First-order magnetic and structural phase transitions in Fe1+ySexTe1−x

We use bulk magnetic susceptibility, electronic specific heat, and neutron scattering to study structural and magnetic phase transitions in ${text{Fe}}_{1+y}{text{Se}}_{x}{text{Te}}_{1ensuremath{-}x}$. ${text{Fe}}_{1.068}text{Te}$ 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 ${text{FeSe}}_{1ensuremath{-}x}{text{Te}}_{x}$ 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 ${text{FeSe}}_{1ensuremath{-}x}{text{Te}}_{x}$ and FeAs-based pnictides are expected to have similar Fermi surfaces and therefore the same spin-density wave AF order.