O 1111 System*

Following the discrovery of the microscopic co-existence of antifermagnetic spin density waves and superconductivity in Ba_1-xK_xFe₂As₂ and the low temperature structural re-entrance to the magnetic C₄ tetragonal phase in Ba_1-xNa_xFe₂As₂, an effort was made to seek similar phases in the related LaFeAs_1-xP_xO 1111 system. Previous work had reported two superconducting domes as well as two distinct magnetic regions. While the low doping magnetic phase had been well characterized by neutron diffraction, the higher doped magnetic region had been deduced from short range ³¹P-NMR measurements. We present a detailed phase diagram with nuclear and magnetic structural characterization based on a combination of x-ray and neutron diffraction, as well as magnetic temperature dependence thoroughly mapped by muon spin resonance. This work demonstrates three distinct magnetic phases evolving from in-plane orthorhombic to in-plane tetragonal to out-of-plane tetragonal permeating the entire phase diagram from the parent LaFeAsO to superconducting LaFeAs_0.2P_0.8O.