IT-9-P-3490 Composite crystal structures of M_xCuO₂ cuprates; (M_x = Li₂, Ca_.83, Sr_.73, Ba_.67, [(Sr/Ca)₂Cu₂O₃]_1/√2, Na)

The M_xCuO₂ cuprates belong to class of composite crystals consisting of two subsystems[1]: „CuO₂-chains“ and „M_x-cations”. An electron microscopy and diffraction study of a number of rare earth cuprates, is presented (Fig.1.) in relation with the corresponding charge ordering that is induced by variable cation valency and nonstoichiometric composition. Level of cation defficiency and the accompanying self doping affects structure modulation, as well as the average Cu-valency; it can range from Cu+2.30 for Ca_.83CuO₂, to Cu+2.66 for Ba_.67CuO₂, and even up to Cu+3.0 for NaCuO₂, or down to Cu+2.0 for Li₂CuO₂. In the case of M_x = Ca_.83, Sr_.73, Ba_.67, [(Sr/Ca)₂Cu₂O₃]1/√2, the two subsystems are mutually incommensurate and modulated along the “chain” direction, while for the end cases of: M_x = Na, Li₂, the structural unit cells are commensurate (Fig. 2.)

The underlying lattices of these subsystems have common a and b parameters while the ratio c_Ch/c_M of their c-parameters along the chain-direction varies with x. For a particular case of M_x=[(Ca/Sr)₂Cu₂O₃]_1/√2, the so-called “chain-ladder” (Sr/Ca)₁₄Cu₂₄O₄₁ compound is well known for its optical and magnetic properties[2],[3]. In this case, the cation subsystem consists of an extended structure: (Sr/Ca)₂Cu₂O₃ -“ladders”. The building unit of the ladders is a pair of cations plus a pair of zigzag edge-sharing CuO₄-squares, Fig. 2, that are connected along “rungs”, so that the cLd period is defined by the CuO₄-square diagonal. For the chains, the CuO₄ building units share opposite edges and the c_Ch period is equal to the CuO₄-square edge. In the case of “chain-ladder” composite structure, the c_Ld/c_Ch ratio is found to vary slightly with cation composition, but is always close to √2, so that the formula [(Ca/Sr)₂Cu₂O₃]_xCuO₂ (x≈1/√2) correctly represents compound's composite structure.

With increasing Ca-substitution the c_Ld/c_Ch ratio varies from 1.44 for pure Sr₁₄Cu₂₄O₄₁, to 1.416 for highly substituted Sr_0.6Ca_13.4Cu₂₄O₄₁. This is accompanied by charge (hole) redistribution between the CuO₂-chains and the Cu₂O₃-ladders[3].

[1] Milat O., Van Tendeloo G., Amelinckx S., Mebhod M., Deltour R. (1992), Acta. Cryst., A48, 618-625.

[2] Vuletić T., Korin-Hamzić B., Ivek T., Tomić S., Gorshunov B., Dressel M., Akimitzu J. (2006), Phys. Rep. 428, 169-258.

[3] 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.