ON THE CATION SUBSTITUTION SITE OF Ca-DOPED LAYERED COBALTITES YBaCo2 O5.5 STUDIED BY NEUTRON DIFFRACTION AND X-RAY ABSORPTION SPECTROSCOPY

AURELIO, G.; JUNQUEIRA PRADO, R.; SALETA, M; SÁNCHEZ, R. D.

Campinas

Conferencia; 23 Reunión Anual de Usuarios LNLS; 2013

The layered compounds RBaCo2 O5+δ (R being a rare earth or yttrium) have been intensively studied in the past few years, as they allow for a wide range of oxygen non-stoichiometry controlling the mixed valence state of Co ions, and therefore many novel physical properties may be tuned. An insteresting variable to explore regards the substitution with other non-magnetic cations. Recently, we reported a neutron diffraction study of the Ba-site substituted cobaltites [1], showing that even a small addition of Sr or Ca in the Ba-site (5 to 10%) produces a dramatic effect on the magnetization of the samples, on the Seebeck coefficient and on the resistivity. However, recent reports on substituted cobaltites have opened up the possibility that the substitution site for Ca atoms might not be obvious. Motin Seikh et al. [2] presented a study of Ca substituting at the Y site, showing very similar magnetic properties as our nominally Ba-site substituted samples. The structure of layered cobaltites has been shown to allow the accommodation of cations with a wide range of ionic radii at the lanthanide site, which in addition to the wide oxygen non- stoichiometry range and three possible oxidation states for Co, renders an extreme versatility to the formation of the compound. Therefore, it can easily accommodate a host ion even if the charge and/or radius of the dopant is quite different. More recently [3], the possibility of any of both substitution sites has been suggested. All these studies motivated the careful synthesis of Ca- and Sr- substituted layered cobaltites with nominal substitution sites at Ba and at Y. We performed X-ray absorption spectroscopy at the Ca and Sr K-edges to study the local environment around the dopant ions and arrived to a picture showing inequivocally that Ca is replaced at the Y site, whereas the Sr substitution site seems to be influenced by the dopant concentration. [1] G. Aurelio et al. J. Phys.: Condens. Matter 23 315403 (2011) and references therein [2] M. Motin Seikh, V. Caignaert, V. Pralong, C. Simon, and B. Raveau, J. of Phys.: Cond. Matter 20, 015212 (2008). [3] T. Sarkar, V. Pralong, and B. Raveau, Physical Review B 83, 214428 (2011).