Structure and charge transfer correlated with oxygen content for a Y <sub/>0.8</sub> Ca <sub/>0.2</sub> Ba <sub/>2</sub> Cu <sub/>3</sub> O <sub/><i>y</i> </sub> ( <i>y</i> = 6.84–6.32) system: a positron studyCao, Shixun; Li, Lingwei; Liu, Fen; Li, Wenfeng; Chi, Changyun; Jing, Chao; Zhang, Jincang · Superconductor Science and Technology · 2005