Collapsed tetragonal phase in Ca Fe1 xCox 2As2 stabilized by pressure Structural studies using single crystal neutron diffraction

Ca Fe1 amp; 8722;xCox 2As2 single crystals with concentration x 0.032, 0.051, and 0.063 were investigated by means of neutron diffraction under hydrostatic pressure. The region of existence of the high temperature tetragonal, low temperature orthorhombic, and pressure induced collapsed tetragonal phases have been determined and the corresponding p T phase diagrams for the three compositions have been constructed. Structural details of the tetragonal phases at various conditions have been determined. The anomaly marking the orthorhombic to tetragonal structural phase transition temperature TS decreases with increasing Co substitution. Similarly to that, the critical pressure of the tetragonal or orthorhombic to collapsed tetragonal transition also decreases with increasing Co doping level. The behavior is highly hysteretic and the differences in critical pressures with increasing and decreasing branches eventually makes the collapsed tetragonal phase stable down to ambient pressure and enables structural studies under identical conditions. Within the error bars on the fourth digit, no modification of the As structural positional parameter is found. Such a result suggests that the change of the As As bond is caused solely by the reduction of the c axis lattice parameter