Structural and Dielectric Properties of SnTiO3, a Putative Ferroelectric

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Structural and Dielectric Properties of SnTiO3, a Putative Ferroelectric Thomas Fix,*,† S.-Lata Sahonta,† Vincent Garcia,‡ Judith L. MacManus-Driscoll,† and Mark G. Blamire† † ‡

Department of Materials Science, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, United Kingdom Unit
e Mixte de Physique CNRS/Thales, 1 Av. A. Fresnel, Campus de l’Ecole Polytechnique, 91767 Palaiseau, France, and Universit
e Paris-Sud, 91405 Orsay, France ABSTRACT: Perovskite SnTiO3 has been predicted to be ferroelectric with properties comparable to BaTiO3. Here we report on the experimental discovery of a new SnTiO3 phase, in the form of thin films grown on various substrates by pulsed laser deposition. They exhibit an unexpected epitaxial relationship, different from the predicted perovskite structure. X-ray diffraction and transmission electron microscopy experiments show that the films grown have an ilmenite structure. The films are insulating but do not display any ferroelectricity at room temperature; however, some films contain a small second phase component consistent with the predicted perovskite phase.

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ost materials with large electric polarization are based on the perovskite structure ABO3, such as BaTiO3 and PbTiO3 (PTO). Environmental considerations are driving research to discover new materials which are lead- and bismuthfree, yet without reduced ferroelectric performance. In the case of PTO, attempts to replace Pb2þ by Ca2þ have the effect of reducing the tetragonality of the material and limiting the ferroelectricity.14 A quick look at the periodic table would suggest that replacing Pb2þ by isoelectronic Sn2þ would give SnTiO3 (SNO) with the perovskite structure. However, only a few compounds containing Sn2þ have been reported so far,57 because Sn4þ is generally more stable. The perovskite SNO structure has been calculated by first principles within density functional theory (DFT) in three theoretical studies.810 All of them predict ferroelectricity with a polarization at least as high as PTO. So far attempts to synthesize SNO have not been successful,8,9 which could suggest that it is a metastable state. The Goldsmith tolerance factor for perovskites is defined as √ t ¼ ðR A þ R O Þ= 2ðR B þ R O Þ where RA, RB, and RO are the ionic radii of A, B, and O respectively, and an accepted range for stability of the perovskite structure is 0.98 < t < 1.02. However, experimentally stable perovskite systems span a much wider range of 0.8 < t < 1.4. For example, tPTO = 1.02 and tBTO= 1.05. The value for perovskite SNO (tSNO = 0.92) does not seem unrealistic in this context.11 Various methods have been suggested to further improve the prospects for stabilizing perovskite SNO; for example, some of the Ba atoms in BTO are replaced by Sn, leading to an increase of the tetragonal-to-cubic phase transition temperature,12 which seems promising for the SNO phase. In this article, we have deliberately avoided the bulk synthesis of pure SNO and only focused on the growth of SNO thin films since there is a long history of stabilizing metastable phases through strain or kinetic effects.13,14 We describe the structure r 2011 American Chemical Society

and properties of a previously unreported material, SNO, but with ilmenite rather than the desired perovskite structure, although a second phase is present which is consistent with the predicted perovskite structure. Films were grown by pulsed laser deposition (PLD) using a KrF laser with a 248 nm wavelength, 10 Hz repetition rate, and a fluence of around 1 J/cm2 on the target at a substrate-target distance of 80 mm. A commercial SrRuO3 (SRO) target was used and SnTiO3 (SNO) - composition targets were made from a stoichiometric mixture of high purity (99.99%) SnO2 and TiO2 powders by milling, presintering at 900 °C for 6 h, pressing and sintering at 900 °C for 6 h. Starting from a base pressure of 107 mbar LaAlO3 (001) (LAO), NdGaO3 (001) (NGO), SrTiO3 (001) (STO), and r-plane R-Al2O3 (ALO) substrates were heated to a temperature of 850 °C for 30 min. The substrates LAO, NGO, and STO were chosen for their chemical stability with respect to Ti and Sn, their high temperature stability, and their perovskite structure with lattice parameters close to the desired perovskite SNO phase. The use of different substrates with slightly different lattice parameters might help to obtain a better epitaxy or stabilize the desired film phase. ALO was used as a control as its crystal structure is radically different from the other substrates. For ferroelectric measurements, a 50 nm thick film of conducting SRO was grown on the substrate at 650 °C in a 102 mbar O2 atmosphere prior to SNO deposition. The SNO film was grown in the same conditions with a thickness of around 80 nm unless specified otherwise, and cooling down was in the same atmosphere as fast as possible (850 to 500 °C in around 5 min time and 500 to 200 °C in around 20 min). The orientation, crystallinity, and lattice parameters of the films were determined by in situ reflective high energy electron diffraction (RHEED), X-ray diffraction (XRD) with Bruker D8 Advance diffractometers (Cu-KR), and transmission electron Received: November 1, 2010 Published: April 12, 2011 1422

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Figure 1. XRD θ-2θ spectra of SNO films grown (a) on LAO (001), NGO (001), STO (001), ALO (1102) substrates; (b) on STO (001) at 350 °C, 650 °C, 850 °C deposition temperature.

microscopy (TEM). Cross-sectional TEM was performed by preparation with mechanical polishing and argon ion milling at 4 kV. Lattice images of the film were taken on a JEOL 4000 EX microscope with a spherical aberration coefficient of 0.9 mm and a point-to-point resolution of 0.17 nm, operating at 400 kV. High angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectrometry (EDX) were also used to study the film composition. To investigate ferroelectric properties, P-E hysteresis loops were measured using a Radiant Precision LC. Piezoresponse force microscopy (PFM) experiments were performed on two different setups including one with Digital Instruments Nanoscope IV at room temperature using CrPt tips at an excitation frequency of 47 kHz and an ac voltage of 1 V and Stanford Research SR830 external lock-in amplifiers. The SRO bottom electrode was accessible either by scratching the film or by covering a part of the electrode with a platinum sheet prior to the deposition of SNO. Conductive tip atomic force microscopy (CT-AFM) experiments were carried out with a Digital Instruments Nanoscope IV setup at room temperature in a nitrogen flow. A laboratory-made high bandwidth (5 kHz) current amplifier was used. The bias voltage between the tip and the bottom electrode was 6 V. Si tips coated with B-doped diamond were used to limit tip wear during scans with a speed of 3 Hz. Figure 1a shows XRD θ-2θ spectra of SNO films grown on LAO (001), NGO (001), STO (001), and r-plane ALO substrates. The substrate and film peaks are indicated, the remaining peaks arise from the setup (there is no monochromator). Only one family of film peaks can be observed for all the substrates,

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suggesting that only one main phase is present in the film. There is a clear trend of progressively increasing d-spacing of the main film peak from LAO to ALO, which will be explained later in the text. The predicted SNO lattice parameters (e.g., from ref 9, a = 0.38 nm, c/a = 1.09) give a mismatch factor with the STO substrate of 0.26%, suggesting an epitaxial relationship of the type [100] LAO (001) || [100] SNO (001). Thus, one would expect a d-spacing of the main peak of around 0.21 nm; however, the measured value is around 0.24 nm. Figure 1b shows XRD θ-2θ spectra of SNO films on STO (001) with a deposition temperature from 350 to 850 °C. While the temperature of 350 °C is too low for epitaxy or phase formation, an increase of the temperature from 650 to 850 °C which is the highest temperature available in the setup does not induce a new phase. The full width at half-maximum of the rocking curves of the main film peak is around 0.7° for the different substrates (which provide a contribution of around 0.1°), suggesting acceptable texture. Clearly, neither the obtained phase nor obtained epitaxial relationship match with the initial predictions. Therefore TEM experiments were performed on a 150 nm thick film in crosssection along NGO [100] (Figure 2a,b) and in plan-view (Figure 2c,d). The diffraction patterns suggest a hexagonal structure in cross-section (Figure 2b), strained along NGO [100] and a rectangular structure in plan-view (Figure 2d). The deviation of diffraction spots from their expected lattice positions in the film reflections indicate small misorientations in adjacent crystallites in the film, which is confirmed by the high resolution image (Figure 2a). The dark and light contrast is caused by electron channelling contrast, which is observed in films which comprise grains with small relative misorientations. The variations in channelling contrast for each grain is consistent with the tilted columnar grain structure visible in Figure 2a and is also consistent with the relatively diffuse diffraction spots in the electron diffraction pattern of Figure 2b), which confirms small relative misorientation of individual grains. Inset in Figure 2a) is an image of the atomic columns of the film viewed along NGO [100]. HAADF STEM did not show any variation in the elemental composition of the film over a 150  200 nm2 area (not shown), and EDX studies revealed a Sn/Ti ratio of 1:1. The SNO films do not have the perovskite structure, whatever its orientation, and, to our knowledge, no previously reported phase based on the composition SnTiO3 matches the XRD, TEM, and RHEED results. We find that the phase is of the ilmenite type with the epitaxial relationship for NGO substrate being [010] NGO (001) || [0001] SNO (2240) —with tetragonal indexing for NGO and hexagonal for SNO—as shown in Figure 3a; the equivalent orientation is obtained for the other substrates. The film reflections from Figure 1 correspond thus to the (2240) reflections. ABO3-type ilmenite has an ordered corundum structure, where each AO6 octahedron layer is inserted between two BO6 octahedron layers. The lattice parameters obtained from TEM diffraction patterns are in triclinic lattice with R = 90°, β = 90°, γ = 120°, a = 0.504 nm, b = 0.519 nm, c = 1.456 nm ( 0.005 (this is not the NGO indexing system). The nonrhombohedral character can be attributed to distortions. Our results are consistent with the study of the formability of 197 perovskite compounds,15 defining an rArB structural map in which SNO would just be at the borderline of the perovskite stability line. As well, calculations in ref 16 indicate that the ilmenite structure is more stable than the perovskite one for this material. In Figure 1a, we show SNO films grown on substrates 1423

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Figure 2. (a) High resolution cross-sectional TEM image; (b) diffraction pattern with NGO [010] zone axis of a 150 nm thick SNO film (hexagonal indexing) grown on NGO (001); (c) plan-view TEM image; (d) plan-view diffraction pattern of a 150 nm thick SNO film (hexagonal indexing) grown on NGO (001).

Figure 3. (a) Sketch showing the epitaxial relationship between SNO and the NGO (001) substrate; (b) evolution of SNO films d-spacing measured in Figure 1 (dfilm) for strained (∼80 nm thick) and fully relaxed films (∼200 nm thick), and in-plane lattice parameters (along [100] and [010] NGO directions) assuming a fully strained growth on LAO (001), NGO (001), STO (001), and ALO (1102) substrates.

with different in plane lattice parameters in order to induce the ilmenite-perovskite transition, which does not seem to occur. Growth of the film at higher temperatures as in Figure 1b is also insufficient to promote the ilmenite-perovskite transition. Figure 3b shows the evolution of the d-spacing of the SNO films obtained from Figure 1, and their in-plane lattice parameters assuming a fully strained growth for the different substrates — which is reasonable according to TEM and experiments varying film thickness. d-spacings are shown for 200 nm thick fully relaxed films and 80 nm fully strained or so films. Most

Figure 4. Reciprocal XRD space maps around the (103) and (013) STO reflections of SNO grown on STO (001).

strikingly, the fully strained d-spacing increases with the in-plane parameters, which implies an apparent negative Poisson ratio. This is not surprising considering the auxetic structure17 of the 1424

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Figure 5. Topographical (a) and local resistance (b) images of the surface of SNO/SRO//NGO by atomic force microscopy at room temperature.

Figure 6. P-E hysteresis loop at 50 K on SNO (200 nm)/SRO//NGO.

film described in Figure 3a. Excluding the ALO data the Poisson ratio is estimated to be around 0.52 by taking the film lattice parameter on LAO substrate as a reference. However, we cannot exclude that strain-induced defects could explain the apparent negative Poisson ratio. Figure 4 shows XRD reciprocal space maps (RSMs) of a film around the (103) and (013) STO substrate reflections. Weak film reflections can be observed which are not consistent with the deduced ilmenite structure of the main phase; the (103) and (104) film reflection family. They are only present in [100] or [010] but not [110] STO orientation, implying a cuboid structure. The inferred lattice parameters of the phase with reference to the substrate are a = b = 0.376 nm and c = 0.418 nm ( 0.005. The epitaxial relationship is [100] STO (001) // [100] phase (001). This matches quite well with ref 9 and suggests that the secondary phase is the desired SNO perovskite phase. The intensities of the peaks are very low; hence corresponding diffractions were observed neither in the θ-2θ spectrum of Figure 1 nor by TEM, suggesting that the perovskite phase is only present in very small quantities. The SNO films grown were insulating when measuring either from two contacts on the top of the film or when the second contact was connected to the bottom SRO electrode. The measured topographic image (Figure 5) obtained by AFM on SNO/SRO//NGO shows a rough SNO surface with root-meansquare roughness over 3 μm2 of around 2.8 nm. The local resistance image of the surface shows that the resistance is over the 1012 range (no current can be detected) with no hot spots. However, no ferroelectric switching or contrast after poling

could be observed in PFM, nor did LC meter measurements reveal ferroelectric loops, even in fully relaxed films. Therefore, no ferroelectricity of the SNO film has been observed at room temperature using NGO, STO, and LAO substrates. Even at low temperature (50 K) P-E hysteresis loops did not provide sign of ferroelectricity but rather paraelectricity as shown in Figure 6. This is consistent with the fact that CdTiO3 for example is wellknown to crystallize into a hexagonal ilmenite structure below 1000 °C and into an orthorhombic distorted perovskite above 1050 °C; the perovskite phase is ferroelectric and the ilmenite phase is not.18 On the other hand, LiNbO3 is ferroelectric with an ilmenite structure;19 thus, further clarification is required.20 In the case of ilmenite SNO, theoretical calculations would be needed to understand if ferroelectricity is expected and if it is not presently observed due to inappropriate orientation of the films or distorsions caused by vacancies or low quality epitaxial growth for instance. Indeed, there are examples where strain or epitaxial relationship has a huge impact on the ferroelectric properties of the thin film.21,22 However, it is unknown yet if the perovskite phase can be made stable or metastable in substantial amounts at room temperature as is the case for several perovskite systems. In conclusion, we have described a new thin film material with composition SnTiO3 and predominantly ilmenite-type structure. While theoretical calculations predict ferroelectricity in the perovskite phase, no ferroelectricity could be observed in the films suggesting either detrimental distortions or orientation or rather the nonferroelectric nature of ilmenite-SNO. Neverthless, there is some evidence that the perovskite phase is present in very small quantities. The fact that we can reproducibly obtain a stable SNO structure provides encouragement for the approach of thin film stabilization, and the presence of a second phase which may be the desired perovskite structure should encourage further work on the ilmeniteperovskite phase transition which might lead to a new efficient green ferroelectric material.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We are grateful to Mary Vickers for fruitful discussions on XRD. Financial support for this work was provided by the EPSRC and the Leverhulme Trust. 1425

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’ REFERENCES (1) Ricardo de L
azaro, S.; Rodrigues de Lucena, P.; Ricardo Sambrano, J.; S
ergio Pizani, P.; Beltr
an, A.; Arana Varela, J.; Longo, E. Phys. Rev. B 2007, 75, 144111. (2) Damjanovic, D.; Gururaja, T. R.; Cross, L. E. Am. Ceram. Soc. Bull. 1987, 66, 699. (3) Jim
enez, B.; Mendiola, J.; Alemany, C.; Del Olmo, L.; Pardo, L.; Maurer, E.; Calzada, M. L.; De Frutos, J.; Gonzalez, A. M.; Fandi~ nto, M. C. Ferroelectrics 1988, 87, 97. (4) Bretos, I.; Ricote, J.; Jim
enez, R.; Mendiola, J.; Jim
enez Riob
oo, R. J.; Calzada, M. L. J. Eur. Ceram. Soc. 2005, 25, 2325. (5) Jeitschko, W.; Sleight, A. W. Acta Crystallogr. B 1974, 30, 2088. (6) Mizoguchi, H.; Wattiaux, A.; Kykyneshi, R.; Tate, J.; Sleight, A. W.; Subramanian, M. A. Mater. Res. Bull. 2008, 43, 1943. (7) Ercit, T. S.; Cerny, P. Can. Mineral. 1988, 26, 899. (8) Konishi, Y.; Ohsawa, M.; Yonezawa, Y.; Tanimura, Y.; Chikyow, T.; Wakisaka, T.; Koinuma, H.; Miyamoto, A.; Kubo, M.; Sasata, K. Mater. Res. Soc. Symp. Proc. 2003, 748, U3.13.1. (9) Matar, S. F.; Baraille, I.; Subramanian, M. A. Chem. Phys. 2009, 355, 43. (10) Uratani, Y.; Shishidou, T.; Oguchi, T. Jpn. J. Appl. Phys. 2008, 47, 7735. (11) Shannon, R. D.; Prewitt, C. T. Acta Crystallogr. B 1969, 25, 925. (12) Suzuki, S.; Takeda, T.; Ando, A.; Takagi, H. Appl. Phys. Lett. 2010, 96, 132903. (13) Kawai, M.; Matsumoto, K.; Ichikawa, N.; Mizumaki, M.; Sakata, O.; Kawamura, N.; Kimura, S.; Shimakawa, Y. Cryst. Growth Des. 2010, 10, 2044. (14) Shimakawa, Y.; Inoue, S.; Haruta, M.; Kawai, M.; Matsumoto, K.; Sakaiguchi, A.; Ichikawa, N.; Isoda, S.; Kurata, H. Cryst. Growth Des. 2010, 10, 4713. (15) Li, C.; Kwan Soh, K. C.; Wu, P. J. All. Comp. 2004, 372, 40. (16) Hautier, G; Fischer, C. C.; Jain, A.; Mueller, T.; Ceder, G. Chem. Mater. 2010, 22, 3762. (17) Lakes, R. Science 1987, 235, 1038. (18) Kabirov, Y. V.; Kulbuzhev, B. S.; Kupriyanov, M. F. J. Struct. Chem. 2001, 42, 815. (19) Mattias, B. T.; Remeika, J. R. Phys. Rev. 1949, 76, 1886. (20) Wang, H.; Zhang, X.; Huang, A.; Xu, H.; Zhu, M.; Wang, B.; Yan, H.; Yoshimura, M. J. Cryst. Growth 2002, 246, 150. (21) Lin, Y.; Chen, X.; Liu, S. W.; Chen, C. L.; Jang-Sik Lee; Li, Y.; Jia, Q. X.; Bhalla, A. Appl. Phys. Lett. 2004, 84, 577. (22) Lin, Y.; Chen, X.; Liu, S. W.; Chen, C. L.; Jang-Sik Lee; Li, Y.; Jia, Q. X.; Bhalla, A. Appl. Phys. Lett. 2005, 86, 142902.

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