Crystal structure and magnetism of layered Ln2Ca2MnNiO8 (Ln = Pr, Nd, Sm, and Gd) compounds

We report the syntheses, crystal structure, and magnetic properties of a series of distorted K2NiF4-type oxides Ln(2)Ca(2)MnNiO(8) (Ln - Pr, Nd, Sm, and Gd) in which LulCa and Mn/Ni atoms randomly occupy the K and Ni sites respectively. The Ln - La compound does not form. These compounds show systematic distortions from the ideal tetragonal K2NiF4 structure (space group I4/mmm) to an orthorhombic structure (space group Pccn) with buckled MO2 (M=Mn/Ni) layers. The degree of distortion is increased as the size of Ln decreases. Based on the magnetic data and X-ray absorption near edge spectra, we assigned Mn-IV and Ni-11. The Curie-Weiss plots of the high temperature magnetic data suggest strong ferromagnetic interactions probably due to Mn-IV_ O-Ni-11 linkages, implying local ordering of Mn/Ni ions to form ferromangnetic clusters in the MO2, layers. At low temperatures below 110-130K, these compounds show antiferromagnetic behaviors because of Mn-IV-O-Mn-IV and/or Ni-11-O-Ni-11 contacts between the ferromagnetic clusters. The Ln = Pr and Nd compounds show additional antiferromagnetic signals that we attribute to the interlayer interactions between the clusters mediated by the Pr3+ and Nd3+ ions in the interlayer spaces. The present compounds show many parallels with the previously reported Ln(2)Sr(2)MnNiO(8) compounds. (C) 2003 Elsevier Inc. All rights reserved.