On the cation substitution site of doped layered cobaltites studied by neutron diffraction and X-ray absorption spectroscopy

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

Buenos Airs

Workshop; LatinAmerican Workshop on Magnetism and Magnetic Materials; 2013

The layered Co-based compound RBaCo2 O5+δ (R being a rare earth or yttrium) has been intensively studied lately, as it allows 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. Of particular interest is the case δ = 0.5, with Co completely in the +3 oxidation state. In this case, oxygen vacancies may become ordered, leading to a superstructure that consists of an ordered array of 50% Co atoms located in oxygen octahedra and 50% in pyramids. One further variable that has been explored regards the substitution with other cations. During the last years we reported several neutron diffraction studies of Ba-site substituted cobaltites Y(Ba1−x Srx )Co2 O5.5 and Y(Ba1−x Cax )Co2 O5.5 , which showed that even small additions of Sr or Ca in the Ba-site produce dramatic effects on the magnetization of the samples, on the Seebeck coefficient and on the resistivity. However, recent reports by other authors have opened up the possibility that the substitution site for Ca atoms might not be obvious. A study was presented of the (Y0.9 Ca0.1 )BaCo2 O5.5 compound which shows very similar magnetic properties as our Y(Ba0.9 Ca0.1 )Co2 O5.5 samples, thus triggering an investigation on the actual substitution site. Moreover, the possibility of any of both substitution sites has been also suggested, which motivated the careful synthesis of Ca- and Sr- substituted layered cobaltites with nominal substitution sites at Ba and Y. To study this, we performed X-ray absorption spectroscopy at the Ca and Sr K-edges to probe 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. The consequences of these findings will be discussed in terms of the macroscopic magnetic properties observed in each case.