PHYSICAL REVIEW B, cilt.86, 2012 (SCI İndekslerine Giren Dergi)
Layered cobaltites RBaCo2O5.5 (R = rare earth and Y) undergo a sequence of magnetic and electronic transitions between antiferromagnetic/ferrimagnetic/paramagnetic and insulating/paramagnetic metallic states with respective transition temperatures T-N (similar to 230-260 K) < T-C (similar to 250-290 K) < T-MI (similar to 360 K). We have synthesized a Nd1-xCaxBaCo2O5.5 series (0 <= x <= 0.2) of cation-[(Nd,Ca)/Ba]-ordered and oxygen vacancy-ordered materials and investigated them by neutron diffraction, magnetization, and electronic and thermal transport. We observe that upon Ca doping, T-N is decreasing to 0 for x = 0.1 and T-C is increasing and coincides with T-MI for x > 0.12, which weakly changes with Ca substitution from similar to 360 to similar to 340 K. This is the largest enhancement of T-C observed for these cobaltites. Unlike hole doping by adding oxygen, Ca doping does not disrupt the cation and oxygen vacancy orderings up to x = 0.20.