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Phase transformation of KNaNb2O6 induced by size effect
Chemical Physics Letters 391 (2004) 288–292 www.elsevier.com/locate/cplett
Phase transformation of KNaNb2O6 induced by size effect Yosuke Shiratori *, Arnaud Magrez 1, Christian Pithan Institut f€ ur Elektronische Materialien, Institut f€ur Festk€orperforschung (IFF), Forschungszentrum J€ulich GmbH, D-52425 J€ulich, Germany Received 1 March 2004; in final form 10 May 2004 Available online 24 May 2004
Abstract A particle size dependent modification in crystal structure of submicron KNaNb2 O6 powders was observed for the first time by X-ray diffraction coupled with Raman spectroscopy. We saw a clear correlation between crystal size and crystallographic structure. Particles larger than 200 nm in diameter consisted of a perovskite type monoclinic phase whereas particles smaller than 200 nm were found to be triclinic. The lattice parameters were refined for both modifications and possible space groups defining the modifications were given. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction In recent years, alkaline niobates such as KNaNb2 O6 (KNN) are gaining increasingly importance, as they represent in comparison to Pb(Zr,Ti)O3 , an alternative and environmentally friendly class of piezoelectric ceramics that do not contain any lead [1]. A drawback, however, is the present low sinter activity of KNN and the difficulty of acquiring dense ceramics by pressureless, natural sintering. Therefore, ultrafine and thus highly sinter active powders are required. Of particular importance is the crystallographic structure of nanocrystalline and submicron KNN since the functional properties such as ferroelectric and piezoelectric response are strongly correlated to structural transformations of the crystal lattice. Usually these physical properties show a strong dependence on crystal size in perovskite type ferroelectrics, which is a phenomenon commonly referred to as size effect [2]. The objective of the present study is to investigate the size effect in KNN by analyzing the crystallographic structure for various particle sizes ranging from the nm to the lm regime. *
Corresponding author. Fax: +492461612550. E-mail address: [email protected] (Y. Shiratori). 1 Present Address: Laboratoire des Nanostructures et des Nouveaux Materiaux Electroniques (LNNME), Ecole Polytechnique Federale de Lausanne (EPFL), CH-1015 Lausanne-EPFL, Switzerland. 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.05.024
Generally, as a result of the small particle size and due to extensive broadening of Bragg reflections, X-ray diffraction alone cannot reveal the complete crystallographic structure of nanocrystalline powders. Here, Raman spectroscopy being very sensitive even to very small distortions of the crystal lattice can give additional evidence for the true structure [3]. One of the very first reports on KNN dates back to Shirane et al. [4], in which a new ferroelectric phase close to the composition of pure KNbO3 was described. Since then several studies reporting on the phase transitions in alkaline niobate powders have been reported. Shen et al. [5–7] analysed very thoroughly the phase transitions in pure KNbO3 and NaNbO3 by temperature and pressure turning Raman spectroscopy. The phase transitions in pure NaNbO3 follow in the order of increasing temperatures the crystal structure sequence that is also observed for many other perovskite type ferroelectrics like BaTiO3 : rhombohedral, orthorhombic, tetragonal and finally cubic above the Curie temperature Tc , all being coupled to different tilts of NbO6 -octahedra [6]. Although the material system of (Kx Na1 x )NbO3 has been known for quite a long time [8], there is scarcely any systematic report on the entire system covering the effect of composition and crystal size on the phase transformation. The present report addresses the effect of crystallite size on phase transformations in the technically interesting composition KNN.
Y. Shiratori et al. / Chemical Physics Letters 391 (2004) 288–292
289
2. Experimental procedure Stoichiometric KNN powders were prepared through the hydrolysis of mixed alkoxides using a water in oil (w/o)-microemulsion as reaction medium. The formation of individual nanoparticles takes place in aqueous micelles, which serve as nanoreactors confining the nucleation and growth of individual particles to the restricted interior space of the micelle. The microemulsion was added to a precursor solution consisting of stoichiometric amounts of niobium pentaethoxide, potassium ethoxide and sodium ethoxide. A more detailed description will be given elsewhere [9]. The principle of the synthesis route has been described by Herrig et al. [10]. In order to remove organic impurities and to promote crystallization, the powders were annealed for 1 h at temperatures between 200 and 1000 °C, respectively. The average particle size was determined by SEM (Carl Zeiss LEO1530 microscope, acceleration voltage of 1.5 kV) and by BET measuring N2 -adsorption. Inductively coupled plasma-optical emission spectrometry showed that the stoichiometry did not change over the range of the annealing temperature applied. X-ray powder diffraction (XRPD) patterns were collected at room temperature using a Philips X’Pert diffractometer (Cu Ka radiation). Raman spectra were measured at room temperature with a Jobin Yvon T64000 spectrometer (Arþ laser excitation with 514.5 nm wavelength and <50 mW power at the samples). The program package Grams/AI (Thermo Galactic) has been used for analysing the obtained spectra. Above 500 °C, no evidence for any organic residues could be detected by Raman spectroscopy [9]. Carbon contents originated from organic and inorganic compounds analyzed by infrared spectroscopy after combustion were <0.1 and <0.001 wt%, respectively, in powders annealed at temperatures higher than 600 °C.
1000 °C (ca. 10 µm) 900 °C 800 °C (ca. 1 µm) 700 °C (200 - 400 nm) 600 °C (50 - 100 nm) 500 °C
400 °C (< 50 nm) Raw powder (ca. 30 nm)
10
20
30
40
50
60
70
80
2θ (°)
Fig. 1. XRPD patterns of KNN powders annealed at various temperatures for 1 h. Particle sizes concluded from BET and SEM are indicated in parenthesis.
3. Results and discussion On annealing several structural changes have been observed as shown in Fig. 1. The raw powders with an average particle size of ca. 30 nm obtained after hydrolysis were amorphous. This amorphous KNN phase was stable up to 400 °C. For annealing temperatures from 600 to 1000 °C, no amorphous phase could be detected as second phase any more; the sharpening of the peaks with increasing temperature reflects the coarsening of the KNN particles upon annealing. The particle size increased from less than 100 nm for an annealing temperature of 600 °C to about 10 lm after annealing at 1000 °C. The most drastic change in particle size and shape occurred between 625 and 700 °C. Fig. 2 shows SEM images of the powders annealed at 625 (a), 650 (b), 675 (c) and 700 °C (d), respectively. The
Fig. 2. SEM images for KNN powders annealed for 1 h at 625 (a), 650 (b), 675 (c) and 700 °C (d).
particle size is smaller than 200 nm in Fig. 2a while the overwhelming part of the particles in Fig. 2d is larger than 200 nm, revealing a well-developed and almost cubic shaped habitus. The powders annealed in the
290
Y. Shiratori et al. / Chemical Physics Letters 391 (2004) 288–292
intermediate temperature range, Fig. 2b and c, present a mixture of fine and coarse particles. Hereafter, we call these powders ‘intermediate powder’. A drastic change was also observed in the evolution of the XRPD peak positions and relative intensities for annealing between 625 and 700 °C. Long-time scanned diffraction patterns with better S/N ratio and wider angular range for these powders are shown in Fig. 3. Above the particle size of 200 nm, the patterns could be indexed by a monoclinic phase (M-type structure) as previously reported for the KNbO3 –NaNbO3 binary system [11]. The refined lattice parameters are aM ¼ 0:40045ð6Þ nm, bM ¼ 0:39452ð2Þ nm, cM ¼ 0:39989ð2Þ nm and bM ¼ 90:345ð2Þ°. Below 200 nm, a new KNN polymorph was observed. Peak splitting indicates a lowering of the crystal symmetry, compared to the Mtype structure (Fig. 3b). Extra reflections were observed at 35.92° (102) and 42.82° (112) in 2h. Their presence requires a new orientation of the lattice. The new
structure could be described by a triclinic symmetry with aT ¼ 0:56696ð6Þ nm, bT ¼ 0:39391ð2Þ nm, cT ¼ 0:55982 ð3Þ nm, aT ¼ 89:837ð6Þ°, bT ¼ 90:670ð2Þ° and cT ¼ 90:273ð6Þ° as lattice parameters (T-type structure). For the intermediate powders, both structures seem to coexist, see Fig. 3b and c. Fig. 4a shows Raman spectra for KNN powders with various particle sizes. The peaks in the region between 0 and 160 cm1 can be assigned to the translational modes of Kþ /Naþ cations and rotations of the NbO6 octahedra due to the similarity to pure alkaline niobates [6]. Internal vibrational modes of the octahedra appear in the wide range from 170 to 900 cm1 . Regarding the equilateral octahedron symmetry (Oh ) of a free NbO 6 ion, three Raman active internal modes (m1 ðA1g Þ, m2 ðEg Þ, m5 ðF2g Þ) are expected to exist in this region [12]. Other
(a)
Raman intensity (a.u.)
> 10 µm ca. 10 µm ca. 1 µm > 200 nm
M
I K+/Na+ NbO6ν6 Rot. 25
ν1 ν5
200
ν2 ν4
< 200 nm
T
< 100 nm
ν3
400 600 Raman shift (cm-1)
ν1+ν5 800
1000
(c)
(b)
T I M I-fM f: subtraction factor 25
200
T
2nd derivative
>10 bands
400
100
500
1000
Raman shift (cm-1) Fig. 3. XRPD patterns of KNN powders annealed for 1 h at 625, 650, 675 and 700 °C (a) and their enlarged parts arranged in order of particle size (b,c). The top and bottom patterns in (a) were used for lattice parameters refinement by full pattern matching.
Fig. 4. Raman spectra of KNN powders with various particle sizes (a) and subtraction between the spectra recorded for the intermediate powder (I) and the sample with M-type modification (M) (b). Savitzky–Golay second derivative for the T-type spectrum is shown in (c).
Y. Shiratori et al. / Chemical Physics Letters 391 (2004) 288–292
Raman inactive modes (m3 ðF1u Þ, m4 ðF1u Þ, m6 ðF2u Þ) can be detected as weak scatterings [6]. Also the vibrational characteristics of the M-type (>200 nm) and the T-type (<200 nm) structures were quite different. Fig. 4b clearly shows how the Raman spectrum obtained for the intermediate powders are composed of exactly two contributions, suggesting a coexistence of both phases. Subtracting the intensities obtained for the M-type polymorph from the ones measured for the intermediate powders resulted in a spectrum which univocally reveals the characteristic bands, which were also obtained experimentally for the T-type polymorph. Based on the XRD extinction rules, three different space groups are possible for the M-type modification: P 2=mðC12h ), P 2ðC12 ) and PmðC1S ). Since only the P 2 and Pm phases have Raman active normal modes, which are 4A + 8B and 8A0 + 4A00 , respectively, [13], the space group P 2=m is no longer considered. On the other hand, two space groups for the T-type modification are possible in the triclinic symmetry: P 1ðC11 ) or P 1ðC1i ). The irreducible representations of the Raman active normal modes for these structures are 27A and 6Ag , respectively. The Raman spectrum of the T-type polymorph shows an extensive broadening of the bands and clearly contains more than 16 bands as deduced from the calculation of the Savitzky–Golay second derivative of the spectrum (Fig. 4c). Therefore, space group P1 is believed to be more suitable for the de-
291
scription of the T-type polymorph. Irreducible representations for the possible structures are summarized in Table 1. Band broadening of the twofold and threefold degenerated modes (m2 ðEg Þ, m5 ðF2g Þ, m1 þ m5 (A1g F2g ¼ F2g )) of the free octahedron, which appear in all NbO3 containing samples, is very significant in the Ttype spectra. Site symmetries of the octahedra in a crystal field cause the splitting of these degenerated modes and other weak bands, which also have degeneracy. If the unit cell contains several octahedra in different site symmetries, splitting can be observed resulting from different crystal fields in the site symmetries. Furthermore, dynamic coupling of the internal modes of octahedra is one possibility of band broadening [14]. If only one octahedron is in the unit cell, this splitting cannot exist. Further evidence such as the refinement of the structure by the Rietveld method will be necessary to determine atomic positions for both the M-type and the T-type modifications in order to understand the transition between them. Our SEM, XRPD and Raman spectroscopy results clearly allowed distinguishing two different structures of KNN depending on particle size. The underlying atomic displacement for the observed phase transformation might be induced by an increase of the internal pressure inside the grain leading to a reduction in the structural stress [15,16]. The increasing internal pressure could be caused by chemical factors such as adsorbed organic
Table 1 Normal mode determination for the possible M- and T-type modifications ( indicates refinable atomic parameters) Atom
x
y
z
Wyckoff notation
Site symmetry
Irreducible representations
M-type P 2 Na/K O1 O2 O3 Nb
1/2 1/2 0 0 0
1/2 0 0 1/2 0
1/2 0 1/2 0 0
1d 1c 1b 1a 1a
C2
A + 2B
CRaman
4A + 8B
Cs
2A0 + A00
CRaman
8A0 + 4A00
C1
3A
CRaman
27A
M-type Pm Na/K O3 O2 O1 Nb T-type P 1 Na/K1 Na/K2 O1 O2 O3 O4 O5 O6 Nb1 Nb2
1/2 0 0 1/2 0
1/2 0 1/4 3/4 3/4 1/4 0 1/2 0 1/2
1/2 1/2 0 0 0
1/2 1/2 0 0 0 0 1/2 1/2 0 0
1/2 0 1/2 0 0
0 1/2 1/4 1/4 3/4 3/4 0 1/2 0 1/2
1b 1b 1a 1a 1a
1a 1a 1a 1a 1a 1a 1a 1a 1a 1a
292
Y. Shiratori et al. / Chemical Physics Letters 391 (2004) 288–292
molecules on the surface [17]. However, the influence of organic impurities should be negligible since a KNN grain of 100 nm in diameter for instance is already quite large with more than 106 unit cells. More likely, an increase in internal pressure is caused by an increase in surface curvature.
4. Conclusions KNaNb2 O6 powders were prepared by microemulsion-mediated synthesis with a wide range of particle size from less than 100 nm to roughly 10 lm. A structural modification induced by crystal size was observed. The critical diameter was about 200 nm. For smaller particles, a new metastable KNN polymorph was obtained which can be described by a triclinic lattice (T-type), while for larger grains the thermodynamically stable structure (monoclinic: Mtype) was found. Acknowledgements We acknowledge Dr. J. Dornseiffer and Dr. F.-H. Haegel for support in powder synthesis, Dr. E. Wessel for SEM studies and Dr. W. Krasser and Dr. W. Sager for fruitful discussions.
References [1] C. Pithan, B. Malic, E. Ringgard, R. Waser, in: International Joint Conference on the Applications of Ferroelectrics 2002 (IFFF 2002), Nara, Japan, 2002 (29B-PI2-5C). [2] M.P. McNeal, S.-J. Jang, R.E. Newnham, J. Appl. Phys. 83 (1998) 3288. [3] A. Magrez, M. Cochet, O. Joubert, G. Louarn, M. Ganne, O. Chauvet, Chem. Mater. 13 (2001) 3893. [4] G. Shirane, R. Newnham, R. Pepinsky, Phys. Rev. 96 (1954) 581. [5] Z.X. Shen, Z.P. Hu, T.C. Chong, C.Y. Beh, S.H. Tang, M.H. Kuok, Phys. Rev. B 52 (1995) 3976. [6] Z.X. Shen, X.B. Wang, M.H. Kuok, S.H. Tang, J. Raman Spectrosc. 29 (1998) 379. [7] Z.X. Shen, X.B. Wang, S.H. Tang, M.H. Kuok, R. Malekfar, J. Raman Spectrosc. 31 (2000) 439. [8] B. Jaffe, W.R. Cook, H. Jaffe, Piezoelectric Ceramics, Academic Press, London, 1971, p. 185. [9] C. Pithan, Y. Shiratori, A. Magrez, C. Ohly, J. Dornseiffer, F.-H. Haegel, R. Waser, in preparation. [10] H. Herrig, R. Hempelmann, Mater. Lett. 27 (1996) 287. [11] M. Ahtee, A.W. Hewat, Acta Cryst. A 34 (1978) 309. [12] G. Herzberg, Molecular Spectra and Molecular Structure: II. Infrared and Raman Spectra of Polyatomic Molecules, Van Nostrand, New York, 1945, p. 122. [13] D.L. Rousseau, R.P. Bauman, S.P.S. Porto, J. Raman Spectrosc. 10 (1981) 253. [14] H. Poulet, J.P. Mathieu, Vibration Spectra and Symmetry of Crystals, Gordon and Breach, New York, 1976 (Chapter 5). [15] G. Skandan, Nanostruct. Mater. 5 (1995) 111. [16] R. Nitsche, M. Rodewald, G. Skandan, H. Fuess, H. Hahn, Nanostruct. Mater. 7 (1996) 535. [17] T. Chraska, A.H. King, C.C. Berndt, Mater. Sci. Eng. A 286 (2000) 169.
Phase transformation of KNaNb2O6 induced by size effect Yosuke Shiratori *, Arnaud Magrez 1, Christian Pithan Institut f€ ur Elektronische Materialien, Institut f€ur Festk€orperforschung (IFF), Forschungszentrum J€ulich GmbH, D-52425 J€ulich, Germany Received 1 March 2004; in final form 10 May 2004 Available online 24 May 2004
Abstract A particle size dependent modification in crystal structure of submicron KNaNb2 O6 powders was observed for the first time by X-ray diffraction coupled with Raman spectroscopy. We saw a clear correlation between crystal size and crystallographic structure. Particles larger than 200 nm in diameter consisted of a perovskite type monoclinic phase whereas particles smaller than 200 nm were found to be triclinic. The lattice parameters were refined for both modifications and possible space groups defining the modifications were given. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction In recent years, alkaline niobates such as KNaNb2 O6 (KNN) are gaining increasingly importance, as they represent in comparison to Pb(Zr,Ti)O3 , an alternative and environmentally friendly class of piezoelectric ceramics that do not contain any lead [1]. A drawback, however, is the present low sinter activity of KNN and the difficulty of acquiring dense ceramics by pressureless, natural sintering. Therefore, ultrafine and thus highly sinter active powders are required. Of particular importance is the crystallographic structure of nanocrystalline and submicron KNN since the functional properties such as ferroelectric and piezoelectric response are strongly correlated to structural transformations of the crystal lattice. Usually these physical properties show a strong dependence on crystal size in perovskite type ferroelectrics, which is a phenomenon commonly referred to as size effect [2]. The objective of the present study is to investigate the size effect in KNN by analyzing the crystallographic structure for various particle sizes ranging from the nm to the lm regime. *
Corresponding author. Fax: +492461612550. E-mail address: [email protected] (Y. Shiratori). 1 Present Address: Laboratoire des Nanostructures et des Nouveaux Materiaux Electroniques (LNNME), Ecole Polytechnique Federale de Lausanne (EPFL), CH-1015 Lausanne-EPFL, Switzerland. 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.05.024
Generally, as a result of the small particle size and due to extensive broadening of Bragg reflections, X-ray diffraction alone cannot reveal the complete crystallographic structure of nanocrystalline powders. Here, Raman spectroscopy being very sensitive even to very small distortions of the crystal lattice can give additional evidence for the true structure [3]. One of the very first reports on KNN dates back to Shirane et al. [4], in which a new ferroelectric phase close to the composition of pure KNbO3 was described. Since then several studies reporting on the phase transitions in alkaline niobate powders have been reported. Shen et al. [5–7] analysed very thoroughly the phase transitions in pure KNbO3 and NaNbO3 by temperature and pressure turning Raman spectroscopy. The phase transitions in pure NaNbO3 follow in the order of increasing temperatures the crystal structure sequence that is also observed for many other perovskite type ferroelectrics like BaTiO3 : rhombohedral, orthorhombic, tetragonal and finally cubic above the Curie temperature Tc , all being coupled to different tilts of NbO6 -octahedra [6]. Although the material system of (Kx Na1 x )NbO3 has been known for quite a long time [8], there is scarcely any systematic report on the entire system covering the effect of composition and crystal size on the phase transformation. The present report addresses the effect of crystallite size on phase transformations in the technically interesting composition KNN.
Y. Shiratori et al. / Chemical Physics Letters 391 (2004) 288–292
289
2. Experimental procedure Stoichiometric KNN powders were prepared through the hydrolysis of mixed alkoxides using a water in oil (w/o)-microemulsion as reaction medium. The formation of individual nanoparticles takes place in aqueous micelles, which serve as nanoreactors confining the nucleation and growth of individual particles to the restricted interior space of the micelle. The microemulsion was added to a precursor solution consisting of stoichiometric amounts of niobium pentaethoxide, potassium ethoxide and sodium ethoxide. A more detailed description will be given elsewhere [9]. The principle of the synthesis route has been described by Herrig et al. [10]. In order to remove organic impurities and to promote crystallization, the powders were annealed for 1 h at temperatures between 200 and 1000 °C, respectively. The average particle size was determined by SEM (Carl Zeiss LEO1530 microscope, acceleration voltage of 1.5 kV) and by BET measuring N2 -adsorption. Inductively coupled plasma-optical emission spectrometry showed that the stoichiometry did not change over the range of the annealing temperature applied. X-ray powder diffraction (XRPD) patterns were collected at room temperature using a Philips X’Pert diffractometer (Cu Ka radiation). Raman spectra were measured at room temperature with a Jobin Yvon T64000 spectrometer (Arþ laser excitation with 514.5 nm wavelength and <50 mW power at the samples). The program package Grams/AI (Thermo Galactic) has been used for analysing the obtained spectra. Above 500 °C, no evidence for any organic residues could be detected by Raman spectroscopy [9]. Carbon contents originated from organic and inorganic compounds analyzed by infrared spectroscopy after combustion were <0.1 and <0.001 wt%, respectively, in powders annealed at temperatures higher than 600 °C.
1000 °C (ca. 10 µm) 900 °C 800 °C (ca. 1 µm) 700 °C (200 - 400 nm) 600 °C (50 - 100 nm) 500 °C
400 °C (< 50 nm) Raw powder (ca. 30 nm)
10
20
30
40
50
60
70
80
2θ (°)
Fig. 1. XRPD patterns of KNN powders annealed at various temperatures for 1 h. Particle sizes concluded from BET and SEM are indicated in parenthesis.
3. Results and discussion On annealing several structural changes have been observed as shown in Fig. 1. The raw powders with an average particle size of ca. 30 nm obtained after hydrolysis were amorphous. This amorphous KNN phase was stable up to 400 °C. For annealing temperatures from 600 to 1000 °C, no amorphous phase could be detected as second phase any more; the sharpening of the peaks with increasing temperature reflects the coarsening of the KNN particles upon annealing. The particle size increased from less than 100 nm for an annealing temperature of 600 °C to about 10 lm after annealing at 1000 °C. The most drastic change in particle size and shape occurred between 625 and 700 °C. Fig. 2 shows SEM images of the powders annealed at 625 (a), 650 (b), 675 (c) and 700 °C (d), respectively. The
Fig. 2. SEM images for KNN powders annealed for 1 h at 625 (a), 650 (b), 675 (c) and 700 °C (d).
particle size is smaller than 200 nm in Fig. 2a while the overwhelming part of the particles in Fig. 2d is larger than 200 nm, revealing a well-developed and almost cubic shaped habitus. The powders annealed in the
290
Y. Shiratori et al. / Chemical Physics Letters 391 (2004) 288–292
intermediate temperature range, Fig. 2b and c, present a mixture of fine and coarse particles. Hereafter, we call these powders ‘intermediate powder’. A drastic change was also observed in the evolution of the XRPD peak positions and relative intensities for annealing between 625 and 700 °C. Long-time scanned diffraction patterns with better S/N ratio and wider angular range for these powders are shown in Fig. 3. Above the particle size of 200 nm, the patterns could be indexed by a monoclinic phase (M-type structure) as previously reported for the KNbO3 –NaNbO3 binary system [11]. The refined lattice parameters are aM ¼ 0:40045ð6Þ nm, bM ¼ 0:39452ð2Þ nm, cM ¼ 0:39989ð2Þ nm and bM ¼ 90:345ð2Þ°. Below 200 nm, a new KNN polymorph was observed. Peak splitting indicates a lowering of the crystal symmetry, compared to the Mtype structure (Fig. 3b). Extra reflections were observed at 35.92° (102) and 42.82° (112) in 2h. Their presence requires a new orientation of the lattice. The new
structure could be described by a triclinic symmetry with aT ¼ 0:56696ð6Þ nm, bT ¼ 0:39391ð2Þ nm, cT ¼ 0:55982 ð3Þ nm, aT ¼ 89:837ð6Þ°, bT ¼ 90:670ð2Þ° and cT ¼ 90:273ð6Þ° as lattice parameters (T-type structure). For the intermediate powders, both structures seem to coexist, see Fig. 3b and c. Fig. 4a shows Raman spectra for KNN powders with various particle sizes. The peaks in the region between 0 and 160 cm1 can be assigned to the translational modes of Kþ /Naþ cations and rotations of the NbO6 octahedra due to the similarity to pure alkaline niobates [6]. Internal vibrational modes of the octahedra appear in the wide range from 170 to 900 cm1 . Regarding the equilateral octahedron symmetry (Oh ) of a free NbO 6 ion, three Raman active internal modes (m1 ðA1g Þ, m2 ðEg Þ, m5 ðF2g Þ) are expected to exist in this region [12]. Other
(a)
Raman intensity (a.u.)
> 10 µm ca. 10 µm ca. 1 µm > 200 nm
M
I K+/Na+ NbO6ν6 Rot. 25
ν1 ν5
200
ν2 ν4
< 200 nm
T
< 100 nm
ν3
400 600 Raman shift (cm-1)
ν1+ν5 800
1000
(c)
(b)
T I M I-fM f: subtraction factor 25
200
T
2nd derivative
>10 bands
400
100
500
1000
Raman shift (cm-1) Fig. 3. XRPD patterns of KNN powders annealed for 1 h at 625, 650, 675 and 700 °C (a) and their enlarged parts arranged in order of particle size (b,c). The top and bottom patterns in (a) were used for lattice parameters refinement by full pattern matching.
Fig. 4. Raman spectra of KNN powders with various particle sizes (a) and subtraction between the spectra recorded for the intermediate powder (I) and the sample with M-type modification (M) (b). Savitzky–Golay second derivative for the T-type spectrum is shown in (c).
Y. Shiratori et al. / Chemical Physics Letters 391 (2004) 288–292
Raman inactive modes (m3 ðF1u Þ, m4 ðF1u Þ, m6 ðF2u Þ) can be detected as weak scatterings [6]. Also the vibrational characteristics of the M-type (>200 nm) and the T-type (<200 nm) structures were quite different. Fig. 4b clearly shows how the Raman spectrum obtained for the intermediate powders are composed of exactly two contributions, suggesting a coexistence of both phases. Subtracting the intensities obtained for the M-type polymorph from the ones measured for the intermediate powders resulted in a spectrum which univocally reveals the characteristic bands, which were also obtained experimentally for the T-type polymorph. Based on the XRD extinction rules, three different space groups are possible for the M-type modification: P 2=mðC12h ), P 2ðC12 ) and PmðC1S ). Since only the P 2 and Pm phases have Raman active normal modes, which are 4A + 8B and 8A0 + 4A00 , respectively, [13], the space group P 2=m is no longer considered. On the other hand, two space groups for the T-type modification are possible in the triclinic symmetry: P 1ðC11 ) or P 1ðC1i ). The irreducible representations of the Raman active normal modes for these structures are 27A and 6Ag , respectively. The Raman spectrum of the T-type polymorph shows an extensive broadening of the bands and clearly contains more than 16 bands as deduced from the calculation of the Savitzky–Golay second derivative of the spectrum (Fig. 4c). Therefore, space group P1 is believed to be more suitable for the de-
291
scription of the T-type polymorph. Irreducible representations for the possible structures are summarized in Table 1. Band broadening of the twofold and threefold degenerated modes (m2 ðEg Þ, m5 ðF2g Þ, m1 þ m5 (A1g F2g ¼ F2g )) of the free octahedron, which appear in all NbO3 containing samples, is very significant in the Ttype spectra. Site symmetries of the octahedra in a crystal field cause the splitting of these degenerated modes and other weak bands, which also have degeneracy. If the unit cell contains several octahedra in different site symmetries, splitting can be observed resulting from different crystal fields in the site symmetries. Furthermore, dynamic coupling of the internal modes of octahedra is one possibility of band broadening [14]. If only one octahedron is in the unit cell, this splitting cannot exist. Further evidence such as the refinement of the structure by the Rietveld method will be necessary to determine atomic positions for both the M-type and the T-type modifications in order to understand the transition between them. Our SEM, XRPD and Raman spectroscopy results clearly allowed distinguishing two different structures of KNN depending on particle size. The underlying atomic displacement for the observed phase transformation might be induced by an increase of the internal pressure inside the grain leading to a reduction in the structural stress [15,16]. The increasing internal pressure could be caused by chemical factors such as adsorbed organic
Table 1 Normal mode determination for the possible M- and T-type modifications ( indicates refinable atomic parameters) Atom
x
y
z
Wyckoff notation
Site symmetry
Irreducible representations
M-type P 2 Na/K O1 O2 O3 Nb
1/2 1/2 0 0 0
1/2 0 0 1/2 0
1/2 0 1/2 0 0
1d 1c 1b 1a 1a
C2
A + 2B
CRaman
4A + 8B
Cs
2A0 + A00
CRaman
8A0 + 4A00
C1
3A
CRaman
27A
M-type Pm Na/K O3 O2 O1 Nb T-type P 1 Na/K1 Na/K2 O1 O2 O3 O4 O5 O6 Nb1 Nb2
1/2 0 0 1/2 0
1/2 0 1/4 3/4 3/4 1/4 0 1/2 0 1/2
1/2 1/2 0 0 0
1/2 1/2 0 0 0 0 1/2 1/2 0 0
1/2 0 1/2 0 0
0 1/2 1/4 1/4 3/4 3/4 0 1/2 0 1/2
1b 1b 1a 1a 1a
1a 1a 1a 1a 1a 1a 1a 1a 1a 1a
292
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molecules on the surface [17]. However, the influence of organic impurities should be negligible since a KNN grain of 100 nm in diameter for instance is already quite large with more than 106 unit cells. More likely, an increase in internal pressure is caused by an increase in surface curvature.
4. Conclusions KNaNb2 O6 powders were prepared by microemulsion-mediated synthesis with a wide range of particle size from less than 100 nm to roughly 10 lm. A structural modification induced by crystal size was observed. The critical diameter was about 200 nm. For smaller particles, a new metastable KNN polymorph was obtained which can be described by a triclinic lattice (T-type), while for larger grains the thermodynamically stable structure (monoclinic: Mtype) was found. Acknowledgements We acknowledge Dr. J. Dornseiffer and Dr. F.-H. Haegel for support in powder synthesis, Dr. E. Wessel for SEM studies and Dr. W. Krasser and Dr. W. Sager for fruitful discussions.
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