Lattice changes in spinel-type iron chromites

J. Phys. CiKm. Solids

LATTICE

Pergamon

Press 1957. Vol. 3. pp. 37-43.

CHANGES

IN SPINEL-TYPE

IRON

CHROMITES

M. N. FRANCOMRE Communication

from the Staff of the Research

Laboratories of The General England

(Received 25 Febwary

Electric

Company Limited,

Wembley,

1957)

Abstract-X-ray powder studies have been made of structure deformations arising on cooling spinet-type iron chromite compounds with compositions of the form FeFez-,Cr,O, below room temperature. Approximate measurements of Curie temperatures have also been carried out over the entire composition range x = 0 to x = 2. A tetragonal distortion with c/a greater than one is first detected at x = 1 *Oand increases in magnitude up to x > 1.3. With higher values of x the deformation is orthorhombic and results from a separate [OOl] expansion and [loo] contraction, giving the sequence cubic -+ tetragonal + orthorhombic on cooling. As x is increased to 2 the second of these deformations predominates and in FeCr,O, the lattice distortion is tetragonal with c/a less than one. The nature and magnitude of the distortions are considered in relation to the probable cation distribution in tetrahedral and octahedral sites. The observed structure changes in these compounds are difficult to explain on the theory of covalent bonding of either octahedral or tetrahedral cations. it is suggested that the deformations may arise from strong negative B-B type interactions of the form Fea+-Fes+, Fes+-Cr8+ or Crs+-Crs+ operating through super-exchange or semicovalentexchange mechanisms.

1. INTRODUCTION

structure deformation is accompanied by a relative expansion in the [OOl] crystallographic direction but for pure ferrous chromite (x = 2) a marked relative contraction in the [OOl] direction (making c/a < 1) is observed. In the present paper the results are given of a more detailed X-ray study of the low-temperature structure changes in these iron chromites, and possible explanations of the phenomena are discussed.

LATTICE distortions have been shown to occur in a number of spinel-type magnetic oxides on cooling through some critical temperature, and are usually associated with magnetic, electrical conductivity or thermal anomalies. Thus in magnetite (FeaO,) the crystal structure has been shown by TOMBS and ROOKSBY to distort from cubic to approximately rhombohedraf symmetry on cooling below -160°C. Most of the other examples of such structure changes reported in the literature appear to involve distortions to tetragonal symmetry. In this class may be listed CoFe,0,,(2) which is slightly distorted at -183OC, and CuFe,0,,(3) %nMn,O,(*) and NiCr,O,@) which undergo similar transitions at appreciably higher temperatures, i.e. 760°C 1025’C and 35°C respectively. Recently some preliminary X-ray observations on tetragonal distortions, produced on cooling to - 183”C, in spinel-type iron chromites possessing the general formula FeFe,,Cr,O,, have been reported.@) The effects are seen in such iron chromites when the value of x lies within the range 1 SGx 6 2. With the composition x = 1.2 the

2. PROBATION

OF SAMPLES

Powder specimens of good homogeneity and purity were prepared both by thermal decomposition of solid solutions of Fe,O,? and CraOs in nitrogen and by reducing such sesquioxide solid solutions with iron powder using a method similar to that reported by RIGBY, LOVELL and GREEN(~) in their preparation of ferrous chromite. The first method consisted in carefully mixing ferric oxide and chromic oxide by milling in a steel ball-mill for 50 hr, or grinding by hand under acetone, and firing the mixture for 4 hr at about lOOO-1100°C in air. X-ray powder photographs of the products showed only a single homogeneous haematite-type phase to be present. The spinel-type iron chromite was then obtained by decomposing the sesquioxide solid solution for 18 37

M.

38

H.

FRANCOMBE

hr at 1250°C in dry, oxygen-free nitrogen. The molar proportions of original oxide used were calculr.ted from the appropriate equation, as in the following example for which x ~~ 1.2. 2.2Fe,O,+O.8Cr,O,

=

1.2Fe,0,+0.8FeCr,O,+jO,. -\i_~ 2Fez+Fe.‘.?Cr,.,04 IL

meter for the spine1 structure varies with x in the formula FeFe,_,Cr,O,. The values obtained for preparations described here differ somewhat from those of YEARIAN et al. in regi:)ns 1 and 2 of the

As values of x were increased beyond 1.2 the thermal decomposition did not proceed to ccmpletion and appreciable quantities of the haematite-type phase remained. For compositions containing more chrcmium i.e. with x ;- 1.2, the solid solutions were reduced by carefully mixing them with the appropriate amounts of iron micropowder (particle size < 1 p) under acetone and firing the mixtures for 24 hr in open silica boats in dry nitrogen at 1250°C. The reaction may be represented by the equation, Fe+(4_3/2x)Fe,O,,

3/2x0,0,

= 3Fe,-J&O,.

Small quantities of boric oxide were used as a flux to promote sintering and to encourage crystal growth. In each case good homogeneous products were obtained which were free from the sesquioxide. Compositions stated in the following sections are in each case nominal and are based on the above reaction equation. 3. EXPERIMENTAL X-ray studies both at room temperature and low temperatures were carried out with 19 cm powder cameras, using either cobalt or chromium radiations. For exposures at low temperatures the techniques employed were, (a) that described by TOMBS(~) in which the powder specimen is bathed in liquid oxygen or nitrogen fed from a nickel capillary tube, and (b) the gas-cooling method described recently by the author.(uj Approximate measurements of Curie temperatures were made (to an accuracy of 5°C) by a method similar to that used by HARWOOD(‘“) in his study of lanthanum manganites. For Curie points above room temperature the powders, contained in small aluminium foil tubes, were heated in a furnace and then allowed to cool slowly. The temperature at which they were attracted by a magnet pole-piece placed close to the mouth of the furnace was recorded by using a copper-constantan thermocouple embedded in the powder. For Curie points below room temperature the furnace was replaced by a Dewar flask containing liquid oxygen or nitrogen. 4. THE

SYSTEM

Fe,O,/FeCr,O,

The chemistry and variation of unit-cell parameters with composition in the system Fe,O,/ FeCr,O, have been studied in some detail by YEARIAN, KORTRIGH.~, and LANGENIIEIM,(~~)who prepared their samples by reducing Fe,OJCr,O, solid solutions in mixed CO/CO, atmospheres. Fig. 1 shows graphically how the a, unit-cell para-

0.4

0

0.8 Nominal

FIG.

1.

Lattice

1.2

1.6

2.0

compositiorxx

parameters FeFe,_,Cr,O,.

of

compositions

x x x Present values. - - - YEARIAN et al.(“)

curve, but for compositions with values of x lying between 0.8 and 2.0 the agreement is fairly close. In any case the general trend of the lattice parameter variation appears to be well substantiated. The curve may be divided into four welldefined linear portions and YEARIANet al. have explained this in terms of the change, in four distinct stages, from the “inverse” ferrite composition of magnetite (Fe3~[Fe2~Fe3+]0,) to the completely “normal” cation arrangement in ferrous chromite (Fe2+[Clt$]04). Thus over region 1 of the curve as x increases the ferric ions on the B sites are probably being replaced by chromic ions. The resulting solid solutions retain a completely inversed structure of the form Fe”+[Fe,f$,Cr’L’ Fe2t]0,. Regions 2 and 3 are two stages over which the cation distribution gradually changes to the normal type, this process being completed at the end of region 3(x - 1.3). All compositions over the last linear region of the curve are assumed to have the normal type of structure represented by the formula Fe2+[Fe,,3_z,Cr3,+]O~. Using these assumptions the approximate cation distributions were calculated along the lines suggested by YEARIAN et al. for compositions x =m1 G2.0 and are shown in Table 1.

LATTICE

CHANGES

IN

SPINEL-TYPE

IRON

39

CHROMITES

Table 1

Approximate cation distribution

Composition x 0 0.1-0.8 1.0 1-2 1.3 1.4 1.45 1.50 1.55 1.60 1.70 1.80 2.0

Approx.

8.396 See Fig. 1. 8.397 8.405 8,406 8.405 8,402 8.400 8.398 8.395 8.391 8.385 8.377

Structure at --183°C Rhombohedral Cubic ?

I

Tetragonal (c/a > 1) 4 Fes+[Fe$Cr$]O,

?‘ Orthorhombic

Tetragonal (c/a < 1) _~___

5. LOW TEMF’ERATURE X-RAY RESULTS

X-ray powder photographs were taken of several different solid solutions in the composition range 0 < x G 2-O with the powder specimen held at -183”C, throughout the exposure. For x = O-1 high-angle X-ray diffraction lines remain sharp and unsplit at - 183°C so that the rhombohedraltype distortion found in magnetite at this temperature has become vanishingly small or does not occur. This is also true of other compositions in the range 0.1 < x G OS. At x = 1.0, however, slight splitting of the X-ray diffraction lines again becomes detectable, this time indicating a new tetragonal-type distortion. With further increase in the chromium content up to x = 1.3 the magnitude of the tetragonal-type distortion is appreciably increased. At x = l-4 there are feeble indications of line-splitting effects rather more complex than those due to a tetragonal structure. At this stage the symmetry is probably more accurately described as pseudo-tetragonal (truly orthorhombic). It now becomes apparent that the observed orthorhombic deformation arises from the combined operation of two dilatation effects, differing in magnitude and directed along different (100) cubic axes. The tetragonal and ortl~orhombic unit-cell dimensions for this series of compositions were calculated from the splitting

of diffraction lines with E h2 of 27 and 32. In general, determinations from the two line groups agreed to within -&O*OOZA,but the limits of accuracy of the absolute values of the lattice parameters are probably a little greater than this.

Nominalcomposition, FIG. 2. Lattice parameters at -183°C

x

for compositions

The graph, Fig. 2, illustrates the manner in which the tetragonal and orthorhombic structurecell dimensions determined at -183°C vary with composition over the range x = O-8 to 2-O. In this graph it can be seen how the tetragonal structure assumed by compositions with x G 1.4 changes progressively as x increases, towards the different tetragonal structure of FeCrsO,, with an axial ratio well below one, which was reported on an earlier occasion.@)

M.

40

H.

FRANCOMBE

5.2. X-ray studies between 20°C and -183°C The variation of lattice parameters with change of temperature has been studied on three of the solid solutions, viz. x = 1.2, 1.6 and 2.0. The results are presented graphically in Figs. 3,4 and 5.

lempemture

FIG. 3. Lattice

parameters of Fe,.,Cr,.,O,.

OQ 8.60 j

8.50

E x 8.40 e 3

8.x)

3 8.20 -200

-180 -160

-140 -120 -100 Temperature

-80

-60 OC

FIG. 4. Lattice parameters of Fe,.4Cr,.aOa.

change is accompanied by a relative contraction instead of expansion in the [OOl] crystallographic direction of the cubic unit cell, the axial ratio becoming less than unity. As can be seen from Fig. 5, the transition is again a fairly gradual one first appearing at about -9O”C, and c/a decreases smoothly with fall in temperature, reaching a value of 0.968 at -183°C. In order to investigate the mechanism by which the orthorhombic deformation is produced on cooling, the composition x = 1.6 was chosen for study at temperatures between 20°C and -183°C. This choice was made chiefly on the grounds that the orthorhombic a,,, b, and c0 parameters at - 183°C differed from each other to a greater extent than for the other solutions in the series. The graph, Fig. 4, shows that the change to orthorhombic symmetry occurs in two steps. Thus on cooling to about - 130°C the structure changes from cubic to tetragonal with an axial ratio greater than one. As the temperature is further lowered the axial ratio increases until at about -165°C a fairly abrupt transition to the orthorhombic structure takes place. The maximum tetragonal axial ratio reached, just above the lower transition temperature, is c/a = 1.013. The lattice distortion effects involved in these two structure transitions seem to be such that a marked relative expansion in the [OOl] crystallographic direction at - 130°C is followed, at - 165°C by a simultaneous relative expansion and contraction in the [OlO] and [loo] directions respectively. 6. MAGNETIC

Temperature

OC

FIG. 5. Lattice parameters of FeCr,O,.

For the x = 1.2 composition the transition in structure from cubic to tetragonal on cooling takes place at about -95°C. The change is continuous and line-splitting effects in the X-ray powder photographs increase gradually as the temperature is decreased down to -183°C. The cia axial ratio attains a value of 1.005 at -183°C. Pure ferrous chromite, FeCr,O, also undergoes a tetragonal deformation on cooling, but here the

MEASUREMENTS

Approximate values of Curie temperatures were determined for the entire range of compositions from pure Fe,O,(x = 0) to FeCr,O,(x = 2). These are presented graphically in Fig. 6 together with available data on structure transition temperatures. The Curie points for Fe,O, and FeCr,O, agree closely with those given by BOZORIII(‘~) and LOTGERIXG(~~) respectively. Although it is not possible to reach any conclusions regarding the nature of the structure deformations from Curie temperatures alone, the graph shows whether the structure transitions occur in the paramagnetic or ferromagnetic region. Also it can be inferred, from the fall in Curie temperature with increase in the value of x, that as the FeCr,O, composition is approached the principal

LATTICE

CHANGES

IN

SPINEL-TYPE

ferrimagnetic interaction occurring between tetrahedral and octahedral cations of the spine1 lattice becomes weaker. 7. INFLUENCE

OF CATION DISTRIBUTION ON D~TORTIONS

Consideration of the lattice parameterjcomposition relationship shown in Fig. 2, in association with the cation distributions listed in Table 1, indicates that the type of distortion of the structure eelI produced on cooling is closely dependent upon the distribution of atoms amongst the A and B sites of the structure. As the value of x increases from 1.0 to l-3 the value of the tetragonal axial ratio increases con siderably. This corresponds, as shown in Table 1 to the gradual removal of Fe3+ ions from the A sites and Fe2+ ions from the 3 sites. At the same time the number of Cr3+ and Fesf ions in the 3 sites increases, the latter reaching a maximum at Fe$$(x = 1.3). AS the chromium content increases further from x < 1.4 to x = 2-O the tetragonal distortion involved changes from a type with C/U greater than unity to one with c/a less than unity. At the same time the overall distortion of the lattice observed at -153°C becomes progressively greater in magnitude. From these observations, even without knowing the origin of the forces causing these deformations, we may conclude that, at least in the composition range x = l-3 to 2.0, the cation distribution on the B sites of the spine1 lattice probably governs the type and magnitude of the lattice distortions produced. 8. DISCUSSION Various explanations have been advanced to account theoretically for the distortions produced in spinel-type structures on cooling. Thus in Fe,@ VERWEYand HAAYMAN@*) concluded, on the basis of electrical conductivity measurements, that ordering of the Fez+ and Fe3+ ions occurs in octahedral sites of the spine1 lattice on cooling below -160°C. VERWEY@~)predicted that such ordering should produce a tetragonal distortion, while BICKFOF&~) suggested that the distortion might be orthorhombic. GOODENOUGHand LoEB@~) have used V~WEY’S model together with the conception of covalent bond formation for the Feaf ions in tetrahedral sites to explain the orthorhombic

IRON

CHROMITES

41

distortion reported by ABRAHAMS and CALHOUN. Unfortunately any such ideas involving ordering of 3d electrons are difficult to extend to structures with mixed B-site cation distributions, such as Fe[CoFe]04 and FeFe,,Cr,O,. GOODENOUGHand LOEB have also proposed an explanation of the tetragonal distortions observed in Mn,O,, CuFesO,, CuCr,O, and ZnMn,O, in terms of covalent bonding effects in the octahedral sites. The tetragonal deformation is assumed to arise from the tendency on cooling for the octahedral cation to form square covalent bonds with four of its nearest neighbour anions and ionic bonds with the other two anions. At the structure transition temperature the ionic bonds become “frozen” into a definite direction in the lattice, which then becomes identified with the c axis of the new tetragonal unit cell. The deformation is positive in the direction of the ionic bonds and thus c/u is greater than one. Chromium ions, however, do not show this tendency and the hypothesis breaks down when applied to NiCr204@) and FeCr,O,, both of which probably possess normal structures. Moreover X-ray studies by BERTAUTand DELORME@~)have shown that CuCr,O, not only possesses a normal cation distribution but also a structure cell with an axial ratio below one. Even if, as has been suggested,(20k the tetragonal deformation of this structure were due to strong covalent bonding of the divdlent Cu ions in the tetrahedral sites, such bonding is not likely to occur for divalent Fe or Ni ions. An alternative interpretation of the effects may possibly be found in antiferromagnetic ordering of spin moments in the spine1 lattice. Thus ROOKSBY and WILLIS(~) have drawn attention to the similarity of the low temperature deformations in Fe,O, (a principal [ll 11expansion) and CoFe,O, (a [OOl] contraction) to those in the corresponding antiferromagnetic monoxides Fe0 and COO. Antiferromagnetic interaction between spin moments in spinels has been considered by YAWT and KITTEL(~~)who suggested a model in which the A and 3 lattices of the spine1 structure could be regarded as composed of sublattices. Negativetype interactions between the sublattices, either for A or B type ions, can be shown to account for apparent anomalies in magnetic behaviour in several of the spinel-type ferrites and [email protected])

42

M.

H.

FRANCOMBE

HASTINGS and CORLISS,(~~) using neutron diffraction techniques, have in fact detected an antiferromagnetic ordering of spin moments on the B sites in normal-type ZnFe,O, at low temperature. Also from specific heat and magnetic data obtained by MCGUIRE, HOWARD, and SMART there seems good reason to suppose that similar ordering effects are responsible for the tetragonal distortion produced on cooling NiCr204.(13) In this substance the structure changes very sharply to tetragonal (c/u > 1) just below 35°C. LOTGERING has made magnetic measurements on FeCr,O, but reports no anomaly similar to that found for NiCr,O,. The structure deformation in FeCr,O,, however, is smaller in magnitude and occurs more smoothly than that in NiCr,O,, so that the magnetic anomaly, if originating from similar causes, might be expected to be somewhat smaller and therefore have been missed. We can interpret the data shown graphically in Figs. 2-5 in terms of deformations comprising anisotropic expansions or contractions directed along cubic (100) axes in the spinel-type unit cell. For compositions in the range x = 0.8 to 1.3 the tetragonal distortion results from a relative expansion along the [OOl] direction of the unit cell, this then becoming the c axis. The magnitude of this effect increases up to x > 1.4, at which composition a second [lOO]-type deformation becomes involved, producing the orthorhombic structure observed at - 183°C. The low-temperature X-ray study of the composition x : 1.6 (Fig. 4) reveals that the second distortion probably occurs as a relative contraction along one of the a axes of the tetragonal unit cell, and at a lower temperature than the first structure change. Further data on the transition temperatures for compositions intermediate between x = 1.6 and 2.0 are not yet available, but the present results indicate that, with increasing chromium content the second distortion effect becomes predominant, the first [OOl]-type expansion gradually disappearing. Also the temperature at which the [loo] contraction occurs rises as the composition FeCr,O, is approached. With pure FeCr,O, the low-temperature structure change arises entirely from this relative contraction, in a direction identified with the orthorhombic a axis. As shown in Fig. 3 the orthorhombic 6, dimension now becomes equal to the c,, dimension and the symmetry may now be

referred to a tetragonal structure cell having its c axis as the deformation direction. It may be possible to explain these structure changes on arguments similar to those used by YAFFET and KITTEL,(~~) by picturing strong negative B-B type interactions to occur between spin moments on interpenetrating sublattices. Over the solid solution series examined such interactions could be of the form Fe3+-Fe3-+, Fe3+-Cr3+ or and would presumably operate by Cr3+-Cr3+ means either of a superexchange@) or semicovalent exchangecl’) magnetic coupling, involving oxygen ions situated between the cations on the sublattices. It is not clear how such antiferromagnetic ordering on the B sites may be affected by changes in the strength of the A-B type interactions or by replacement of Fe3+ by Cr3+ ions on the B sites as x increases in value from 0.8 to 2.0. From the fact that the ferrimagnetic Curie temperature falls throughout the series with increasing chromium content (Fig. 6) it appears that the A-B type interactions become progressively weakened. The maximum tetragonal distortion (at -183°C) with an axial ratio greater than one appears to be reached with a composition of x N 1.3. This probably corresponds to a maximum number of Fe3+ ions in the B sites (Table 1) and thus to a maximum Fe3f-Fe3+ or Fe3+-Cr3+ interaction in these sites. As x is further increased the Cr3+-Cr3+ type interaction increases, coinciding with the strengthening of the [loo] contraction and with the removal of the relative [OOl] expansion.

Nominal composition, x FIG. 6. Curie temperatures FeFe,-,Cr,O,.

a

forcompositions

x x x Curie temperatures. Structure transition temperatures.

LATTICE

CHANGES

IN

SPINEL-TYPE

Reference to the graph, Fig. 6, shows that for compositions with x < 1.4 the structure changes in the ferromagnetic region, i.e. below the temperature at which the principal ferrimagnetic ordering effect between spin moments on the spine1 A and B sites is assumed to occur. If, as has been suggested above, the deformation originates from strong negative B-B interactions, it must be concluded that below the Curie temperature, in these solid solutions, states of ferrimagnetism and antiferromagnetism coexist. A similar condition might arise in Fe,O, and CoFe,O, where the structure deformations also occur in the ferromagnetic region. It is obviously desirable to supplement these studies with careful thermal and magnetic measurements, particularly of susceptibility and magnetic moments. Further work along the lines described above could be attempted on NiCr,O, and CuCr,O, compositions in which the B-site CrS+ ions are replaced by small amounts of Fe3+ ions. It would also be interesting to study the effect produced on the tetragonal structure deformation of NiCr,O, by replacing Ni2+ ions both by Fez+ ions and also by a non-magnetic ion such as ZrPf or Mg2+. Observations on the resulting changes in the deformations should enable firmer conclusions to be reached concerning their origin. REFERENCES 1. TOMBSN. C. and ROOKSBYH. P. Acta Cryst. 4,474 (1951).

IRON

CHROMITES

43

2. ROOK~BYH. P. and WILLIS B. J. M. Nature, Land. 172, 1054 (1953). E. F. r. Phys. Radium 12, 252 (1951). 3. BERTAUT 4. MACONB. Amer. Min. 32, 426 (1947). 5. DELORMBC. R. C.R. Acad. Sci., Paris 241, 1588 (1955). M. H. and ROOK~BYH. P. Nature, Lond. 6. FRANCOMBE 178, 586-587 (1956). 7. RIGBY G. R., LOVELL G. H. B., and GREEN A. T. Iron and Steel Inst. Spec. Rep. No. 32. p. 93 (1946). 8. TOMBSN. C. 7. Sci. Instrum. 29. 364 (1952). fi. H. ‘J. Sci. Inns&m. 34, 35 11957). 9. FRANCOMBE 10. HARWOODM. G. Proc. Phys. Sot. B68, 586-592 (1955). J. M., and LANCENHEIM 11. YEARIANH. J., KORTRIGHT R. H..? Chem. Phys. 22, 1196-1198 (1954). p.246. Nostrand, 12. BOZORTHR. M. Fewomagnetism New York (1951). 13. LOTGERINGF. K. Philips Res. Rep. 11, 190-249 (1956). 14. VERWEYE. J. W. and HAAYMANP. W. Physica 8,979 (1941). 15. VERWEYE. J. W. Nature, Lond. 144, 327 (1939). L. R. Phys. Rev. 78, 449 (1950). 16 BICKFORD J. B. and LOEB A. L. Phys. Rev. 98, 17. GOODENOUGH 391-408 (1955). 18. ABRAHAMSS. C. and CALHOUNB. A. Acta Cryst. 6, 105 (1953). 19. BERTAUT E. F. and DELORME C. C.R. Acad. Sci., Paris 239. 504-505 (1954). 20. GOODENOIJCHJ. B. P&ate communication (1956). 21. YAFET Y. and KITTEL C. Phvs. Rev. 87. 290 (1952). 9, 295, 3i1, 405 22. GORTER E. W. Philips Res:Rep. (1954). 23. HASTINGSJ. M. and CORLISSL. M. Phys. Rev. 102, 146C (1956). 24. MCGUIRE T. R., HOWARD L. N., and SMART J. S. Cevakc Age 60, 22 (1952). 25. KRAMERSH. A. Physica 1, 182 (1934).