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

Significance: Higher carrier mobility can contribute to a larger power factor, so it is important to identify effective means for achieving higher carrier mobility. Since carrier mobility is governed by the band structure and the carrier scattering mechanism, its possible enhancement could be obtained by manipulating either or both of these. Here, we report a substantial enhancement in carrier mobility by tuning the carrier scattering mechanism in n-type Mg ₃ Sb ₂ -based materials. The ionized impurity scattering in these materials has been shifted into mixed scattering by acoustic phonons and ionized impurities. Our results clearly demonstrate that the strategy of tuning the carrier scattering mechanism is quite effective for improving the mobility and should also be applicable to other material systems.

We use neutron scattering to study the spin and lattice structures of single crystal and powder samples of ${\text{Na}}_{1-\delta }\text{FeAs}$ $\left(T_c=23\text{K}\right)$. Upon cooling from room temperature, the system goes through a series of phase transitions: first changing the crystal symmetry from tetragonal to orthorhombic at 49 K, then ordering antiferromagnetically with a spin structure similar to that of LaFeAsO and a small moment $\left(0.09\pm 0.04{\mu }_B\right)$, and finally becoming superconducting below about 23 K. These results confirm that antiferromagnetic order is ubiquitous for the parent compounds of the iron arsenide superconductors and suggest that the separated structural and magnetic phase-transition temperatures are due to the reduction in the $c$-axis exchange coupling of the system.