Crystal structure and magnetism of layered Ln₂Ca ₂MnNiO₈ (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 ₂Ca₂MnNiO₈ (Ln=Pr, Nd, Sm, and Gd) in which Ln/Ca 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^II. The Curie-Weiss plots of the high temperature magnetic data suggest strong ferromagnetic interactions probably due to Mn^IV-O-Ni^II 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^II-O-Ni^II 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 Ln₂Sr ₂MnNiO₈ compounds.