Coincident structural and magnetic order in BaFe2(As1-x Px)2 revealed by high-resolution neutron diffraction

We present neutron diffraction analysis of BaFe2(As1-xPx)2 over a wide temperature (10 to 300 K) and compositional (0.11≤x≤0.79) range, including the normal state, the magnetically ordered state, and the superconducting state. The paramagnetic to spin-density wave and orthorhombic to tetragonal transitions are first order and coincident within the sensitivity of our measurements (∼0.5 K). Extrapolation of the orthorhombic order parameter down to zero suggests that structural quantum criticality cannot exist at compositions higher than x=0.28, which is much lower than values determined using other methods, but in good agreement with our observations of the actual phase stability range. The onset of spin-density wave order shows a stronger structural anomaly than the charge-doped system in the form of an enhancement of the c/a ratio below the transition.