Magnetoelastic coupling in the phase diagram of Ba1−xKxFe2As2as seen via neutron diffraction

We report a high resolution neutron diffraction investigation of the coupling of structural and magnetic transitions in Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$. The tetragonal-orthorhombic and antiferromagnetic transitions are suppressed with potassium doping, falling to zero at $x\lesssim 0.24$. However, unlike Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$, the two transitions are first order and coincident over the entire phase diagram, with a biquadratic coupling of the two order parameters. The phase diagram is refined showing that the onset of superconductivity is at $x=0.133$ with all three phases coexisting until $x⩽0.24$.