TEM Study of Two-Dimensional Incommensurate Modulationin

The best approximation of the incommensurate superstructure is by a commensurate lattice ax' = axt + 2ayt; ay' = −2axt + ayt; c' = ct (axt, ayt, ct:...
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TEM Study of Two-Dimensional Incommensurate Modulationin Layered La2-2xCa1+2xMn2O7 (0.6 < x < 0.8) Leonid A. Bendersky,*,† Ian D. Fawcett,‡ and Martha Greenblatt‡ Metallurgy Division, NIST, 100 Bureau Drive, Stop 8554, Gaithersburg, Maryland 20899-8554, and Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854 Received January 12, 2004. Revised Manuscript Received June 10, 2004

Ruddlesden-Popper n ) 2 compounds La2-2xCa1+2xMn2O7 with 0.6 e x e 0.8 were synthesized by a citrate gel technique and studied by transmission electron microscopy. Electron diffraction identified the presence of eight satellite reflections around fundamental spots of the basic tetragonal I4/mmm structure. The satellite reflections belong to two sets of two-dimensional incommensurate modulation, and the sets are rotated with respect to each other around [001] by ∼29°. The modulation is sensitive to electron-beam irradiation and rapidly degrades under a focused beam. In situ heating experiments indicated the existence of possible tetragonal-to-incommensurate phase transition around 350 °C, although the presence of the modulation persists up to 500 °C. High-resolution imaging proves that the modulation in the real space is truly two-dimensional and incommensurate and not a combination of one-dimensional commensurate modulations. Such modulation has never been observed before for La-Ca-Mn-O or any other perovskite system. The best approximation of the incommensurate superstructure is by a commensurate lattice ax′ ) axt + 2ayt; ay′ ) -2axt + ayt; c′ ) ct (axt, ayt, ct: lattice vectors of the tetragonal I4/mmm; ax′, ay′, c′: lattice vectors of the proposed approximant). Such superstructure could be satisfied with a 4:1 two-dimensional ordering in a (001) plane. For the studied compositions, such 4:1 ordering is plausibly described by charge ordering between Mn4+ and Mn3+ ions. Structural models of the charge ordering between Mn4+ and Mn3+ ions are suggested.

1. Introduction Mixed-valence perovskite manganates have been a subject of intense research because of their complex magnetic and electric transport properties, and especially due to their colossal magnetoresistance (CMR) behavior.1,2 In these materials with ABO3 stoichiometry, the 12-coordinated A-sites usually have mixed occupancy with 3+ (lanthanides) and 2+ (alkaline earth) cation valences (e.g., La3+ and Sr2+). In the O3 stoichiometric phase, the mixed A-site occupancy induces a mixture of manganese Mn3+ and Mn4+ on the octahedral B-sites. The search for materials with improved CMR effect lead to the study of layered manganates, in particular, Ruddlesden-Popper (RP) structures with (AO)(ABO3)n stoichiometry. Such RP structures are comprised of perovskite-like ABO3 layers n-octahedra thick (n ) integer) separated by a rock-salt AO layer.3 The RP (n ) 2) La1+xSr2-xMn2O7 phase, in particular, has shown CMR near the combined metal-insulator and ferromagnetic (FM) Curie transition for 0.32 < x 0.5.8-10 In this study, La2-2xCa1+2xMn2O7 RP phases with 0.6 e x e 1.0 were synthesized by a citrate gel technique.9 The 0.6 e x e 1.0 range is in the electron-doped region, with a majority of Mn ions having 4+ valences. For these compounds, distinctly different magnetic susceptibility and conductivity behaviors were measured for compositions below and above x ) 0.8. In the region 0.6 e x e 0.8, the magnetic susceptibility has a pronounced inflec(4) Moritomo, Y.; Asamitsu, A.; Kuwahara, H.; Tokura, Y. Nature 1996, 380, 141. (5) Battle, P. D.; Green, M. A.; Laskey, N. S.; Millburn, J. E.; Murphy, L.; Rosseinsky, M. J.; Sullivan, S. P.; Vente, J. F. Chem. Mater. 1997, 9, 552. Sloan, J.; Battle, P. D.; Green, M. A.; Rosseinsky, M. J.; Vente, J. F. J. Solid State Chem. 1998, 138, 135. (6) Mitchell, J. F.; Argyriou, D. N.; Jorgensen J. D. In Colossal Magnetoresistive Oxides; Tokura, Y., Ed.; Gordon and Breach Science Publishers: Langhorne, PA, 2000; Chapter 2.2. (7) Asano, H.; Hayakawa, J.; Matsui, M. Appl. Phys. Lett. 1996, 68, 3638. Asano, H.; Hayakawa, J.; Matsui, M. Phys. Rev. B 1997, 56, 5395. (8) Li, J. Q.; Jin, C. Q.; Zhao, H. B. Phys. Rev. B 2001, 64, 20405R. (9) Fawcett, I. D.; Greenblatt, M.; Croft, M.; Bendersky, L. A. Phys. Rev. B 2000, 62, 6485. (10) Bendersky, L. A.; Chen, R. J.; Fawcett, I. D.; Greenblatt J. Solid State Chem. 2001, 157, 309.

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TEM Study of La2-2xCa1+2xMn2O7 (0.6 < x < 0.8)

tion at ∼280 K, presumably due to charge ordering, and antiferromagnetic (AF) order develops at lower temperatures (∼150-200 K) with quasi-two-dimensional (2D) fluctuation effects above the ordering temperature. For the 0.825 e x < 1.0 range, 2D magnetic coupling is observed, and at ∼115 K the system spontaneously orders antiferromagnetically, but with a residual FM moment (canted antiferromagnetism (CAF)). X-ray powder diffraction for all compositions yielded tetragonal I4/mmm symmetry. Structural imaging using transmission electron microscope (TEM) confirmed that the structures consist predominately of n ) 2 layers, with minor intergrowth of n ) 3 layers. Rietveld refinement on the tetragonal structure identified preferential occupancy of the smaller Ca2+ ion on the nine-coordinated A-site of the rock salt layer. Similar redistribution of A-site cations was observed for other RP compounds.5,11 The magnetic study of La2-2xCa1+2xMn2O7 compounds in the region 0.6 e x e 0.8 identified the possibility of charge/orbital (CO) ordering, which leads to insulating behavior.9 Therefore, the authors suggested that a detailed transmission electron microscopy (TEM) study is needed to identify diffraction effects related to structural distortions, which should accompany the CO ordering of Mn3+ and Mn4+ ions. Previously, CO ordering-related modulations were identified for perovskites (e.g., La1-xCaxMnO3 with x > 0.512 and Ln0.5Ca0.5MnO3 (Ln ) Pr, Nd, Sm, Eu, and Gd)13,14) and layered R-P compounds with n ) 1 (e.g., (La,Sr)2NiO415 and (Pr,Ca)2MnO416) and n ) 2 (e.g., LaSr2Mn2O717 and La2-2xCa1+2xMn2O78). In the distorted (Ln1-xCax)MnO3 perovskites (with Pnma space group), the appearance of onedimensional modulation (stripes) with k ) δ[100]*O (O, orthorhombic Pnma) was observed by TEM at temperatures below 300 K.12 The scaling parameter, δ, of the modulation vector depends on the composition as δ ) 1 - x; hence, the modulation is in general incommensurate. The charge/orbital ordering between Mn3+ and Mn4+ is nicely accounted for by the value and compositional dependence of δ. Careful synchrotron X-ray and neutron powder diffraction studies of La0.5Ca0.5MnO3 confirmed the electron diffraction results and established the Jahn-Teller distortions of the Mn3+O6 octahedra and the orientation of dz2 orbitals.18 For LaSr2Mn2O7, the one-dimensional (1D) modulation with k ) δ[110]*t (t, tetragonal) was observed below 200 K.17 In a recent paper, we reported TEM structural studies of La2-2xCa1+2xMn2O7 in the 0.825 e x < 1.0 compositional range.10 The structure was shown to be an (11) Seshadri, R.; Martin, C.; Hervieu, M.; Raveau, B. Chem. Mater. 1997, 9, 270. (12) Chen, C. H.; Cheong, S.-W. Phys. Rev. Lett. 1996, 76, 4042. Chen, C. H.; Cheong, S.-W.; Hwang, H. Y. J. Appl. Phys. 1997, 81, 4326. (13) Rao, C. N. R. J. Phys. Chem. B 2000, 104, 5877. (14) Barnabe, A.; Hervieu, M.; Martin, C.; Maignan, A.; Raveau, B. J. Appl. Phys. 1998, 84, 5506. (15) Chen, C. H.; Cheong, S.-W.; Cooper, A. S. Phys. Rev. Lett. 1993, 71, 2461. Cheong, S.-W.; Hwang, H. Y.; Chen, H.; Batlogg, B.; RuppJr. L. W.; Carter, S. A. Phys. Rev. B 1994, 49, 7088. (16) Autret, C.; Retoux, R.; Hervieu, M.; Raveau, B. Chem. Mater. 2001, 13, 4745. Ibarra, M.; Retoux, R.; Hervieu, M.; Autret, C.; Maignan, A.; Martin, C.; Raveau, B. J. Solid State Chem. 2003, 170, 361. (17) Li, J. Q.; Matsui, Y.; Kimura, K.; Tokura, Y. Phys. Rev. B 1998, 57, 1. (18) Radaelli, P. G.; Cox, D. E.; Marezio, M.; Cheong, S.-W. Phys. Rev. B 1997, 55, 3015. Radaelli, P. G.; Cox, D. E.; Capogna, L.; Cheong, S.-W.; Marezio, M. Phys. Rev. B 1999, 59, 14440.

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orthorhombic Cmc21, a superstructure of the tetragonal I4/mmm, which apparently forms by a low-temperature phase transition. The structural transition occurs by the combination of octahedral tilt in the perovskite layers. Similar structural behavior was also found recently for the end-member, Ca3Mn2O7, of the series Can+1MnnO3n+1.19,20 Here, we report a TEM study of La2-2xCa1+2xMn2O7 compounds with compositions ranging between x ) 0.6 and x ) 0.8. This is the range of compositions where the transition to a charge and orbital ordering state at ∼280 K was identified in the magnetic susceptibility measurements.9 The main observation of this study is the presence of 2D-incommensurate modulations, with two near-orthogonal modulation vectors normal to the tetragonal axis. To the best of our knowledge, such a set of modulations has not been observed for the manganates or other oxide structures. 2. Experimental Section Samples of La2-2xCa1+2xMn2O7 were prepared from La2O3, CaCO3, and Mn(NO3)2 by a citrate gel technique. Details of the synthesis are described in Fawcett et al.9 The powder was sintered into pellets at 1250 °C for 24 h and quenched to room temperature (RT) in air. Four sol-gel-prepared specimens with compositions x ) 0.6, 0.66, 0.7, and 0.8 were studied in this work. TEM specimens were prepared from sintered pellets by a conventional method involving grinding, dimpling, and ion thinning. The specimens were examined with a JEM3010 and a Phillips 430 TEM microscope operated at 300 and 200 kV, respectively. [Certain commercial equipment, instruments, or materials are identified in this paper in order to specify the experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the materials or equipment identified are necessarily the best available for the purpose.] In situ heating and cooling experiments were performed using double tilt specimen holders in the Phillips 430 microscope. The TEM specimens investigated here were polycrystalline single-phase samples; however, a small volume fraction of an intergranular amorphous phase was typically present. The presence of a large fraction of C, in addition to La, Ca, Mn, and O, in the amorphous phase was determined qualitatively from energy-dispersive spectroscopy (EDS) of the TEM specimen. The relative height of the EDS peaks of La, Ca, and Mn was similar to that of the crystalline phase. The presence of the La, Ca, Mn, O, and C amorphous phase introduces some uncertainty about the precise composition of the crystalline phase.

3. Experimental Results 3.1. Electron Diffraction. Figure 1a shows a selected area electron diffraction (SAED) pattern typically observed for the x ) 0.7 and 0.8 specimens. The pattern is indexed as [0 0 1] of the tetragonal I4/mmm RP structure, which was proposed for the La2-2xCa1+2xMn2O7 compound.9 In the pattern eight satellite reflections surround each fundamental reflection of the I4/mmm structure. For the x < 0.7 samples, there are continuous arches of diffuse intensity instead of satellite reflections, Figure 1b. Our discussion will be focused mainly on the study of phases showing sharp satellite reflections. (19) Bendersky, L. A.; Greenblatt, M.; Chen, R. J. Solid State Chem. 2003, 174, 418. (20) Guiblin, N.; Grebille, D.; Leligny, H.; Martin, C. Acta Crystallogr. C 2002, 58, i3. Okamoto, P. R.; Thomas, G. Acta Metall. 1971, 19, 825.

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Figure 1. [001] SAED patterns of La(2-2x)Ca(1+2x)Mn2O7 compounds with (a) x ) 0.7 and (b) x ) 0.66. A star of eight satellites surrounds each fundamental spot. Additional diffuse spots on the (a) pattern were seen occasionally and only for the thinnest areas and therefore are believed to be surfacerelated artifacts.

Figure 2. Schematic representation of [0 0 1] SAED pattern. Large gray circles represent fundamental reflections of the basic tetragonal structure. Smaller circles represent satellite reflections of two sets (filled and open circles), each consisting of two near-orthogonal kIx/kIy and kIIx/kIIy vectors. D labels a diffuse intensity corresponding to k1x + k1y. C labels a diffuse intensity corresponding to the (1 0 1) reflection and is probably visible because of intensity streaks in the c*-direction.

For clarity, a schematic representation of the essential diffraction features of the [0 0 1] SAED pattern is shown in Figure 2. In the drawing, the fundamental reflections are shown as large circles and the satellite reflections as small circles. The k vectors of the satellite reflections have a length corresponding to the 0.86 nm periodicity. According to the microdiffraction from small regions and high-resolution imaging (see later discussion), the star of eight satellites consists of two sets, each having near orthogonal k vectors kIx - kIy and kIIx - kIIy (filled and open circles in Figure 2). Apparently only first-order harmonics of the k vectors are present, thus suggesting a sinusoidal modulation. Additional features of the [0 0 1] pattern in Figure 2 are the presence of diffuse spots (labeled as D) and a weak (1 0 0) spot (labeled as C and forbidden for I4/mmm). Location of the D-type diffuse intensity roughly corresponds to the sum (kIx + kIy). The (1 0 0)-type spots correspond to the intersection of intensity streaks (in the c-direction) in the first-order Laue zone with the zero-order Laue zone reciprocal plane. The streaks are a result of the presence of (001) intergrowth defects.

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Figure 3. (a) High-resolution image taken at [0 1 0] orientation from the x ) 0.7 specimen. The plane of the image includes (a) long c-axis along which the streaking of intensity typically occurs. (b) An enlarged high-resolution image showing the correspondence between the phase contrast and the projected RP structure. A lower part of the image has n ) 2 stacking, whereas an upper part has n ) 3.

The angle between the two satellite sets is ∼29° (i.e., the angle between kIy and kIIy). The length and direction of the k vectors cannot be approximated by a small rational number based on the I4/mmm lattice. Therefore, it is suggested that the observed satellite reflections belong to two rotational variants of a twodimensional incommensurate modulation. A sequence of [h k 0] SAED patterns obtained by tilting around the [0 0 1] c-axis confirmed the reciprocal lattice consisting of fundamental spots of the I4/mmm structure surrounded by the incommensurate satellites in a (00l) plane. 3.2. In Situ Cooling and Heating Experiments. We performed in situ cooling experiments for x ) 0.7 specimens in a liquid N2 cooled stage. In the cooling experiment, a single grain was oriented to the [0 0 1] zone axis, and a series of SAED patterns from the same region were recorded at intervals of 50 °C, from room temperature down to -150 °C. Analysis of the SAED patterns recorded shows that there is no noticeable change in position of the satellite reflections. The reflections appear to be somewhat sharper at lower temperatures. In heating experiments, the x ) 0.7 specimen was heated in situ up to 500 °C, and a series of [0 0 1] SAED patterns from a single oriented grain were recorded. Complete disappearance of satellite reflections was not observed at these temperatures. A noticeable change occurred above 300 °C (on a temperature controller) when the well-separated pairs of satellite spots merged and formed dumbbell-like diffuse arches. 3.3. High-Resolution Imaging. Figure 3a shows a high-resolutionTEM (HRTEM) image taken at [1 0 0] orientation with near-Scherzer defocus. The plane of the image includes the long c-axis, along which the streaking of intensity is typically occurring. According to the image simulations for the ideal RP I4/mmm structure, white dots correspond to columns of alternating Mn (Bsite) and (La,Ca) (A-site) ions over a wide range of defocus-thickness conditions (including the Scherzer defocus). Using this structure-image relationship, we can identify the n-value of a perovskite layer. Figure 3b shows an enlarged portion of the image with an overlaid projected structure. Typically, the majority of the structure has n ) 2 layers, but occasional intergrowth of the n ) 3 layers (and rarely with n > 3) is observed and cause intensity streaking.

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Figure 5. (a) [0 0 1] HRTEM image of the x ) 0.7 specimen obtained from a thick part of a TEM specimen (200 keV). (b) A filtered image obtained by masking FFT (c) with circular apertures and applying inverse FFT. Dashed lines in (a) and (b) delineate a boundary between two rotational domains, I and II, with different k-vectors (shown in (c)).

Figure 4. (a) [0 0 1] HRTEM images taken from the same region of the x ) 0.7 specimen. The left-side image was taken very fast after the initial electron beam focusing, while a rightside image was taken after 4 min of focused beam irradiation; (b) FFT of the HRTEM images; (c) images formed by inverse FFT procedure using only Fourier components included between the two dotted circles shown in (b).

HRTEM imaging of the modulated superstructure was performed for the [0 0 1] orientated grains of the x ) 0.7 and 0.8 specimens. Although the sharpness of the satellites on SAED patterns suggests a well-developed long-range order of the modulation, surprisingly most of the HRTEM images showed a disordered superstructure. The reason for that was found in the rapid degradation of the superstructure when the electron beam was focused for obtaining a high-resolution image (as compared to the divergent, low-intensity beam used for the SAED recording). Such degradation is shown in Figure 4a, where two high-resolution images recorded from the same region but in different exposure conditions are shown. The left-side image was taken immediately after the beam was focused, whereas the right-side image was recorded after having the beam focused on the region of imaging for 4 min. Both images show an underlying lattice of the basic tetragonal structure, whereas a square lattice of the superstructure (a gradually varying intensity contrast) is seen only on the first image. Figure 4b shows a fast Fourier transform (FFT) of these images. For the short-exposed HRTEM image, the FFT shows well-developed satellite reflections (predominately a single set kIx - kIy) and the fundamental {110} spots. In the 4-min-exposed image the satellite reflections are replaced by diffuse intensity lobes aligned in the 〈1 1 0〉 directions.

To locate two variants with different modulations, medium-resolution imaging using 200 keV (to prevent the degradation under beam) was employed for a thick part of the TEM specimen. Although the quality of the HRTEM image was not very good, Figure 5a, the FFT and inverse FFT of the image confirmed the presence of the variants, Figure 5b,c. An inverse FFT image (obtained by masking the FFT pattern with circular apertures) in Figure 5b clearly shows two modulation lattices, each rotated from the other 28°. Dashed lines in Figure 5a,b delineate a boundary between the two rotational domains II and I. 4. Discussion The TEM results demonstrate the existence of a 2D incommensurate modulation for La2-2xCa1+2xMn2O7 specimens with 0.6 < x < 0.8. The existence of incommensurate modulations in different manganates, including layered RP structures, has been reported in the recent literature.12-17 However, the type of modulations found in our study is significantly different from those observed previously and has not been reported thus far. According to the previous reports, the modulation reflections result from the structural distortions associated with charge (of Mn4+ and Jahn-Teller Mn3+) and orbital (between differently oriented dz2 orbitals of Mn3+) ordering. In most cases the ordering results in onedimensional stripe patterns, with a periodicity related to the compound’s composition, namely, to the fraction of A-site ions x and correspondingly to the Mn4+/Mn3+ratio R (R ) x/(1 - x). The incommensurability of the modulation vector for the (Ln1-xCax)MnO3 (x g 0.5) compounds was explained with the help of HRTEM by the presence of discommensuration (domain walls separating small domains of commensurate modulation). The degree of the incommesuration is sensitive to the degree of oxidation and to the effective size of lanthanide ions.14 In the case of La2-2xCa1+2xMn2O7 studied here, both electron diffraction and structural imaging show the presence of a truly two-dimensional (2D) modulation in the (001) plane. The modulation manifests itself in electron diffraction by the appearance of eight sharp superlattice spots around each fundamental reflection.

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Figure 6. Schematic representation of the superlattice reflections in [001] SAED pattern, where (a) is the two sets of incommensurate reflections according to the experimental observations. (b) Approximation of incommensurate reflections with a commensurate lattice (two variants) having an orthogonal base with 1/5{210} vectors.

The k vectors of the spots appear to be incommensurate with respect to the basic tetragonal lattice of the n ) 2 RP structure. Somewhat similar electron diffraction, with four pairs of superlattice reflections around each fundamental reflection, was observed for the layered RP structure (n ) 1) of La2-xSrxNiO4 (0.0 < x