Crystal Chemistry and Sub-Solidus Phase Relations in (La,Re) 2 CuO 4 Systems
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CRYSTAL CHEMISTRY AND SUB-SOLIDUS PHASE RELATIONS IN (La,RE) 2CuO 4 SYSTEMS Joseph F. Bringley, Steven S. Trail, Michael McElfresh and Bruce A. Scott IBM Research Division, Thomas J. Watson Research Center Yorktown Heights, NY 10598 ABSTRACT La 2 _.RExCUO 4 (RE = Nd-Y) solid solution systems have been investigated to determine the factors stabilizing the T (La 2CuO 4), T' (Nd 2CuO 4) and hybrid T*-type structures. A simple ionic model with a perovskite-like tolerance factor is found to accurately define the existence field of each. Metastable T* phases are observed for the larger RE cations Nd, Gd, Eu. The structure is quite stable for RE = Dy, Tb, but does not occur at all for the smallest rare earths. Oxygen activity plays a role in T' and T* phase formation. INTRODUCTION There are three closely related structure-types with stoichiometry RE 2CuO 4, each containing isolated sheets of either 4-fold (T'-structure), 5-fold (T*-structure) or 6-fold (T-structurc) Cu-O coordination. For the largest rare-earth cation (La), RE 2CuO 4 crystallizes in a slightly distorted K2NiF 4 T-structure [1,2]. La 2CuO 4 contains perovskite-like sheets of elongated CuO 6 octahedra, sharing corners in the (001) planes and separated by rock-salt-like La-O layers in which La is nine-fold coordinated by oxygen. Hole doping into the T-structure is required to produce superconductivity. Rare-earth ions of intermediate size (Pr-Gd) form the T'-structure of Nd 2CuO 4 [3]. Here, two-dimensional square-planar CuO 2 sheets share corners in (001) planes and are separated by NdO 2 fluorite-type layers. The rare-earth ion is now eight-fold coordinated. Compounds with the T'-structure superconduct only when electrondoped [4,5].
A third RE 2CuO 4 structure, denoted T*, is observed with certain A-site rare earth and alkaline earth cations (e.g., LaGd. 8Sr. 2CuO 4). This structure can be considered an "amalgam" of T and T' [6,7] in which sheets of CuO5 square-pyramids share corners in (001) planes and are separated along (001> by alternating RE-O rock-salt and REO 2 fluorite layers. This creates two crystallographically independent rare-earth ion sites: a nine-fold coordinated "T-site," and an eight-fold coordinated "T'-site." The majority of known T* phases require Sr to stabilize the structure. Two are known which contain only rare earth cations [8-10]. The Sr-doped materials become superconducting (T,-30 K) after annealing in high-pressure oxygen [11-13]. In an effort to understand the crystal chemistry and phase stability of the T, T' and T*-phases, the systems La 2 _xRExCuO 4 (RE = Nd-Y) have been investigated. These mixed rare earth systems allow us to evaluate how ionic size and ordering affect structure and stability. The results are explained within the framework of a simple ionic model in which a perovskite-like tolerance factor (t) is found to be a remarkable predictor of T, T' and T* stability limits. EXPERIMENTAL Powdered mixtures of rare-earth and copper oxides, or coprecipitated hydroxides, were heated in Pt crucibles for 24h, ground, pel
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