First-order magnetic and structural phase transitions in Fe1+y Sex Te1-x

We use bulk magnetic susceptibility, electronic specific heat, and neutron scattering to study structural and magnetic phase transitions in Fe1+y Sex Te1-x. Fe1.068 Te exhibits a first-order phase transition near 67 K with a tetragonal-to-monoclinic structural transition and simultaneously develops a collinear antiferromagnetic (AF) order responsible for the entropy change across the transition. Systematic studies of the FeSe1-x Tex system reveal that the AF structure and lattice distortion in these materials are different from those of FeAs-based pnictides. These results call into question the conclusions of present density-functional calculations, where FeSe1-x Tex and FeAs-based pnictides are expected to have similar Fermi surfaces and therefore the same spin-density wave AF order.