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Structure modulation and charge ordering in selfdoped MxCuO2 crystals

An electron diffraction study of a number of rare earth cuprates MxCuO2, is presented (for Mx = Ca.83, Sr.73, Ba.67, [Sr/Ca2Cu2O3].70). These cuprates belong to class of composite crystals consisting of two subsystems [1]: „CuO2-chains“ and „Mx“. These subsystems are interpenetrated, mutually modulated, and usually incommensurate along the chain direction. The lattices of these subsystems have common a and b parameters while the ratio of parameters along chains cM/cCh varies with 1/x. For Mx = [(Ca/Sr)2Cu2O3]0.7, it is the case of “chain-ladder” compound: (Sr/Ca)14Cu24O41. It is well known because of exotic interaction, ordering and dynamics of electron holes and spins within either of the two constituting subsystems [2], [3]. Various level of self doping (also related to nonstoichiometry x) affects Cu-valency to vary in the range from +2.66 for Ba.67CuO2, to +2.30 for Ca.85CuO2, and even to +3.0 in NaCuO2, and +2.0 in Li2CuO2. In the case of [(Sr/Ca)2Cu2O3].70CuO2, the average Cu valency is +2.25, but for all excess charge distributed in the CuO2-chains, the Cu valency should be: +2.6 on „chains“ (+2.0 on „ladders“). In addition Cu+2 carries spin ½ while Cu+3 is nonmagnetic. La for Sr substitution strongly affects charge distribution and spin depletion in CuO2-chains [4], resulting with no excess charge in La6Ca8Cu24O41. On the other hand, our electron diffraction study of „chain-ladder“ compounds reveals that it is only the amplitude of displacive modulation of basic crystal structure that is slightly affected by charge and spin ordering [5]. [1] Milat O., Van Tendeloo G., Amelinckx S., Mebhod M., Deltour R. (1992), Acta. Cryst., A48, 618-625. [2] T. Ivek, T. Vuletić, B. Korin-Hamzić, O. Milat, and S. Tomić, at all (2008) Phys. Rev. B78, 205105. [3] T. Vuletić, B. Korin-Hamzić, T. Ivek, S. Tomić, B. Gorshunov, M. Dressel, J. Akimitzu (2006), Phys. Rep. 428, 169-258. [4] Ilakovac V., Gougoussis C., Calandra M., Brookes N. B., Bisogni V., Chiuzbaian S. G., Akimitsu J., Milat O., Tomic S., Hague C. F. (2012), Phy. Rev. B 85, 075108. [5] O. Milat, K. Salamon, S. Tomić, T. Vuletić, T. Ivek (2011) Proc. 10th MCM 2011, Urbino, Italy, ed. E. Falcieri, SISM 2011, pp 41-42.

composite crystal; electron diffraction; charge ordering

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