Crystallochemistry of Fe-Based Superconductors: Interplay Between Chemical, Structural and Physical Properties in the Fe(Se,Te) and 1111-Type Systems

The physical properties of the crystalline phases are strongly dependent on the symmetric arrangement of the atomic species in the solid, the distribution of the intervening chemical bonds, and the overall chemical composition. In the Fe(Se,Te) system, the parent compound Fe <inf>1+y</inf>Te crystallizes in the tetragonal system at room temperature, but on cooling different structural transitions are observed, depending on the amount of the Fe-interstitial content. In particular, the amount of the interstitial Fe strongly controls the amplitude of the different symmetry-breaking modes intervening in the structural transitions. On the other hand, these modes are progressively suppressed by the substitution of Te with Se. The structural transition characterizing the 1111-type systems exhibit significant differences: the displacive mode that should drive the symmetry-breaking is not involved in any atomic displacement, but an abrupt increase of lattice microstrain occurs along the tetragonal hh0 direction in LnFeAsO compounds just above the structural transformation; this strain is then suppressed by symmetry breaking. It is generally accepted that above a critical amount of electron doping (induced by F substitution), the tetragonal to orthorhombic transition is suppressed. Recently, we found strong experimental evidences that the orthorhombic distortion is actually strongly reduced with doping, but not suppressed, reconciling the apparently contradictory findings reported in literature for the tetragonal to orthorhombic transition in electron and hole-doped compounds.