Syntheses and Magnetic Properties of Layered LnSrMn 0.5 Ni 0.5 O 4 (Ln ) La, Pr, Nd, Sm, Gd) Compounds Kunpyo Hong, † Young-Uk Kwon,* ,† Duk-Kyun Han, † Jeong-Soo Lee, ‡ and Sung-Hyun Kim § Department of Chemistry, Sungkyunkwan University, Suwon, 440-746, Korea, Korea Atomic Energy Research Institute, Taejon, 350-600, Korea, and Department of Chemistry, Konkuk University, Seoul, 143-701, Korea Received February 16, 1999. Revised Manuscript Received April 14, 1999 K 2 NiF 4 -type layered compounds LnSrMn 0.5 Ni 0.5 O 4 (Ln ) La, Pr, Nd, Sm, Gd) were synthesized, and their magnetic properties were studied. Both Curie-Weiss fits and X-ray absorption near-edge structure (XANES) of Mn and Ni atoms indicate that the oxidation states of Mn and Ni are +4 and close to +2, respectively. The oxidation state of Ni is assigned primarily as Ni II , although there is increasing contribution from Ni III as the size of Ln increases. These compounds show overall antiferromagnetic behavior in their magnetic susceptibility vs temperature plots. The data in the paramagnetic regions indicate that there are ferromagnetic contributions by showing positive Weiss temperatures, probably because the Mn IV and Ni II atoms are locally ordered to form ferromagnetic regions. The antiferro- magnetic behavior of the La compound is attributed to the interactions among these ferromagnetic regions within the MO 2 (M ) Mn, Ni) layers. In the Pr and Nd compounds, there are additional local maxima at lower temperatures that are attributed to the interlayer antiferromagnetic ordering between the ferromagnetic regions and the Pr/Nd ions. Low- temperature neutron diffraction data of Pr and Nd samples show a magnetic Bragg peak that is absent in the La sample. The Sm and Gd compounds show still different behaviors from the La-Nd compounds that can be explained with the reduced interlayer interactions due to the small ionic sizes and the different magnetic properties of Sm and Gd ions. Introduction B-site mixed perovskite oxides of the general formula LnMn 0.5 M 0.5 O 3 (Ln ) rare earths, M ) Co, Ni, Cu) have attracted continued interest over decades because these compounds are ferromagnetic while the parent perovs- kite compounds are either antiferromagnetic or Pauli paramagnetic. 1 In fact, this group of compounds con- stitutes a rare class of ferromagnetic perovskite oxides along with the colossal magnetoresistance Ln 1-x A x MnO 3 (A ) divalent cations) 2 and SrRuO 3 3 while most of the other perovskites are antiferromagnetic or paramag- netic. However, the details of the electronic states of the metal ions and the mechanism of the ferromagnetism remain controversial. 4-10 The evolution of the physical properties with the dimensionality of the structure is another important theme in solid-state science. Therefore, there has been much effort to synthesize 2D Ruddelsden-Popper ((AO)- (ABO 3 ) n , n ) 1, 2, ...) type compounds to compare with the 3D perovskites for the physical properties. 11,12 We have been interested in whether the ferromagnetic nature of the perovskite compounds LnMn 0.5 Ni 0.5 O 3 can be maintained if their 2D derivatives are formed. Very recently, Millburn et al. reported on LaSrMn 0.5 Ni 0.5 O 4 compound in the n ) 1K 2 NiF 4 structure (Figure 1) for the synthesis and magnetic properties. 13 Contrary to the analogous perovskite LaMn 0.5 Ni 0.5 O 3 , this compound exhibits an antiferromagnetic ordering. On the basis of the magnetic measurements and the crystal structure analysis as compared with that of LaSrCr 0.5 Ni 0.5 O 4 , they claimed to observe high spin Ni III ions in their com- pound. We have extended their studies by substituting La with other rare earths. In this paper, we report our results on B-site mixed LnSrMn 0.5 Ni 0.5 O 4 (Ln ) La, Pr, Nd, Sm, and Gd) compounds for the synthesis, structure, and magnetic properties. * Corresponding author. † Sungkyunkwan University. ‡ Korea Atomic Energy Research Institute. § Konkuk University. (1) Blasse, G. J. Phys. Chem. Solids 1965, 26, 1969. (2) Wollan, E. O.; Koehler, W. C. Phys. Rev. 1955, 100, 545. (3) Jonker, G. H. Physica 1956, 22, 707. (4) Rao, C. N. R.; Cheetham, A. K.; Mahesh, R. Chem. Mater. 1996, 8, 2421 and references therein. (5) Longo, J. M.; Raccah, P. M.; Goodenough, J. B. J. Appl. Phys. 1968, 39, 1327. (6) Sonobe, M.; Kichizo, A. J. Phys. Soc. Jpn. 1992, 61, 4193. (7) Asai, K.; Sekizawa, H.; Iida, S. J. Phys. Soc. 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