Comments pubs.acs.org/cm
Comment on “Frustrated Octahedral Tilting Distortion in the Incommensurately Modulated Li3xNd2/3−xTiO3 Perovskites” The diameter of the collection aperture used was 2 mm, which cannot be considered a small one. Therefore, these working conditions guarantee that the EELS spectra are not severely affected by diffraction effects. Even considering possible artifacts coming from diffraction effects in the spectra, working at 300 keV, the variation in La intensity, mostly in phase with the stripes length, gives relevant evidence that composition variations of this element are present. Moreover, diffraction effects have stronger influence on atomic resolution experiments, like in the articles in references 53 and 54, which are not our case. EELS experiments in NaLaMgWO6 and KLaMnWO6 support periodic variation of the La content in the crystals with a periodicity similar to the one observed in the modulation of the crystal structure by SAED and HRTEM. This suggests that the modulation of the crystal structure is associated, at least partially, with a compositional modulation. We are not trying to offer an absolute quantitative analysis of the La composition variation but a qualitative one that is supported by other experimental data in our work. In addition to the EELS experiments, the HAADF-STEM images of KLaMnWO6 (Figures 4, 5, and 11 in reference 42) show faint but clear Z-contrasts, also supporting compositional variations within the crystal. In fact, Abakumov et al. acknowledge that the chemically sensitive HAADF-STEM image of Li0.36Nd0.547TiO3 shows faint contrast modulations. The faint Z-contrasts are clear in the HAADF-STEM image shown in Figure 4e in their article. However, we indeed agree with the opinion that separation into two phases, which are significantly different in composition, should give strong Zcontrasts. The point is that compositional modulation is not necessary similar to phase separation. Phase separation is a more rigid concept but compositional modulation can occur in a crystal with an average composition close to NaLaMgWO6 and periodic distribution of nanoregions with different Na and La content. In our first article on NaLaMgWO6 (reference 44), we propose different models of possible phase separation but we do conclude that the modulation of the crystal structure is related to “a repeating pattern of La-rich and La-poor stripes, each 6ap unit cells wide but further study is needed to definitively identify the compositional modulation responsible for the formation of stripes”. We also point out that “in addition to the compositional modulation, twinning of the octahedral tilt system and the corresponding twinning of the B-site cation displacements (particularly tungsten) almost certainly also contribute to the image contrast differences (stripes) seen in the HRTEM”. More details of the twinning are given in reference 43. In our study of KLaMnWO6, in addition to the EELS and HAADF-STEM results, energy filtered transmission
A. M. Abakumov et al. have recently published an article entitled “Frustrated Octahedral Tilting Distortion in the Incommensurately Modulated Li3xNd2/3−xTiO3 Perovskites” (Chem. Mater. 2013, 25, 2670). This work provides new insights into the understanding of the origin of the crystal structure modulation of these perovskite-type oxides. The author’s picture of incommensurate octahedral tilting is an impressive piece of work and an important piece of the puzzle, one that advances our understanding of these complex structures. However, there is no reason why a composition modulation cannot also accompany the modulation in octahedral tilting in the AA′BB′O6 compounds that we have studied. In fact, the data that have been published lend strong support for that conclusion. All the features associated with the superstructure are explained in terms of displacive modulations associated to the octahedra tilting system, discarding previous conclusions centered on compositional nanoscale phase separation into Nd2/3TiO3 and Li1/2Nd1/2TiO3 domains in the compounds of this Li3xNd2/3−xTiO3 system. On the basis of their results, the authors of this article state that they “doubt the existence of a compositional phase separation in other layered A-site ordered perovskites with incommensurate superstructures, which are strikingly similar to that in Li3xNd2/3−xTiO3”. In particular, they disagree with the suggestion of compositional modulation in combination with twinning of the octahedral tilting system in AA′BWO6 (A = La, Ce, Nd, A′ = Na, K, B = Mg, Mn) reported in references 42−46. However, they admit that “nanometerscale mapping of the La concentration with electron energy loss spectroscopy (EELS) in NaLaMgWO6 and KLaMnWO6 demonstrates a variation of the La content with a periodicity matching that of the incommensurate modulation, thus causing an unresolved controversy (references 42, 44)”. Despite this, they mention the fact that the interpretation of EELS data in terms of chemical composition (qualitative or quantitative) can be misleading in the presence of severe diffraction effects, particularly if a small collection aperture is used. As authors of the articles in references 42−44 and 46, we would like to make the following remarks about our EELS experiments, which confirm compositional modulation in NaLaMgWO6 and KLaMnWO6. The spectral data sets were acquired as line scans, with an electron probe size diameter of 0.3 nm, a collection time of 2 s, a dispersion of 0.5 eV/pixel, and a collection semiangle of β ≈ 8.9 mrad. In order to minimize plural-scattering effects we worked with the thinnest crystals we could find and as close to their edge as possible, since plural-scattering effects are stronger as the crystal grows thicker. (We cannot acquire low-loss and core-loss line-scans at the same time with the EELS equipment installed in the JEM3000F microscope employed in our experiments. Therefore, trying to acquire two different line-scans in the same area is not reliable and would introduce real artifacts in the convoluted core-loss signal). © 2013 American Chemical Society
Received: October 13, 2013 Revised: December 10, 2013 Published: December 19, 2013 1286
dx.doi.org/10.1021/cm403365d | Chem. Mater. 2014, 26, 1286−1287
Chemistry of Materials
Comments
electron microscopy (EFTEM) images taken at the La-M4,5 edge and the Mn-L2,3 edge also support the idea that the compositional modulation is associated with the La/K sublattice as no contrast is seen for Mn. We even suggest two different models for defining the relationship between compositional modulation and octahedral tilt twinning (Figure 10 in reference 42): one of the models is based on the idea that the A-site coordination environment will be slightly different at the tilt twin boundary than it is away from the boundary. On the other hand, if the in-phase tilts form in order to relieve strains associated with the subtly different lattice parameters of the La-rich and La-poor regions, then model 2 seems more plausible. The HAADF-STEM seems to be in better but not complete agreement with model 2, and therefore, we did not rule out a modulation that is a hybrid between models 1 and 2. In summary, our results on AA′BWO6 (A = La, Ce, Pr, A′ = Na, K, B = Mg, Mn) (references 42−44 and 46) experimentally support that these compounds both have a compositional modulation of the A-site cations which is coupled with a twinning of the octahedral tilt system. The compositional modulation likely does not involve a phase separation, but rather a periodic distribution of nanoregions with different Na and La content, which in combination with the twinning of the octahedral tilt system plays an important role in stabilizing formation of these complex arrangements. However, more work is required to accomplish the exact composition of the different areas of the structure and, therefore, the complete description of the modulation of the crystal structure.
Susana García-Martín† Esteban Urones-Garrote† Graham King*,‡ Patrick Woodward§ †
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Departamento de Química Inorgánica, Facultad de Ciencias Químicas, Universidad Complutense, Madrid 28040, Spain ‡ Lujan Neutron Scattering Center, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States § Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210-1185, United States
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*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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dx.doi.org/10.1021/cm403365d | Chem. Mater. 2014, 26, 1286−1287
