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Commensurate superstructures in the [(Ca/Sr)2Cu2O3][CuO2]x≈√2 composite crystal

An electron microscopy and diffraction study of the nominal (Sr/Ca)14Cu24O41 compound is presented. We propose an alternative formula: [(Ca/Sr)2Cu2O3][CuO2]x≈√2 that better represents this incommensurate composite crystal structure. Namely, composite crystals are a class of long range ordered solids that are composed of two or more subsystems, each one with its own lattice and cell symmetry. For a series of compounds with widely used formula (Sr/Ca)14Cu24O41 (Ca for Sr isostructural substitution), the constituting subsystems are: (i) Sr/Ca)2Cu2O3 -“ladders”, and ; (ii) CuO2 -“chains”[1][2]. The lattices of these subsystems have common a and b parameters while being incommensurate along c-axis. With increasing Ca-substitution the cLd/cChratio varies from 1.44 for pure Sr14Cu24O41, to 1.416 for highly substituted Sr0.6Ca13.4Cu24O41[3]. This is accompanied by charge (hole) redistribution between the CuO2-chains and the Cu2O3-ladders. More holes reside in the CuO2-chains for higher cLd/cCh ratio [4].

composite crystal; modulated structure; electron diffraction

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