Temperature-induced Phase Transitions in Micro-, Submicro-, and

J. Phys. Chem. C 2007, 111, 18493-18502

18493

Temperature-induced Phase Transitions in Micro-, Submicro-, and Nanocrystalline NaNbO3 Yosuke Shiratori,*,† Arnaud Magrez,‡ Werner Fischer,§ 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 Institut fu¨r Energieforschung (IEF-1), Forschungszentrum Ju¨lich GmbH, D-52425 Ju¨lich, Germany ReceiVed: May 16, 2007; In Final Form: September 13, 2007

Phase transitions in micro-, submicro-, and nanocrystalline NaNbO3 were investigated by temperature-tuning Raman spectroscopy and X-ray powder diffraction method. Three powders with different average particle size showed successive phase transitions within the measured temperature range from -150 to 450 °C. The temperature characteristics of Raman active phonons in microcrystalline NaNbO3 corresponded the one reported for bulk NaNbO3, which transforms with increasing temperature from the ferroelectric N into the antiferroelectric P phase and finally above 373 °C (Tm3) into the antiferroelectric R phase. Submicrocrystalline NaNbO3, which takes the noncentrosymmetric orthorhombic Pmc21 structure at room temperature, transformed into a pseudocubic structure at 333 °C (Ts3). Nanocrystalline NaNbO3 showed a diffused phase transition from an orthorhombic Pmma structure to a high-temperature phase at around 180 °C (Tn2). For micro- and submicrocrystalline NaNbO3, hysteretic phase transition behavior was found for the temperature characteristics of specific phonons. On the other hand, the characteristics obtained for nanocrystalline NaNbO3 were much more diffused and did not show any hysteretic effect. Crystal structure refinements of the X-ray powder diffraction patterns using the Rietveld method demonstrated a hysteretic deformation of the a-b plane for microcrystalline NaNbO3 around Tm3 and of the b-c plane for submicrocrystalline NaNbO3 around Ts3. The temperature dependence of the primitive perovskite volumes showed a very small hysteresis for microcrystalline NaNbO3 but a clear one for submicrocrystalline NaNbO3. Lattice distortion of the submicrocrystalline Pmc21 structure from a cubic perovskite lattice induced a particularly large contraction of parameter c around Ts3 with increasing temperature, which resulted in a decrease of the primitive cell volume. This transition showed a first-order type character, which may relate to a ferroelectric-antiferroelectric transition. Rearrangement of the NbO6 octahedra induces a transition from an orthorhombic into a pseudocubic structure.

1. Introduction Piezoelectric ceramic materials are widely applied in many electromechanical components, such as actuators, sensors, and ultrasonic transducers. They are used in modern vibrational sensors, for ultrasonic medical diagnostics, underwater acoustics, piezoelectric motors, such as micropumps, and finally in atomic force microscopy. Due to its strong piezoelectric response the compound Pb(Zr,Ti)O3 (PZT) is most widely employed at present. Recently, however, concerns about the environmental impact,1 biocompatibility, and toxicity of this material due to its high lead content (usually more than 60 wt %), have led to the development of alternative lead-free ceramics. Among these, especially, alkaline niobates, based on the perovskite type oxide (K,Na)NbO3, have attracted much attention.2,3 In this context, the characterization and processing of nanocrystalline powders with improved sintering activity make up part of the emerging and rapidly growing field of nanoscience and -technology. In the particular case of nanocrystalline polar functional oxides, many properties, such as electrical conductivity, dielectric * 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. Telephone: +81-3-5841-7330. Fax: +81-3-5841-7332. E-mail: [email protected]. † IFF-Forschungszentrum Ju ¨ lich. ‡ LNNME-EPFL. § IEF-1-Forschungszentrum Ju ¨ lich.

permittivity, and spontaneous polarization, distinctly depend on the crystallite size, a phenomenon commonly referred to as “size effect”. So far we reported novel size-induced phase transitions for (K0.5,Na0.5)NbO3 (KNN)4 and NaNbO3 (NN).5,6 The KNN crystallites transform from a monoclinic phase (M-type structure) into a triclinic one (T-type structure) at a critical size of 200 nm with decreasing particle size. The coarse NN powders take an orthorhombic Pbcm structure. Powders as fine as 70 nm present a structure described in the centrosymmetric Pmma space group. At an average particle size of 200-400 nm, the crystallites stabilize in a noncentrosymmetric Pmc21 structure. Bulk NaNbO3 itself and solid solutions with LiNbO3 or KNbO3 are known as antiferroelectric and ferroelectric materials, respectively, at ambient temperature and pressure.7 Electrically polarized NaNbO3 ceramics have a metastable ferroelectric phase.8 Recently we demonstrated that pure NaNbO3 crystallizes in a ferroelectric phase at a specific particle size range in the order of several hundred nanometers. Such submicrocrystalline powders are expected to show enhanced piezoelectric response. Currently piezoresponse force microscopy revealed enhancement of piezoelectricity.9 In the present study, we investigated the crystallite-size dependence of the temperature characteristics of transitions

10.1021/jp0738053 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/29/2007

18494 J. Phys. Chem. C, Vol. 111, No. 50, 2007

Shiratori et al. respectively. Figure 1 shows an illustration of these structures, which were presented in ref 5. The Pbcm structure corresponds to the bulk P phase,10 which gives 60 Raman active modes represented by 15Ag + 17B1g + 15B2g + 13B3g. The Pmc21 symmetry is a possible structure for NN ceramics treated in a strong electric field.8,11 This is a ferroelectric (FE) phase due to its noncentrosymmetric structure. On the other hand, submicrocrystalline NN stabilizes in this FE phase without any dopants and pretreatment such as an electric field. This phase has 57 Raman acitve modes represented by 16A1 + 13A2 + 12B1 + 16B2. The A1, B1, and B2 modes are simultaneously infrared active. The Pmma structure, which has been refined for the nanopowders, is a new polymorph in NaNbO3, with a centrosymmetric unit cell. Its Raman active modes are represented by 8Ag + 5B1g + 7B2g + 10B3g. Table 1 shows the results of the factor group analysis12 used as a basis for the Raman spectroscopic study. One can consider two types of vibrational modes, external (Na+ and NbO6- translations and NbO6librations) and internal (stretching and bending in NbO6 octahedra) modes. External modes generate specific Raman scattering peaks at wavenumbers below the region for the internal modes.

Figure 1. Illustrations of the structural differences between (a) micro-, (b) submicro-, and (c) nanocrystalline NaNbO3. Particle size ranges obtained from XRPD and Raman spectroscopy are also indicated. Reprinted with permission from ref 5. Copyright 2005 American Chemical Society.

based on the results of temperature-tuning Raman spectroscopy and X-ray powder diffraction (XRPD) measurements. In this paper, we present successive phase transitions for micro-, submicro-, and nanocrystalline NaNbO3. Novel hysteretic characteristics of these phase transitions are reported for the first time, and their mechanisms are discussed. Raman Active Modes of Three NaNbO3 Polymorphs. According to our previous work,5 micro-, submicro-, and nanocrystalline NaNbO3 powders with an average particle diameter of 1.1 µm, 280 nm, and 70 nm, respectively, stabilize in the orthorhombic Pbcm (D2h11, no. 57), Pmc21 (C2V2, no. 26), and Pmma (D2h5, no. 51) structures at room temperature,

2. Experimental Section Stoichiometric micro-, submicro-, and nanocrystalline NaNbO3 powders were prepared through microemulsion mediated synthesis followed by subsequent annealing treatments. They will be referred to as m-NN, s-NN, and n-NN, respectively, in this paper. Details of the synthesis procedures were reported elsewhere.13 All synthesized powders were thoroughly analyzed with respect to crystallinity, phase purity, chemical composition, and crystallite size using XRPD, inductively coupled plasma with optical emission spectroscopy, and scanning electron microscopy.5 Raman spectra at various temperatures were recorded using a Jobin Yvon T64000 Raman spectrometer equipped with a charge coupled device detector. A 514.5 nm laser line (BeakLok 2060, Spectra-Physics Co.) was used to irradiate the powdery samples, which were mounted on a temperature tunable stage

TABLE 1: Factor Group Analysis for Optical Modes in Micro-, Submicro-, and Nanocrystalline NaNbO3 modes external modes

Na+ and NbO6- translations acoustic modes Raman active modes NbO6- librations Raman active modes

internal modes Raman active modes external modes


internal modes Raman active modes external modes


internal modes Raman active modes

irreducible representation

no. of Raman active modes

m-NaNbO3, Pbcm (D2h11), Z ) 8 6Ag + 5Au + 7B1g + 6B1u + 6B2g + 7B2u + 5B3g + 6B3u B1u + B2u + B3u 6Ag + 7B1g + 6B2g + 5B3g 3Ag + 3Au + 3B1g + 3B1u + 3B2g + 3B2u + 3B3g + 3B3u 3Ag + 3B1g + 3B2g + 3B3g 6Ag + 5Au + 7B1g + 6B1u + 6B2g + 7B2u + 5B3g + 6B3u 6Ag + 7B1g + 6B2g + 5B3g

24

s-NaNbO3, Pmc21 (C2v2),

7A1 + 5A2 + 5B1 + 7B2 A 1 + B1 + B2 6A1 + 5A2 + 4B1 + 6B2 3A1 + 3A2 + 3B1 + 3B2 3A1 + 3A2 + 3B1 + 3B2 7A1 + 5A2 + 5B1 + 7B2 7A1 + 5A2 + 5B1 + 7B2

12 24

Z)4

n-NaNbO3, Pmma (D2h5), Z ) 4 4Ag + Au + B1g + 4B1u + 3B2g + 4B2u + 4B3g + 3B3u B1u + B2u + B3u 4Ag + B1g + 3B2g + 4B3g Ag + 2Au + 2B1g + B1u + 2B2g + B2u + B3g + 2B3u Ag + 2B1g + 2B2g + B3g 3Ag + 2B1g + 5B1u + 2B2g + 3B2u + 5B3g + 4B3u 3Ag + 2B1g + 2B2g + 5B3g

21 12 24

12 6 12

Temperature-Induced Phase Transitions in NaNbO3

J. Phys. Chem. C, Vol. 111, No. 50, 2007 18495

Figure 2. Temperature evolution of Raman spectra obtained for (a) micro-, (b) submicro-, and (c) nanocrystalline NaNbO3 on heating.

(THMS600, Linkam Scientific Co.) with a temperature stability of (0.1 °C. An OLYMPUS BX41 optical microscope equipped with a 40× objective was used to focus the laser beam on the samples. Raman scatterings were collected in backscattering geometry through an entrance slit of 100 µm and diffracted on a grating with 1800 grooves mm-1 under triple subtractive configuration. XRPD was performed on a D5000 diffractometer (Bruker AXS GmbH, Germany) using Cu-KR radiation. The diffractometer was equipped with a HDK S1 high-temperature chamber (Buehler GmbH, Germany) driven at air and a diffracted beam graphite monochromator. Powder samples were deposited from an ethanol slurry as about 20 µm thick layers onto a DC heated, 100 µm thick Pt strip mounted between two power supply rods. The Pt strip was kept during heating under a slight stress to compensate its elongation and in order to ensure a stable height sample position. The temperature was controlled by a Pt/Pt-10Rh thermocouple spot-welded to the Pt strip in the middle of the rare side. The length of the sample surface illuminated by X-rays was about 10 mm. It has been kept centrally between the two power supply rods. Additionally, to minimize the temperature gradient across the sample, the Pt strip was surrounded by a 270° circular radiation shield thermally isolated from the chamber body. The maximum temperature error across the illuminated sample area was estimated to be about 20 °C. Scanning was performed in Bragg-Brentano parafocusing geometry in the angle window 20-80° (2θ) with 0.02° step width for temperatures 250-450 °C in steps of 10 °C during the heating and the cooling cycles. For temperature stabilization at every temperature level the samples were kept for 10 min prior to the start of data collection. The temperature ramp was set to 1 °C/s. Lattice parameters were calculated from XRPD patterns using the Rietveld method (software X’Pert Plus, Panalytical/The Netherlands). Revised crystal structure data5,6 were used as starting models. The Rietveld method had to be applied to exclude the systematic error caused by slight remaining fluctuations in sample height due to elongation/contraction of the Pt heating strip. The addition of a reference material to the sample powder was not used to avoid reactions at elevated temperature. Error bars marked in plots represent the sum of estimated accidental errors and uncertainties of Rietveld results caused by the counting statistics and/or curve fitting.

3. Results and Discussion 3.1. Temperature-Evolution of Raman Spectra. Figure 2 shows Raman spectra measured for m-, s-, and n-NN at various temperatures on heating. The different spectral fine structures specific for each particle size gradually diminish with increasing temperature, and finally all three samples show a similar Raman scattering profile, which corresponds to the bulk high-temperature phase.14,15 3.1.1. Translational Modes. Figure 3 shows the temperatureevolution of the Raman spectra in the low-wavenumber region recorded for the three powders. m-NN indicates successive spectral transitions with increasing temperature (Figure 3a). At -150 °C, the profile is believed to originate from a mixture of the low-temperature FE phase N (rhombohedral R3c, C3V6), which gives a strong sharp peak at 82.1 cm-1 (3),16 and the AF phase P (orthorhombic Pbcm, D2h11). Its complicated profile originating from the phase mixture gradually simplifies, and two main peaks at 61 and 74 cm-1, which are assigned to the translational modes between Na+ and NbO6- ions,16 dominate around room temperature. According to the factor group analysis (Table 1), the Raman active translational modes are represented to be 6Ag + 7B1g + 6B2g + 5B3g. The present results indicate that two major interactions between Na+ and NbO6- ions due to two different Na+ ions occupying different sites exist in this system. At high temperatures above 250 °C, the splitting gradually dissolves and finally this mode highly softens down to about 40 cm-1 at around 400 °C (Figure 3a). A clear spectral change is found between 350 and 400 °C, which is an indication for the transition from the orthorhombic P into the orthorhombic R phase. These characteristics agree very well with the results obtained for coarse powders as reported by Shen et al.14,16 In contrast to the work of these authors, however, our measurements clearly revealed for the first time a hysteretic effect in the evolution of the Raman spectra at high temperature. On cooling, the spectral profile drastically changes between 350 and 300 °C. s-NN shows a similar temperature evolution (Figure 3b) compared to m-NN except for its slightly simplified and broadened spectral structures at each temperature. A drastic spectral change is found between 300 and 350 °C on heating and between 300 and 250 °C on cooling. Here also a significant hysteresis is observed. The high-temperature spectral profiles are the same as those for m-NN. On the other hand, n-NN shows a completely different characteristic (Figure 3c) compared to


Shiratori et al.

Figure 3. Temperature evolution of Raman spectra in the wavenumber region below 340 cm-1 obtained for (a) micro-, (b) submicro-, and (c) nanocrystalline NaNbO3 on heating and cooling. Arrows in a and b indicate spectral transitions.

the other two powders. At room temperature, the Raman active translational modes of this powder (Pmma) are represented to be 4Ag + B1g + 3B2g + 4B3g, which is much simpler in comparison to the cases of the two other polymorphs. According to recent high-pressure Raman spectroscopy, which emphasizes the separation of the translational bands,17 the peak at the lowest wavenumber region consists of two components. Therefore, two major Na+-NbO6- interactions due to different types of Na+ ions at different sites exist also in the n-NN at room temperature. Above 150 °C, fine structures of the spectra continuously diminish. The transition behavior is highly diffuse, but the

profiles at low and high temperatures are completely different, which indicates the existence of successive phase transitions. Figure 4 shows wavenumber shifts of two intense translational modes on heating and cooling sequences. m-NN indicates a drastic shift at 370 °C on heating (Figure 4a). Curve fitting gives different peak traces for the cooling sequence. On cooling the high-temperature characteristic transforms into the low-temperature one at 315 °C. This spectral transition corresponds to the P-R transformation. To the best of our knowledge, such hysteretic phase transition behavior has never been reported yet for NaNbO3. For s-NN (Figure 4b), a similar hysteretic transition



Figure 5. Temperature characteristics of specific NbO6- librational modes obtained for micro- (red), submicro- (blue), and nanocrystalline (green) NaNbO3 on heating (closed circles) and cooling (open circles).

Figure 4. Temperature characteristics of specific translational modes obtained for (a) micro-, (b) submicro-, and (c) nanocrystalline NaNbO3 on heating (b) and cooling (O).

from the room-temperature noncentrosymmetric Pmc21 phase into a high-temperature phase is also demonstrated in the temperature range from 230 to 360 °C. On the other hand, the translational modes of n-NN (Figure 4c) almost linearly shift to lower wavenumbers without any hysteresis. 3.1.2. Librational Modes. According to Shen et al.,16 bands within the range from 100 to 160 cm-1 can be assigned to NbO6- librational modes. They found that some specific bands in the bulk NN soften on cooling to the orthorhombic P(AF)rhombohedral N(F) phase transition point (ca. -75 °C). As indicated in Table 1, NbO6- librational modes are represented to be 3Ag + 3Au + 3B1g + 3B1u + 3B2g + 3B2u + 3B3g + 3B3u at a C1 site in a D2h unit cell (m-NN), 3A1 + 3A2 + 3B1 + 3B2 at a C1 site in a C2V unit cell (s-NN), and Ag + 2Au + 2B1g + B1u + 2B2g + B2u + B3g + 2B3u at a Csyz site in a D2h unit cell (n-NN). The numbers of Raman active librational modes are 12, 12, and 6 for m-, s-, and n-NN, respectively. It should be noted that in the present study it is difficult to distinguish all possible modes due to complex band overlapping and coupling. However a much simpler spectrum profile of n-NN in the region of librational modes (Figure 3) can be explained mainly by the half-number of modes in comparison with the other two powders. Figure 5 shows the temperature characteristics of specific librational modes obtained for the three powders. m-NN shows shifts of the split bands to higher wavenumbers with increasing temperature from -150 to 120 °C. These bands converge to one peak at 130 °C, and this converged peak subsequently shifts to higher wavenumbers up to 280 °C. Yuzyuk et al.15 reported that the room-temperature monoclinic phase (Pm), which was

concluded on the basis of the results of a synchrotron XRD study, transforms into a incommensurate (INC) phase at 137 °C. They also reported a transition from Pm to orthorhombic Po phase at 187 °C through the INC phase, which is a disordered phase between the other two structures. This transition is believed to be associated with a rotation of NbO6 octahedra. Peak convergence around 125 °C obtained for our m-NN can be related to the rotation of NbO6 octahedra. Here we call the phases stabilized below and above 125 °C P1 and P2 phases, respectively. From 290 to 370 °C, the band shifts to lower wavenumbers and a drastic and abrupt shift occurs at 373 °C, which corresponds to the P-R transition.14 Here again one can clearly see a hysteresis in the shift. On cooling, this band drastically shifts to higher wavenumbers from 325 °C to 290 °C. Below 290 °C, the band shifts to lower wavenumbers through splitting at 130 °C corresponding to the P2 to P1 transition. For s-NN, a similar tendency is revealed. On heating, a gradual shift to lower wavenumbers starts at 285 °C and a drastic one occurs at 333 °C. On cooling, a drastic shift to higher wavenumbers starts at 307 °C and a subsequent shift continues down to 245 °C. n-NN shows a much more diffused temperature characteristic of the NbO6- librational mode in comparison with the coarser powders. Hysteretic behavior cannot be observed for this powder. The average transition temperature from the room-temperature phase into the high-temperature phase is 180 °C. 3.1.3. Internal Vibrations. Under the assumption of an equilateral octahedral structure (Oh) of a free NbO6- ion, the internal vibrational modes of the octahedra can be represented as A1g (ν1) + Eg (ν2) + 2F1u (ν3, ν4) + F2g (ν5) + F2u (ν6). The ν1, ν2, and ν3 modes originate from stretching motions, and the others are bending ones. A1g (ν1), Eg (ν2), and F2g (ν5) modes are Raman active, 2F1u (ν3, ν4) modes are IR active, and F2u (ν6) mode is silent for the perfect octahedron structure. At specific site symmetry, doubly and triply degenerated modes can split and Raman inactive modes can be active. However, generally Raman spectroscopy gives rather intense scattering from the ν1, ν2, and ν5 modes even in a crystal field with a lower symmetry than the Oh point group. Spectral profiles in the regions of bending and stretching modes (160-450 and 500-750 cm-1, respectively) show a complex splitting. They broaden and converge with increasing temperature, which indicates that the structure of the octahedra approaches the perfect Oh structure at high temperature (Figure 2). Figure 6 shows the temperature characteristics of the ν1, ν2, and ν3 modes obtained for the three powders. For m- and s-NN, the wavenumbers of the ν3 mode clearly shift to lower values


Shiratori et al.

Figure 7. Temperature characteristics of the combined ν5 + ν1 mode obtained for (a) micro-, (b) submicro-, and (c) nanocrystalline NaNbO3 on heating (b) and cooling (O).

Figure 6. Temperature characteristics of internal stretching modes obtained for (a) micro-, (b) submicro-, and (c) nanocrystalline NaNbO3 on heating (b) and cooling (O).

and their intensity decreased with increasing temperature (Figure 2). At high temperatures, the peak fitting procedure did not always give excellent reproducibility not only due to band broadening but also because of the convergence of the three modes. However, it is clear that significant changes of the temperature characteristics occur around 370, 330, and 200 °C for m-, s-, and n-NN, respectively. The wavenumber-temperature curves for m- and s-NN can be divided into four regions within the measuring temperature range (Figure 6a,b). For m-NN, the four regions correspond to stability regions of the N (FE) + P1 (AF), P1 (AF), P2 (AF), and R (AF) phases, respectively. The corresponding three transition temperatures separating these stability fields are called Tm1, Tm2, and Tm3. s-NN shows successive transitions from phase I to IV through II and III phases. The transition temperatures are referred to as Ts1, Ts2, and Ts3, respectively. On the other hand, the temperature characteristic of n-NN can be categorized into three specific regions separated by two specific temperatures, Tn1 and Tn2 (See Figure 6c). In summary, significant structural changes occur at high temperatures (Tm3, Ts3, Tn2), but other minor structural changes, which may relate to slight NbO6- rotation,15,18 exist at temperatures below 200 °C. Plots of the wavenumber versus temperature for the ν5 + ν1 combination mode are shown in Figure 7. m-NN indicates two transition points around 120 (Tm2) and 350 °C (Tm3). A similar tendency can be demonstrated around 70 (Ts2) and 335 °C (Ts3) for s-NN. A very slight change of slope is found between 100

Figure 8. Spectral profiles of the ν4 mode recorded for micro-, submicro-, and nanocrystalline NaNbO3 at (a) 30 and (b) -150 °C.

and 200 °C for n-NN. Local symmetries are easily changeable through tilting of NbO6 octahedra in the NaNbO3 polymorphs; however, a significant structural transition only occurs at high temperatures. Especially for m- and s-NN, hysteretic temperature characteristics of lattice vibrations are revealed. Figure 8 shows Raman scattering of the ν4 modes obtained for the three powders at -150 and 30 °C. Although the ν4 mode in equilateral NbO6 octahedra is Raman inactive, this mode is detectable for m-NN, s-NN, and n-NN but its intensity is very weak because of their pseudocubic structures. It should be stressed that no contamination between different types of particles can be confirmed for all powders since their peak profiles are completely different from each other. At 30 °C the



Figure 9. Temperature evolution of the ν4 mode obtained for (a) micro-, (b) submicro-, and (c) nanocrystalline NaNbO3 on heating.

triply degenerated ν4 mode in a free NbO6 octahedron (F1u) on an Oh point group splits into 3A and 2A′ + A′′ modes on C1 (m- and s-NN) and Csyz (n-NN) site symmetries, respectively.12 The factor groups for 3A modes are the Raman active A1, A2, B1, and B2 modes in the Pmc21 (C2V2) crystal field of s-NN. On the other hand, the factor groups for 3A (and 2A′ + A′′) modes in a D2h crystal field (m- and n-NN) are Raman active Ag, B1g, B2g, and B3g modes and Raman inactive Au, B1u, B2u, and B3u modes. m-NN shows a splitting into four bands; however, for n-NN a clear disappearance of one of the ν4 modes at ca. 377

cm-1 is observed. This result is due to a pseudocubic perovskite primitive lattice in n-NN at room temperature. Figure 9 shows the temperature evolution for the ν4 modes. One of the ν4 modes of m- and s-NN around 375 cm-1 disappears around 100 and 120 °C and another band at ca. 435 cm-1 quenched around 370 and 320 °C, respectively. The former spectral transition corresponds to slight NbO6- rotation (transitions from P1 to P2 and from II to III for m- and s-NN, respectively), and the latter one corresponds to the transition into high-temperature phases R and IV for m- and s-NN,


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TABLE 2: Transition Temperatures (°C) of Micro-, Submicro-, and Nanocrystalline NaNbO3 Obtained from Temperature-Tuning Raman and XRPD Measurementsa sample

transition

method

m-NaNbO3 Raman XRPD s-NaNbO3 Raman XRPD n-NaNbO3b Raman

Tm1: N f P1 ca. -50

Tm2: P1 f P2 100 to 120

Ts1: I f II -80 to -20

Ts2: II f III 50 to 120

Tn1: I f II ca. 0

Tn2: II f III 180 (180)

Tm3: P2 f R 373 (325) 355 (310) Ts3: III f IV 333 (307) 310 (280)

a Temperatures in parentheses were determined from cooling sequences. b No XRPD measurements available.

Figure 10. XRPD patterns of (a) micro- and (b) nanocrystalline NaNbO3 during heating from 250 (lowest) to 450 °C (middle) and the subsequent cooling to 250 °C (uppermost pattern), recorded in temperature intervals of 10 °C.

respectively. n-NN shows a weak scattering profile around 430 cm-1 above 150 °C, which indicates that the perovskite unit structure is nearly cubic but local lattice distortion still exists. Phase transition temperatures, which are determined from the analysis of the external and internal modes of NaNbO3, are summarized in Table 2. Additionally the transition temperatures Tm3, Ts3, and Tn2 obtained from the XRPD results are included. 3.2. Temperature-Evolution of XRPD Patterns. To reveal the phase transition mechanisms especially for m- and s-NN, which indicated a hysteresis of the local phase transition around Tm3 and Ts3, respectively, Rietveld refinements using a roomtemperature structural basis were performed for the XRPD patterns obtained for the three types of NN powders. Figure 10 shows the evolution of the XRPD patterns for (a) m- and (b) n-NN during heating from 250 to 450 °C in steps of 10 °C and subsequent cooling. In m-NN the unit cells of both modifications (P2 and R) do not differ very much, so that remarkable changes in the powder patterns are visible only at Bragg angles above 60°, where the angular resolution becomes

Figure 11. Temperature characteristics of the perovskite formula parameters (PFPs) obtained for (a) micro- and (b) submicrocrystalline NaNbO3 on heating (closed circles and solid lines) and cooling (open circles and dotted lines) and for (c) nanocrystalline NaNbO3 on heating.

better. The observed phase transition is fully reversible. There is a thermal hysteresis of about 50 °C. No phase transition can be detected by XRPD for n-NN. 3.2.1. Temperature Characteristics of the Lattice Parameters. Figure 11 shows the temperature dependence of the perovskite formula parameters (PFPs) obtained for NN powders with different crystallite sizes. Once we use PFPs, it is rather easy to image distortion from the cubic perovskite structure. The PFP corresponds to the distance between the centers of two successive NbO6 octahedra in the three directions of the space. Therefore, they can be obtained from the lattice constants using the relations a/21/2, b/21/2, c/4; a/2, b/21/2, c/21/2; and a/21/2, b/2, c/21/2 for m-NN, s-NN, and n-NN structures, respectively, at room temperature. We call PFPs derived from parameters a, b, and c PFPa, PFPb, and PFPc, respectively. The temperature characteristics of the perovskite primitive volume (V) for the three powders, which were calculated from the PFPs, are shown



Figure 13. Results of differential thermal analysis obtained for micro(red), submicro- (blue), and nanocrystalline (green) NaNbO3 on heating with a heating rate of 7 °C/min.

Figure 12. Temperature characteristics of the primitive volume obtained for (a) micro- and (b) submicrocrystalline NaNbO3 on heating (closed circles and solid lines) and cooling (open circles and dotted lines) and for (c) nanocrystallline NaNbO3 on heating.

in Figure 12. It should be noted that for n-NN only measurements upon heating were performed since this powder did not show any significant transitions of phonon parameters above 250 °C, but it is important to know temperature evolution of the perovskite primitive lattice at high temperature. Both the PFPs and V of n-NN linearly increase with increasing temperature (Figures 11c and 12c). Hysteretic change of parameters a and b of the Pbcm lattice (m-NN) is revealed by XRPD. On the other hand parameter c does not show any significant hysteresis (Figure 11a). The transition temperatures of both parameters a and b are 355 °C on heating and 310 °C on cooling. This transition corresponds to the P2-R phase change and is in good accordance with our Raman results. Above the transition temperature, PFPa and PFPb coincide being slightly larger than PFPc. Due to the compensation of the temperature characteristics of the lattice parameters, the primitive volume almost linearly changes with increasing and decreasing temperature and shows only very minor hysteresis (Figure 12a). These results indicate that rearrangement

of NbO6 octahedra, which transform atomic interactions in the a-b plane, is a critical structural change for P2-R transition. In the case of the noncentrosymmetric Pmc21 structure, transformation of the b-c plane into a diametric shape with hysteresis is revealed. The drop of PFPc with increasing temperature is much larger in comparison with the increase of PFPb (Figure 11b). Consequently these characteristics coupled with an almost linear characteristic of PFPa induce a decrease of V at 310 °C despite increasing temperature (Figure 12b). In turn, the volume increases at 280 °C with decreasing temperature. Only the Pmc21 structure refined for s-NN shows a clear hysteresis of the temperature-volume characteristic. At room temperature5 and 250 °C, n-NN and s-NN have the smallest and the largest volume, respectively (Figure 12). Above 360 °C the temperature characteristics of V for the three powders almost coincide with each other. The behavior of volumetric expansion for m-NN and s-NN becomes close to that for n-NN above P2-R and III-IV transitions, respectively. The phase transition temperatures obtained from temperature-tuning Raman and XRPD measurements are summarized in Table 2. Although the transition points are clearly revealed from Raman spectroscopy and XRPD especially for m-NN and s-NN at hightemperature (Tm3: P2-R and Ts3: III-IV, respectively), differential thermal analysis does not show any presence of latent heat for these transition points (Figure 13). Even for n-NN, which can transform from Pmma into Pmc21 around 600 °C and from Pmc21 into Pbcm around 900 °C5 with particle coarsening, no sharp peak appears. In the case of NaNbO3 powders, it is difficult to detect transition latent heats due to their slight structural changes. Table 3 lists the linear thermal expansion coefficients of the lattice parameters (LTEC) and the volumetric thermal expansion coefficients of perovskite primitive cell (VTEC) calculated from linear regressions fitted for the temperature characteristics (Figures 11 and 12). LTEC values in three directions obtained for n-NN are in the same order. It should be marked that LTEC for parameter b in m-NN (P2-phase) and the one for parameter c in s-NN (III-phase) are negative. The negative thermal expansion is one of the indications for octahedra rearrangement, which induces the transition into equilateral octahedra. The microscopic thermal expansion coefficients of m-NN exhibit smooth behavior in the a and c directions, while there is a discrete discontinuity in the b direction from contraction below the P2-R transition to an expansion above it (Figure 11a, Table 3). This observation suggests that the structural transformation is mainly due to a change of the atomic bonding in the b direction caused by octahedra rearrangement. The same finding can be obtained for s-NN, where the phase transition induces

18502 J. Phys. Chem. C, Vol. 111, No. 50, 2007 TABLE 3: Linear and Volumetric Thermal Expansion Coefficients (LTEC and VTEC) of Micro-, Submicro-, and Nanocrystalline NaNbO3 Obtained from Temperature-Dependent XRPD Measurementsa sample

phase

m-NaNbO3 P2 R s-NaNbO3 III IV n-NaNbO3 III

105LTEC (K-1) a b 1.9 (1.9) -0.8 (-0.9) 2.0 (2.0) 1.8 (1.7) b c 1.0 (-) -0.9 (-) 1.4 (1.3) 1.3 (1.8) a c 1.9 2.0

c 2.8 (2.8) a 3.3 (3.0) b 1.7

105 × VTEC (K-1) 4.6 (4.5) 6.3 (6.3) 3.4 (-) 5.9 (6.0) 5.8

a

Values in parentheses were determined from cooling sequences. LTEC, (lRT)-1 dl/dT; VTEC, (VRT)-1 dV/dT, where lRT and VRT are the lattice parameters and the perovskite primitive volume at room temperature.5,6 Errors of the values listed in this table are below 0.2 K-1.

changes of atomic interaction in the c direction. The expansion behavior for n-NN, which has a pseudocubic structure within the measuring temperatures, is almost isotropic. LTEC and VTEC of n-NN originate almost only from pure thermal expansion of crystal lattice. Here a nonnegligible tendency is anisotropic expansion for m-NN and s-NN in R and IV phases, respectively (see LTEC values in Table 3), despite not only n-NN showing almost isotropic expansion but also VTEC values for the R phase of m-NN, the IV phase of s-NN, and the III phase of n-NN are almost the same with each other. Such behavior will induce a tetragonal phase for m-NN and s-NN at temperatures much higher than the P2-R (m-NN) and III-IV (s-NN) transition points. Actually, bulk NN takes the tetragonal T2 phase (575-641 °C) just before the cubic phase.14 The III (FE)-IV transition of s-NN is characterized by hysteretic transitions of (1) specific phonons, (2) lattice parameters b and c, and (3) unit cell volume at Ts3. It is believed that the transition is of first order and contains a displacive-type character. A Curie temperature around 300 °C has been reported for NaNbO3 ceramics poled under a high electric field.8 Ts3 in the present case corresponds to the Curie point observed for the poled ceramics. Again it should be emphasized that the present s-NN (FE, Pmc21) is stabilized by particle size. 4. Conclusions Temperature-induced successive transitions for micro-, submicro-, and nanocrystalline NaNbO3 were substantiated by temperature-tuning Raman spectroscopy and X-ray powder diffraction. The phase transition behavior observed for microcrystalline NaNbO3 on heating agreed well with the one reported for bulk NaNbO3; however, remarkable hysteretic temperature characteristics of specific phonons and lattice parameters, which are perpendicular to the c-axis, were demonstrated for the first time for the orthorhombic P-R phase transition (around 350 °C). The structural transformation is mainly due to a change of the atomic interaction in the b direction. The hysteretic temperature characteristic of perovskite primitive volume was almost negligible due to compensation of linear expansion and

Shiratori et al. contraction in three directions. Submicrocrystalline NaNbO3, for which details of the temperature characteristics of Raman active phonons and lattice parameters have never been reported, showed successive phase transitions with a hysteretic transition at high temperature (around 300 °C). Rearrangement of NbO6 octahedra resulted in a hysteretic volume change. The structural transformation is mainly due to a change of the atomic interaction in the c direction. Nanocrystalline NaNbO3 indicated diffused transition behavior. Its pseudocubic structure, which has a much smaller degree of octahedra tilting from the cubic lattice than for the other two polymorphs, showed almost isotropic expansion behavior, which supports octahedra reorientation through rotation as a possible mechanism for the phase transitions in NaNbO3. Acknowledgment. We gratefully thank Dr. Ju¨rgen Dornseiffer (Institut fu¨r Chemie und Dynamik der Geospha¨re Tropospha¨re (ICG-2), Forschungszentrum Ju¨lich GmbH, Ju¨lich, Germany) and Dr. Franz-Hubert Haegel (NanDOx, Germany) for powder synthesis and Dr. Frank Tietz (Institut fu¨r Energieforschung (IEF-1), Forschungszentrum Ju¨lich GmbH, Ju¨lich, Germany) for DTA measurements. Y.S. sincerely acknowledges the Alexander von Humboldt Foundation (Bonn, Germany) for granting financial support through a Research Fellowship. References and Notes (1) Lead Free Piezoceramics Based on Alkaline Niobates, EU-funded project GRD1-2000-25682 (Fifth Framework Project: Mar. 1, 2001 to Feb. 29, 2004). (2) Cross, E. Nature 2004, 432, 24. (3) Saito, Y.; Takao, H.; Tani, T.; Nonoyama, T.; Takatori, K.; Homma, T.; Nagaya, T.; Nakamura, M. Nature 2004, 432, 84. (4) Shiratori, Y.; Magrez, A.; Pithan, C. Chem. Phys. Lett. 2004, 391, 288. (5) Shiratori, Y.; Magrez, A.; Dornseiffer, J.; Haegel, F.-H.; Pithan, C.; Waser, R. J. Phys. Chem. B 2005, 109, 20122. (6) Shiratori, Y.; Magrez, A.; Dornseiffer, J.; Haegel, F.-H.; Pithan, C.; Waser, R. J. Phys. Chem. B 2006, 110, 16801. (7) Jaffe, B.; Cook, W. R.; Jaffe, H. Piezoelectric Ceramics; Academic Press: London, 1971; p 185. (8) Reznitchenko, L. A.; Turik, A. V.; Kuznetsova, E. M.; Sakhnenko, V. P. J. Phys.: Condens. Matter 2001, 13, 3875. (9) Shiratori, Y.; Magrez, A.; Kasezawa, K.; Kato, M.; Ro¨hrig, S.; Peter, F.; Pithan, C.; Waser, R. J. Electroceram., published online (DOI: 10.1007/ s10832-007-9032-7). (10) Xu, H.; Su, Y.; Balmer, M. L.; Navrotsky, A. Chem. Mater. 2003, 15, 1872. (11) Cross, L. E.; Nicholson, B. J. Philos. Mag. 1955, 46, 453. (12) Rousseau, D. L.; Bauman, R. P.; Porto, S. P. S. J. Raman Spectrosc. 1981, 10, 253. (13) Pithan, C.; Shiratori, Y.; Dornseiffer, J.; Haegel, F.-H.; Magrez, A.; Waser, R. J. Cryst. Growth 2005, 280, 191. (14) Wang, X. B.; Shen, Z. X.; Hu, Z. P.; Qin, L.; Tang, S. H.; Kuok, M. H. J. Mol. Struct. 1996, 385, 1. (15) 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. (16) Shen, Z. X.; Wang, X. B.; Kuok, M. H.; Tang, S. H. J. Raman Spectrosc. 1998, 29, 379. (17) Shiratori, Y.; Magrez, A.; Kato, M.; Kasezawa, K.; Pithan, C.; Waser, R. Manuscript in preparation. (18) 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.

Temperature-induced Phase Transitions in Micro-, Submicro-, and

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