The phase diagram of BaFe$_2$(As$_{\mathrm{1-x}}$P$_{x})_2$ as determined by neutron diffraction

The iron-arsenides are a now famous family of high-$T_{\mathrm{c}}$ superconductors where the superconducting state is stabilized by suppressing a magnetic ground state in a parent compound. The phenomenon is quite robust, and BaFe$_{2}$As$_{2}$, for example, can be made superconducting either by applying pressure or by electron, hole, or isovalent doping. The isovalently doped BaFe$_{2}$(As$_{\mathrm{1-x}}$P$_{x})_{2}$ materials are particularly interesting because it is not obvious what is driving the suppression of the SDW and enhancing $T_{\mathrm{c}}$. The driving force has been variously ascribed to chemical pressure, changes in polarity of the Fe-(As,P) bond, and other even more subtle chemical effects. Moreover, reports on various general features in the iron-arsenide phase diagram---such as short-range nematic order and the separation of the N\'{e}el transition ($T_{\mathrm{N}})$ and the structural transition ($T_{\mathrm{s}})$---remain contradictory and underexplored. We have undertaken a detailed neutron diffraction study of the phase diagram in order to clarify some of the ambiguities. We find that $T_{\mathrm{s}} =$ $T_{\mathrm{N}}$ and that the superconducting dome rises more sharply than for the aliovalently doped materials. Moreover, the $T$ dependence of the structural and magnetic order parameters and a discontinuous increase in $c$/$a$ below $T_{\mathrm{N}}$ suggest a first order phase transition.