Pressure-Induced Phase Transitions in Micro-, Submicro-, and

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J. Phys. Chem. C 2008, 112, 9610–9616

Pressure-Induced Phase Transitions in Micro-, Submicro-, and Nanocrystalline NaNbO3 Yosuke Shiratori,*,† Arnaud Magrez,‡ Minoru Kato,§ Kunihiro Kasezawa,§ Christian Pithan,† and Rainer Waser† Institut fu¨r Elektronische Materialien, Institut fu¨r Festko¨rperforschung (IFF), Forschungszentrum Ju¨lich GmbH, D-52425 Ju¨lich, Germany, Laboratoire des Nanostructures et des NouVeaux Mate´riaux Electroniques (LNNME), Ecole Polytechnique Fe´de´rale de Lausanne (EPFL), CH-1015 Lausanne-EPFL, Switzerland, and Department of Applied Chemistry, Faculty of Science and Engineering, Ritsumeikan UniVersity, 1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan ReceiVed: February 11, 2008

Pressure-induced successive phase transitions of micro-, submicro-, and nanocrystalline NaNbO3 particles were investigated by high-pressure Raman spectroscopy. Microcrystalline NaNbO3, which stabilizes in an orthorhombic Pbcm structure at ambient pressure, showed bulk-like successive transitions at around 2, 6, and 9 GPa. The transitions were essentially reversible but some specific bands became sharper after pressure release, indicating that the positions of the ions in the perovskite lattice were rearranged. Submicrocrystalline NaNbO3 (orthorhombic Pmc21 at ambient pressure) also showed phase transitions at around 2, 6, and 10 GPa on pressing. The pressure characteristic above 6 GPa was similar to that for microcrystalline NaNbO3. The transition was almost reversible on pressing up to 2 GPa. Pressing above 2 GPa and subsequent pressure release resulted in a bulk-like spectral profile for specific sites of the powder. Nanocrystalline NaNbO3 (orthorhombic Pmma at ambient pressure) showed a diffused and completely reversible transition behavior. Above 6 GPa, all powders with different crystallite sizes showed a similar pressure-evolution of the spectra. The lattice distortion induced by further pressing after touching of Na+ and NbO6- ions is comparable for all powders. On the other hand, a major difference of the pressure characteristics among the three types of powders was revealed for pressures below 6 GPa. Tilted NbO6 octahedra start to reorient at around 2 GPa with alteration of Na+-NbO6- interactions. Remarkable spectral changes at ca. 2 GPa were observed for submicrocrystalline powders, which have the lowest crystal symmetry among possible polymorphs. The phase stability of the Pmma structure in the nanocrystallline NaNbO3 could not be explained by internal pressure in fine particles. 1. Introduction NaNbO3 (NN) shows several successive phase transitions from the ferroelectric (FE) rhombohedral N phase (T < -113 °C) to the paraelectric (PE) cubic phase (>641 °C) through several other polymorphs.1 At room temperature NN takes an antiferroelectric orthorhombic P phase (-113 °C < T < 373 °C). Raman spectroscopy has been used to investigate successive transitions of NN. The temperature characteristic of bulk NN has also been studied intensively.2–6 On the other hand, pressureinduced phase transitions were only reported by Shen et al.7 They found a first-order phase transition at 7 GPa and another significant structural change at 12 GPa, which was prospected as a transition into the PE phase. The former transition was proved by softening of specific NbO6 internal modes and the latter one was indicated by a remarkable spectral change at high pressure. Another parameter, which can induce structural phase transitions, is the crystallite size.8,9 NN shows three polymorphs, orthorhombic Pbcm, Pmc21, and Pmma structures, for different * Corresponding author. Present address: Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. E-mail: [email protected]. Phone: +81-3-5841-7330. Fax: +81-3-5841-7332. † IFF-Forschungszentrum Ju ¨ lich. ‡ LNNME-EPFL. § Ritsumeikan University.

ranges of crystallite diameter, d < 70 nm, 200 nm < d < 400 nm, and d > 600 nm, respectively. We thoroughly studied the temperature evolution of the Raman spectra for these three polymorphs.10 Micro- and submicrocrystalline NN (m-NN and s-NN) show a hysteretic spectral change at around 350 and 300 °C, respectively. Nanocrystalline NN (n-NN) reveals a diffused spectral change without hysteretic behavior. Although hysteretic transitions of lattice parameters are also observed for m-NN and s-NN, significant hysteretic expansion and contraction of the unit cell volume are found only for s-NN having a polar structure with the largest unit cell volume among the three different polymorphs. Successive transitions of all polymorphs are completely reversible in the temperature range from -150 to 450 °C. In the present study we report on pressure-induced successive transitions for m-, s-, and n-NN. Changes of ionic interactions between Na+ and NbO6-, the rotation of NbO6 octahedra, and internal vibrations under hydrostatic pressure are studied by pressure tuning Raman spectroscopy at a constant temperature since mechanical stress is an important thermodynamic parameter affecting materials properties of ceramics. Existing interactions and thermodynamic stability of possible polymorphs are discussed. 2. Experimental Section m-, s-, and n-NN were prepared through microemulsion mediated synthesis and subsequent annealing treatments at 1000,

10.1021/jp801234g CCC: $40.75  2008 American Chemical Society Published on Web 06/11/2008

Pressure-Induced Phase Transitions in Crystalline NaNbO3 800, and 500 °C for 12 h, respectively, as described in detail elsewhere.11 These three powders had an average crystallite size of 70 nm, 280 nm, and 1.1 µm, respectively.9 A large number of data points at various pressures were collected for the wavenumber region above 152 cm-1, using a NRS-1000DT micro-Raman spectrometer (JASCO Co.) equipped with an Andor CCD detector under backscattering geometry. A clamped-type diamond anvil cell was used to apply pressures up to 16 GPa. The samples were pressed together with ruby tips and a pressure medium consisting of a methanol-ethanol mixture with a volume ratio of 4:1 between the two diamonds (culet-diameter: 0.8 mm) mediated by a gasket of stainless steel (thickness: 0.5 mm; hole diameter: 0.4 mm). Laser radiation with a wavelength of 532 nm and a power below 10 mW at the sample site (LD pumped Nd:YVO4 laser, Showa optronics Co.) were used to excite the samples. Pressure calibration was performed from the shifts of the R1 fluorescence line of the ruby tips. Raman spectra at low wavenumbers (10 to 150 cm-1) were additionally recorded with a Jobin Yvon T64000 spectrometer equipped with a light microscope unit (OLYMPUS BX41) and a Spectrum-One CCD detector under triple subtractive configuration in backscattering geometry. A gas membrane driven microscope diamond anvil cell (Diacell Products) was used to apply the hydrostatic pressure to the powder samples. Culetdiameter, gasket thickness, and diameter were 0.6, 0.5, and 0.3 mm, respectively. The pressure-medium and pressure-calibration procedures were the same as in the case for the high wavenumber region. The powders were excited with laser radiation (514.5 nm, < 10 mW at the sample site) from a BeamLok 2060 argon ion laser (Spectra-Physics Co.). All the obtained Raman spectra were fitted with Lorentzian functions with use of Grams/ AI software (Thermo Galactic Co.). 3. Results and Discussion 3.1. Microcrystalline NaNbO3. m-NN crystallizes in the point group Pbcm.9 Figure 1 shows the pressure-evolution of Raman spectra in the wavenumber range from 150 to 800 cm-1 obtained for m-NN. According to Shen et al.,7 the bands located at 160-190, 190-300, 370-440, ca. 560, ca. 600, and ca. 670 cm-1 can be assigned to the V6 (bending), V5 (bending), V4 (bending), V2 (stretching), V1 (stretching), and V3 (stretching) modes, respectively. The first remarkable spectral change in the region of 180-250 cm-1 occurs at around 2 GPa (ambient-pressure phase: APm f high-pressure phase I: HPm I). Broadening of the V6 and V5 modes (up to 350 cm-1) and the V1-V3 modes (500 - 700 cm-1) occurs at around 6 GPa (HPm I f HPm II). Finally another highpressure profile was revealed by a significant intensification of the bending modes (up to 550 cm-1) and a drastic shape change of the stretching modes (570 - 700 cm-1) at around 9 GPa (HPm II f HPm III). Present pressure-evolutions are similar to those reported for bulk NN in the literature.7 Figure 2 shows the pressure characteristics of peak positions obtained for the V1-V3, V5, and V6 modes. According to Figure 2a, the slopes of the pressure characteristic change at around 2 GPa (APm f HPm I) and subsequently the V1-V3 modes converge at around 6 GPa (HPm I f HPm II). Further convergence is found at around 9 GPa (HPm II f HPm III). The Pbcm structure of m-NN can be contracted by rotation of octahedra since according to our previous work9 a more compact polymorph (Pmma) exists, in which the most significant structural difference is the extent of octahedra rotation about the longest crystal axis. Above 9 GPa the slopes of the pressure

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Figure 1. Pressure-evolution of Raman spectra obtained for microcrystalline NaNbO3. The spectra for the phases at ambient and high pressures, APm, HPm I, HPm II, and HPm III, are shown as black, blue, red, and green curves, respectively.

characteristic strongly increase (Figure 2a). This indicates that a repulsive force drastically increases after completion of rotational rearrangement and Na+-NbO6- touching. Pressing above 9 GPa finally induces a significant distortion of lattices. Above 10 GPa, the effect of the nonhydrostatic condition should be noted for spectral changes.7 On the other hand, for the V6-V5 region (170-350 cm-1) at high pressures one can see changes of the pressure characteristics at around 2 GPa and band convergence at around 6 and 9 GPa, respectively (Figure 2b), although these modes show a very complicated spectral change on pressing. Also for this spectral region, a remarkable increase of the slopes of the pressure characteristics is found above 9 GPa. Above 9 GPa, rotations of octahedra along a specific axis do not contribute to phase transitions due to the mutual hindrance of their motions. Spectral changes below 6 GPa can be explained by reorientation of NbO6 octahedra through their rotation and accompanying octahedra distortion induced by changes of Na+-NbO6- interactions. The spectra obtained before pressing up to 15.8 GPa and after pressure release are approximately consistent with each other. It is believed that the Pbcm structure is thermodynamically stable since the spectral fine structures are reversible on pressing up to 15.8 GPa, at which rotation of octahedra is deactivated due to closed packing of Na+ and NbO6- ions. However, a slightly sharper band shape is found for the stretching modes (500-700 cm-1) after pressure release. The scattering profile in the range of 200-300 cm-1 also slightly deviates from the spectrum

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Figure 2. Pressure characteristics of internal vibrations obtained for microcrystalline NaNbO3: (a) V1, V2, and V3 modes and (b) V5 and V6 modes. The phases at ambient and high pressures are indicated as APm, HPm I, HPm II, and HPm III, respectively.

before pressing. These results indicate that the atomic positions are rearranged by hydrostatic pressing. 3.2. Submicrocrystalline NaNbO3. In the particle size range from 200 to 400 nm, NN crystallizes in the polar polymorph Pmc21.9 High-pressure Raman spectroscopy on lattice vibrations of s-NN will reveal unique pressure characteristics since the Pmc21 structure has not only polarity but also the largest unit cell volume among possible polymorphs at room temperature.9 The extent of octahedra rotation is much larger in comparison with other polymorphs. Figure 3 shows the pressure-evolution of Raman spectra obtained from measurements up to 15.9 GPa. Peaks at ca. 200 cm-1 are drastically intensified at around 2 GPa (APs f HPs I). Subsequently the spectral fine structures become obscure and the V3 mode completely disappears above 6 GPa (HPs I f HPs II). At around 10 GPa, a peak at ca. 260 cm-1 is marked and all remaining bands become more pronounced (HPs II f HPs III). Slight spectral irreversibility is revealed after pressure release. The pressure characteristics of peak positions obtained for s-NN are shown in Figure 4. One can see similar pressure characteristics to those obtained for m-NN. Remarkable transitions occur at around 2, 6, and 10 GPa, respectively. Above 10 GPa, the slopes of characteristics strongly increase. Figure 5 shows the spectra obtained at ca. 4.5, 7, and 16 GPa for m-NN and s-NN. The spectra obtained from two different measurements (first and second runs) are shown for s-NN at 4.5 and 7 GPa. Except for the sharpness and spectral profiles of the V5 + V1 mode, the spectra obtained for m-NN and s-NN at 7 GPa have a similar profile. A certain crystallite may be subject to a stress, which is different from that in other crystallites, since s-NN consists of isolated particles and aggregates.9 Local stresses, which can enhance anharmonicity of vibrations, contribute to wavenumber shifts of specific phonons and transitions of their coupling behavior. The scattering profile of the stretching modes (V1-V3) obtained from the first run is different from that for the second

Figure 3. Pressure-evolution of Raman spectra obtained for submicrocrystalline NaNbO3. The phases at ambient and high pressures, APs, HPs I, HPs II, and HPs III, are shown as black, blue, red, and green curves, respectively.

run. In addition, for the first run two sharp peaks appeared in the region of the combined V5 + V1 mode (∼900 cm-1) at pressures above 2.9 GPa (see insets of Figure 5). The C-O stretching mode of ethanol, which is one component of the liquid pressure medium, is located at 880 cm-1.12 Therefore in the case of high-pressure measurements, the profile at around 900 cm-1 becomes asymmetric since the C-O stretching mode of ethanol and the combined V5 + V1 mode of NN overlap. However, it is clear that the sharp peaks (bottom spectra in insets of Figure 5) cannot be assigned to ethanol. The appearance of the sharp peaks is explained by enhancement of the split V5 + V1 combination mode represented by F2g (triply degenerated) under equilateral octahedron symmetry of NbO6. One of the components of the degenerated mode at the lowest wavenumber completely overlaps with the C-O stretching mode of ethanol. Figure 6 shows pressure characteristics of the two components at higher wavenumber. Changes of slopes are found at around 6 and 10 GPa. The transition points correspond to those revealed for internal modes (Figure 4). This result supports that the split bands arise from the degenerated V5 + V1 combination mode. It is believed that harmonicity and coupling behavior of specific bands are sensitive to local crystal structures induced by local stresses in powders. The characteristics for m-NN (Figure 2) and s-NN (Figure 4) indicate that high-pressure characteristics of both powders are similar and that a main difference occurs only in the pressure range below 6 GPa. Figure 7 shows the pressure-evolution of the low-wavenumber region on pressing up to (a) 1.0, (b) 1.9, and (c) 6.1 GPa, respectively. A spectrum recorded for m-NN at ambient pressure

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Figure 4. Pressure characteristics of internal vibrations obtained for submicrocrystalline NaNbO3: (a) V1, V2, and V3 modes and (b) V5 and V6 modes. The phases at ambient and high pressures are indicated as APs, HPs I, HPs II, and HPs III, respectively.

is also shown in Figure 7c (top). A pressure up to 2 GPa induces a clear split of the Na+-NbO6- translational mode into three components (1), indicating a transition of Na+-NbO6- interactions. Peak contours become much broader by further pressing up to 6.1 GPa (Figure 7c). The intensity of two peaks (3) in the region of the NbO6 rotational modes strongly decreases above 6 GPa. This is due to hindrance of octahedra rotation after their reorientation and subsequent touching of Na+ and NbO6- ions. A complete reversibility of the spectral fine structure after pressing and subsequent pressure release is observed only for pressing up to 1 GPa (Figure 7a). On the other hand, pressing up to 1.9 GPa deconvolutes the spectrum after pressure release (Figure 7b). No additional peak appears but a clearer separation of these components forming this peak can be observed. Pressing up to 6.1 GPa gives mainly three different pressure-evolutions with decreasing pressure (see spectra in Figure 7c). The spectrum for a randomly selected site (site 1) is the same as that before pressing. The profile at another randomly selected site (site 2) is deconvoluted by pressing and subsequent pressure release. Splitting of the band at around 120 cm-1 and a shift of the band at around 140 cm-1 to higher wavenumber cannot be seen for site 2 after pressure release but they can be found for site 3. The profile for site 3 looks like that for m-NN at ambient pressure (see the spectrum on top of Figure 7c). These results suggest that reorientation of NbO6 octahedra by a hydrostatic pressing up to 6.1 GPa and subsequent pressure release induces a transformation into the Pbcm structure on certain parts of powders but on other sites the Pmc21 structure is still kept after pressure release. Figure 8 shows plots of wavenumbers for the band at around 120 cm-1, which were collected from four measurement runs. Band splitting with decreasing pressure from a pressure over 2 GPa is clearly seen, indicating that a significant atomic rearrangement

Figure 5. Raman spectra obtained for micro- and submicrocrystalline NaNbO3 at 4.5, 7, and 16 GPa. For submicrocrystalline NaNbO3, spectra obtained by two different subsequent experimental runs are shown. The insets show spectra enlarged for the spectral region of the combined V5 + V1 mode.

Figure 6. Pressure characteristics of the split V5 + V1 combination modes obtained from one of the measurements for submicrocrystalline NaNbO3.

occurs above 2 GPa. Partially existing Pbcm structure after pressing above 2 GPa and subsequent pressure release causes splitting of the band at 120 cm-1. As mentioned above, the pressure response of s-NN is characterized by the transition at 2 GPa. Temperature characteristics of s-NN are completely reversible within the temperature range of -150 to 450 °C, which realizes four different phases;10 however, pressing above 2 GPa can result in irreversibility of spectral profiles. 3.3. Nanocrystalline NaNbO3. Below a particle size of 70 nm NN takes the point group Pmma. In this section, pressure characteristics of specific phonons of n-NN are discussed compared with the results for m-NN. Figures 9 and 10 show

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Figure 7. Pressure-evolution of the low-wavenumber region obtained for submicrocrystalline NaNbO3 on pressing up to (a) 1.0, (b) 1.9, and (c) 6.1 GPa, respectively. The spectrum obtained for the microcrystalline NaNbO3 at ambient pressure is shown on the top of panel c.

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Figure 9. Pressure-evolution of Raman spectra obtained for nanocrystalline NaNbO3.

Figure 8. Plots of specific peak positions in the low-wavenumber region obtained for submicrocrystalline NaNbO3 on pressing (b) and depressing (O), respectively.

the pressure-evolution of Raman spectra and pressure characteristics of some specific phonons obtained for n-NN, respectively. The scattering intensity at around 200 cm-1 (b) gradually increases with increasing pressure and a peak at around 250 cm-1 (O) develops above ca. 4 GPa (Figure 9). Evidence of the transitions is obscure due to the highly diffused characters of phonons (Figure 10). In the case of the stretching modes (Figure 10a), the slope for the V1 mode slightly changes at around 2.4 GPa. ν1 and ν3 modes shift to higher wavenumbers at around 7 GPa. On the other hand, it is difficult to assess the transitions from the pressure response of the bending modes (Figure 10b) because of the broadband shape and rather diffused characteristics. Figure 11 shows the scattering profiles obtained for n-NN at (a) 8.0 and (b) 15.9 GPa with the spectra obtained for m-NN at 8.2 and 15.8 GPa. As shown in Figure 11a,b, the features of both spectra observed for m-NN and n-NN are similar but the extent of broadening and the band shape of stretching modes around 600 cm-1 are different. In n-NN, a variety of correlational size of lattice vibrations may induce band broadening. As mentioned in section 3.2, the shape of the stretching modes is changeable by laser-focused positions. Above 10 GPa, nonhydrostatic condition is another possible factor, which induces additional band shifts and broadening. At any rate, since the number of Raman scattering peaks and their positions are very sensitive to the local crystallographic

Figure 10. Pressure characteristics of internal vibrations obtained for nanocrystalline NaNbO3: (a) V1, V2, and V3 modes and (b) V5 and V6 modes.

structure, we conclude that m-NN and n-NN intrinsically have the same local crystallographic structures above 8 GPa. According to the discussion in sections 3.1 and 3.2, rotation of NbO6 octahedra is already hindered at 8 GPa therefore m-NN and n-NN may show the same pressure characteristic relating to distortion of octahedra at higher pressures. From the comparison between the spectra before pressing and after pressure release, any significant difference cannot be found for

Pressure-Induced Phase Transitions in Crystalline NaNbO3

Figure 11. Raman spectra obtained for nanocrystalline NaNbO3 at (a) 8.0 and (b) 15.9 Gpa. Spectra obtained for microcrystalline NaNbO3 at 8.2 and 15.8 GPa are shown by dotted curves below spectra a and b.

Figure 12. Pressure-evolution of the low-wavenumber region obtained for nanocrystalline NaNbO3. A spectrum obtained after pressure release is indicated by a dotted curve below the spectrum obtained before pressing (ambient pressure).

n-NN (Figure 9). The complete reversibility of temperature10 and pressure characteristics for this powder indicates that the Pmma structure of n-NN is thermodynamically stable. It should be noted that pressure-induced sintering does not occur under present experiments because the phase transitions observed for n-NN are completely reversible. If the powders would sinter under pressurization, the crystallites would be subject to intergranular stresses and the spectra should be affected by that. However, this is not the case. Figure 12 shows pressure-evolution of spectra at the lowwavenumber region obtained for n-NN. Pressing at pressures between 0.4 and 2.5 GPa induces a clear separation of Na+-NbO6- translational modes into two components (1). With this clear split of the Na+-NbO6- translational modes, the intensity of NbO6 librational modes (3) decreases. Reorientation of NbO6 octahedra occurs below 2.5 GPa and the

J. Phys. Chem. C, Vol. 112, No. 26, 2008 9615 rotation is hindered at high pressure resulting in a further distortion of octahedra. These results obtained for the lowwavenumber region also support the above-mentioned NbO6reorientaion model. 3.4. Summary of Phase Stability. m-NN shows a bulk-like pressure characteristic. The first transition (APm f HPm I), which may be related to octahedra reorientation, occurs at around 2 GPa. Touching of Na+ and NbO6- ions can take place at around 6 GPa (HPm I f HPm II). Finally distortion of octahedra due to further pressing is enhanced at about 10 GPa (HPm II f HPm III). m-NN reversibly transforms from the ambient pressure phase (APm) f HPm I f HPm II f HPm III. Pressure release from HPm III induces a slight spectral change from the initial spectra. Very local disorder of atomic positions in the initial powders is believed to be ordered by hydrostatic pressure. s-NN, in which NbO6 octahedra significantly rotate about the longest crystal axis, reveals a drastic transition at ca. 2 GPa and a similar pressure characteristic above 6 GPa to that for m-NN. Pressing above 2 GPa and subsequent pressure release induce an irreversible spectral transformation into a bulk-like scattering profile for specific sites of powders. The phase of s-NN at ambient pressure APs transforms into HPs I f HPs II f HPs III. Pressing below 2 GPa (keeping APs) and subsequent pressure release causes slight spectral changes from the initial spectra as in the case for m-NN, which can be explained by local ordering of atoms. Temperature characteristics from -150 to 450 °C are completely reversible.10 However, irreversibility is found for the transition from APs into HPs I. Pressure-induced reorientation and accompanying distortion of NbO6 octahedra can induce the transition into a bulk-like Pbcm structure. n-NN shows a completely reversible and diffused phase transition. At 8 GPa, the spectral feature is approximately consistent with that for m-NN obtained at the same pressure range. n-NN transforms from APn into HPn III through diffused successive phase transitions. Therefore, it is difficult to distinguish transition pressures. The extent of octahedra rotation about the longest crystal axis is the smallest among the three possible polymorphs and the Pmma structure has a pseudocubic perovskite lattice.9,10 This indicates n-NN is already compressed under ambient pressure. Further pressing by hydrostatic pressure does not induce a drastic octahedra reorientation. However, once Na+ and NbO6- ions touch each other, distortion of NbO6 octahedra occurs in the same manner as that for m- and s-NN. Since the successive transitions of n-NN by temperature10 and pressure are completely reversible, the Pmma structure is a thermodynamically stable polymorph. Application of a hydrostatic pressure does not change local order of NbO6 octahedra after pressure release. Internal pressure in fine particles has been discussed as one of the possible mechanisms for size-induced phase transformation.13 According to the Laplace equation, an internal pressure (p) in a spherical particle is expressed by 2γ/r, where γ and r are the surface energy of a material and the radius of a particle. The new polymorph revealed for n-NN (orthorhombic Pmma) with an average particle size of about 70 nm has the smallest unit cell volume in comparison with the other polymorphs.9 Internal pressure in fine particles with an average particle diameter of 70 nm can be estimated to be around 60 MPa by using a general value of surface energy for oxides 1 N/m.13 Therefore, even if we consider a giant surface energy due to a large curvature, internal pressure in fine particles should be substantially lower than 1 GPa. For m-NN at pressures up to 1 GPa, we cannot detect any scattering profile that corresponds

9616 J. Phys. Chem. C, Vol. 112, No. 26, 2008 to n-NN. Consequently, the effect of internal pressure, which is variable during crystal growth from the amorphous phase, is not a major mechanism of phase stability. 4. Conclusions Phase transitions of micro-, submicro-, and nanocrystalline NaNbO3 induced by hydrostatic pressure were investigated by pressure tuning Raman spectroscopy. All polymorphs showed comparable spectral fine structures at high pressures over 6 GPa. The high-pressure phase above 10 GPa has distorted perovskite lattice induced by further pressing subsequent to touching of Na+ and NbO6- ions. Significant reorientation of NbO6 octahedra starts at about 2 GPa. This reorientation induces an irreversibility of the transition for the submicrocrystalline NaNbO3, which has a polar crystal structure and the largest cell volume (with the largest degree of octahedra rotation) among possible polymorphs. The powder partially transforms into a bulk-like structure by pressing over 2 GPa and subsequent pressure release. Nanocrystalline NaNbO3 showed diffused and completely reversible successive phase transitions. Stabilization of the Pmma structure could not be explained by internal pressure in fine particles. Acknowledgment. We gratefully thank Dr. Ju¨rgen Dornseiffer (Institut fu¨r Chemie and Dynamik der Geospha¨re Tropospha¨re (ICG II), Forschungszentrum Ju¨lich GmbH, Ju¨lich, Germany) and Dr. Franz-Hubert Haegel (NanDOx, Germany) for powder synthesis. One of the authors (Y.S.) sincerely

Shiratori et al. acknowledges the Alexander von Humboldt Foundation (Bonn, Germany) for granting financial support through a Research Fellowship. References and Notes (1) Jaffe, B.; Cook, W. R.; Jaffe, H. Piezoelectric Ceramics; Academic Press: London, 1971; p 185. (2) Wang, X. B.; Shen, Z. X.; Hu, Z. P.; Qin, L.; Tang, S. H.; Kuok, M. H. J. Mol. Struct. 1996, 385, 1. (3) Shen, Z. X.; Wang, X. B.; Kuok, M. H.; Tang, S. H. J. Raman Spectrosc. 1998, 29, 379. (4) Lima, R. J. C.; Freire, P. T. C.; Sasaki, J. M.; Ayala, A. P.; Melo, F. E. A.; Filho, J. M.; Serra, K. C.; Lanfredi, S.; Lente, M. H.; Eiras, J. A. J. Raman Spectrosc. 2002, 33, 669. (5) Bouziane, E.; Fontana, M. D.; Ayadi, M. J. Phys.: Condens. Matter 2003, 15, 1387. (6) Yuzyuk, Yu. I.; Simon, P.; Gagarina, E.; Hennet, L.; Thiaudie`re, D.; Torgashev, V. I.; Raevskaya, S. I.; Raevskii, I. P.; Reznitchenko, L. A.; Sauvajol, J. L. J. Phys.: Condens. Matter 2005, 17, 4977. (7) Shen, Z. X.; Wang, X. B.; Tang, S. H.; Kuok, M. H.; Malekfar, R. J. Raman Spectrosc. 2000, 31, 439. (8) Shiratori, Y.; Magrez, A.; Pithan, C. Chem. Phys. Lett. 2004, 391, 288. (9) Shiratori, Y.; Magrez, A.; Dornseiffer, J.; Haegel, F.-H.; Pithan, C.; Waser, R. J. Phys. Chem. B 2005, 109, 20122. (10) Shiratori, Y.; Pithan, C.; Waser, R.; Fischer, W.; Magrez, A. J. Phys. Chem. C 2007, 111, 18493. (11) Pithan, C.; Shiratori, Y.; Dornseiffer, J.; Haegel, F.-H.; Magrez, A.; Waser, R. J. Cryst. Growth 2005, 280, 191. (12) Schindler, W. Chem. Phys. 1978, 31, 345. (13) Uchino, K.; Sadanaga, E.; Hirose, T. J. Am. Ceram. Soc. 1989, 72, 1555.

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