Lattice collapse and quenching of magnetism in CaFe2As2 under pressure:   A single crystal neutron and x-ray diffraction investigation
  Single crystal neutron and high-energy x-ray diffraction have identified the phase lines corresponding to transitions between the ambient-pressure tetragonal (T), the antiferromagnetic orthorhombic (O) and the non-magnetic collapsed tetragonal (cT) phases of CaFe2As2. We find no evidence of additional structures for pressures up to 2.5 GPa (at 300 K). Both the T-cT and O-cT transitions exhibit significant hysteresis effects and we demonstrate that coexistence of the O and cT phases can occur if a non-hydrostatic component of pressure is present. Measurements of the magnetic diffraction peaks show no change in the magnetic structure or ordered moment as a function of pressure in the O phase and we find no evidence of magnetic ordering in the cT phase. Band structure calculations show that the transition results in a strong decrease of the iron 3d density of states at the Fermi energy, consistent with a loss of the magnetic moment. 
A.I. Goldman, A. Kreyssig, K. Prokes, D.K. Pratt, D.N. Argyriou, J.W.   Lynn, S. Nandi, S.A.J. Kimber, Y. Chen, Y.B. Lee, G....