Crystal structure determination by high resolution electron microscopy for (Sr 0.86 Nd 0.14 ) 3 Cu 2 O 5 intergrown with
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I.
INTRODUCTION
HIGH pressure synthesis has attracted much attention as a means of preparing the infinite-layer (IL) superconducting cuprates, which are considered as the parents of superconductors with a chemical formula of A2B2Can21CunO2n14 (A 5 Bi, Tl; and B 5 Sr, Ba).[1] The composition range for forming the IL structure can be extended drastically.[2] So far, the compounds of (Ba, Sr)CuO2(Tc 5 60 to 90 K),[3] (Sr, Nd)CuO2 (Tc 5 40 K),[4] and (La, Sr)CuO2 (Tc 5 34 K)[5] were synthesized under high pressure. Additionally, some new phases coexist stably with the IL structure at high pressure. Among those, two series reported by Hiroi et al.[6,7] are of perovskite-like structure and hence have drawn much attention. One is the series of superconducting compounds Srn11CunO2n111d (n 5 1, 2, 3, . . .) with Tc 5 70 to 100 K. It possesses tetragonal symmetry with lattice parameters of a 5 0.3764 nm, c 5 1.2548 nm for n 5 1; a 5 0.39 nm, c 5 2.10 nm for n 5 2; and a 5 0.39 nm, c 5 2.78 nm for n 5 3.[6] The other series contains nonsuperconducting compounds Srn21Cun11O2n (n 5 3, 5, . . .) which belong to the orthorhombic system with lattice parameters of a ' 0.39 nm, b ' n 3 a, and c ' 0.35 nm.[7] The crystal structure of the two series is closely related with that of the IL SrCuO2 unit cell, and furthermore, the structure of both series degenerates to the IL structure of SrCuO2 when n becomes infinite. In the present article, a new phase (Sr0.86Nd0.14)3Cu2O5, which is intergrown with the IL structure, is reported. The crystal structure of the new phase is determined by high resolution electron microscopy (HREM) and selected area electron diffraction (SAED). II.
EXPERIMENTAL
The nominal composition of the compound is Sr0.86Nd0.14CuO2. The synthesis procedure of the sample is Y.G. PENG, formerly with Laboratory of Atomic Imaging of Solids, Chinese Academy of Sciences, is now Senior Researcher with the National Key Laboratory of Advanced Composites, Beijing Institute of Aeronautical Materials, Beijing 100095, P.R. China. F.H. LI and Y.S. YAO, Professors, are with the Institute of Physics, Chinese Academy of Sciences, Beijing 100080, P.R. China. Manuscript submitted July 8, 1996.
METALLURGICAL AND MATERIALS TRANSACTIONS A
similar to that described in a former article,[8] but the annealing temperature is 650 7C rather than 1000 7C. The resistivity-temperature dependence was determined by a four-probe method, and the result reveals that the sample does not exhibit superconductivity. Ion-milled specimens were prepared for electron microscopic observation. A JEM-200CX transmission electron microscope and a JEM2000EX (Cs 5 0.7 mm) high resolution one, both equipped with a top-entry goniometer stage and operated at 200 kV, were used for SAED and HREM investigations.
III.
S PHASE AND ITS UNIT CELL
A new secondary phase, called S phase hereafter, was found to be intergrown with or embedded in the IL phase Sr0.86Nd0.14CuO2, as shown in the lattice image in Figure 1. The new S phase and the IL phase are labeled S and IL in th
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