Effect of heat treatment on microstructure evolution of haematite derived from synthetic goethite

Materials Science and Engineering, A 149 ( 1991 ) 121 - 127

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Effect of heat treatment on microstructure evolution of haematite derived from synthetic goethite G. N. Kryukova*, S. V. Tsybulya, L. E Solovyeva, V. A. Sadykov, G. S. Litvak and M. P. Andrianova Institute of Catalysis, Siberian Branch of the U.S.S.R. Academy of Sciences, Prospekt Akademika Lavrentieva 5, Novosibirsk 630090 (USSR) (Received April 15, 1991 ; in final form July 19, 1991 )

Abstract The evolution of the microstructure of a-Fe203 derived from synthetic a-FeOOH in a wide temperature range (400-1100 °C) has been systematically investigated using X-ray powder diffractometry and transmission electron microscopy. It was found that during the dehydration of a-FeOOH at 400 °C the a-Fe203 structure with a small amount of residual OH groups is formed. After heating at 500 °C the specimen has been shown to possess a stoichiometric composition. Further heating to 900 °C causes the gradual disappearance of the planar faults observed in low temperature specimens, followed by the formation of the (100) twins and surface steps enriched by potassium, chlorine and aluminium impurities.

1. Introduction The goethite-haematite ( a - F e O O H - a - F e 2 0 3 ) transf o r m a t i o n has attracted much scientific and technological interest over recent years. This process is very important in the preparation of some catalysts, pigments and so on. The decomposition of a - F e O O H has been studied previously [1-8]. The main results of these investigations can be summarized as follows. After heating at 300 °C a - F e O O H transforms into a-Fe203. Throughout this dehydration reaction the crystal habit is conserved, but simultaneously a system of fine pores develops in the bulk of the a-Fe203 crystal due to loss of water molecules from the structure of the starting material. On raising the temperature to 500 °C, the system of fine pores disappears. In contrast with the above results Wolska [9] concluded that the transformation is not direct, but an intermediate phase forms called "hydrohaematite". From data obtained by X-ray powder diffractometry, it was found that this third phase has the structure of aFe203 with a partial substitution of O H groups for oxygen molecules and the presence of cation vacancies as charge compensators. However, such an interpretation should be justified only on the basis of direct structure refinement. *Author to whom all correspondence should be addressed. 0921-5093/91/$3.50

There is also some debate as to whether nonuniform broadening of the X-ray lines can be ascribed to the shape anisotropy of the o~-Fe203 particles [7]; either the presence of imperfections or strains, or faults occurring at low temperatures, result in anomalous diffraction effects [3]. However, little is known in detail about the defect structure of synthetic a-FeEQ, especially over a wide range of calcination temperatures (500-1100 °C). Nevertheless, it is now well established that the deviation from the regular structural arrangement affects substantially the physical and chemical properties of the materials. The objective of this study was to investigate the structure and microstructure of synthetic a-Fe203 derived from a - F e O O H over a wide temperature range. The X-ray powder diffraction (XPD) method and transmission electron microscopy (TEM) were used for this purpose.

2. Experimental details Specimens were prepared by the thermal decomposition of ct-FeOOH at 400 °C for 4 h followed by calcination in the temperature range from 500 to 1100 °C for 1 h at every temperature step. Microchemical analysis revealed that impurities were present in the order of 0.01 wt.% in the specimens. © 1991--Elsevier Sequoia, Lausanne

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G. N. Kryukova et aL / Microstructure evolution of a-Fe203

XPD patterns were obtained with a H Z G - 4 diffractometer using Cu Kfl radiation. The scan region was 2 0 = 19°-120 °, and the profile step width was 0.05 °. The profile step counting time was 90 s. The computer program system Polycrystal [10] was used to determine the structural parameters (atomic coordinates, occupation of positions etc.). A crystal structure refinement procedure using the modified Rietveld method [11] was also performed. Particle sizes D were calculated by means of the Debye-Scherrer equation D = KMA(2 O) cos 0, where K is a constant equal to 0.9; 2 is the X-ray wavelength used; A(20) is the broadening of the spectral line chosen, calculated from the expression [A(20)] 2 =/~exp -/-/~.sd (where Hexp is the measured linewidth of the specimen and Hsd the linewidth measured under the same conditions with a standard substance); and 0 is the diffraction angle of the line considered. An a-Fe203 specimen annealed at 1100 °C was used as the standard; the line broadening for this sample was caused only by the experimental conditions. All the specimens were examined in a JEM-100CX electron microscope. The resolution limit of the machine is about 4 A, the accelerating potential 100 kV. Specimens were prepared by dispersing samples in ethanol; then a drop of the suspension was dried on a carbon film supported on a copper grid. Thermogravimetric analysis of the sample under investigation was carried out using a Q-1500D device at a heating rate of 10 °C min- ~in air.

3. Results and discussion

3.1. X P D analysis The crystal structure of ct-Fe203 is described by the space group R3c (hexagonal); the parameters of the unit cell are a = 5.0356(1) A and c = 13.7489(7) A [12]. Fe and O atoms are placed in the (0,0,z) and (x,0,¼) positions respectively, where z = 0 . 3 5 and x = 0 . 3 1 [13]. The structural parameters (i.e. lattice constants, atomic coordinates, occupation of atomic positions) were refined for a-Fe203 specimens annealed at 400, 500, 800 and 1100 °C. The total amount of iron cation vacancies was found to be equal to only 2-3% for the ct-Fe20 3 specimen annealed at 400 °C. The observed amount of vacancies in the structure of this sample is close to the limiting error of the XPD method. Nevertheless, it is significant that the presence of vacancies has not been detected for all the specimens calcinated at higher temperatures. Thermogravimetric analysis revealed the weight loss (about 2%) during the heating of the low temperature specimen from 400 to 800 °C. According to the XPD data, the chemical composition of this sample follows approxi-

mately the formula Fel.9502.85(OH)0.15, because it seems likely that charge compensation is achieved by partial substitution of O H groups for oxygen atoms in the oxygen sublattice of a-FezO3. This suggests that the structure of the low temperature specimen possessed 1.5 wt.% water. This assumption compared favourably with the thermogravimetric data. All other specimens annealed at high temperatures are believed to have the composition a-Fe203. The interatomic distances for the specimens under investigation are listed in Table 1. From these results it can be seen that, although the Fe-Fe interatomic distances are the same for all the specimens, the two nonequal F e - O distances become closer together as the calcination temperature rises, i.e. 1.90-2.18 A (400 °C) in comparison with 1.95-2.10 A (1100 °C). At the same time, the difference between two neighbouring oxygen positions is increased from 2 . 8 2 - 2 . 9 5 A (400°C) to 2 . 6 4 - 3 . 0 5 A (1100°C). Therefore the degree of distortion of the oxygen octahedron approaches that of the perfect a-Fe203. It should be noted that synthetic a-Fe203 shows non-uniform broadening of the XPD lines in a wide range of calcination temperatures (400-1000 °C)(see Fig. 1 ). For the a-Fe203 specimen annealed at 1100 °C the broadening of the X-ray peaks is caused only by the experimental conditions. Results of the particle size determination using different reflections are given in Table 2. This table shows that the particle sizes obtained from X-ray analysis of(110) and (030) reflections are bigger than those of (012) and (104). Duvigneaud and Derie [7] reported similar results; they postulated that the non-uniform broadening of the XPD lines could be ascribed to the anisotropy of the a-Fe203 thin platelets with the most developed (001) faces and minimal thickness in the [001] direction as indicated in Fig. 2(a). Consequently, the (hkl) particle sizes are: D012 = 1.27t, Di04 = 1.35t, Du6 = 1.89t etc., where t is the platelet thickness. From these equations the t value could be calculated. However, the authors of ref. 7 did not take into consideration the (006) X-ray reflection because of its weakness. Yet this study was TABLE 1 Interatomic distances in a-Fe203 specimens annealed at various temperatures Calcination temperature (°C)

Distance (A) Fe-O (3)

Fe-O~ (3)

O-O

400 500 800 1100

1.90 1.93 1.93 1.95

2.18 2.14 2.13 2.10

2.82-2.95 2.73-3.00 2.72-3.00 2.64-3.05

Numbers in parentheses denote the number of definitive interatomic distances in the structure of material under investigation.

G. N. Kryukova et el.

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Microstructure evolution of a-Fe :O,~

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(104) ~110)

2UO~

j

~012I

2'~:

24

~ ~ 1 1 1 3 )

(024)

28

Fig. 1. A portion of a typical XPD spectrum of the a-Fe203 specimen annealed at 400 °C.

TABLE 2 Comparison of(hkl) particle sizes for various calcination temperatures Calcination temperature (°C)

(hkl) particle size (A) 012

104

110

4l)0 500 600 700 800

160 220 350 430 700

160 230 350 440 620

430 430 591) 570 900 >

006 600 580 580 580 1500

c N

113

030

116

430 380 540 540 1100

370 360 510 540 1200

340 330 390 520 880

able to obtain reliable data for this weak reflection using a prolonged counting time in the experimental procedure. O u r results showed that the (006) diffraction line has a minimal broadening in c o m p a r i s o n with other diffraction maxima; therefore the corresponding particle size is large (see Table 2). T h e a b o v e - m e n t i o n e d considerations lead us to conclude that the real a-Fe203 crystallite consists of sheets which are disoriented with respect to each other along the [001] direction. Figure 2(b) shows a particle with such layer disorder. Guinier [14] analysed similar planar faults occurring in banded structures; in this case the broadening of the (001) peaks is caused by the thickness of the whole crystal whereas the broadening of the (hkl) reflections is associated with the thickness of one layer. T h u s the a p p r o a c h taken in this analysis allowed us to calculate the thickness of the whole a Fe203 particle as well as that of each separate sheet (or the n u m b e r of planar faults). T h e s e results are listed in Table 3.

(a)

Fig. 2. The a-Fe203 particle with (a) a regular and (b) a disordered arrangement of sheets (see text for further discussion).

TABLE 3 Number of planar faults in a-Fe203 particles Calcination Thickness Thickness Number of Probability temperature of separate of a-FeeO3 planar of planar (°C) sheet (A) particle(A) faults faults presence 400 500 600 700 800

140 170 250 320 510

68(I 580 580 580 > 1500

4.9 3.4 2.3 1.8 --

0.098 0.081 0.055 0.043 --

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Microstructure evolution of a-Fe_,O~

By comparing the calculated values from Table 3, it can be seen that the thickness of the whole a-Fe203 particle does not change in the temperature range from 400 to 700 °C, and the thickness of each separate sheet increases, reflecting the process of the disappearance of the planar fault. Some decrease in the whole particle thickness for the specimen calcinated at 500 °C may be related to the annealing of the fine pores in the bulk of the low temperature sample. After further heating at 800 °C the a-Fe203 crystallites coalesce and give rise to large particles more than 1000A in thickness. For the sample annealed at l l00°C, X-ray line broadening is not observed because of the increase in the particle sizes up to 1/~m. Thus, from the data obtained with a-Fe203 specimens by XPD analysis, we reached the conclusion that the anomalous broadening of the X-ray lines is associated with (a) the shape anisotropy of the particles and (b) the presence of the planar faults in the crystallite structure. 3.2. T E M observation

The best-developed plane of a-FeOOH particles was found to be of (100) type. Figure 3(a) shows a typical electron diffraction pattern of the a-FeOOH crystallite. After heating at 400 °C the [100] zone of a-FeOOH transforms into the [001] zone of a-Fe203 as is evident from the electron diffraction pattern (Fig. 3(b)). aFe203 is formed in a close orientation relationship with the original a-FeOOH, the a, b and c axes of the orthorhombic cell of R-FeOOH becoming c, a and (110) directions of the hexagonal cell of a-Fe203. Therefore the c axis of a-Fe203 is normal to the bestdeveloped plane of the particles. These findings are consistent with previously published results [2, 3]. The morphology of the crystals does not change during the dehydration: it consists mainly of thin platelets 300 A wide by 1 /~m long (Fig. 4). Of particular interest in this low temperature specimen is the occurrence of the narrow (about 20/k) voids (arrowed in Fig. 4). After further heating at 500 °C these voids transform into closed spherical pores by surface diffusion and coalescence of the material. The calcination at 600 °C intensifies the internal sintering, a-Fe203 particles are characterized by a mosaic structure: sharp reflexes on the selected-area diffraction pattern (Fig. 5) are transformed into small arcs, indicating slightly misoriented crystalline blocks. An increase in the temperature to 700 °C leads to the appearance of dislocations in the structure of the a-Fe203 crystallites. Screw dislocations with Burgers vector b = (1120){0001 } and dislocation network are arrowed in Fig. 6. The development of the dislocation

Fig. 3. Selected-areaelectron diffraction patterns: (a)[100] zone axis of the original a-FeOOH; (b)[001] zone axis of a-Fe203 (the heating temperature was about 400 °C).

network seems to be caused by misorientation between the sintering particles, especially when the crystallites coalesce along the lateral faces. Consequently, the particle width increases up to 1000 A. On raising the temperature from 700 to 900 °C the dislocation network disappears with the simultaneous formation of twins and macrosteps due to the recrystallization process. The micrographs of twin boundaries and macrosteps are given in Fig. 7(a) and Fig. 7(b) respectively. The electron diffraction pattern (see the inset in Fig. 7(a)) shows that the twin plane is (100). X-ray energy-dispersive analysis has revealed the presence of the impurities potassium, aluminium and chlorine in the vicinity of the twin boundaries. Spectra obtained from the twin region, and a twin-free part of the same a-Fe203 crystal, are given in Fig. 8(a) and Fig. 8(b) respectively. It should be noted that the original a-FeOOH was obtained by precipitation from FeCI3 solution with the addition of KOH. According to microchemical analysis data the amount of aluminium admixture in the parent

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Microstructure evolution of a-Fe :O~

125

Fig. 4. Typical image of the a-Fe~O~ particle annealed at 400 °C. The voids are arrowed.

Fig. 7. Micrographs of (a) the (100) twin boundaries and {b) surface steps in an a-Fe20~ specimen after heating at 900 °C. The electron diffraction pattern from the twin region is inset in (a). Fig. 5. The micrograph and selected-area electron diffraction pattern of an a-Fe203 crystallite after calcination at 600 °C.

Fig. 6. The micrograph of the dislocations (arrowed) in the structure of an a-Fe203 particle (the calcination temperature was about 700 °C).

FeCI 3 was equal to 0.1 wt.%. T h e r e f o r e potassium, aluminium and chlorine impurities should be included in the bulk of the low temperature a-Fe203 particles. Treatment of this specimen at a higher temperature causes the migration of impurities from the bulk of the material into the region of twin boundaries and macrosteps. These surface steps remain stable after further heating from 900 to 1100 °C because of the presence of the impurities. Thus T E M observation provided details of the aFe203 structure, mainly in the final step of the thermal treatment of the specimens under investigation. Undoubtedly, the XPD analysis gave more helpful information on the a-Fe203 defect structure for the low temperature region of the specimen calcination. Systematic investigations carried out simultaneously by use of these methods have thus made possible the direct elucidation of the microstructure evolution of a-Fe203 derived from synthetic a - F e O O H over a broad temperature range.

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G. N. Kryukova et al.

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Microstructure evolution of a-Fe20 ~

~e~

because of the partial substitution of O H groups for oxygen atoms. The dehydration transformation is topotactic, and forms a fine pore structure. (2) Further heating at 500-600 °C leads to the disappearance of the cation vacancies (or O H groups). The F e - O interatomic distances, as well as the O - O distances, change in comparison with the analogous parameters in the above specimen. However, they are not yet identical with the distances in the perfect aFe203 structure. The pore structure collapses; an internal sintering occurs which brings about greater crystal homogeneity, mainly as a result of the disappearance of the basal planar faults observed in the particles of the specimen annealed at 400 °C. After calcination at 700 °C, a-Fe203 particles sinter with the development of a dislocation network. (3) Raising the temperature from 800 to 1000 °C leads to the annealing of the dislocations and intensifies the recrystallization process, resulting in the appearance of coherent twins and surface steps. Only after the full formation of the twin boundaries, with their simultaneous enrichment by impurities of aluminium, potassium and chlorine, do the Fe-O interatomic distances, as well as O - O distances, become equal to those for the ideal a-Fe203 arrangement.

FeK0

~Ka ,IK a

clK~ {°cu

0

(a)

Fe~

4. Conclusions

FeK0 OKa

.g0

~ ~,g~ev

(b)

A study has been made of the effect of thermal treatment on the microstructure formation of a-Fe203 over a wide temperature range. It was shown that a stoichiometric a-Fe203 structure is formed after heating of the sample at 500 °C. The microstructure of the specimens annealed at temperatures below 800 °C is characterized by the presence of planar faults in the (001) basal plane of a-Fe203. In the temperature region from 900 to 1100 °C the real structure formation is determined by the distribution of the potassium, chlorine and aluminium impurities between the bulk of a-Fe203 particles, twin boundaries of (100) type and surface steps.

Fig. 8. X-ray energy-dispersive spectra obtained (a) from the twin region and (b) from the twin-free part of the a-Fe203 crystal. The specimen was annealed at 900 °C. References

Several trends can be discerned from our observations, as follows. (1) The structure of a-Fe203 with a small (2%) amount of iron cation vacancies (or O H groups) is formed after dehydration of the original a - F e O O H at 400 °C. The Fe-Fe interatomic distances are equal to those for the perfect a-Fe203 structure, whereas the F e - O and O - O interatomic distances are not ideal

1 M. H. Framcomble and H. P. Rooksby, Clay Miner. Bull., 4 (1959) 1. 2 K. Lotgering, J. lnorg. Nucl. Chem., 9(1959) 113. 3 J. Lima-de-Faria, Z. Kristallogr., 119(1963) 176. 4 F. Watari, J. van Landuyt, E Delavignette and S. Amelinckx, J. Solid State Chem., 29(1979) 137. 5 E Watari, E Delavignette and S. Amelinckx, J. Solid State Chem., 29(1979) 417. 6 E Watari, P. Delavignette, J. van Landuyt and S. Amelinckx, J. Solid State Chem., 48 ( 1983) 49.

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Microstructure evolution of a-Fe~_O¢

7 P. H. Duvigneaud and R. Derie, J. Solid State Chem., 34 (1980) 323. 8 H. Naono and R. Fujiwara, J. Colloid Interface Sci., 73 (1980) 406. 9 E. Wolska, Z. Kristallogr., 154 ( 1981 ) 69. 10 S. V. Tsybulya and L. P. Solovyeva, Appar. Metody Rentgenovskogo Anal., 38 (1988) 46-61 (in Russian).

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11 S.V. Tsybulya, Thesis, Novosibirsk, 1989. 12 JCPDS, Powder Diffraction File No. 33-664. 13 R. L. Blake, R. E. Hesseiric, T. Zoltai and L. Lingler, Am. Mineral., 51 (1966) 123. 14 A. Guinier, Theorie et Technique de la Radiocristallographie, Dunod, Paris, 1956, p. 294.