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Comments Comment on “ALaMn2O6-y (A ) K, Rb): Novel Ferromagnetic Manganites Exhibiting Negative Giant Magnetoresistance” The manganese oxide “KLaMn2O6-y ” was reported as part of a series of new manganese oxides ALaMn2O6-y with A ) K, Rb, and Cs by Ramesha et al.1 It was proposed to adapt a new ordered superstructure of perovskite, with a tetragonal 2ap × 2ap supercell structure with at∼7.75 Å and ct ∼ 7.10 Å (ap ) lattice parameter of the perovskite subcell; subscript t refers to the tetragonal supercell). It was also proposed to be ferromagnetic and show giant magnetoresistance behavior. The proposed structure contains ordered oxygen vacancies giving an alternation of layers of MnO6 octahedra with layers containing MnO5 square based pyramids. The existence of the “KLaMn2O6-y” phase was used to support the possibility to induce Mn(III)/Mn(IV) mixed valency and the related properties through anion deficiency in ordered perovskites of the type ALaMn2O6. However, the experiments presented in the current comment, using scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), electron diffraction (ED), and X-ray diffraction (XRD), show that the phase “KLaMn2O6-y” in fact does not exist and that the experimental data reported by Ramesha et al. belong to a mixture of two different phases, the La0.82K0.18MnO3 perovskite and the K-birnessite K0.49MnO2 · yH2O phase. The KLaMn2O6-y sample was synthesized using a Pechini route from Mn(NO3)2, La(NO3)3, and K2CO3 (“Reakhim”, “pure for analysis” grade). A stoichiometric amount of K2CO3 together with citric acid and ethylene glycol were added to a solution of Mn and La nitrates. The solution was evaporated at 60 °C, and the gel was decomposed at 600 °C for 5 h in air. The obtained powder was pressed into pellets which were placed into powder of the same composition to prevent potassium evaporation and annealed at 1000 °C for 30 h in air. The X-ray diffraction pattern of the resulting sample corresponds to the XRD pattern of “KLaMn2O6-y”1 (Figure 1). Bright reflections on this pattern can be indexed using a tetragonal unit cell with parameters at ) 7.743(2) Å and ct ) 7.037(1) Å as proposed by Ramesha et al.,1 but with poor indexation quality as reflected by the low value of the Smith and Snyder criterion F(15) ) 5.2 (0.024, 118).2 It will be argued in the following paragraphs that the indexation proposed by Ramesha et al. is not correct. SEM images of the KLaMn2O6-y sample (Figure 2) clearly show the presence of crystallites with two different morphologies. Small particles with a submicrometer size are present, along with large crystals with an average size of several micrometers. EDX analysis of both types of particles separa† ‡
University of Antwerp. Moscow State University.
(1) Ramesha, K.; Smolyaninova, V. N.; Gopalakrishnan, J.; Greene, R. L. Chem. Mater. 1998, 10, 1436. (2) Smith, G. S.; Snyder, R. L. J. Appl. Crystallogr. 1979, 12, 60.
Figure 1. Powder XRD pattern of the KLaMnO6-y sample. Top tick bar corresponds to K2MnO4, the middle one to birnessite-type KMO phase, and the bottom one to the perovskite-type LKMO phase.
Figure 2. SEM image of the KLaMnO6-y sample. Crystals of different morphologies are clearly visible.
Figure 3. Electron diffraction patterns of the main zones of the perovskitetype LKMO phase.
tely show that the small particles have the composition La0.82(5)K0.18(2)MnOx (LKMO), while the larger crystals contain only a negligable amount of lanthanum and have a composition K0.49(4)MnOx (KMO). This clearly indicates that the KLaMn2O6-y sample consists of at least two different phases. To determine the nature of these phases, we obtained electron diffraction patterns of both types of particles. The correspondence between the particles studied by TEM and those studied
10.1021/cm900298a CCC: $40.75 2009 American Chemical Society Published on Web 04/14/2009
Comments
Chem. Mater., Vol. 21, No. 9, 2009 2001
Figure 4. Electron diffraction patterns of the birnessite-type KMO phase. Diffuse intensity lines are marked with arrows.
by SEM was established through their EDX spectra in TEM as well as in SEM. The ED patterns of the LKMO phase (Figure 3) can be indexed on a perovskite based unit cell with the parameters found in literature for La0.8K0.2MnO3, that is, a ) 5.51 Å and c ) 13.38 Å; space group R3jc.3 For reasons of comprehensibility, the indices shown on Figure 3 are those of the perovskite subcell. These zones will correspond in the R3jc cell to respectively [001] and [421j]. The ED patterns of the KMO phase (Figure 4) can be indexed assuming a hexagonal structure of a K-rich form of birnessite, KxMnO2+δ · yH2O,4 with lattice parameters a ) 2.88 Å and c ) 12.90 Å (space group P63/mmc; Figure 4). Extra spots at the n/3{h h 2jh 0} positions are visible on the [0001] ED pattern of the KMO phase. These spots originate from the intersection of the [0001] reciprocal lattice plane with the lines of diffuse intensity clearly visible on the [1j100] ED pattern. The lattice parameters and space symmetry correspond to a 2H1 polytype of birnessite.5 The diffuse intensity lines indicate a short-range order present between successive (loosely bound) hexagonal planes. Using the combination of these two main phases and taking into account the presence of a minor amount of K2MnO4, the XRD pattern of the KLaMn2O6-y sample can be completely indexed (see the result of the LeBail fit in Figure 1). The lattice parameters of the phases were refined to a ) 5.5091(1) Å and c ) 13.3930(4) Å for the LKMO phase and a ) 2.8812(3) Å c ) 14.1202(2) Å for the KMO phase. A comment should be made on the value of the c-parameter of the KMO phase determined from the ED and XRD data. The c-parameter derived from the ED experiment is approximately 12.9 Å, which is considerably smaller than c ) 14.1202(2) Å obtained from the indexation of the XRD pattern. This is explained by the difference in experimental conditions between the X-ray diffraction experiment and the electron diffraction experiments, of which the latter are necessarily performed under deep dynamic vacuum, which causes dehydration of the birnessite phase. This decrease in c-parameter upon dehydration of κ-birnessite was proven by Gaillot et al.6 where an extensive study of the variation of the cell parameters upon variation of the experimental conditions is presented, showing a shift of the 002 reflection from 7.044 Å down to 6.397 Å in the case the sample was studied under deep vacuum conditions (3) Boudaya, C.; Laroussi, L.; Dahri, E.; Joubert, J. C.; Cheikh-Rouhou, A. J. Phys.: Condens. Matter 1998, 10, 7485. (4) Kim, S. H.; Kim, S. J.; Oh, S. M. Chem. Mater. 1999, 11, 557. (5) Drits, V. A.; Lanson, B.; Gaillot, A.-C. Am. Mineral. 2007, 92, 771. (6) Gaillot, A.-C.; Drits, V. A.; Manceau, A.; Lanson, B. Microporous Mesoporous Mater. 2007, 98, 267.
of 10-5 Pa due to dehydration. Birnessite is known as a natural mineral of varying composition containing interlayer water molecules. It seems unlikely that water would incorporate in the crystal lattice of birnessite during annealing at 1000 °C, but it is possilble that ambient air moisture influenced the sample during cooling and storage in air. The ferromagnetism and giant magnetoresistivity observed in “KLaMn2O6-y” can most probably be explained by the presence of La0.82K0.18MnO3, which is near the limiting point of the La1-xKxMnO3 solid solution.7 It is ferromagnetic with a Curie temperature around 340 K7 while a Tc ) 327 K was reported for “KLaMn2O6-y”.1 Moreover, it is known that the Tc strongly depends on the exact K and oxygen content. The measurements on the bulk ceramic samples of that phase display an MR effect of the same magnitude as in the case of the alkaline earth substituted La manganites,3 which is the same behavior as that reported for the “KLaMn2O6-y” phase.1 This investigation also creates some doubts about another phase reported by Ramesha et al.,1 indexed with similar cell parameters and similar properties as “KLaMn2O6-y”, that is, “RbLaMn2O6-y”, that might actually be a two phased mixture too. In conclusion, it is found that the “KLaMn2O6-y” sample reported by Ramesha et al. to be a single phase consists of two main phases, the La0.82K0.18MnO3 perovskite and K-birnessite K0.49MnO2 · yH2O. All experimental data, as well as the reported physical properties, are in support of this statement. Acknowledgment. This work was supported in frame of the RFBR Projects 07-03-01136a, 07-03-00664-a, and 06-0390168-a and by the IAP VI program of the Belgian government. Supporting Information Available: Experimental details (PDF). This material is available free of charge via the Internet at http:// pubs.acs.org.
Joke Hadermann,*,† Artem M. Abakumov,† Senne Van Rompaey,† Aleksey S. Mankevich,‡ and Igor E. Korsakov‡ EMAT, UniVersity of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium, and Department of Chemistry, Moscow State UniVersity, 119992 Moscow, Russia ReceiVed February 2, 2009 CM900298A (7) Zhong, W.; Chen, W.; Ding, W. P.; Zhang, N.; Hu, A.; Du, Y. W.; Yan, Q. J. J. Magn. Magn. Mater. 1999, 195, 112.
