Chem. Mater. 2007, 19, 3575-3580
3575
Structural Singularities in Ferroelectric Sr2NaNb5O15 E. Garcı´a-Gonza´lez,*,† A. Torres-Pardo,† R. Jime´nez,‡ and J. M. Gonza´lez-Calbet† Departamento de Quı´mica Inorga´ nica, Facultad de Quı´micas, UniVersidad Complutense, Madrid 28040, Spain, and Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, Madrid 28049, Spain ReceiVed May 14, 2007. ReVised Manuscript ReceiVed May 16, 2007
The structural features of the ferroelectric compound Sr2NaNb5O15 have been carefully studied. A detailed investigation has been performed by means of room temperature (RT) and high temperature (HT) powder X-ray diffraction (XRD), selected area electron diffraction (SAED), high resolution electron microscopy (HREM), and impedance spectroscopy. Comparison is done with the barium homologue compound and the two RT ferroelectric phases can be structurally distinguished. The use of electron diffraction in combination with HREM has revealed the formation of a superlattice structure of tetragonal tungsten bronze (TTB) at RT. While in Ba2NaNb5O15 the incommensurate nature of this RT phase seems to be doubtless, the Sr2NaNb5O15 ferroelectric phase displays a commensurate superstructure of the TTB structure-type. The RT XRD pattern can be indexed in the Im2a space group. The transition to the paraelectric phase occurs at 518 K, and the material can be indexed in the centrosymmetric P4/mbm space group. From the Rietveld refinement of this HT phase, disordered distribution of Na and Sr in the A positions of the structure seems to occur, as opposite to the barium compound where barium atoms show clear preference for the largest A2 sites.
Introduction Tetragonal tungsten bronze (TTB)-type oxides compose a large and extensively studied family of materials. The term “tetragonal tungsten bronze” originally utilized for the nonstoichiometric compound KxWO3 (x ) 0.4-0.6)1 was further extended to all the compounds showing a similar AxBO3 structure. The tungsten bronze skeleton can be regarded as being a derivative of the perovskite in which the BO3 octahedral frame is transformed to create three different types of cavities, square, pentagonal, and triangular tunnels, available for cation inclusion.2 A1, A2, and C are the notations of the different sites in the crystal structure (see Figure 1). A wide variety of cation substitution is possible in the TTB structure. By substituting tungsten for other high valence transition metals, the term groups a large number of functional materials possessing electrooptic, ferroelectric, pyroelectric and piezoelectric properties. The size and type of substituted ions on the different available sites and the amount of disorder have a significant effect on the dielectric properties of these materials.3 From a dielectric point of view, ferroelectric compounds are divided into two groups: the classical and the relaxor ferroelectrics.4 In the TTB structure, the coexistence of cations is favorable both at A and at B sites, and disorder in cationic repartition is associated to the * Corresponding author. Tel.: +34-913944518. Fax: +34-913944352. Email:
[email protected]. † Universidad Complutense. ‡ Instituto de Ciencia de Materiales de Madrid.
(1) Magne´li, A. Ark. Kemi 1949, 1, 269. (2) Hyde, G.; O’Keeffe, M. Acta. Crystallogr. 1973, A29, 243. (3) Simon, A.; Ravez, J. C. R. Chim. 2006, 9, 1268 (see also references therein). (4) Samara, G. A. J. Phys.: Condens. Matter 2003, 15, R367.
Figure 1. Schematic representation of the structural skeleton of the TTBtype structure projected on the ab plane. Square, pentagonal, and triangular tunnels available for cation inclusion can be observed. The unit cell of both the basic tetragonal substructure as well as the orthorhombic one have been outlined (dotted and solid lines, respectively).
relaxor behavior.5 Among the TTB type oxides, the ferroelectric quaternary niobates are of particular relevance. The solid solution system (Sr/Ba)5Nb10O306,7 is one of the most widely investigated where barium and strontium atoms occupy only five of the six A1 and A2 positions and the C sites remain empty. Monovalent cations can also be substituted leading to the so-called “filled TTB” where all A positions are occupied. This is the case of Ba2NaNb5O158 and Sr2KNb5O15.9 The barium strontium sodium niobate system (Ba1-xSrx)2NaNb5O15 (BSNN) has also attracted much attention as a (5) Hornebecq, V.; Elissalde, C.; Weill, F.; Villesuzanne, A.; Menetrier, M.; Ravez, J. J. Appl. Crystallogr. 2000, 33, 1037. (6) Jamieson, P. B.; Abrahams, S. C.; Bernstein, J. L. J. Chem. Phys. 1968, 48, 5048. (7) Fang, T.-T.; Chen, E.; Lee, W. J. J. Eur. Ceram. Soc. 2000, 20, 527.
10.1021/cm071303w CCC: $37.00 © 2007 American Chemical Society Published on Web 06/19/2007
3576 Chem. Mater., Vol. 19, No. 14, 2007
system with large electrooptic effect and second harmonic generation. Both the ferroelectric properties and the structural aspects of Ba2NaNb5O15 (BNN), which is one of the end members in the BSNN system, have been studied extensively using different diffraction and imaging techniques.8,10-13 The other end member of the system, Sr2NaNb5O15 (SNN), was initially described14-16 as an orthorhombically distorted TTB structure (cell parameters x2aTTB × x2aTTB × cTTB), and in spite of a later report17 which described it as adopting the basic tetragonal cell, it is currently accepted. Because the study of the strontium compound has received less attention, especially in which structural aspects refer to it, and given the technological relevance of the system, we have carried out a detailed structural study of this composition by means of room temperature (RT) and high temperature (HT) powder X-ray diffraction (XRD), selected area electron diffraction (SAED), and high-resolution electron microscopy (HREM) and its dielectric characterization. Comparison is done with the barium homologue compound, and the two RT ferroelectric phases can be structurally distinguished. Experimental Section High purity SrCO3 (99.9%, Merck), Na2CO3 (99.5%, Merck), and Nb2O5 (99.99%, Aldrich) were used for the preparation of polycrystalline Sr2NaNb5O15 by the conventional ceramic technique. Stoichiometric amounts of the starting materials were mixed in an agate mortar and heated in platinum crucibles at 1173 K for 24 h. The resulting powders were reground in an agate mortar, pelleted, heated in air at 1523 K for 2 days, and finally quenched to RT. RT powder XRD patterns were collected on a Panalytical X’PERT PRO ALPHA 1 diffractometer with a Ge(111) primary beam monochromator prealigned for Cu KR1 radiation and provided with a X’Celerator fast detector. HT powder XRD data at air were carried out on a Panalytical X’PERT PRO θ θ-diffractometer supplied with a X’Celerator fast detector and Ni β-filter with Cu KR radiation. Effective step time of 500 s and a step size of 0.017 and 0.033 (°/2θ) were used to record data for Rietveld analysis18 at RT and HT, respectively. Samples for transmission electron microscopy were ultrasonically dispersed in n-butanol and transferred to carbon coated copper grids. SAED experiments were carried out on a PHILIPS CM20FEG SuperTwin electron microscope. HREM was performed on a JEOL JEM300FEG electron microscope. Simulated images were calculated by the multislice method using the MacTempas software package. Crystal by crystal chemical composition was determined (8) Jamieson, P. B.; Abrahams, S. C.; Bernstein, J. L. J. Chem. Phys. 1969, 50, 4352. (9) Lanfredi, S.; Cardoso, C. X.; Nobre, M. A. L. Mater. Sci. Eng. B 2004, 112, 139. (10) Toledano, J. C.; Schneck, J. Solid State Commun. 1975, 16, 1101. (11) Lin, J. P.; Bursill, L. A. Acta Crystallogr. 1987, B43, 49. (12) Labbe, P.; Leligny, H.; Raveau, B.; Schneck, J.; Toledano, J. C. J. Phys.: Condens. Matter 1990, 2, 25. (13) Van Tendeloo, G.; Amelinckx, S.; Manolikas, C.; Shulin, W. Phys. Status Solidi A 1985, 91, 483. (14) Van Uitert, L. G.; Levinstein, H. J.; Rubin, J. J.; Capio, C. D.; Dearborn, E. F.; Bonner, W. A. Mater. Res. Bull. 1968, 3, 47. (15) Giess, E. A.; Scott, B. A.; Burns, G.; O’Kane, D. F.; Segmu¨ller, A. J. Am. Ceram. Soc. 1969, 52 (5), 276. (16) Ravez, J.; Budin, J.-P.; Hagenmuller, P. J. Solid State Chem. 1972, 5, 239. (17) Morin, D.; Colin, J.-P.; Le, Roux, G.; Pateau, L.; Toledano, J. C. Mater. Res. Bull. 1973, 8, 1089. (18) Rodrı´guez-Carvajal, J.; Roisnel, T. FullProf, WinPLOTR, and accompanying programs. http://www-llb.cea.fr/fullweb/powder.htmf (1999).
Garcı´a-Gonza´ lez et al. by energy-dispersive spectrometry (EDS) X-ray microanalysis carried out on both a PHILIPS CM20 FEG Super Twin electron microscope supplied with an EDAX analyzer DX4 (super-ultrathin window (resolution ≈ 135 eV)) and a JEOL JEM300FEG microscope equipped with an ISIS 300 X-ray microanalysis system (Oxford Instruments) with a LINK “Pentafet” EDS detector. From this analysis, crystals showed a constant composition with a Nb:Sr ratio in agreement with the nominal composition. Sodium was present in all the crystals, but the content was not quantified by this technique. The average cationic composition of the sample, Sr1.97Na1.03Nb5O15, was determined on a JEOL 8900 “Super Probe” electron probe microanalyzer with five wavelength-dispersive spectrometers operating at 20 kV and 50 µA. Dielectric properties were measured on a HP 4284A impedance analyzer. Measurements were carried out as a function of temperature (80-750 K) at selected frequencies between 102 and 106 Hz on disk-shaped specimens of 13 mm diameter and 0.9 mm thickness sintered in air at 1473 K for 24 h (density > 95%). Gold paste (Dupont QG 150) sintered at 1123 K was used as an electrode.
Results and Discussion The structural references provided in the literature for Sr2NaNb5O15 single crystals14-16 describe this crystal phase as an orthorhombic cell with parameters a ≈ b ≈ 1.75 nm and c ≈ 0.389 nm (x2aTTB × x2aTTB × cTTB) similar to that of Ba2NaNb5O15.8 Figure 2 shows the low-angle region of the RT powder XRD pattern corresponding to the sample of nominal composition Sr2NaNb5O15. Even though at first glance the main reflections could be indexed on the basis of the orthorhombic cell described above (space group No. 35, Cmm2),14-16 several significant diffraction maxima of low intensity cannot be assigned to such a unit cell. A minor impurity (about 3%) was observed which was assigned to the perovskite-type oxide NaNbO3 (JCPDS 01-089-6652), but the presence of a certain amount of strontium in this perovskite-type phase cannot be ruled out provided the existence of Na1-xSrx/2NbO3 solid solution.17 The use of electron diffraction in combination with HREM revealed the barium homologue as forming a superlattice structure of TTB at RT.11,13 In BNN, the incommensurate nature of this RT orthorhombic phase seems to be doubtless, and it appears in combination with ferroelastic properties.19 Correlation between incommensurate modulation and macroscopic polarization in the barium compound led us to search for deviations from the average TTB structure in the strontium crystal phase. In this sense, the microstructure of the sample was further analyzed by transmission electron microscopy. Figure 3a corresponds to the SAED pattern along the [001]ORT zone axis. Besides the main reflections of the TTB substructure, weaker diffraction maxima located at (h/2 k/2 0)TTB can be observed, indicating an orthorhombic distortion of the TTB basic unit cell with parameters a ≈ b ≈ x2×aTTB (Miller indices in all figures refer to the ∼x2aTTB × ∼x2aTTB × ∼cTTB orthorhombic cell). The corresponding HREM micrograph is shown in Figure 4. The contrast of the black and white dots resembles very well the features of the TTB basic unit cell, and the three types of tunnels building up (19) Schneck, J.; Denoyer, F. Phys. ReV. B 1981, 23, 383.
Structural Singularities in Sr2NaNb5O15
Figure 2. (a) RT powder XRD pattern of Sr2NaNb5O15 in the low-angle region. Asterisk (*) shows the most intense reflection of the impurity phase. (b and c) Enlarged regions 1 and 2 including the profile fit to the 1.7 nm × 1.7 nm × 0.4 nm unit cell (space group Cmm2). (d and e) Enlarged regions 1 and 2 including the profile fit to the 3.5 nm × 3.5 nm × 0.8 nm unit cell (space group Im2a). Parts b and c show extra diffraction maxima which do not belong to the Cmm2 unit cell. Note in parts d and e the presence of several well-defined peaks with at least one of the h, k, or l Miller indices being an odd number. These reflections confirm the doubling of the three a, b, and c cell parameters in respect to the Cmm2 unit cell.
the structure can be clearly observed. By tilting around the orthorhombic axes, corresponding SAED patterns along [100]ORT and [101h]ORT zone axes are obtained, showing extra diffraction maxima located at (0 k/2 l/2) and (h/2 k l/2), respectively (Figure 3b,c). These results indicate the formation of unit cell of parameters a ≈ 3.5 nm, b ≈ 3.5 nm, and c ≈ 0.79 nm (i.e., ∼2x2aTTB × ∼2x2 × aTTB × ∼2cTTB) for the phase under study. The same lattice periodicity was previously described for the ferroelectric material Ba2NaNb5O15,11,13 where the use of electron microscopy to detect weak superlattice reflections allowed a space group to be assigned for the RT form. In this case, the superstructure consists of four orthorhombic unit cells of approximate dimensions 1.7 nm × 1.7 nm × 0.4 nm one displaced in respect to the other by a vector 1/2 [001] along both [100]ORT and [010]ORT directions.11 This ordered displacement takes place only at low temperature in Ba2NaNb5O1520 and it gives rise to a periodic antiphase boundary each 1.7 nm which originates the doubling of a and b orthorhombic axes as well as the c-axis. At RT,
Chem. Mater., Vol. 19, No. 14, 2007 3577
however, the structure shows incommensurability which has been attributed to the irregular arrangement of the antiphase boundaries.19,21,22 In this sense, the extra diffraction maxima rows observed for Sr2NaNb5O15 at (0 k/2 l/2) and (h/2 k l/2) (Figures 3b and 3c, respectively), show equally spaced spots indicating the formation of a commensurate superstructure of TTB. Figure 5 shows the high-resolution image of a crystal of Sr2NaNb5O15 projected along the [100]ORT direction. The doubling of the c-axis in respect to the c-parameter of the tetragonal cell can be clearly seen because the periodicity of the crystallographic planes along [001] is 0.79 nm. The basic measured distance along the [010]ORT direction is 0.89 nm which corresponds to the d110 spacing of the basic tetragonal structure; however, multiple of this periodicity can be observed as crystal thickness grows, and the periodicity of 3.5 nm is easily identified. From the observation of the black and white contrast inside the unit cell, the ordered displacement of the 1.7 nm × 0.4 nm blocks by a vector 1/2 [001] is clearly seen as the origin of the new cell multiplicity. Therefore, by considering the structural model described for the homologous Ba2NaNb5O15, periodic antiphase boundaries would be responsible for the periodicity of 3.5 nm in Sr2NaNb5O15 showing, in this case, a commensurate phase at RT. The diffraction conditions observed by tilting a series of electron diffraction patterns about the a* and b* axes are compatible with both centrosymmetric Imma (space group No. 74) and the noncentrosymmetric Im2a (space group No. 46) space groups. On the basis of the previously published results on the ferroelectric behavior of this material,14,23 we have performed the dielectric characterization on Sr2NaNb5O15. Figure 6 shows the temperature dependence of the real part of the relative dielectric constant (K′) measured at 10 kHz. The obtained profile, which is in agreement with the measurements previously reported, displays a sharp maximum of K′ at Tc ) 518 K without relaxation on frequency increasing (see inset in Figure 6). From the shape of the maximum, the temperature dependence of K′ obeys a Curie-Weiss law above Tc as shown by the linear 1/K′ versus temperature response in Figure 8. The Curie-Weiss constant value (∼1 × 105 K) and the positive value (470 K) of the extrapolation of the linear fit to 1/K′ ) 0 indicate a ferroelectric-paraelectric displacive phase transition. Therefore, the ferroelectric behavior of SNN led us to describe its structure at RT in the noncentrosymmetric Im2a space group as the unit cell ∼2x2aTTB × ∼2x2aTTB × ∼2cTTB. Such a unit cell was used to fit the profile of the RT XRD data (Figure 2a), and all diffraction maxima which could not be assigned with the Cmm2 space group (parameters ∼x2×aTTB × ∼x2×aTTB × ∼cTTB) nicely matched with the reflections of a unit cell of parameters a ) 3.49812(1) nm, b ) 3.49294(1) nm, and c ) 0.779042(2) nm (profile matching of selected 2θ ranges to both unit cells can be compared in Figure 2b-d). (20) Verwerft, M.; Van, Tendeloo, G.; Van Landuyt, J.; Amelinckx, S. Ferroelectrics 1988, 88, 27. (21) Manolikas, C. Phys. Status Solidi A 1981, 68, 653. (22) Barre, S.; Mutka, H.; Roucau, C. Phys. ReV. B 1988, 38, 9113. (23) Kimura, M.; Minamikawa, T.; Ando, A.; Sakabe, Y. Jpn. J. Appl. Phys. 1997, 36, 6051.
3578 Chem. Mater., Vol. 19, No. 14, 2007
Garcı´a-Gonza´ lez et al.
Figure 3. SAED patterns of Sr2NaNb5O15 taken along (a) [001]ORT, (b) [100]ORT, and (c) [101h]ORT zone axes. (Miller indices and zone axes directions refer to the orthorhombically distorted cell.)
Figure 4. HREM micrograph of a crystal of Sr2NaNb5O15 taken along the [001]ORT zone axis. The corresponding Fourier transform is shown in the inset.
Approximate atomic coordinates derived from the asymmetric unit of the above-described cell and reasonable positions for the oxygen atoms found by calculation of interatomic distances were used as input data for simulation of high-resolution images. As can be observed in Figure 5, a nice fit between calculated and experimental images is obtained. It is important to mention that the asymmetric unit of the above structural model contains 102 atoms from which 66 are oxygen atoms. In this sense and provided the large size of the asymmetric unit, the refinement of the atomic positions from the XRD data was not performed. Dielectric measurements performed in Sr2NaNb5O15 showed that the material undergoes a single transition above RT (T ) 518 K) as observed in Figure 6. HT XRD patterns were collected to identify the HT phase. Initial measurements were done by collecting data from 300 to 1520 K at intervals of 50 K trying to observe all possible structural changes in the material. Because no changes were observed above 620 K, data for structural refinement were collected from 300 to 720 K at intervals of 10 K. Figure 7a shows the sequence of patterns recorded as temperature increases. The structural refinement by the Rietveld method was performed on the XRD data collected at 720 K. The
Figure 5. HREM micrograph of a crystal of Sr2NaNb5O15 taken along the [100]ORT zone axis. The corresponding Fourier transform is shown in the inset. The calculated image has been included (defocus ) 30 nm, thickness ) 10 nm).
Structural Singularities in Sr2NaNb5O15
Chem. Mater., Vol. 19, No. 14, 2007 3579
Figure 8. Plot of 1/K′ as a function of temperature measured at 100 kHz and 1 MHz. The Curie-Weiss law fit is obtained for the Sr2NaNb5O15 material.
Figure 6. Temperature dependence of K′ in Sr2NaNb5O15. Inset shows a zoom on the temperature range 300-700 K.
Figure 9. Powder XRD pattern of Sr2NaNb5O15 at 720 K. Experimental, calculated, and difference XRD profiles are shown. Asterisk (/) in the inset shows the most intense reflection of the impurity phase. Table 1. Crystallographic Data for Sr2NaNb5O15 at 720 K
Figure 7. (a) Temperature-dependent powder diffraction data for Sr2NaNb5O15 in the isoline plot. (b, c, and d) Enlargement of three different zones in three-dimensional representation (from 300 K back to 720 K front).
experimental, calculated, and difference powder XRD profiles are shown in Figure 9. Fractional atomic coordinates belonging to the centrosymmetric P4/mbm space group (127) were used as input data, by location of atoms in the atomic positions of a unit cell of periodicity 1.2 nm × 1.2 nm × 0.4 nm like the one previously described for Ba2NaNb5O1524 and corresponding to the basic TTB structure. The ordered disposition of sodium atoms in A1 sites and strontium atoms in the A2 sites of the basic TTB skeleton was initially considered following their distribution in BNN, but it could not reproduce the profile of intensities of the diffraction pattern. In this way, sodium and strontium atoms were randomly distributed in the A1 and A2 sites of the structure. Table 1 summarizes the crystallographic parameters (24) Scott, J. F.; Hayward, S. A.; Miyake, M. J. Phys.: Condens. Matter 2005, 17, 5911.
space group a (nm) c (nm) Biso (Å2) Rp Rwp Rexp RB χ2
P4/mbm (No. 127) 1.243205(6) 0.389839(2) 0.93(4) 2.85 3.89 2.66 7.48 2.13
of Sr2NaNb5O15 at 720 K. The tendency to a disordered cationic distribution in the A1 and A2 sites can be deduced from the refined site occupancy factors (Supporting Information, Table S-1). The presence of strontium atoms in 2a positions (A1 sites), which are only occupied by sodium atoms in the barium compound, can be interpreted as the trend of sodium and strontium to be located preferably in the perovskite-type tunnels. The remaining sodium and strontium atoms occupy the largest A2-sites. Size similarity between sodium and strontium atoms25 can be the cause of the disordered distribution, as opposite to BNN, where (25) Shannon, R. D. Acta Crystallogr. 1976, A32, 751.
3580 Chem. Mater., Vol. 19, No. 14, 2007
barium atoms show clear preference for the largest A2 sites. In this sense, it would not be unreasonable to think that disorder in A1 and A2 sites is also present in the RT phase. With this fact in mind, we have simulated the RT high-resolution images by considering both ordered distribution of sodium and strontium as well as their random location, but contrast of the images for the thickness range considered does not account for the difference. In TTB-type structures, the main structural distortions responsible for the phase transitions take place on the ab plane because of the cooperative tilt and rotation of NbO6 octahedra. In Sr2NaNb5O15, the displacive transition from mm2 to 4/mmm point symmetry on heating changes the orientation of the unit cell from ∼2x2×aTTB × ∼2x2×aTTB × ∼2cTTB to aTTB × aTTB × cTTB, and, therefore, reflections of the type (hk0) are expected to be mainly influenced in comparison with (00l) or (hkl) diffraction maxima. Effectively, as can be seen in Figure 7a and enhanced in Figure 7b-d, the structural transition is better observed from 2θ values for which d-spacing of (hk0) nearly coincides with (00l) and (h′k′l) reflections (Figure 7b-d, where h, k, and l indices referred to the RT phase). In summary, detailed structural investigations have been performed in Sr2NaNb5O15 in comparison with its homologue Ba2NaNb5O15. RT ferroelectric phases of both compounds can be structurally distinguished. Electron microscopy data in combination with the electric characterization performed
Garcı´a-Gonza´ lez et al.
has shown that the strontium compound displays a commensurate superlattice of the TTB structure type as opposite to the incommensurate crystal phase of BNN. The paraelectric HT phase is similar in both compounds, although a trend to a disordered location of sodium and strontium in the A1 and A2 sites seems to occur in Sr2NaNb5O15, as deduced from the structural refinement performed. Further studies are in progress to investigate the structural transition occurring at low temperature to establish the parallelism with the structural behavior of the barium compound. Acknowledgment. Financial support from DGICYT in Spain through Project Nos. MAT2004-01248 and MAT2004-02014 is gratefully acknowledged. Authors are grateful to the Centro de Difraccio´n de Rayos X (U.C.M.) and to the Centro de Microscopı´a Electro´nica Luis Bru (U.C.M.) for facilities, to Dr. Emilio Matesanz for helpful assistance, and to Dr. U. Amador for fruitful discussions. Supporting Information Available: Crystallographic data in CIF format for Sr2NaNb5O15 and summarizing Table S-1 showing final atomic positions and site occupancy factor at 720 K (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM071303W
