Structural complexity in AA′MM′O6 Perovskites. A Transmission Electron Microscopy

Study

Susana García-Martín1, Esteban Urones-Garrote1, Meghan C. Knapp2, Graham King2 and Patrick M. Woodward2

1Departamento de Química Inorgánica, Facultad de Ciencias Químicas, Universidad Complutense, Madrid-28040, Spain.

2The Ohio State University, Department of Chemistry, 100 West 18th Avenue, Columbus, OH 43210-1185, USA

ABSTRACT

We have prepared five perovskite compounds which have two different A-site cations and examined the ordering of the A-site lattice through powder X-ray diffraction, TEM, HRTEM, HAADF-STEM, STEM-EELS. NaLaTi2O6 and NaLaZr2O6 show no indication of cation ordering by XRD but TEM analysis reveals that the A-site cations are ordered into layers in nanometer domains which have perpendicular orientations of the layers. NaLaMgTeO6 has larger domains which can be detected by XRD. XRD has revealed that both NaLaMgWO6 and KLaMnWO6 have nearly complete long range layered ordering of the A-site cations. TEM microscopy has further shown that a compositional modulation exists in both compounds. NaLaMgWO6 organizes into alternating stripes ~24Å wide that are compositionally distinct from each other. KLaMnWO6 shows similar behaviour except with a chessboard pattern.

INTRODUCTION

Oxides with the perovskite structure (stoichiometry ABO3) are among the most interesting

type of inorganic functional materials. Substitutions at either the A- or the B-sites modify the properties and the crystal structure creating new possibilities for materials design. Ordering of cations also plays an important role on the physical properties and structure of substituted perovskites. Ordering of A-site cations is relatively rare compared to the numerous examples of B-site cation ordering reported in the literature and is usually found in non-stoichiometric perovskites (1, 2). When A-site cations do order, they adopt a layered arrangement rather than rock salt ordering, which is the most common form of the B-site cation ordering. These oxides can also display other interesting microstructural effects. A beautiful example is the observation of spontaneous phase separation to form nano-chessboard superlattices in the (Nd2/3-xLi3x)TiO3 system (3).

We have prepared a significant number of stoichiometric perovskites, AA′B2O6 and AA′BB′O6, with various degrees of A-site cation layered ordering in combination with B-site cation rock salt ordering in the case of the quintinary phases (4, 5). To characterize the crystal microstructure of these materials, a study by means of selected area electron diffraction (SAED) and high resolution electron microscopy (HRTEM) has been carried out. We report the results here.

Mater. Res. Soc. Symp. Proc. Vol. 1148 © 2009 Materials Research Society 1148-PP15-05

EXPERIMENT

NaLaTi2O6, NaLaZr2O6, NaLaMgTeO6, NaLaMgWO6 and KLaMnWO6 were prepared

from Na2CO3 (Fisher 99.9%), K2CO3 (Baker 99.9%), La2O3 (G. Frederick Smith 99.99%), MnO (Cerac 99.9%), MgO (Allied), TeO2 (Cerac), TiO2 (GFS), ZrO2 (Johnson Matthey) and WO3 (Cerac 99.99%). Synthesis was aided by the use of BWO4 ternary oxides as precursors. A 5% excess of the alkali metal carbonates, Na2CO3 or K2CO3, were used to account for the high-temperature volatility of these substances. Reactants were heated to 900 º C for 6 h and then to a final annealing temperature ranged from 1050 to 1100 ºC (4, 5). KLaMnWO6 was prepared by first heating to 800 °C in air and then to 1000 ºC under a dynamic flow of forming gas (5% H2, 95% N2). X-ray powder diffraction patterns were collected on a Bruker D8 diffractometer (40 kV, 50 mA, Cu Kα1 radiation) equipped with a Ge 111 incident beam monochromator and a Braun linear position sensitive detector.

For transmission electron microscopy the samples were ground in n-butyl alcohol and ultrasonically dispersed. A few drops of the resulting suspension were deposited in a carbon-coated grid. SAED studies were performed with an electron microscope JEOL 2000FX (double tilt ±45º) working at 200kV. High Resolution Transmission Electron Microscopy (HRTEM) and Scanning TEM-Electron Energy-Loss Spectroscopy (STEM-EELS) studies were carried out with a JEM 3000F microscope operating at 300 kV (double tilt ±20º) (point resolution 1.7 Å), fitted with an ENFINA 1000 spectrometer and a JEOL Annular Dark Field (ADF) detector. DISCUSSION

The powder X-ray diffraction patterns of NaLaMgTeO6, NaLaMgWO6 and KLaMnWO6

are characteristic of the perovskite-type structure with extra reflections associated with both layered ordering of A-site cations and rock salt ordering of B-site cations. Powder XRD patterns of NaLaTi2O6 and NaLaZr2O6 do not show evidence of A-cation ordering (4, 5). In other cases, superlattice reflections arising from A-site cation ordering (½ (001)p and ½ (201)p) are weakened and/or broadened by partial loss of order. This is the case for NaLaMgTeO6, for which less than 10% of layered Na/La ordering is deduced from Rietveld refinement of the crystal structure (4).

Fig. 1 shows SAED patterns along different zone axis of NaLaTi2O6 and NaLaZr2O6. The patterns have been indexed according to the ideal perovskite structure (Pm-3m symmetry and ap

~ 4 Å). SAED patterns of NaLaMgTeO6 (not shown) are similar to those of NaLaZr2O6 but with differences on the intensity of the reflections associated with both B-site cation ordering and octahedral tilting. Despite the absence of evidence for A-site cation ordering in the XRD powder patterns, both ½ (001)p and ½ (201)p superlattice reflections, which are related to the layered ordering of A-site cations, appear in the SAED patterns of NaLaZr2O6. Diffuse scattering is detected at these positions in the patterns of NaLaTi2O6.

Fig. 1: SAED patterns of a crystal of NaLaTi2O6 (a, b, c, d) and NaLaZr2O6 (e, f, g, h) along the [111]p, [010]p, [001]p and [11 2 ]p zone axes. Fig. 2a shows the HRTEM image corresponding to the [001]p zone axis of NaLaTi2O6.

The image is characteristic of perovskite structure but in addition to lattice fringes with √2ap periodicity, contrast differences indicating 2ap periodicity are observed in small regions of the crystal along either the [1 0 0]p or the [0 1 0]p directions (regions with two or three times the size of the unit cell dimensions). Therefore, the crystal is formed by microdomains of the √2ap × √2ap × 2ap cell with perpendicular orientation of the long 2ap axis. The size of the ordering microdomains is expected to be significantly bigger for NaLaMgTeO6, as confirmed in the HRTEM image of this oxide along [001]p zone axis (Fig. 2b). Therefore, NaLaMgTeO6 has a higher degree of A-site cation ordering than NaLaZr2O6 or NaLaTi2O6. This type of domain formation (domains with perpendicular orientation of structure) leads to the appearance of ½ (100)p and ½ (010)p superlattice reflections (diffuse scattering at these positions in the case NaLaTi2O6).

The ½ (ooo)p reflections (“o” and “e” refer to odd and even Miller indices, respectively, when indexed on a simple cubic perovskite cell) (6, 7), such as ½ (111)p derive intensity from out-of-phase octahedral tilting, but in the case of NaLaMgTeO6 most of their intensity comes from rock salt ordering of Mg and Te. In addition to this, ½ (ooe) h≠k reflections, such as ½ (312) and ½ (130) in the pattern along the [001]p zone axis can be seen in the patterns of NaLaZr2O6 and NaLaMgTeO6. In most perovskites these reflections are evidence of in-phase tilting of the octahedra about the c-axis (8). However, the presence of A- and B-site cation ordering in combination with out-of-phase tilting leads to cation displacements that also contribute to these peaks.

110p

101p

001p

100p 100p

010p 111p

- a b c d

110p

110p

101p

001p

100p

010p

100p

111p

110p -

e f g h

a

Fig. 2: HRTEM images of a crystal of NaLaTi2O6 (a) and NaLaMgTeO6 (b) along the [001]p zone axis

Interestingly enough, the SAED patterns corresponding to the NaLaMgWO6 and KLaMnWO6 samples show very peculiar features in addition to the reflections characteristic of both A- and B-cation ordering. Fig. 3 (a and b) show the SAED patterns along the [001]p zone axis of a crystal of NaLaMgWO6 and KLaMnWO6 respectively. Satellite reflections around the main ones are observed.

Fig. 4a presents a HRTEM image while Fig. 4b is an HAADF-STEM image, both corresponding to the [001]p zone axis of a crystal of NaLaMgWO6. Fig. 4c and d show the corresponding HRTEM and HAADF-STEM images of a crystal of KLaMnWO6. The satellite reflections are due to a modulation of the crystal structure. The distance between satellites is related to the periodicity of the modulation (12ap along both [100]p and [010]p directions for NaLaMgWO6). Contrast differences formed by dark stripes intercalated with bright ones, both of

2ap

2ap

[100]p

[010]p

b

2ap

[100]p

[010]p

a

2ap

Fig. 3: SAED patterns along the [001 ]p zone axis of a crystal of NaLaMgWO6 (a) and KLaMnWO6 (b).

100p

010p

a

100p

010p

b

them of 6ap in width, are observed along two perpendicular directions corresponding to two domains of the crystal. These contrast differences are consistent with the 12ap periodicity along [100]p and [010]p directions shown in the SAED pattern. With KLaMnWO6 contrast differences forming a kind of chessboard, somewhat similar to the ones observed in (Nd2/3-xLi3x)TiO3 samples (3), appear in the HRTEM image. HAADF-STEM images provide contrast directly related to Z and practically free of diffraction effects (9). The HAADF-STEM images shown in Figure 4b and d show contrast differences with the same periodicity as the contrast variations observed in the corresponding HRTEM images of the crystals (Figure 4a and c), indicating that both NaLaMgWO6 and KLaMnWO6 exhibit a compositional modulation.

To directly probe this, STEM-EELS line-scans were acquired perpendicular to the stripes of the STEM image of a crystal of NaLaMgWO6, collecting at the La-M4,5 edge. Fig. 5 shows a periodic variation of the La content that agrees well with the corresponding ADF-STEM image intensity profile, as previously reported (10). Therefore, the light and dark stripes in the HRTEM images have different lanthanum contents. STEM-EELS studies for KLaMnWO6 are in progress. In addition to the compositional modulation, twinning of the octahedral tilt system almost certainly also contribute to the image contrast differences seen in the HRTEM images.

12ap

12ap

[100]p

[010]p

a

12a

b

[100]p

[010]p

c d

Fig. 4: HRTEM images of the [001]p zone axis of a crystal of NaLaMgWO6 (a) and KLaMnWO6 (c). HAADF-STEM images of the [001]p zone axis of a crystal of NaLaMgWO6 (b) and KLaMnWO6 (d).

CONCLUSIONS

SAED and HRTEM studies on NaLaTi2O6, NaLaZr2O6 and NaLaMgTeO6 indicate that

the crystals are formed by perpendicularly oriented domains of layered Na/La order. The size of the domains is slightly bigger for the NaLaMgTeO6 sample than for the NaLaTi2O6 and NaLaZr2O6, both of which present microdomain formation that is not detected by powder X-ray diffraction. NaLaMgWO6 and KLaMnWO6 exhibit two distinctly different compositional modulations on the basic perovskite structure. In the case of NaLaMgWO6 the stripe modulation is associated with a periodic variation of the La concentration. Further studies are needed to elucidate the details of the chessboard-type compositional modulation seen in KLaMnWO6.

ACKNOWLEDGMENTS

P. M. Woodward, M. C. Knapp and G. King thank the NSF for funding through a Materials World Network grant, MWN-0603128. S. García-Martin and E. Urones-Garrote thank the Spanish MEC for funding the projects MAT2007-64486-C07-04 and CAM for the project MATERYENER S 505/PPQ/0358. Authors also thank the Microscopy Centre Luis Bru from U.C.M. for technical assistance. REFERENCES

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Chem. Soc. 2008, 130 (45), 15028.

0 5 10 15 20 25 30 35 40 450 5 10 15 20 25 30 35 40 45

a

b

12ap In

ten

sit

y (

a.u

.)

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Fig. 5: Variation of the intensity of the La-M4,5 edge signal, obtained from the STEM-EELS line-scan (a). Contrast intensity variation of the ADF-STEM image perpendicular to the observed fringes of a crystal of NaLaMgWO6

12ap