Superconductivity in LaFeAsPxO: Effect of chemical pressures and bond covalency Cao Wang, Shuai Jiang, Qian Tao, Zhi Ren, Yuke Li, Linjun Li, Chunmu Feng, Jianhui Dai, Guanghan Cao and Zhu-an Xu EPL, 86 4 (2009) 47002 Published online: 26 May 2009 DOI: 10.1209/0295-5075/86/47002