Study of phase components of La 1.5 Ca 1.5 Mn 2 O 7
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Z. Zhang Beijing Laboratory of Electron Microscopy, Institute of Physics, Center for Condensed Matter Physics, Chinese Academy of Sciences, P.0. Box 2724, Beijing 100080, People’s Republic of China (Received 30 November 2000; accepted 28 April 2001)
The manganate with nominal composition La1.5Ca1.5Mn2O7, which is regarded as a single-phase compound with layered perovskite structure in the literature, was prepared using a standard ceramic process. The structures and morphology of the manganate were investigated by x-ray diffraction, transmission electron microscopy (TEM), scanning electron microscopy, and energy-dispersive x-ray microanalysis. However, no direct evidence of layered Sr3Ti2O7-type structure was observed in TEM experiments; instead, we observed multiphase mixtures of an orthorhombically distorted perovskite phase as majority and cubic perovskite phase as minority, as well as a small amount of calcium oxide. The measured magnetic and transport properties of this manganate arise mainly from the presence of hole-doped multiphase perovskite manganates. These physical properties demonstrated again the correctness of our phase component analysis.
I. INTRODUCTION
The layered manganates R2−2 x A1+2 x Mn2O7 (R is a rare-earth trivalent ion, and A is a divalent cation) with Sr3Ti2O7-type structure, which exhibit colossal magnetoresistance (CMR) and a variety of fascinating physical properties, have become a subject of intense interest during recent years.1–3 As an important CMR compound family, the layered manganates show strong anisotropy in physical properties in comparison to the ABO3-type materials. This anisotropy arises from the presence of the [(R, A)O] NaCl-type block, which separates the paired MnO2 sheets forming bilayers. The insertion of the NaCl-type block leads to a longer C axis and makes the Mn–O–Mn interaction which governs the physical properties of these systems into two-dimensions. Kimura et al.4 reported the anisotropy in resistivity between the out-plane ( c ) and in-plane ( ab ) components in La1.4Sr1.6Mn2O7 single crystals by a factor as large as 10 3 at room temperature. More important is that La1.4Sr1.6Mn2O7 single crystals showed 50% magnetoresistance effect at a field as low as 0.3 T,3 which is much promising for the applications of CMR materials. So far the reports on the structure and properties of these layered manganates are focused on the La2−2x Sr1+2 x Mn2O7 series,5–9 while the La2−2 x Ca1+2 x Mn2O7 series has not yet received considerable attention. Asano et al.10 reported that the sample of nominal composition La2−2 x Ca1+2 x Mn2O7 (x ⳱ 0.25) crystallized into a layered Sr 3 Ti 2 O 7 J. Mater. Res., Vol. 16, No. 7, Jul 2001
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typestructure with I4/mmm space group (tetragonal: a ⳱ b ⳱ 3.872 Å, c ⳱ 19.248 Å) and exhibits metallic ferromagnetism. The large magnetoresistance (MR) effect (⌬/0 ⳱ 64% at a field of 1.8 T) was observed at a temperature (77 K), which is much lower than its magnetic transition temperature (215
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