Magnetic and Structural Phase Diagram of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$

It is well known that the partial substitution of Ba by K in Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ causes a steep suppression of both the antiferromagnetic and tetragonal-orthorhombic transitions, leading to the onset of superconductivity over a large substitution range peaking at 38 K for x = 0.4. We report high resolution neutron powder diffraction results, which show that the magnetic and structural transitions are coincident over the entire phase diagram, in contrast to Ba(Fe$_{1-x}$,Co$_{x})_{2}$As$_{2}$. Volume discontinuities show that the combined transitions are first-order. The superconducting phase diagram has been refined with greater precision and a narrow region of phase coexistence have been delineated.