Low temperature synthesis, structure and magnetic properties of La_0.85(Na_1–xK_x)_0.15MnO₃ perovskites: the role of A cation size disparity in the electronic properties of mixed-valence manganates

Abstract

Single-phase perovskites in the solid solution series La_0.85(Na_1–xK_x)_0.15MnO₃ (0≤x≤1) have been obtained using a soft treatment, which makes possible strict stoichiometric control. X-Ray powder diffraction patterns of these compounds have been completely indexed with a rhombohedral perovskite cell. The crystal structures have been refined in space group R-3c, in the hexagonal setting, from room-temperature data. Substitution of Na⁺ by larger K⁺ ions produces a cell expansion and a decrease in the structural distortion from the ideal cubic structure. The critical temperature for the paramagnetic-ferromagnetic transition, T_c, is found to be practically constant, ca. 333 K, along the entire series. This behaviour is unexpected, taking into account previous correlations established for the alkaline-earth La_0.7(Ca_1–xSr_x)_0.3MnO₃ series (T_c increases with the mean size of cations at the A positions, 〈r_A〉) which expands over a similar 〈r_A〉 range. We can therefore discuss these results in terms of two counterweighting contributions: increasing T_c values could be expected as a consequence of the increase with x of the 〈r_A〉 value, but the concomitant disorder introduced at the A positions [as represented by the variance of the A cations radial distribution, σ²(〈r_A〉)] would cause a decrease in T_c. An approach to the understanding of the contribution that the electronic energy makes to this last effect is advanced.

References

1. (a) R. M. Kusters, J. Singleton, D. A. Keen, R. McGreevy and W. Hayes, Physica B, 1989, 155, 362; (b) R. von Helmot, J. Wecker, B. Holzapfel, L. Schultz and K. Samwer, Phys. Rev. Lett., 1993, 71, 2331; (c) S. Jin, T. H. Tiefel, M. McCormack, R. A. Fastnacht, R. L. Ramesh and H. Chen, Science, 1994, 64, 413.
2. (a) Z. Zener, Phys. Rev., 1951, 82, 403; (b) P. W. Anderson, Phys. Rev., 1955, 100, 675; (c) P. G. De Gennes, Phys. Rev., 1960, 116, 141.
3. (a) P. Schiffer, A. P. Ramirez, W. Bao and S. W. Cheong, Phys. Rev. Lett., 1995, 75, 3336; (b) A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido and Y. Tokura, Phys. Rev. B, 1995, 51, 14103.
4. (a) A. Maignan, V. Caignert, Ch. Simon, M. Hervieu and B. Raveau, J. Mater. Chem., 1995, 5, 1091; (b) H. Y. Hwang, S. W. Cheong, P. G. Radaelli, M. Marezio and B. Batlogg, Phys. Rev. Lett., 1995, 75, 914; (c) R. Mahesh, R. Mahendiran, A. K. Raychaudhuri and C. N. R. Rao, J. Solid State Chem., 1995, 120, 204; (d) J. Fontcuberta, B. Martinez, S. Piñol, J. L. García-Muñoz and X. Obradors, Phys. Rev. Lett., 1996, 76, 1122.
5. (a) L. M. Rodriguez-Martinez and J. P. Attfield, Phys. Rev. B, 1996, 54, R15622; (b) L. M. Rodriguez-Martinez and J. P. Attfield, Phys. Rev. B, 1998, 58, 2426; (c) J. P. Attfield, Chem. Mater., 1998, 10, 3239.
6. (a) F. Damay, C. Martin, A. Maignan and B. Raveau, J. Appl. Phys., 1997, 82, 6181; (b) B. Raveau, A. Maignan, C. Martin and M. Hervieu, Chem. Mater., 1998, 10, 2641.
7. J. P. Attfield, A. L. Kharlanov and J. A. McAllister, Nature, 1998, 394, 157.
8. J. Fontcuberta, B. Martinez, S. Piñol, J. L. García-Muñz and X. Obradors, Phys. Rev. B, 1997, 55, R668.
9. J. Xu, W. H. Tang and J. K. Liang, J. Phys.: Condens. Matter, 1998, 10, 1387.
10. (a) Y. Ng-Lee, F. Sapiña, E. Martinez-Tamayo, J. V. Folgado, R. Ibañez, D. Beltrán, F. Lloret and A. Segura, J. Mater. Chem., 1997, 7, 1905; (b) T. Boix, F. Sapiña, Z. El-Fadli, E. Martinez, A. Beltrán, J. Vergara, R. J. Ortega and K. V. Rao, Chem. Mater., 1998, 19, 1569; (c) J. Vergara, R. J. Ortega-Hertogs, V. Madurga, F. Sapiña, Z. El-Fadli, E. Martinez, A. Beltrán and K. V. Rao, Phys. Rev. B, in press.
11. Samples in the series La_0.7(Ca_1–χSr_χ)_0.3MnO₃(with χ= 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) and La_0.7Ba_0.3MnO₃, which are used for comparison, were obtained by using the same preparative procedure.
12. J. Rodriguez-Carvajal, FULLPROF Program, personal communication.
13. V. Primo, DRXWin and CreaFit version 2.0, graphical and analytical tools for powder XRD patterns, Powder Diffr., 1999, 14, 70.
14. Transition temperatures for La_0.7Ba_0.3 MnO₃ and samples in the series La_0.7(Ca_1–χSr_χ)_0.3MnO₃ were also obtained by thermogravimetric experiments, except for the χ= 0.0 and 0.2 samples (which have transition temperatures lower than 300 K). In these last cases, the transition temperatures were determined by ac susceptibility measurements performed using a Lake Shore Cryotonics Inc. Model 7000 ac susceptometer (temperature range 100–325 K, frequency and exciting field, 333 Hz and 1 Oe, respectively). All the experimental results concerning these compounds are in good agreement with those previously reported in the literature for analogous compositions (ref. 4).
15. J. A. M. Van Roosmalen, E. H. P. Cordfunke, R. B. Helmholdt and H. W. Zandbergen, J. Solid State Chem., 1994, 110, 100.
16. A. H. Morris, The Physical Principles of Magnetism, Robert E. Kreiger Publishing Company, New York, 1980.
17. R. D. Shannon, Acta Crystallogr., Sect. A, 1976, 32, 751.
18. R. G. Pearson, Inorg. Chem., 1988, 27, 734.
19. F. Millange, V. Caignert, B. Domengès and B. Raveau, Chem. Mater., 1998, 10, 1974.
20. W. Archibald, J. S. Zhou and J. B. Goodenough, Phys. Rev. B, 1996, 53, 14445.
21. (a) P. G. Radaelli, G. Iannone, M. Marezio, H. Y. Hwang, S. W. Cheong, J. D. Jorgensen and D. N. Argyriou, Phys. Rev. B, 1997, 56, 8265; (b) V. Caignert, E. Suard, A. Maignan, Ch. Simon and B. Raveau, C. R. Acad. Sci., Ser. IIb, 1995, 321, 515.
22. J. M. De Teresa, M. R. Ibarra, J. García, J. Blasco, C. Ritter, P. A. Algarabel, C. Marquina and A. del Moral, Phys. Rev. Lett., 1996, 76, 3392.
23. In fact, structural disorder in mixed valence manganates can be produced by the presence of at least two different cations at the A sites or, alternatively, be due to the presence of cationic vacancies at the B sites. The present discussion, as well as Attfield's analysis, is centered on the former source of structural disorder, which is consistent with the absence of cationic vacancies (δ≈0) in the series we are dealing with. The effect of disorder produced by cationic vacancies is discussed elsewhere (ref. 10).
24. (a) L. Sheng, D. Y. Xing, D. N. Sheng and C. S. Ting, Phys. Rev. Lett., 1997, 79, 1710; (b) L. Sheng, D. Y. Xing, D. N. Sheng and C. S. Ting, Phys. Rev. B, 1997, 56, 7053.
25. In fact, by assuming this approach, the electronic energy variations associated with the decrease in the Mn–O bond length due to the FM–PM transition are being neglected, and the change in the electronic energy associated with the transition is approximated by considering that only the energy of the more external (conduction band) states is modified. Such a simplification finds experimental support in previous results such as those of Radaelli et al. in ref. 21(a).
26. Q. Li, J. Zhang, A. R. Bishop and C. M. Soukoulis, Phys. Rev. B, 1997, 56, 4541.