Comments pubs.acs.org/cm
Reply to Comment on “Frustrated Octahedral Tilting Distortion in the Incommensurately Modulated Li3xNd2/3−xTiO3 Perovskites” n their Comment Garcı ́a-Marı ́n et al. provide experimental details of the EELS measurements in ref 42 and 44 in Abakumov et al. (Chem. Mater. 2013, 25, 2670) and argue that these data support the presence of compositional modulation in the cation-ordered NaLaMgWO6 and KLaMnWO6 double perovskites. We highly appreciate the thorough and systematic work on the AA′BB′O6 perovskites done by Garcı ́a-Marı ́n et al., but in our opinion extra experimental details provided in their Comment do not make the case fully compelling. Although we do not per-se rule out that the interpretation of the EELS data in ref 42 and 44 is correct, in our Reply we would like to summarize aspects that remain controversial and require further investigation. First, we would like to stress that EELS (and EFTEM) data should be treated with extreme caution when affected by diffraction effects. Garcı ́a-Marı ́n et al. mention that their working conditions and, in particular, the collection semiangle of 8.9 mrad “guarantees that the EELS spectra are not severely affected by diffraction effects”. This collection semiangle should be compared with the characteristic scattering angles of the LaM4,5 edge and the Mn-L2,3 edge, which are around 7.8−8.2 mrad (200 kV, slightly depending on the beam convergence angle in the STEM mode and on the energy loss). The characteristic scattering angle of these ionization edges is thus only marginally smaller than the collection semiangle used in the experiments. As the angular scattering distribution is quite broad, a significant part of the energy loss associated with the La and the Mn edges does not contribute to the EELS signal, thus making the signal vulnerable to diffraction effects. Even more importantly, channeling and diffraction effects can substantially affect the intensity of the core-loss EELS signal (see, e.g., Muller et al.2), both on the atomic and on the nanoscale. The authors of the Comment apparently did not apply deconvolution of the core-loss spectra with the corresponding low-loss spectra using, e.g., the Fourier ratio method. Considering the strong modulation in the bright-field STEM signal (see, e.g., Figure 4c in Abakumov et al.), a deconvolution with the low-loss spectra is essential to minimize effects that stem from the modulation of the low loss. The elastic mean free path is substantially smaller than the inelastic mean free path associated with a core loss. Hence, in order to quantitatively or at least semiquantitatively interpret the coreloss signal, a deconvolution with the low-loss part is essential. Otherwise, multiple scattering effects can lead to artificial chemical modulations in the core-loss signal (see, e.g., Egerton1). Second, we are still puzzled by the problem of charge balance ́ in the compositionally modulated AA′BB′O6 perovskites. ciaMariń et al. have shown in ref 44 that the compositional modulation is restricted to the A sublattice. For example, in NaLaMgWO6 the Mg:W ratio should stay the same all over the crystal, and only the Na and La occupancies can be modulated with the periodicity of ∼12ap (ap = perovskite subcell parameter). Maintaining charge balance requires the La-poor
I
© 2014 American Chemical Society
regions to have the nominal composition of NaLaMgWO6, whereas the La-rich regions should be understood as Na1−3xLa1+xMgWO6, resulting in overall Na deficiency. Therefore, the Na/La ratio deviates from 1:1 in the average composition, but no convincing evidence for this change in the chemical composition has been reported so far. In fact, the Na/La ratio in the bulk could be the most direct probe of the putative compositional modulation. This ratio will measure the extent of the modulation and put an upper limit on the compositional difference between the La-poor and La-rich regions. Finally, we believe that this fruitful exchange of opinions calls for advanced structural studies of the modulated AA′BB′O6 perovskites. We have shown that a quantitative structure refinement of Li3xNd2/3−xTiO3 is feasible through a combination of superspace crystallography and modern diffraction techniques, including high-resolution synchrotron X-ray and neutron powder diffraction or, ultimately, diffraction on single crystals. Resolving the occupational and displacive modulations in cation-ordered perovskites will be the key to understanding these challenging materials.
Artem M. Abakumov*,† Rolf Erni‡ Alexander A. Tsirlin§ †
■
EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium ‡ Electron Microscopy Center, Empa, Swiss Federal Laboratories for Materials Science and Technology, CH-8600 Dübendorf, Switzerland § National Institute of Chemical Physics and Biophysics, 12618 Tallinn, Estonia
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
REFERENCES
(1) Egerton, R. F. Electron Energy-Loss Spectroscopy in the Electron Microscope, 3rd ed.; Springer: 2011. (2) Muller, D. A.; Fitting Kourkoutis, L.; Murfitt, M.; Song, J. H.; Hwang, H. Y.; Silcox, J.; Dellby, N.; Krivanek, O. L. Microsc. Microanal. 2008, 14 (Suppl. 2), 132.
Received: January 2, 2014 Published: January 3, 2014 1288
dx.doi.org/10.1021/cm500005d | Chem. Mater. 2014, 26, 1288−1288
