Superconductivity in LaFeAs$_{1-x}$P$_{x}$O: effect of chemical   pressures and bond covalency
  We report the realization of superconductivity by an isovalent doping with phosphorus in LaFeAsO. X-ray diffraction shows that, with the partial substitution of P for As, the Fe$_2$As$_2$ layers are squeezed while the La$_2$O$_2$ layers are stretched along the c-axis. Electrical resistance and magnetization measurements show emergence of bulk superconductivity at $\sim$10 K for the optimally-doped LaFeAs$_{1-x}$P$_{x}$O ($x=0.25\sim0.3$). The upper critical fields at zero temperature is estimated to be 27 T, much higher than that of the LaFePO superconductor. The occurrence of superconductivity is discussed in terms 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