The substitution of Ca for rare earth in RBa2Cu3O7-δ (R = rare earth) generates excess holes in the 123 system, and Tc is suppressed due to the over doping effect. It is also known that the Ca doping cointroduces oxygen vacancies which reduces the number of generated holes. However, there is still some disagreement on the site where the oxygen vacancy is created, the plane or the chain site. There are also some arguments about the site which Ca occupies, the R or the Ba site. We performed the element specific extended x-ray absorption fine structure study at the Ca K edge, Gd and Ho LIII edge for a series of R1-χ,CaχBa2Cu3O7-δ (R = Gd, Ho) samples withχ= 0,0.1 and 0.2. Our data indicate that Ca is located in the R site, there is no observable change of oxygen content in the CuO2 plane and that the increased Ca concentration causes a higher disorder in the local environment around the rare earth site.
Chinese Journal of Physics=中國物理學刊 36(2-I)，pp.114-119