W lattice a rearrangenemt of the O'- and Baz+ is necessary. It is hard to believe that during such a process parts of the oxygen frame remain intact. It seems to us much more likely that the topotactieal reaction VII starts with an epitaxial growth of MezWcrystals on the surfaces of the M crystals. The spacings of the oxygen layers of M, MezW, MezZ and Me, Y are practically equal and parts of these lattices have even the same atomic arrangement (e.g. the M and Mez W lattices contain layers having spinel structure with an identical atomic arrangement. The only difference is that the cation sites are occupied by Fe z+ in M and by Fe z+ and Me z+ in Me,W). Therefore the conditions for an epitaxial growth for M, MezW etc. (and S) on each other are expected to be very favourable. Such an epitaxial growth is observed during reaction XX.
3. Intermediate products (Table 2) A topotactieal reaction leads to the final product via several intermediate products which appear during the heat treatment o f the pellet. B = BaFe204 and S = MeFe~O~ are formed at relatively low temperatures (below 900°C). At a higher temperature (1000-1100°C) M and Me2 Y a p p e a r . The final products Me2 W a n d MezZ are generally formed only after a longer firing at a still higher temperature (1250°C) by the.reactions:
M + 2S ---} Me 2 W and M + Me 2 Y--* Me~Z During reaction V I I I a double conversion f r o m Zn2Z and Fe2Os into M and S takes place between 1000 and 1100°C. Since the Zn2Z crystals, which were oriented in the magnetic field, have disappeared at temperatures above 1100°C and well-oriented MeoW does not start to a p p e a r until 1250°C is reached, the conservation of the orientation must take place via the M phase. Reaction X (Table 1) gives a well-oriented final product although the initial mixture contains only a small a m o u n t (15 weight per cent) o f the oriented c o m p o n e n t M and a large a m o u n t (42weight per cen0 of non-oriented C o / z Z n z / Z . T h e p r e s e n c e o f the latter c o m p o n e n t evidently causes no f o r m a t i o n o f a considerable a m o u n t of nonoriented final product. This m a y be explained as follows. F r o m an analysis o f the intermediate reaction products it was found that a double conversion Me~Z + 8Fe203 --* 3 M + 2MeFe~O 4 at temperatures between 1000 and 1100°C occurs here also, just as during reaction VIII. The large a m o u n t of M formed in this way will grow on the o r i e n t e d - M crystals present in the initial mixture (reaction of type (c)). Then, at 1250°C, well-oriented M e ~ W i s f o r m e d from the oriented M phase and S, just like during the reactions V I I and VIII. In connexion with the above-mentioned mechanism of the reactions VIII-X we investigated the conversion of CozZ and FezOz into M and CozW (reaction XI, Table 1). The final product consists of non-oriented M and oriented Co,W. From an analysis of the intermediate reaction products it was found that the reaction takes place as given by equation XI at temperatures between 1000 and 1100°C and that no spinel phase appears during the reaction. Thus the double conversion
108
F . K . LOTGER1NG
XI differs from the double conversions that take place during the reactions VIII-X in the orient'atiom of M and W; the reaction mechanism is also different. This difference of mechanism may be caused by the fact that the double conversions during the reactions VIII-X proceed in the presence of a $ phase (Table 2), which may promote further S formation, whereas no S phase is present when reaction XI starts.
Acknowledgements--The author is greatly
indebted to Mr. J. VEnnE~T for carrying out the experiments and to Dr. E. W. GORT~Rand L~. C. KooY for valuable discussions and suggestions.
100
to 108. PergamonPress Ltd. Printed In Northern Ireland
TOPOTACTICAL REACTIONS WITH FERRIMAGNETIC OXIDES HAVING HEXAGONAL CRYSTAL STRUCTURES--II F. K. LOTGERING Philips Research Laboratories, N. V. Philips' Gloeilampenfabrieken Eindhoven, The Netherlands (Received i 7 November 1959; in revisedform 4 February 1960) Abstract--A number of reactions between grains of non-oriented and magnetically oriented oxides of the system BaO-MeO-Fe2Oa are described. Such conversions, for which the term "topotactical reactions" was introduced in a previous paper, can yield polycrystalline final products with oriented crystals. The reaction mechanism and the influence of sintering on the degree of orientation of the final product is studied for some topotactical reactions.
lN the BaO-MeO-Fe2Oa system (Fig. 1), in which Me = Co, Ni, Zn etc., a.o. the ferrimagnetic compounds M : BaFe12019, MezW = BaMe2FelsO~ 7, Me~Z = Ba 2 Me2Fe24041 and Me2Y := BazMe.,Me2FelzOz2, having closely related hexagonal Fez03
BoO
~0
Fit,. I.- Composition diagram of the system BaO-MeO-Fe~Oa (Me 2+ = Fe 2+, Mn s+, Co z+ etc.) BaFel~.O19(M), BasMe:Fe.~404t(Z), Ba2Me2Fex.oO22(Y), BaMe~Fe160~7(W), MeFe~O4(S) and BaFe±O4(B). The compounds Ba2MezFe28Oao and BaaMe2Fe3eO60, which also exist, will not be mentioned m this paper and are therefore omitted in the diagram.
crystal structures, occur. (1} The grains of these magnetically hard oxides can be oriented by means of a magnetic field. This leads to the possibility of studying crystal orientation phenomena during reactions between oriented and non-oriented grains in mixtures of oxide powders. As described in a previous paper (2~(referred to as paper I) many of such reactions lead to polycrystalline materials with oriented grains. An example of such a "topotactical" reaction is: BaFe12019 ~ + 2 C o 0
+ 2 F e 2 0 3 --* BaCo2FeleO27
(t) j. SMIT and H. P. J. WUN, Ferrites. Philips" Technical Library, Eindhoven; John Wiley, New York (1959). G. H. JO>~KER,XVI- Congres international de chimie pure et appliquee, 1957. M~moires pr~sent~es h la Section de Chimie Min~rale, Soci~t~ d'Editions d'Enseignernent Sup~rieur, Paris, p. 117 (1958). ~sj F. K. LOTGEalNG,J. htorg. Nucl. Chem. 9, 113 (1959). 100
Topotactical reactions with ferrimagnetic oxides having hexagonal crystal structures--I I
or
101
M ~ + 2CoO + 2Fe~O3 --* Co~W
The hexagonal M crystals in the initial mixture are oriented in a magnetic field with their c-axes mutually parallel (indicated by the arrow). The reaction takes place by firing the mixture at 1250°C. The final product consists of sintered, polycrystalline Co2W, the hexagonal crystals of which show the same crystal orientation as the M crystals had in the initial mixture. A conservation of crystal orientation during a solid state reaction has, as far as we know, never been described in the literature. However, similar, but not exactly the same phenomena, are known. E.g. WESTRnCand ZWIETERING(8) observed that the reduction of a F%O4 single crystal gives a conglomerate of iron crystals with a preferred orientation. This reaction takes place between a solid and a gas phase and not, as in our case, between solid phases. The Experimental part of this paper gives (1) the degree of orientation for a number of reactions between oriented and nonoriented grains, (2) the analysis of the intermediate reaction products of some of these reactions; (3) the results of some marker experiments and (4) experiments concerning the influence of sintering on the crystal orientation, The experimental results (1) (2) and (3) will be used for the discussion. EXPERIMENTAL 1. Degree of orientation
Table 1 gives the reactions investigated between oriented and non-oriented grains and the degree of crystal orientation of the final products. We refer to paper 1 for details of the preparation techniques and for the determination of the orientation factor f from the X-ray diffraction pattern of a sample ( f = 0 a n d f = 1 correspond to no orientation and complete orientation, respectively). For the sake of brevity the equations are formulated with BaO and CoO instead of BaCO8 and CoCOs, which actually were used as raw materials. Further we introduce the symbols S for MeFe~O, and B for BaFeiO4.
The initial mixtures of the reactions V, VI and X contain Col/~Zs/sZ as a second ferrimagnetic component. The Col/IZns/sZ crystals, which have a low magnetic anisotropy, (2) are not oriented in a magnetic field so that the initial mixture contains only one oriented crystal phase. Most of the reactions investigated are conversions of M, Me~W, Me~Z or Me2Y into each other so that the crystal structure of the final product is closely related to, or identical with, that of the oriented component in the initial mixture. The question arises whether oriented material can be obtained if this is not the case. We therefore tried to prepare BaFe~O,, BaFeOl_~ (~ < 0.5) and TiFe~Os, the crystal structures of which are completely different from those of M, Me2 W etc., from the ferrimagnetic hexagonal compounds by the reactions XXI-XXIII. From each of the initial mixtures two pellets were pressed, one in the presence and one in the absence of a magnetic field. The two pellets were fired in the same way. Reactions XXI and XXII occurred as anticipated by the equations. The reaction product of XXIII consisted of the pseudobrookite phase TiFe205 together with a small amount of unidentified material. A comparison of the X-ray diffraction patterns of the magnetically oriented and the non-oriented pellets did not give any indication for crystal orientation of the BaFe20~, BaFeOs_0 or TiFesO5 phases in the oriented samples. 2. Intermediate reaction products
For some reactions given in Table 1 the intermediate products that lead from the initial mixture to the final, product were investigated in the following way. A sample was heated up slowly (200-300°C/hr) to a certain temperature (900, 1000, 1100 or ~s~R. W~STRIKand P. ZwxE'rgmNo,Proc. Kon. Ned. Akad. v. Wetenschappen B $6, No. 5, 492 (1953).
102
F . K . LOTGERING
TABLE
I . - - R E A C T I O N S BETWEEN ORIENTED A N D N O N - O R I E N T E D G R A I N S , THE DENSITY A N D CRYSTAL ORIENTATION OF THE F I N A L P R O D U C T S *
No. I
II llI
Reaction ~ M ~ + ~ B a O + ÷-~CoO + ~-~'~TiOs~ BaTil. a Co~,x Fes.~ O~ ½Co~Z.t + ]BaO + ZnO + 6F%Oa ~ Cox.oZnl.oZ -~Zn~Z.t + -~ BaO + CoO + 6Fe20 a --~ Cot.oZnx.oZ
Density (g/cma) 4.5 4-2 5.1
Orientation very good ( f = 0.97) good i f = 0.8) good i f = 0.7)
IV
~a B aF%O 4 + ~ C o O ~o-6CO2YI (35 wt ~ ) + y~ + ~-SF%Os ~ Cos Y
4"4
very good
V
g C o ~a Z nai Z ( 3 3 w t % ) + _~r,o ~,~ ~xZn~t a -v~, ( 3 6 w t ~ ) + -~BaO + 4F%Os -~ Co½Zn~Z
5.0
non-oriented
VI
~ C o - ~ Z n ] Y . t ( 3 6 w t % ) + -5~Co~Zn~Z ~ ~ 8 (33 wt %) + -~-~BaO + aZo~CoO + -~o~ZnO + -~R ~ ~FesO~ --~ C o i5 Z n ia Y
5"0
very good ( f = 0.91)
VII
M ~' + CoO + ZnO + 2F%O8--,- Cot.oZnt.oW
4.3
VIII
~ZnzZ ~" + CoO + ~aZ nO + 4F%Oa---~ Col.oZnl.oW
2.8
IX
]Co~Z ~ + -~CoO + ~-ZnO + 4F%O~ -- Coo.sZnl.,W
3-3
X
~M
very good ( f = 0.89) good (f= 0-84) good ( f = 0.86)
i' (15 wt %) + ~Co-~Zn-~Z(42 wt ~o) q i~CoO av + -~Fe2Os --* Cot.etZn0.as W
( f = 0.95)
(f=o)
3.3
good ( f = 0.80) M non-oriented. Co2 W oriented good i f = 0.7) poor ( f = 0.1) poor ( : = O.I)
XI
CozZ.I. + 8F%Os ~ 2M + CozI'V
2.7
XII
Co~W.L + 2BaO + 4F%Oa --* Co2Z
4.6
XIII
2M t + B~O + CoO -k ZnO ---*Cot.0ZnroZ
4.8
XIV
CosY.t + BaO + 6F%Oa ~ Co~Z
4.9
XV
~CosW-L + -~BaO + -~CoO ~ CosY
5.1
XVI
½ZnsZ ~ + ½BaO + ZnO ~ Zn s Y
5" 1
XVII
~Co2Z.L + ½BaO + CoO ~ CozY
4.9
XVIII
2ZnsZ ~ + 8ZnO ~ 3Zn2 Y + 6ZnFesOa
5.2
( f = 0.8) poor ( f = 0.2) good ( f = 0.6) poort
XIX
2 Co2Z-L + 8CoO --~ 3Cosy + 6CoF%O4
4"8
moderate1"
XX
Co2Z1 + BaF%O, + 5Fe.Os ~ CosZ + M
--
M moderatet
XXI
¼M ~' + ~BaO ~ BaF%Oa
XXII
"ax~M 1' + ~x~a2BaO + (~ -- ½6) 02 --* BaFeOs'_,~
xxIII
Fe2IIW ~ + 10TiO2 + ~O2 --*-9TiF%O 5 + BaTiOs
good
* The ferrimagnetic grains in the initial mixture were oriented in a static ( t ) or rotating (2.) magnetic field. All samples were fired 3 hr at 1250°C in oxygen. For meaning of symbols M, M%W etc. see introduction and Fig. 1. f ' f c o u l d not be determined with sufficient accuracy for samples with more than one phase,
Topotactical reactions with ferrimagnetic oxides having hexagonal crystal structures--II
103
1200°C) in oxygen. As soon as the desired temperature was reached, the pellet was quenched and the reaction products were identified by X-ray analysis. The results for four reactions are given in Table 2. From the phases found at each temperature the partial reactions indicated by arrows in Table 2 can be derived. Since the X-ray reflexions of phases present in small amounts may escape observation, it is not possible to reconstruct the partial reactions in great detail. The principal reactions, however, could be recognized clearly. VII, VIII,
T A B L E 2.--INTERMEDIATE P R O D U C T S OF THE REACTIONS
Reaction VII Heated up to M + CoO + ZnO + 2FesOs - *
COl.
M + FesOs + ZnO + CoO I 900 °
I000 °
1100° 1200° 1250° with annealing
I
Z+
[
900 °
Fe~Os
q-
I
I
I
Z(s) -k FesOs(s) + S(s) Z(s) -k FesOs(s) -{- S(s) k__l
M(s) + S(s) M(s) + S(s) [__l
M(s) -k S(s) M(s) + S(s) i I
W(s)
W(s)
Reaction XII Co~WJ. q- 2BaO -k 4FesO8 -* CoeZ
Reaction XVI ½ZnaZ ~' q- ½BaO q- ZnO --* Zn s Y
W + BaO + FesOs
Z q- BaO -k ZnO
W(a) -I- B(s) -{- FesOs(m) -{- S(w) + M(vw) W(s) + B(s) -k FesOs(m q- S(w) q- M(vw) l i. I
,
,J,
i
Z(s) "k B(s) q- ZnO(m) Z(s) -~ B(s) -[- ZnO(m)
I
I
1100 °
w(s) + B(s) + y(m) + S(w) + M(m) I L
Z(s) + B(m) + ZnO(m) + Y(s) L I
1200°
W(w) + Y(s) + M(s) + Z(cw)
e(vw) + Y(s)
1250° with annealing
I*
-{- Z n O
CoO
M(s) + FesOs(w ) + $(s) M(s) -{- FesOs(w) q- S(s)
I 1000°
XII A N D X V I , T A B L E
Reaction VIII ½ZntZ + CoO + ½ZnO + 4FetOa-~ o Znz. o W Col.0Znl.o W
Z(s)
Y(s)
* The X-ray intensities of the phases are indicated by (s) strong; (m) moderate; (w) weak; (vw) very weak. From Table 2 it is seen that e.g. reaction VII proceeds as follows. ZnO, CoO and Fe~O3 react and give a spinel phase S at temperatures below 900°C. From 900 ° to 1200 ° the reaction product, which exists mainly of M and S, remains practically unchanged. The final product is formed from M and S at 1250 ° after heating for some hours at 1250°C.
3. Marker experiments Two sintered pellets of different compounds were brought into contact. The contact surfaces were ground flat carefully. The boundary was marked by platinum wires (diameter 30/~) placed in grooves (depth 30/~) in one of the pellets. The pellets were heated 24 hr at 1250°C and cooled. They could be separated easily and under a stereoscopic microscope it was observed that the position of the wires was unchanged with respect to the boundary. (The error in the estimated position of the wire was much smaller than the diameter of the wire.) X-ray diffraction patterns of the surface layers of the two pellets gave the following results.
104
F.K. LOTGEIUNG
In the cases BaFesOJFeiOs, BaFe~=O~,/CoFetO, and Ba=ZnaFes,O,x/ZnO (i.e. reactions of a Ba compound with a compound free from Ba), the reaction products were present exclusively on the pellets of the Ba compound. For the reactions BaFex=O~,/Ba=ZnsFel=Oasand BaFesOJBaCo=Fex,O s7 (i.e. for which both components contain Ba) the reaction products were found on both pellets. From these experiments we conclude that the large and heavy Ba + ions diffuse much more slowly than the small and light Co +, Zn + and Fe a+ ions under circumstances present during topotactical reactions. 4. The influence of sintering on the degree of orientation S l ~ r r s et al. ~4) observed on oriented polycrystalline M material that the degree of orientation increases with increasing heating time and temperature, which shows that the oriented crystals grow at the expense of the wrongly oriented ones. This was attributed to a lowering of the surface energy. We studied this phenomenon on the following topotactical reactions:
x Z n , Z + ( 1 - x) Co~Z--* Zn~Co~_~T Co,Z, has a preferential plane of magnetization whereas Zn~Z has a preferential direction and a magnetic anisotropy 4--5 times weaker than that of Co~Z. 18~ In a rotating magnetic field cs~the Co2Z crystals arc oriented with their c-axes mutually parallel (the "correct" orientations) whereas the c-axes of the Zn,Z crystals are oriented at random in a plane perpendicular to the c-axes of the Co~Z crystals (the "incorrect" orientations). The "correctly" and "incorrectly" oriented crystals increase and decrease the orientation factor f, respectively. Pellets of mixtures with x = 0; 0.15; 0.30; 0.50 and 0.70 were pressed in the presence of a rotating field andfwas determined from the X-ray diagrams. Then the pellets were heated for 16 hr at 1250°C in oxygen, furnace cooled a n d f w a s determined again. The samples were well sintered and had densities of 5.1-5.2. Thefvs. x curves are given in Fig. 2. For all samplesfis increased by siutering and the increase is considerable for the higher ZnsZ contents. Thus the "correctly" oriented crystals are grown at the expense of the "incorrectly" oriented ones by sintering for a long time. Using the same experimental procedure, the increase of fduring sintering was determined as a function of the sintering time for the mixture x = 0.5. From the result, given in Table 3, it is seen that the improvement of the orientation increases with increasing density. We therefore think that the improvement of orientation by sintering during a topotactical reaction may be important, but only in the case of a well-siutered final product (with a density of at least 4.5). DISCUSSION We divided the reactions I - X X (Table 1) into four kinds, the prototypes o f which are: (a) M ~ + C o O q- Z n O + 2F%Oa
- * Col.0Znl.0 W (reaction VII)
(b) ½Co~Z.I. + I B a O + Z n O + 6F%Os
--* Col.0Znl.o Z (reaction II)
(c) 2o-7-6C%YJ. + ZlB10+ x~CoO + ~ F e s O s --* Cos Y (reaction IV) (d) Co~Z.L q- B + 5F%O s
--* C % Z + M (reaction XX)
In p a p e r I a topotactical reaction was distinguished f r o m an eptitaxial crystal growth by the fact that the oriented c o m p o n e n t in the initial mixture, which determines the orientation of the final product, takes p a r t in the reaction and disappears completely. (a) and CO) are covered by this definition o f topotactical reaction whereas (c) and (d) are not. The difference between (a) and CO) is that the crystal structures o f the oriented c o m p o n e n t in the initial mixture and o f the final p r o d u c t is equal for Co), but different for (a). During (c) oriented Cos Yis f o r m e d in the presence o f oriented Cos Y crystals i.e. a simple case o f crystal growth. In reaction (d) an a m o u n t o f oriented M is ~ A. L. S~-trrs, G. W. RAI"tm~qAUand G. H. Wear.R, Philips Techn. Rev. 16, 141 (1954).
Topotactical reactions with ferrimagneticoxideshaving hexagonalcrystal structures--II
105
formed in the presence of oriented CoBZ crystals, which does not take part in the reaction i.e. an epitaxial growth of M on C%Z takes place.
I. Reaction of the types Co) and (c) (I-VI, Table 1) If the initial mixture contains a well oriented phase with the crystal structure of the final product, we found a highly oriented final product in all cases, investigated. i.o
O.E
f (>4
02 ! 0
c~
02
04
06
0.8
z
Fio. Z--Orientation factors f for CotZ-Zn~, mixtures oriented in a rotating magnetic field.
(a) Beforefiring; (b) after 16 hr firingat 1250°C.
The high degree of orientation of reaction (c) means that the oriented C o s y crystal present in the initial mixture act as a substrate for the growth of the Cos Y produced so that the amount of oriented material increases during the reaction. Apparently practically no new Cos Y nuclei are formed on the BaO, CoO or FesOs crystals (or on their partial reaction products like B or S). This behaviour may be TABLE3.--INcrease OF THE ORIENTATIONYACTORfDURING DIFFERENT ~ 1 " ~ ^ l ~ e w r s ov ^ mrrtn~ oF Co,Z + Zn~Z (l:l), oRn~creD IN AROTATINGFIELD Non-fired ~mple: fo
Heat treatment: heated up slowly to
0"4 0"3 0"3
I100°C, cooled rapidly 1250°C, cooled rapidly 1250°C, i hr 1250°C furnace cooled 1250°C, 3 hr 1250°C furnace cooled 1250°C, 16 hr 1250°C furnace cooled
0.3 0.5
fired sample density I f 3"4
f -- f0
4-6
0"4 0"5
0.0 0.2
4"9
0"5
0.2
5"0
0"6
0.3
5"1
0.7c6~
0.2o~
connected with the fact that BaO, CoO etc. have crystal structures and compositions completely different from Cos Y. Then the question arises what happens ff the reaction mixture contains two components like MesZ and M% Y that have closely related crystal structures. This case was investigated by means of the reactions V and VI. Final product crystals grown on the non-oriented C01/2Zs/2Z cannot be oriented, whereas the formation of an oriented final product takes place ff the oriented C%Zn=_ e Y phase acts as a
106
E K. LOTGERINO
substrate. Reactions V and VI gave non-oriented and very well oriented final products* respectively. This pronounced difference indicates that the component with the crystal structure of the final product acts preferentially as a substrate for the final product, even if a component with a closely related structure is present in the reaction mixture. Reaction VI is essentially similar to reaction XVI, both being the formation of Mes Yfrom Me2Z, BaO etc. From the appearance of an Me~ Y phase during reaction XVI after firing for a short time at relatively low temperatures of 1100-1200°C (see Table 2) it is seen that nucleation of Me2 Y takes
place easily. It seems to us therefore rather surprising that during reaction VI the segregation of Cos/,Zns/4Y takes place mainly on the Cov4Zns/4Y crystals present in the initial mixture. The formation of well-oriented final products by topotactical reactions of the type (b) can be easily understood from the above mentioned preference for crystal growth by the assumption that the reaction (b) (or II, Table I) starts with an epitaxial growth of ZnzZ on the oriented Co2Z crystals. Such an eptaxial process differs, however, hardly from crystal growth because the substrate (CozZ) and the segregate (Zn2Z) are practically the same materials (the difference being the kind of Me ~+ ions, which constitute only 1/13 part of the small cations) A mixed Cov0Znv0Z crystal is then formed by a diffusion of the Co s+ and Zn ~+ ions through a composite grain consisting of Co2Z and ZnzZ.
2. Reactions of the type (a) (VII-XIX, Table 1) In contrast to the (b)-type, not all (a)-type reactions give well-oriented final products (Table 1). In particular, the reactions XIII, XIV and XVI gave poorly oriented final products. (These reactions we carried out several times. It was verified by X-ray analysis of the unfired pellets that the ferrimagnetic phase in the initial mixtures were indeed well oriented). During reaction (a) (or VII, Table 1) nucleation of a new phase CopoZnx. 0 W takes place. The production of highly oriented C01.oZnx.oW indicates that the oriented M phase acts mainly as a substrate for the COl.0Znl.o W crystals. For the formation of CopoZnl.0W nuclei on M grains a transport of MeO and FelOn from the other reaction components toward the M grains is necessary whereas a transport of BaO in the opposite direction must take place for nucleation on the other reaction components, which do not contain Ba. The observed preference for nucleation on the M grains may, therefore, be explained by assuming that the Ba 2+ ions diffuse much more slowly than the Me 2+ and Fe z+ ions. This assumption was indeed confirmed by the marker experiments (see Experimental Part 3). Not all the experimental data can, however, be explained in this way. The pressence of BaO in the initial mixture should promote the formation of a non-oriented final product. Thus the reactions VII-IX are expected to give better oriented final products than the reactions XII-XVII. Such a tendency is indeed observed. The formation of well-oriented final products by the reactions XII and XV and of a poorly oriented Zn 2 Y by reaction XVIII are exceptions, however, for which we are not able to give any explanation. KEDESDY and DRUKALSKY~s~ proposed an atomic mechanism of the formation of a mixed crystal (having NiO-structure) from a NiO and a ZnO crystal. Oxygen atoms diffuse from the ZnO to the * The high degree of orientation of the final product VI is not caused by sintering (c.f. Experimental Part 4). This was checked by heating the mixture up to 1250°C and cooling the pellet rapidly. The sample was poorly sintered (d = 4-3) and still very well oriented (f = 0.92). ts~ H. KEDESDYand A. DRUKALSKY,J. Amer. Chem. Soc. 76, 5941 (1954).
Topotactieal reactions with ferrimagnetic oxides having hexagonal crystal structures--II
107
NiO crystal and build up a new oxygen layer on the NiO surface. Then a diffusion of Zn atoms from the ZnO into the NiO crystal follows. According to this picture the oxygen "frame" of the NiO lattice remains intact and increases layer for layer during the reaction. This process would lead to a topotactically oriented mixed crystal with the orientation of the original NiO crystal. A similar mechanism for e.g. the formation of a MezW nucleus on a M crystal (reaction VIl) would explain nicely the orientation of the MesW crystal. Such a mechanism, which seems to us quite possible in the ease of the mixed NiO-ZnO crystal, is, however, very unlikely for our much more complicated reaction. Although the crystal structures of Mez W and M are closely related, the oxygen frame and the positions of the Ba+ ions in this frame are different for both structures. For a transition of the M into the Me1 W lattice a rearrangenemt of the O'- and Baz+ is necessary. It is hard to believe that during such a process parts of the oxygen frame remain intact. It seems to us much more likely that the topotactieal reaction VII starts with an epitaxial growth of MezWcrystals on the surfaces of the M crystals. The spacings of the oxygen layers of M, MezW, MezZ and Me, Y are practically equal and parts of these lattices have even the same atomic arrangement (e.g. the M and Mez W lattices contain layers having spinel structure with an identical atomic arrangement. The only difference is that the cation sites are occupied by Fe z+ in M and by Fe z+ and Me z+ in Me,W). Therefore the conditions for an epitaxial growth for M, MezW etc. (and S) on each other are expected to be very favourable. Such an epitaxial growth is observed during reaction XX.
3. Intermediate products (Table 2) A topotactieal reaction leads to the final product via several intermediate products which appear during the heat treatment o f the pellet. B = BaFe204 and S = MeFe~O~ are formed at relatively low temperatures (below 900°C). At a higher temperature (1000-1100°C) M and Me2 Y a p p e a r . The final products Me2 W a n d MezZ are generally formed only after a longer firing at a still higher temperature (1250°C) by the.reactions:
M + 2S ---} Me 2 W and M + Me 2 Y--* Me~Z During reaction V I I I a double conversion f r o m Zn2Z and Fe2Os into M and S takes place between 1000 and 1100°C. Since the Zn2Z crystals, which were oriented in the magnetic field, have disappeared at temperatures above 1100°C and well-oriented MeoW does not start to a p p e a r until 1250°C is reached, the conservation of the orientation must take place via the M phase. Reaction X (Table 1) gives a well-oriented final product although the initial mixture contains only a small a m o u n t (15 weight per cent) o f the oriented c o m p o n e n t M and a large a m o u n t (42weight per cen0 of non-oriented C o / z Z n z / Z . T h e p r e s e n c e o f the latter c o m p o n e n t evidently causes no f o r m a t i o n o f a considerable a m o u n t of nonoriented final product. This m a y be explained as follows. F r o m an analysis o f the intermediate reaction products it was found that a double conversion Me~Z + 8Fe203 --* 3 M + 2MeFe~O 4 at temperatures between 1000 and 1100°C occurs here also, just as during reaction VIII. The large a m o u n t of M formed in this way will grow on the o r i e n t e d - M crystals present in the initial mixture (reaction of type (c)). Then, at 1250°C, well-oriented M e ~ W i s f o r m e d from the oriented M phase and S, just like during the reactions V I I and VIII. In connexion with the above-mentioned mechanism of the reactions VIII-X we investigated the conversion of CozZ and FezOz into M and CozW (reaction XI, Table 1). The final product consists of non-oriented M and oriented Co,W. From an analysis of the intermediate reaction products it was found that the reaction takes place as given by equation XI at temperatures between 1000 and 1100°C and that no spinel phase appears during the reaction. Thus the double conversion
108
F . K . LOTGER1NG
XI differs from the double conversions that take place during the reactions VIII-X in the orient'atiom of M and W; the reaction mechanism is also different. This difference of mechanism may be caused by the fact that the double conversions during the reactions VIII-X proceed in the presence of a $ phase (Table 2), which may promote further S formation, whereas no S phase is present when reaction XI starts.
Acknowledgements--The author is greatly
indebted to Mr. J. VEnnE~T for carrying out the experiments and to Dr. E. W. GORT~Rand L~. C. KooY for valuable discussions and suggestions.