- Email: [email protected]
Room temperature synthesis and characterization of perovskite compounds
Solid State Ionics 154 – 155 (2002) 393 – 398 www.elsevier.com/locate/ssi
Room temperature synthesis and characterization of perovskite compounds Kenji Toda a,*, Saori Tokuoka a, Kazuyoshi Uematsu b, Mineo Sato b b
a Graduate School of Science and Technology, Niigata University, Niigata 950-2181, Japan Department of Chemistry and Chemical Engineering, Niigata University, Niigata 950-2181, Japan
Accepted 10 March 2002
Abstract A new soft chemical method has been developed to remove the rock-salt type layers in the Ruddlesden – Popper-type layered perovskite at room temperature. The rock-salt type (KF) blocks in the layered perovskite, K2NbO3F, were selectively dissolved into water to give three-dimensional perovskite, KNbO3, at room temperature. In the alkali metal aqueous solution, the ionexchange reaction of parent layered compound with coexisting ions in the alkaline solution results in the new perovskites containing the other metal ions for the A-site. The development of this synthetic route could lead to interesting new metastable perovskite materials at low temperature. D 2002 Published by Elsevier Science B.V. PACS: 81.30.-t Phase diagrams and microstructures developed by solidification and solid – solid phase transformations Keywords: Soft chemistry; Layered perovskite; Topotactic reaction; Cubic perovskite; Niobate
1. Introduction An ideal perovskite compound of formula ABO3 has the Pm3¯m symmetry. The large A-site atom is at the centre of a cube with 12 coordinating O atoms, whereas the small radius B-site cation is octahedrally coordinated to 6 O atoms. Since the perovskite structure is capable of incorporating a large number of atoms, the perovskites show many useful properties, such as ionic conductivity [1], superconductivity [2], giant magnetoresistance [3] and ferroelectricity [4]. *
Corresponding author. Department of Chemistry and Chemical Engineering, Faculty of Engineering, Niigata University, 8050 Ikarashi 2-nocho, Niigata 950-2181, Japan. Tel./fax: +81-25-2626771. E-mail address: [email protected] (K. Toda).
Therefore, the perovskite compounds are expected to be key materials in the various material science fields. Many useful perovskite materials are produced from high-temperature reactions (T >700 K) to accelerate the slow solid – solid diffusion. Such a high-temperature reaction results in the formation of thermodynamically stable phases. Alternatively, soft chemical (Chimie Douce) reactions are carried out under moderate conditions (typically T < 700 K). The soft chemical methods such as ion exchange, intercalation and de-intercalation are very useful for modifying the electronic structure (hole or electron doping) and design of new metastable compounds. The Chimie Douce reactions are topotactic, meaning that the rigid frameworks of the precursors are preserved in the product. The only disadvantage to these soft chemical routes is that these topochemical methods require the appropriate
0167-2738/02/$ - see front matter D 2002 Published by Elsevier Science B.V. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 5 7 8 - 7
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K. Toda et al. / Solid State Ionics 154 – 155 (2002) 393–398
precursors for the metastable products. Most important principle of soft chemical routes is ‘‘searching the precursors before constructing the products’’. The topochemical transformations of the Ruddlesden– Popper-type layered perovskites to the defective perovskite or layered perovskite phase have already been demonstrated [5,6]. However, these low temperature materials synthesis produce poorly crystalline materials. In order to resolve the problem, we demonstrated a simple and unique solution synthetic method for the perovskite-related compound. The rock-salt type KF blocks in the layered perovskite, K2NbO3F [7], with a K2NiF4-type structure were selectively dissolved in water to give a three-dimensional perovskite, KNbO3, at room temperature. At low temperature, this synthetic route could lead to interesting metastable perovskite materials, which are not possible to prepare using high-temperature routes. The mechanism of the phase transformation was characterized by elemental analysis and crystallographic techniques.
2. Experimental High-purity KF, K2CO3 and Nb2O5 powders were weighed out in the ratio KF/K2CO3/Nb2O5 = 2.6:1:1, intimately ground and pressed into disk-shape pellets of 10 mm in diameter under a pressure of 30 MPa for 10 min. A 30% molar excess of KF was added to compensate for loss due to volatilization. The pellets were heated at 1063 K for 1 h in air. The precursors, K2NbO3F, were cooled to room temperature and thoroughly ground. Topochemical reaction of the layered perovskite precursor was carried out in distilled water by stirring the powder at room temperature for 2 – 96 h. Simple test tubes or test cups can be used as the reaction vessels. The completeness of topochemical reaction was demonstrated by electron probe microanalysis (EPMA) and powder X-ray diffraction (XRD). Phase purity and powder X-ray data for structural analysis were collected on a Rigaku RAD-rA diffractometer. Rietveld structure refinement was carried out with RIETAN97 [8]. The products were characterized by X-ray fluorescence (XRF), IR spectroscopy and thermal analysis (TG-DTA). Scanning electron microscopy (SEM) is used to observe the morphology of products.
The ion-exchange reactions were carried out by stirring the layered precursors in a saturated LiCl solution at room temperature for several days. The products were washed with deionized water and characterized by X-ray fluorescence (XRF), Atomic absorption spectroscopy (AAS), IR spectroscopy and thermal analysis (TG-DTA).
3. Results and discussion Fig. 1 shows the XRD patterns of K2NbO3F before and after immersion in water. The XRD pattern of the sample before immersion is in accord with the previous literature (a = 0.3956 and c = 1.3670 nm) [7]. K2NbO3F crystallized the tetragonal in space group I4/ mmm with a = 0.39560(1) and c = 1.37010(5) nm. The powder XRD data showed preferred orientation indicating stronger peaks of (002) and (006) than the other peak. The SEM image of the particle before immersion is shown in Fig. 2. The crystallites of layered perovskite, K2NbO3F, have the crystal size of 10 –30 Am with a highly anisotropic plate-like morphology. Strong preferred orientation for the layered precursor is in accord with the SEM observation. On the other hand, the XRD pattern of the sample after immersion shows a formation of SrTiO3-like pseudo-cubic perovskite at room temperature (Fig. 1). The absence of
Fig. 1. XRD patterns of (a) before and (b) after immersion in water.
K. Toda et al. / Solid State Ionics 154 – 155 (2002) 393–398
395
Fig. 2. SEM photographs of layered perovskite, K2NbO3F, before immersion in water.
superstructure peaks in the powder XRD pattern indicates statistical occupation of the potassium ions for the available A-sites of perovskite lattice. The EPMA data indicate that all of the fluoride ions and half of the interlayer potassium ions were removed by the stirring in the water. From XRF analysis, K/Nb ratios of the soft chemical samples were found to be a nearly stoiochiometric ratio of K/Nb = 1 ( F 0.05). It can be
concluded that the sample after immersion is the perovskite compound, KNbO3. The size of particle after immersion in water was determined by SEM data shown in Fig. 3. The platy particles with the size of f5 Am are observed in the samples after immersion. Although the particle size decreased with the progress of the topochemical reaction, the plate-like morphology of layered precursor is retained in the perovskite
Fig. 3. SEM photographs of SrTiO3-type perovskite, KNbO3, after immersion in water.
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Fig. 4. XRD pattern fitting for KNbO3 prepared by the soft chemical reaction. The calculated and observed patterns are shown by the top by the solid line and dots, respectively. The vertical marks and middle show the positions calculated for Bragg reflections. The trace on the bottom is a plot of the difference between the calculated and the observed intensities.
product. This powder synthesized from the soft chemical route had a smaller particle size, which may be advantageous for subsequent sintering of product or fabrication of an oriented thin film [9]. As shown in Fig. 4, the Rietveld refinement showed that the crystal structure of the soft chemical sample (reaction time 96 h) was essentially changed from that of the KNbO3 prepared by the high-temperature processing [10]. Soft chemical sample is indexed on a tetragonal (pseudo-cubic) in space group P4mm with the lattice constants a = 0.400(4) nm and c = 0.402(4) nm. The crystallographic data are listed in Table 1. Structural refinement based on a cubic space group Pm3¯m (Rwp = 11.2%) showed poor pattern fitting than that of the tetragonal space group P4mm. In spite of the fact that the orthorhombic form is the thermodynamically stable form for the perovskite KNbO3 at room temperature, the crystalline products in the soft chemical route have a tetragonal symmetry. Similar behavior is also seen in the BaTiO 3 hydrothermally synthesized at low temperature [11 –13]. This effect is ascribed to a considerable content of water incorporated in the perovskite lattice [11]. However, the TGDTA and IR data show the absence of water in the tetragonal KNbO3 perovskite synthesized by the soft
chemical route. Room temperature XRD patterns of solid state reaction and soft chemical KNbO3 product annealed at 1173 K for 24 h and 1273 K for 3 h are shown in Fig. 5. The XRD pattern revealed a thermodynamically stable orthorhombic symmetry in the soft chemical KNbO3 powders annealed at 1273 K. Therefore, the tetragonal (pseudo-cubic) form synthesized from the soft chemical route is a thermodynamically metastable form. On the other hand, the soft chemical KNbO3 synthesized for short reaction time (2 h) decomposed to a mixture of K3Nb7O19 and Nb2O5 above 875 K. This instability is related to a large number of defects in the perovskite lattice. The mech-
Table 1 Crystallographic data of KNbO3 synthesized by the soft chemical route Atoms
Site
g
x
y
K 1a 1.0 0.0 0.0 Nb 1b 1.0 0.5 0.5 O (1) 1b 1.0 0.5 0.5 O (2) 2c 1.0 0.5 0.0 Space group P4mm (no.99) a = 0.400(4) nm, c = 0.402(4) nm Rwp = 8.49%, Rp = 5.87%, Re = 4.25%,
z
B 10
0.031 (2) 0.5 0.077 (3) 0.496 (3)
0.5 (1) 0.68 (3) 0.3 (1) 0.3 (1)
2
RI = 2.72%, RF = 1.76%
/nm2
K. Toda et al. / Solid State Ionics 154 – 155 (2002) 393–398
Fig. 5. XRD patterns of KNbO3: (a) solid state reaction, (b) soft chemical KNbO3 annealed at 1273 K, (c) soft chemical KNbO3 annealed at 1173 K, (d) soft chemical KNbO3.
anism of topochemical reaction is illustrated in Fig. 6. When a layered perovskite, K2NbO3F, is immersed in the water at room temperature, the rock salt type structure was selectively dissolved in the water. Subsequent to the release of fluoride ion, restacking of perovskite blocks is possible. This reaction is able to occur owing to the rich solubility of KF in the water (50.4 wt.%). To the best of our knowledge, no such phase transformation of a layered perovskite at room temperature has been found up to now. The effect of coexisting ions on the formation of three-dimensional framework from the layered perovskite in the other alkali metal aqueous solutions was also studied. The elemental analysis after topochemical reaction suggested that the ion-exchange reactions of the parent compound with coexisting alkali metal ions in the suspension consisting of the layered precursor powder and the saturated alkali metal (Li or Na) aqueous solution produce the new perovskites containing the other metal ions at room temperature. For example, the AAS data of lithium ion-exchanged phases indicate that approximately 20– 35 mol% of
397
the potassium ions in the three-dimensional perovskite was removed by ion exchange in the saturated LiCl aqueous solution. The ion-exchanged samples also keep a cubic perovskite structure. A new perovskite compound, K1 xLixNbO3, became unstable at high temperature as in the case of the metastable KNbO3. The lithium-exchanged K1 xLixNbO3 decomposed at 778 K to give a mixture of LiNbO3 and an impurity phase. Although the exchanges in all cases are incomplete, the ion-exchange reactions in aqueous solutions at room temperature are an extremely effective process for the synthesis of metastable phases. If the solution of small multivalent cations such as Pb2 +, Bi3 +, Eu3 + or Ti4 + were used as reaction media, it may be possible to exchange the A-site ions (favourable to the large cations with high coordination numbers) for the highly charged cations with the low coordination numbers. This transformation has a topotactic character because the perovskite block is preserved. It is unusual for small cations with low coordination number to occupy the perovskite A site with high coordination number. Such a small cation could be incorporated in a perovskite lattice through
Fig. 6. Topochemical transformation from a layered perovskite to a three-dimensional perovskite.
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K. Toda et al. / Solid State Ionics 154 – 155 (2002) 393–398
this topochemical synthetic method. These novel compounds with small A-site cations may be promising candidates for the new ionic conductor, superconductor, ferroelectrics and photocatalytic materials. The layered precursor, K2NbO3F, has the Ruddlesden– Popper-type structure, which is the most widely known layered perovskite group [14]. The Ruddlesden– Popper phase can be described as an intergrowth structure between the perovskite layer and the rock-salt block. We expect that new topochemical synthetic method could be generalized and extended to the synthesis of metastable three-dimensional perovskite by the selective dissolution of the rock-salt block from the tailored Ruddlesden – Popper phase. In fact, our preliminary work demonstrates that some of layered perovskite oxyhalogenide systems in liquid media show selective dissolution at the interlayer blocks. These studies are in progress and the results will be reported in forthcoming papers. Therefore, the soft chemical routes containing the selective dissolution from layered perovskite precursors will become a general and powerful tool for the low-temperature synthesis of metastable perovskites.
4. Conclusion In this paper, we demonstrated the first example of topochemical synthetic route to the three-dimensional perovskite materials from a layered perovskite at room temperature. The cubic KNbO3 can be synthesized from the layered perovskite, K2NbO3F, via a topochemical reaction at room temperature for only 2 h. The XRD and EPMA study indicated that the rock-salt type KF blocks in the layered perovskite, K2NbO3F, were selectively dissolved in water to give a threedimensional perovskite, KNbO3. This soft chemical procedure can be applied to the other metastable perovskite materials as well, allowing for efficient one-step preparation. The combination of the selective
dissolution and ion-exchange reaction of parent compound with the coexisting ions in the aqueous solutions produces the new metastable perovskites containing the coexisting ions. These reactions provide a simple and unique route to new perovskite materials that cannot be prepared by conventional ceramic processing requiring high temperature and long reaction times.
Acknowledgements This work was supported by the ‘‘Research for the Future, Preparation and Application of Newly Designed Solid Electrolytes (JSPS RFTF96P00102)’’ program from the Japan Society for the Promotion of Science (JSPS).
References [1] Y. Inaguma, C. Liquan, M. Itoh, T. Nakamura, T. Uchida, H. Ikuta, M. Wakihara, Solid State Commun. 86 (1993) 689. [2] H. Fukuoka, T. Isami, S. Yamanaka, Chem. Lett. (1997) 703. [3] Y. Morimoto, A. Asamitsu, H. Kuwahara, Y. Tokura, Nature 380 (1996) 141. [4] W.A. Schulze, J.V. Biggers, L.E. Cross, J. Am. Ceram. Soc. 61 (1978) 46. [5] J. Gopalakrishnan, V. Bhat, Inorg. Chem. 26 (1987) 4299. [6] V. Thangadurai, G.N. Subbanna, J. Gopalakrishnan, Chem. Commun. (1998) 1299. [7] F. Galasso, W. Darby, J. Phys. Chem. 66 (1962) 1318. [8] Y.-I. Kim, F. Izumi, J. Ceram. Soc. Jpn. 102 (1994) 401. [9] K.S. Han, S.W. Song, H. Fujita, M. Yoshimura, Solid State Ionics 135 (2000) 273. [10] V.A. Shuvaeva, K. Yanagi, K. Yagi, K. Sakaue, H. Terauchi, Solid State Commun. 106 (1998) 335. [11] D. Hennings, S. Schreinemacher, J. Eur. Ceram. Soc. 9 (1992) 41. [12] K. Kajiyoshi, N. Ishizawa, M. Yoshimura, J. Am. Ceram. Soc. 74 (1991) 369. [13] P.K. Dutta, J.R. Gregg, Chem. Mater. 4 (1992) 843. [14] S.N. Ruddlesden, P. Popper, Acta Crystallogr. 11 (1958) 54.
Room temperature synthesis and characterization of perovskite compounds Kenji Toda a,*, Saori Tokuoka a, Kazuyoshi Uematsu b, Mineo Sato b b
a Graduate School of Science and Technology, Niigata University, Niigata 950-2181, Japan Department of Chemistry and Chemical Engineering, Niigata University, Niigata 950-2181, Japan
Accepted 10 March 2002
Abstract A new soft chemical method has been developed to remove the rock-salt type layers in the Ruddlesden – Popper-type layered perovskite at room temperature. The rock-salt type (KF) blocks in the layered perovskite, K2NbO3F, were selectively dissolved into water to give three-dimensional perovskite, KNbO3, at room temperature. In the alkali metal aqueous solution, the ionexchange reaction of parent layered compound with coexisting ions in the alkaline solution results in the new perovskites containing the other metal ions for the A-site. The development of this synthetic route could lead to interesting new metastable perovskite materials at low temperature. D 2002 Published by Elsevier Science B.V. PACS: 81.30.-t Phase diagrams and microstructures developed by solidification and solid – solid phase transformations Keywords: Soft chemistry; Layered perovskite; Topotactic reaction; Cubic perovskite; Niobate
1. Introduction An ideal perovskite compound of formula ABO3 has the Pm3¯m symmetry. The large A-site atom is at the centre of a cube with 12 coordinating O atoms, whereas the small radius B-site cation is octahedrally coordinated to 6 O atoms. Since the perovskite structure is capable of incorporating a large number of atoms, the perovskites show many useful properties, such as ionic conductivity [1], superconductivity [2], giant magnetoresistance [3] and ferroelectricity [4]. *
Corresponding author. Department of Chemistry and Chemical Engineering, Faculty of Engineering, Niigata University, 8050 Ikarashi 2-nocho, Niigata 950-2181, Japan. Tel./fax: +81-25-2626771. E-mail address: [email protected] (K. Toda).
Therefore, the perovskite compounds are expected to be key materials in the various material science fields. Many useful perovskite materials are produced from high-temperature reactions (T >700 K) to accelerate the slow solid – solid diffusion. Such a high-temperature reaction results in the formation of thermodynamically stable phases. Alternatively, soft chemical (Chimie Douce) reactions are carried out under moderate conditions (typically T < 700 K). The soft chemical methods such as ion exchange, intercalation and de-intercalation are very useful for modifying the electronic structure (hole or electron doping) and design of new metastable compounds. The Chimie Douce reactions are topotactic, meaning that the rigid frameworks of the precursors are preserved in the product. The only disadvantage to these soft chemical routes is that these topochemical methods require the appropriate
0167-2738/02/$ - see front matter D 2002 Published by Elsevier Science B.V. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 5 7 8 - 7
394
K. Toda et al. / Solid State Ionics 154 – 155 (2002) 393–398
precursors for the metastable products. Most important principle of soft chemical routes is ‘‘searching the precursors before constructing the products’’. The topochemical transformations of the Ruddlesden– Popper-type layered perovskites to the defective perovskite or layered perovskite phase have already been demonstrated [5,6]. However, these low temperature materials synthesis produce poorly crystalline materials. In order to resolve the problem, we demonstrated a simple and unique solution synthetic method for the perovskite-related compound. The rock-salt type KF blocks in the layered perovskite, K2NbO3F [7], with a K2NiF4-type structure were selectively dissolved in water to give a three-dimensional perovskite, KNbO3, at room temperature. At low temperature, this synthetic route could lead to interesting metastable perovskite materials, which are not possible to prepare using high-temperature routes. The mechanism of the phase transformation was characterized by elemental analysis and crystallographic techniques.
2. Experimental High-purity KF, K2CO3 and Nb2O5 powders were weighed out in the ratio KF/K2CO3/Nb2O5 = 2.6:1:1, intimately ground and pressed into disk-shape pellets of 10 mm in diameter under a pressure of 30 MPa for 10 min. A 30% molar excess of KF was added to compensate for loss due to volatilization. The pellets were heated at 1063 K for 1 h in air. The precursors, K2NbO3F, were cooled to room temperature and thoroughly ground. Topochemical reaction of the layered perovskite precursor was carried out in distilled water by stirring the powder at room temperature for 2 – 96 h. Simple test tubes or test cups can be used as the reaction vessels. The completeness of topochemical reaction was demonstrated by electron probe microanalysis (EPMA) and powder X-ray diffraction (XRD). Phase purity and powder X-ray data for structural analysis were collected on a Rigaku RAD-rA diffractometer. Rietveld structure refinement was carried out with RIETAN97 [8]. The products were characterized by X-ray fluorescence (XRF), IR spectroscopy and thermal analysis (TG-DTA). Scanning electron microscopy (SEM) is used to observe the morphology of products.
The ion-exchange reactions were carried out by stirring the layered precursors in a saturated LiCl solution at room temperature for several days. The products were washed with deionized water and characterized by X-ray fluorescence (XRF), Atomic absorption spectroscopy (AAS), IR spectroscopy and thermal analysis (TG-DTA).
3. Results and discussion Fig. 1 shows the XRD patterns of K2NbO3F before and after immersion in water. The XRD pattern of the sample before immersion is in accord with the previous literature (a = 0.3956 and c = 1.3670 nm) [7]. K2NbO3F crystallized the tetragonal in space group I4/ mmm with a = 0.39560(1) and c = 1.37010(5) nm. The powder XRD data showed preferred orientation indicating stronger peaks of (002) and (006) than the other peak. The SEM image of the particle before immersion is shown in Fig. 2. The crystallites of layered perovskite, K2NbO3F, have the crystal size of 10 –30 Am with a highly anisotropic plate-like morphology. Strong preferred orientation for the layered precursor is in accord with the SEM observation. On the other hand, the XRD pattern of the sample after immersion shows a formation of SrTiO3-like pseudo-cubic perovskite at room temperature (Fig. 1). The absence of
Fig. 1. XRD patterns of (a) before and (b) after immersion in water.
K. Toda et al. / Solid State Ionics 154 – 155 (2002) 393–398
395
Fig. 2. SEM photographs of layered perovskite, K2NbO3F, before immersion in water.
superstructure peaks in the powder XRD pattern indicates statistical occupation of the potassium ions for the available A-sites of perovskite lattice. The EPMA data indicate that all of the fluoride ions and half of the interlayer potassium ions were removed by the stirring in the water. From XRF analysis, K/Nb ratios of the soft chemical samples were found to be a nearly stoiochiometric ratio of K/Nb = 1 ( F 0.05). It can be
concluded that the sample after immersion is the perovskite compound, KNbO3. The size of particle after immersion in water was determined by SEM data shown in Fig. 3. The platy particles with the size of f5 Am are observed in the samples after immersion. Although the particle size decreased with the progress of the topochemical reaction, the plate-like morphology of layered precursor is retained in the perovskite
Fig. 3. SEM photographs of SrTiO3-type perovskite, KNbO3, after immersion in water.
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Fig. 4. XRD pattern fitting for KNbO3 prepared by the soft chemical reaction. The calculated and observed patterns are shown by the top by the solid line and dots, respectively. The vertical marks and middle show the positions calculated for Bragg reflections. The trace on the bottom is a plot of the difference between the calculated and the observed intensities.
product. This powder synthesized from the soft chemical route had a smaller particle size, which may be advantageous for subsequent sintering of product or fabrication of an oriented thin film [9]. As shown in Fig. 4, the Rietveld refinement showed that the crystal structure of the soft chemical sample (reaction time 96 h) was essentially changed from that of the KNbO3 prepared by the high-temperature processing [10]. Soft chemical sample is indexed on a tetragonal (pseudo-cubic) in space group P4mm with the lattice constants a = 0.400(4) nm and c = 0.402(4) nm. The crystallographic data are listed in Table 1. Structural refinement based on a cubic space group Pm3¯m (Rwp = 11.2%) showed poor pattern fitting than that of the tetragonal space group P4mm. In spite of the fact that the orthorhombic form is the thermodynamically stable form for the perovskite KNbO3 at room temperature, the crystalline products in the soft chemical route have a tetragonal symmetry. Similar behavior is also seen in the BaTiO 3 hydrothermally synthesized at low temperature [11 –13]. This effect is ascribed to a considerable content of water incorporated in the perovskite lattice [11]. However, the TGDTA and IR data show the absence of water in the tetragonal KNbO3 perovskite synthesized by the soft
chemical route. Room temperature XRD patterns of solid state reaction and soft chemical KNbO3 product annealed at 1173 K for 24 h and 1273 K for 3 h are shown in Fig. 5. The XRD pattern revealed a thermodynamically stable orthorhombic symmetry in the soft chemical KNbO3 powders annealed at 1273 K. Therefore, the tetragonal (pseudo-cubic) form synthesized from the soft chemical route is a thermodynamically metastable form. On the other hand, the soft chemical KNbO3 synthesized for short reaction time (2 h) decomposed to a mixture of K3Nb7O19 and Nb2O5 above 875 K. This instability is related to a large number of defects in the perovskite lattice. The mech-
Table 1 Crystallographic data of KNbO3 synthesized by the soft chemical route Atoms
Site
g
x
y
K 1a 1.0 0.0 0.0 Nb 1b 1.0 0.5 0.5 O (1) 1b 1.0 0.5 0.5 O (2) 2c 1.0 0.5 0.0 Space group P4mm (no.99) a = 0.400(4) nm, c = 0.402(4) nm Rwp = 8.49%, Rp = 5.87%, Re = 4.25%,
z
B 10
0.031 (2) 0.5 0.077 (3) 0.496 (3)
0.5 (1) 0.68 (3) 0.3 (1) 0.3 (1)
2
RI = 2.72%, RF = 1.76%
/nm2
K. Toda et al. / Solid State Ionics 154 – 155 (2002) 393–398
Fig. 5. XRD patterns of KNbO3: (a) solid state reaction, (b) soft chemical KNbO3 annealed at 1273 K, (c) soft chemical KNbO3 annealed at 1173 K, (d) soft chemical KNbO3.
anism of topochemical reaction is illustrated in Fig. 6. When a layered perovskite, K2NbO3F, is immersed in the water at room temperature, the rock salt type structure was selectively dissolved in the water. Subsequent to the release of fluoride ion, restacking of perovskite blocks is possible. This reaction is able to occur owing to the rich solubility of KF in the water (50.4 wt.%). To the best of our knowledge, no such phase transformation of a layered perovskite at room temperature has been found up to now. The effect of coexisting ions on the formation of three-dimensional framework from the layered perovskite in the other alkali metal aqueous solutions was also studied. The elemental analysis after topochemical reaction suggested that the ion-exchange reactions of the parent compound with coexisting alkali metal ions in the suspension consisting of the layered precursor powder and the saturated alkali metal (Li or Na) aqueous solution produce the new perovskites containing the other metal ions at room temperature. For example, the AAS data of lithium ion-exchanged phases indicate that approximately 20– 35 mol% of
397
the potassium ions in the three-dimensional perovskite was removed by ion exchange in the saturated LiCl aqueous solution. The ion-exchanged samples also keep a cubic perovskite structure. A new perovskite compound, K1 xLixNbO3, became unstable at high temperature as in the case of the metastable KNbO3. The lithium-exchanged K1 xLixNbO3 decomposed at 778 K to give a mixture of LiNbO3 and an impurity phase. Although the exchanges in all cases are incomplete, the ion-exchange reactions in aqueous solutions at room temperature are an extremely effective process for the synthesis of metastable phases. If the solution of small multivalent cations such as Pb2 +, Bi3 +, Eu3 + or Ti4 + were used as reaction media, it may be possible to exchange the A-site ions (favourable to the large cations with high coordination numbers) for the highly charged cations with the low coordination numbers. This transformation has a topotactic character because the perovskite block is preserved. It is unusual for small cations with low coordination number to occupy the perovskite A site with high coordination number. Such a small cation could be incorporated in a perovskite lattice through
Fig. 6. Topochemical transformation from a layered perovskite to a three-dimensional perovskite.
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K. Toda et al. / Solid State Ionics 154 – 155 (2002) 393–398
this topochemical synthetic method. These novel compounds with small A-site cations may be promising candidates for the new ionic conductor, superconductor, ferroelectrics and photocatalytic materials. The layered precursor, K2NbO3F, has the Ruddlesden– Popper-type structure, which is the most widely known layered perovskite group [14]. The Ruddlesden– Popper phase can be described as an intergrowth structure between the perovskite layer and the rock-salt block. We expect that new topochemical synthetic method could be generalized and extended to the synthesis of metastable three-dimensional perovskite by the selective dissolution of the rock-salt block from the tailored Ruddlesden – Popper phase. In fact, our preliminary work demonstrates that some of layered perovskite oxyhalogenide systems in liquid media show selective dissolution at the interlayer blocks. These studies are in progress and the results will be reported in forthcoming papers. Therefore, the soft chemical routes containing the selective dissolution from layered perovskite precursors will become a general and powerful tool for the low-temperature synthesis of metastable perovskites.
4. Conclusion In this paper, we demonstrated the first example of topochemical synthetic route to the three-dimensional perovskite materials from a layered perovskite at room temperature. The cubic KNbO3 can be synthesized from the layered perovskite, K2NbO3F, via a topochemical reaction at room temperature for only 2 h. The XRD and EPMA study indicated that the rock-salt type KF blocks in the layered perovskite, K2NbO3F, were selectively dissolved in water to give a threedimensional perovskite, KNbO3. This soft chemical procedure can be applied to the other metastable perovskite materials as well, allowing for efficient one-step preparation. The combination of the selective
dissolution and ion-exchange reaction of parent compound with the coexisting ions in the aqueous solutions produces the new metastable perovskites containing the coexisting ions. These reactions provide a simple and unique route to new perovskite materials that cannot be prepared by conventional ceramic processing requiring high temperature and long reaction times.
Acknowledgements This work was supported by the ‘‘Research for the Future, Preparation and Application of Newly Designed Solid Electrolytes (JSPS RFTF96P00102)’’ program from the Japan Society for the Promotion of Science (JSPS).
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