Electrochemical and high pressure superoxygenation of YCuO 2+x and LaCuO 2+x delafossites
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Electrochemical and high pressure oxygenation experiments are described which extend the range of intercalated oxygen content beyond that accessible in 1 atm O 2 treatments for the triangular planar cuprate delafossites YG1O2+* and LaCuO 2+x , with the intention of increasing the hole doping, and filling vacancies in the intercalated oxygen lattice to decrease electrical resistivities. Stabilization of the 2H polytype of YCuO 2 through 1% Ca substitution for Y and oxidation conditions for obtaining the pure orthorhombic intercalated oxygen superlattice form of YCu02.5o are also described. Temperature dependent resistivities for some of the superoxygenated compositions are reported; metallic behavior was not observed.
I. INTRODUCTION The electronic and magnetic properties of the square planar C u 2 + - O 2 array, which is at the heart of layered copper oxide superconductors, are of considerable current theoretical and experimental interest. Triangular planar lattices of Cu 2+ and oxygen, on the other hand, which might also be expected to display interesting physical properties, due to the spin 1/2 state of Cu 2+ , the frustrated spin geometry, and the special nature of C u - 0 bonding, were not known until recently.1 In that study, it was shown that the YCuO 2 and LaCuO 2 delafossites, which consist of alternating triangular planes of LnO 6 octahedra and 0 - C u 1 + - 0 sticks, shown in Fig. 1, could be oxidized under a limited set of conditions at 1 atm oxygen pressure to form the new oxidized phases YCuO 2+;c , 0.50 =£ x =£ 0.55 and LaCuOa+x, 0.57 =£ x =£ 0.64. The oxygen is intercalated into the Cu plane much as it is intercalated into a similar plane in Ba 2 YCu 3 0 6 , with the difference that the overall geometry is triangular rather than square. At the oxygen stoichiometries obtained, the copper is formally oxidized to valence 2 + and higher. The materials do indeed display unusual magnetic properties, which are currently under detailed investigation.2 Recent band structure calculations3 have shown that (in the absence of strong electronic correlations) the materials are expected to be metallic conductors at sufficiently high oxygen loading, particularly near stoichiometry LnCuO2.67 where 2/3 of the available oxygen sites can be filled in the Cu plane in an ordered manner to yield the hoped for two-dimensional fully connected C u - O array.4 The materials in the initial study were semiconducting. With the intention of inducing metallic conductivity in the triangular C u - 0 array, we have attempted to increase the oxygen contents of the phases by both ambient temperature electrochemical and elevated temperature 314
http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 2, Feb 1994
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high pressure oxygen treatment methods. The results are reported here. II. SYNTHESIS AND CONVENTIONAL OXIDATION Polycrystalline samples of the reduced form LaCuO 2 delafossite were prepared as described in Ref. 1, from La2O3 and Cu2O powders pressed into pill form and fired in Zr gettered N 2 at 1000
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