Magnetoelastic coupling in the phase diagram of Ba1-xKxFe2As2 as seen via neutron diffraction.

We report a high resolution neutron diffraction investigation of the coupling of structural and magnetic transitions in Ba1-xKxFe2As2. The tetragonal-orthorhombic and antiferromagnetic transitions are suppressed with potassium doping, falling to zero at x≲0.24. However, unlike Ba(Fe1-xCox)2As2, 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. [ABSTRACT FROM AUTHOR]