First-order magnetic and structural phase transitions in Fe_1+ySe_xTe_1−x

We use bulk magnetic susceptibility, electronic specific heat, and neutron scattering to study structural and magnetic phase transitions in Fe_1+ySe_xTe_1−x. Fe_1.068Te 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 FeSe_1−xTe_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 FeSe_1−xTe_x and FeAs-based pnictides are expected to have similar Fermi surfaces and therefore the same spin-density wave AF order.