Neutron Powder Diffraction Study on the Non-Superconducting Phases of ThFeAsN_1-xO_x (x=0.15, 0.6) Iron Pnictide

We use neutron powder diffraction to study the non-superconducting phases of ThFeAsN_1-xO_x with x=0.15, 0.6. In our previous results of the superconducting phase ThFeAsN with T_\rm c=30 K, no magnetic transition is observed by cooling down to 6 K, and possible oxygen occupancy at the nitrogen site is shown in the refinement Europhys. Lett. 117 (2017) 57005. Here in the oxygen doped system ThFeAsN_1-xO_x, two superconducting regions (0\leqslant x \leqslant 0.1 and 0.25\leqslant x \leqslant 0.55) are identified by transport experiments J. Phys.: Condens. Matter 30 (2018) 255602. However, within the resolution of our neutron powder diffraction experiment, neither the intermediate doping x=0.15 nor the heavily overdoped compound x=0.6 shows any magnetic order from 300 K to 4 K. Therefore, while it shares the common phenomenon of two superconducting domes as most 1111-type iron-based superconductors, the magnetically ordered parent compound may not exist in this nitride family.