Structure and Charge-Transfer Mechanism in Y1−xCaxBa2Cu3O7−δ Through Direct Doping

Here we study the effect of Ca doping on the charge-transfer mechanism of polycrystalline YBa2Cu3O7−δ compound. The samples of composition Y1−xCaxBa2Cu3O7−δ (x=0.00,0.05,0.10,0.20 and 0.30) are synthesized through standard solid-state reaction route. Carrier doping is controlled by annealing of samples in oxygen and subsequently in reducing atmosphere. Samples are investigated using resistivity, dc magnetization (M–T) and magnetization with field (M–H) measurements. With increase of Ca the transition temperature (Tc) decreases in oxygenated samples, whereas the same increases in reduced samples. Further reduction of samples at higher temperatures (>600 °C) though results in non-superconducting nature up to Ca concentration of x=0.20, the x=0.30 sample is superconducting below 30 K. This provides a remarkably simple and effective way to study the relationship between structure, superconductivity, and associated electronic properties. Variation in Cu1–O4, Cu2–O4 and Cu2–O2 bond lengths with oxygen content, is seen through the structural refinement of XRD pattern. The effective coordination of Cu2 atom with oxygen changes with the change in these bond lengths and hence the holes in the CuO2 planes. The charge-transfer mechanism from CuOx chains to CuO2 planes and thus effective hole doping is discussed in the context of the results observed.