  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.
