Magnetoelastic Coupling in the Phase Diagram of Ba1-xKxFe2As2
  We report a high resolution neutron diffraction investigation of the coupling of structural and magnetic transitions in Ba1xKxFe2As2. The tetragonal-orthorhombic and antiferromagnetic transitions are suppressed with potassium-doping, falling to zero at x <~ 0.3. However, unlike Ba(Fe1xCox)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. 
Sevda Avci, Omar Chmaissem, Eugene A. Goremychkin, Stephan Rosenkranz,   John-Paul Castellan, Duck-Young Chung, Ilya S. Todo...
cond-mat.supr-con