Crystal Structure of β-Fe2O3 and Topotactic Phase Transformation to

Jan 2, 2013 - Graduate School of Natural Science and Technology, Okayama ... College of the Arts, Kurashiki University of Science and the Arts, 2640 ...
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Crystal Structure of β‑Fe2O3 and Topotactic Phase Transformation to α‑Fe2O3 Teruaki Danno,† Daisuke Nakatsuka,† Yoshihiro Kusano,*,‡ Hiroshi Asaoka,† Makoto Nakanishi,† Tatsuo Fujii,† Yasunori Ikeda,§ and Jun Takada† †

Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Kita-ku, Okayama 700-8530, Japan College of the Arts, Kurashiki University of Science and the Arts, 2640 Nishinoura, Tsurajima-cho, Kurashiki-shi, Okayama 712-8505, Japan § Institute for Chemical Research, Kyoto University, Uji, Kyoto-fu 611-0011, Japan ‡

ABSTRACT: β-Fe2O3 is the scarce polymorph of Fe2O3 phases and is transformed easily into α-Fe2O3 at high temperature. However, its crystal structure and the transformation mechanism to α-Fe2O3 are still unclear because of the difficulty in obtaining monophasic βFe2O3 crystals. We established a synthesis method of the monophasic β-Fe2O3. It was synthesized by a two-step reaction: heating a mixture of Na2SO4 and Fe2(SO4)3 in air at 250 °C to form NaFe(SO4)2, and subsequent heating the resultant phase with NaCl in air at 500 °C. The crystal structure was refined to a bixbyite-type cubic structure (Ia3)̅ with a = 9.4039(1) Å by the Rietveld method. Single crystalline β-Fe2O3 particles of approximately 1 μm in size were topotactically transformed into single α-Fe2O3 crystals. Electron diffraction analysis revealed the crystallographic orientation relationships between β-Fe2O3 and αFe2O3 to be [100]β//[0001]α, [010]β//[1010̅ ]α, and [001]β//[12̅ 10̅ ]α.

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heating at 350 °C in air.18 We have since succeeded in the transformation of the pure β-Fe2O3 phase to α-Fe2O3 by heating at around 500 °C. Here, we report further results regarding the synthesis of β-Fe2O3 and crystallographic studies on β-Fe2O3 and α-Fe2O3.

INTRODUCTION Iron oxides are among the most important metal oxides for industrial materials. They exhibit various attractive physical and chemical properties due to differences in the valence state of their ions and/or crystal structure.1 Ferric oxide (Fe2O3) has four polymorphs; α-, β-, γ-, and ε-Fe2O3.1 α-Fe2O3 is a rhombohedral system and is used as a raw material for magnetic materials and cathodes for lithium ion-batteries, and as a color pigment in art.2−6 γ-Fe2O3 has a spinel structure and is used as a magnetic storage material due to its ferromagnetic properties.7,8 ε-Fe2O3 is isostructural with AlFeO3 and GaFeO3, and has begun to attract much attention for its potential as a magnetic material.9,10 In contrast, β-Fe2O3 has a bixbyite (Fe,Mn)2O3-type structure and exhibits antiferromagnetism with a Néel temperature (TN) of 119 K.11−13 This oxide is much less known than the other polymorphs because it is an intermediary metastable phase in the evolution to αFe2O3.13−17 Therefore, it has been thought to be difficult to obtain monophasic β-Fe2O3. We reported that β-Fe2O3 was formed by the reaction between Fe2(SO)4 and NaCl, and was subsequently transformed into needle-like α-Fe2O3 particles, which is an attractive material for magnetic storage applications.13 On this account, we have examined more efficient synthesis methods for βFe2O3 to study its intrinsic physical and chemical properties, and the crystallographic relation between β-Fe2O3 and α-Fe2O3. As a result, β-Fe2O3 was formed via a NaFe(SO4)2 intermediate phase, and fine β-Fe2O3 particles (ca. 100 nm) with a low Néel temperature of TN = 113 K were successfully synthesized by © XXXX American Chemical Society

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EXPERIMENTAL METHODS

A monophasic product of β-Fe2O3 was prepared from reagent grade NaCl, Na2SO4, and Fe2(SO4)3. NaFe(SO4)2 was first obtained by evaporating an aqueous solution containing Na2SO4 and Fe2(SO4)3. NaFe(SO4)2 was then mixed with NaCl in a 1:2 ratio, placed in an alumina crucible and heated at 500 °C in air for 1 h. After being cooled to room temperature, the product was dispersed in distilled water and filtered to remove NaFe(SO4)2 and Na3Fe(SO4)3. The obtained βFe2O3 was heated at 650 °C in air for 15−120 min. The products were identified using powder X-ray diffraction (XRD; Rigaku RINT2000) with monochromatic Cu Kα radiation. The RIETAN-FP program was used for refinement of the crystal structure.19 Samples for XRD analysis were mixed with Si powder as an internal standard to calibrate the diffraction angles. Microstructural observations were conducted using scanning electron microscopy (SEM; Hitachi S-4300) and transmission electron microscopy (TEM; Topcon EM-002B).

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RESULTS AND DISCUSSION NaFe(SO4)2 is a key material for the formation of β-Fe2O3 by heating with NaCl at 500 °C in air. The product is then dispersed in distilled water to remove NaFe(SO4)2 and Received: October 11, 2012 Revised: December 21, 2012

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Na3Fe(SO4)3.20 Figure 1 shows an XRD pattern of the residual particles with the Rietveld refinement result. The sample

Figure 1. XRD patterns of the β-Fe2O3 particles (crosses) and Rietveld refinement result (solid line). The difference curve is shown as a blue solid line. The possible Bragg reflections of β-Fe2O3 and αFe2O3 are indicated by the upper and lower bars, respectively. The inset shows the refined crystal structure of β-Fe2O3.

obtained had high crystallinity, evidenced by the small full width at half-maximum (fwhm) of the diffraction peaks, and the separation of the Kα1 and Kα2 lines was clearly observed in the low angle region. The room temperature XRD pattern was successfully fitted as a bixbyite-type cubic structure (Ia3̅) with a = 9.4039(1) Å. The reliability factors, Rwp and S (= Rwp/Re), were 6.99% and 1.62, respectively. The R-Bragg factor (RB) and R-structure factor (RF) were 0.806 and 0.510%, respectively. The structural and thermal parameters refined from the XRD data are listed in Table 1, and the refined crystal structure is illustrated in the inset of Figure 1. The refinement indicated the presence of a small amount of α-Fe2O3 (2.2 wt %) as a secondary phase, as shown in Figure 1 (lower vertical lines). Figure 2 shows SEM and TEM images of the β-Fe2O3 sample. The SEM image shows cubic-shaped β-Fe2O3 particles with side lengths of approximately 1 μm, which is also confirmed by the TEM image shown in Figure 2b. The [001] electron diffraction (ED) pattern in the inset of Figure 2b indicates that the particles are single β-Fe2O3 crystals with {100} facets. Figure 2c shows the same particle as in Figure 2b, but tilted by ca. 55° from the [001] direction. The hexagonal symmetry of the ED pattern shown in the inset indicates the [111] zone axis of β-Fe2O3. A hexagonal shape with corner angles of 120° is expected when a cubic particle is viewed along one of its diagonal directions. However, in this image, only one of the corner angles appears to be ca. 120° (left part of panel c), which indicates that this particle is not perfectly cubic but instead has a shape similar to that shown in the lower inset, which was typical of all of the particles examined. The transformation of β-Fe2O3 into α-Fe2O3 has been reported to begin at 500 °C, with completion at 600 °C.14,15 However, it seems that this transformation is dependent on particle size, specific surface area, and/or the presence of

Figure 2. (a) SEM image of β-Fe2O3. (b) [001] TEM image of βFe2O3 with the corresponding ED pattern. (c) [111] TEM image obtained by tilting by approximately 55° from the [001] direction with the corresponding ED pattern. The inset shows a schematic illustration of the probable particle shape.

impurities.13,14,21 The particle size of the pure β-Fe2O3 obtained in this study (ca. 1 μm) was much larger than that reported in the literature,22 which suggests that β-Fe2O3 would transform into α-Fe2O3 at relatively high temperatures. The transformation temperature of β-Fe2O3 produced in the present study began at 500 °C and was completed at 700 °C. A heating temperature of 650 °C was employed because the formation ratio of α-Fe2O3 was controllable at that temperature. Figure 3 shows XRD patterns of samples heated at 650 °C in air for 15− 120 min. The intensity of reflections associated with α-Fe2O3 increased with the heating time. After being heated for 120 min (Figure 3h), the β-Fe2O3 reflections disappeared and all reflections were assigned to α-Fe2O3. It should be emphasized that no polymorphic phase was observed throughout the transformation. In addition, although the heating temperature of 650 °C was relatively high, the transformation progressed slowly. Figure 4 shows the change in the lattice parameters of βFe2O3 and α-Fe2O3, and the change in the lattice volume of αFe2O3 as a function of the α-Fe2O3 formation ratio determined from Figure 3. Each rate of change was normalized according to the corresponding Joint Committee on Powder Diffraction Standards (JCPDS) values. The relative formation ratio of αFe2O3 (xi) was evaluated as xi = Iα/(Iα + Iβ) using the XRD intensities for the (011̅2) plane of α-Fe2O3 and the (211) plane of β-Fe2O3. The a lattice parameter for β-Fe2O3 was constant

Table 1. Positional and Thermal Parameters for β-Fe2O3 Obtained from XRD Data atom

site

x

y

z

occupancy

Biso (Å2)

Fe1 Fe2 O

8a 24d 48e

0 0.2844(0) 0.1428(1)

0 0 0.1319(1)

0 0.25 0.9057(1)

1 1 1

0.40(1) 0.38(3) 0.44(2)

B

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Figure 5. SEM image of monophasic α-Fe2O3 particles. The inset shows a [0001] zone-axis TEM image of a single α-Fe2O3 crystal with the corresponding ED pattern.

supposed that this transformation occurs via a topotactic process. γ-Fe2O3 is reported to transform into α-Fe2O3 at around 400 °C. Kachi et al. reported that the orientation relation between γ-Fe2O3 and α-Fe2O3 was (111)γ//(0001)α and [110̅ ]γ// [011̅0]α, and that the transformed sample contained twins on either the basal or (011̅0) planes.23 However, the α-Fe2O3 obtained in this study was twin-free, as evidenced by the ED pattern. ED measurements were conducted to clarify the topotactic mechanism. Figure 6 shows ED patterns taken from partly transformed samples. The reflections in Figure 6a were indexed as belonging to the [100] zone axis for β-Fe2O3, which corresponds to the [0001] zone axis for α-Fe2O3. The crystallographic relations deduced here are (010)β//(101̅0)α and (001)β//(1̅21̅0)α, which are consistent with the relation d(004)β ≈ d(1̅21̅0)α, i.e., aβ ≈ 2aα. The ED pattern in Figure 6b shows that the [001] zone axis for β-Fe2O3 corresponds to the [1̅21̅0] zone axis for α-Fe2O3, which indicates an additional relation of (100)β//(0001)α, which is consistent with the relation 3aβ ≈ 2cα, i.e., aβ ≈ 2cα/3. Figure 6c shows an ED pattern obtained from another particle, which indicates the relation d(101)β ≈ 4d(1̅21̅6)α. For ease of understanding, schematic illustrations of these ED patterns are given in Figure 6d−f. From these results, the crystallographic relations between βFe2O3 and α-Fe2O3 can be expressed as [100]β//[0001]α, [010]β//[101̅0]α, and [001]β//[1̅21̅0]α, which confirms that βFe2O3 is topotactically transformed into α-Fe2O3. The lattice distortion of α-Fe2O3 was considered next. Figure 6 shows that the crystallographic relations between β-Fe2O3

Figure 3. XRD patterns of products obtained by heating β-Fe2O3 at 650 °C for (a) 0, (b) 15, (c) 20, (d) 30, (e) 45, (f) 60, (g) 90, and (h) 120 min.

for all samples (Figure 4a); however, the a and c lattice parameters for α-Fe2O3 in Figure 4b changed significantly as the formation ratio of α-Fe2O3 increased. When xi is 0.1, the c lattice parameter is 13.793 Å, which is larger than the standard value of 13.749 Å (JCPDS No. 33-0664). In contrast, the a lattice parameter is 5.027 Å, which is smaller than the standard value of 5.036 Å (JCPDS No. 33-0664). It should be noted here that the lattice volume of α-Fe2O3 shown in Figure 4c is the same as that calculated using the JCPDS values. As xi increased, both parameters tended toward the JCPDS values. Figure 5 shows an SEM image of the monophasic α-Fe2O3 particles. The particle shapes and sizes are the same as for the original particles prior to heat treatment. The hexagonal symmetry of the ED pattern observed along the [0001]α zone axis in the inset indicates that the particle is single crystalline αFe2O3. These results reveal that a single β-Fe2O3 crystal is transformed into a single α-Fe2O3 crystal; therefore, it is

Figure 4. Change in the lattice parameters of (a) β-Fe2O3 and (b) α-Fe2O3, and (c) change in the lattice volume of α-Fe2O3 as a function of the formation ratio of α-Fe2O3. Each rate of change was normalized based on the corresponding JCPDS values. C

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space group Ia3̅ and two types of distorted FeO6 octahedra in the unit cell.1 Note here that the oxygen positions in β-Fe2O3 are not consistent with those in α-Fe2O3, although the O−O distance (ca. 2.3 Å) along the a-axis direction of β-Fe2O3 is equal to that along the c-axis direction of α-Fe2O3. Figure 8a

Figure 8. Schematic illustrations of (a) a single FeO6 slab sliced parallel with the c-plane of α-Fe2O3 and (b) a FeO6 slab sliced perpendicular to the a-axis of β-Fe2O3.

Figure 6. ED patterns taken from the partly transformed sample showing that the (a) [100], (b) [001], and (c) [101̅] zone axes for βFe2O3 correspond to the (a) [0001], (b) [1̅21̅0], and (c) [12̅11] zone axes for α-Fe2O3. Corresponding schematic illustrations of these patterns are shown in d−f, where filled triangles and circles indicate the reflections from β-Fe2O3 and α-Fe2O3, respectively, and crosses represent double diffractions.

shows a single FeO6 slab sliced parallel with the c-plane of αFe2O3, and Figure 8b shows a single FeO6 slab sliced perpendicular to the a-axis of β-Fe2O3. Comparing the FeO6 arrangements in both crystals, a FeO6 octahedron in α-Fe2O3 is linked by edge-sharing to three other FeO6 to form a 6-fold symmetry, while a FeO6 octahedron in β-Fe2O3 is linked to two FeO6 by edge-sharing and two FeO6 by corner-sharing. When the FeO6 rows shift to the [01̅0] direction with the 1/4 unit cell of β-Fe2O3 (a and b rows), 1/2 (c and d), 3/4 (e and f), 1 (g and h), and then all FeO6 octahedrons are linked by edgesharing, so that the FeO6 arrangement is the same as that of αFe2O3 shown in Figure 8a. Considering the unchanged particle morphology shown in Figure 5, the FeO6 rows shown with arrows in Figure 8b could shift to the arrow directions with the 1/4 unit cell (ca. 0.235 nm). Accordingly, the topotactic atomic transformation between β-Fe2O3 and α-Fe2O3 is produced by a major shift of atoms along the a-planes of β-Fe2O3.

and α-Fe2O3 are d(400)β ≈ d(112̅0)α and 3aβ ≈ 2cα. According to JCPDS, d(400)β and d(112̅0)α are 2.350 and 2.519 Å, respectively; therefore, d(112̅0)α is larger than 2d(200)β. On the other hand, d(0006)α = 2.292 Å is smaller than d(400)β = 2.350 Å. Therefore, the decrease in the a parameter and increase in the c parameter shown in Figure 4 for α-Fe2O3 can be explained by the difference in the lattice dimensions. For the partly transformed samples, very interesting magnetic behavior resulted from the lattice distortion,24 which will be reported elsewhere in the near future. Here, we discuss the transformation mechanism of β-Fe2O3 to α-Fe2O3. Figure 7 shows a comparison of the structures of α-

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SUMMARY The synthesis and refinement of the β-Fe2O3 crystal structure and its transformation into α-Fe2O3 were investigated. Monophasic cubic β-Fe2O3 crystals with side lengths of approximately 1 μm were obtained by the reaction of NaCl and NaFe(SO4)2. The crystal structure of β-Fe2O3 was successfully refined using the Rietveld method. β-Fe2O3 was transformed into α-Fe2O3 topotactically by heat treatment, and the crystallographic orientation relationships between β-Fe2O3 and α-Fe2O3 were determined as [100]β//[0001]α, [010]β// [101̅0]α, and [001]β//[1̅21̅0]α. We consider this transformation to correspond to a special form of topotaxy that has not been reported to date. The present results are expected to be a useful contribution to further the understanding of the chemical and physical properties of β-Fe2O3 and for studies on the crystallographic relationships between other iron oxides.

Figure 7. Schematic representations of the (a) α-Fe2O3 and (b) βFe2O3 crystal structures. Panel (a) shows the view along the [1̅010]α direction, which is the bβ axis before heating. Panel (b) shows the view along the aβ axis.

Fe2O3 viewed along the [101̅0] direction, which is the bβ axis before heating (Figure 6), and β-Fe2O3 viewed along the aβ axis. γ-Fe2O3 transforms into α-Fe2O3 topotactically and this transformation occurs comparatively smoothly above 400 °C, because the arrangement of oxygen atoms on the face-centered cubic lattice along the [111] direction in γ-Fe2O3 is similar to that along the [0001] direction in the hexagonal close-packed lattice of α-Fe2O3. α-Fe2O3 with a corundum structure is constructed of a hexagonal close-packed oxygen sublattice with only two-thirds of the octahedral sites filled with Fe ions.1 In contrast, β-Fe2O3 has a body centered cubic structure with the

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AUTHOR INFORMATION

Corresponding Author

*Tel & Fax: +81 86 440 1051. E-mail: [email protected]. Web site: http://www.kusa.ac.jp. Notes

The authors declare no competing financial interest. D

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ACKNOWLEDGMENTS This study was financially supported by a Special Grant for Education and Research (2008.4-2013.3) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan, and by a Grant-in-Aid for Scientific Research (KAKENHI, No. 24550240) from the Japan Society for the Promotion of Science.

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REFERENCES

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