The recent discovery of superconductivity in $(\mathrm{Ba},\mathrm{K}){\mathrm{Fe}}_2{\mathrm{As}}_2$, which crystallizes in the ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$ (122) structure as compared with the LnFeAsO (Ln is lanthanide) systems that possess the ZrCuSiAs (1111) structure, demonstrates the exciting potential of the FeAs-based materials for high-$T_C$ superconductivity. Here we report neutron diffraction studies that show a tetragonal-to-orthorhombic distortion associated with the onset of $\mathbf{q}=(101)$ antiferromagnetic order in ${\mathrm{BaFe}}_2{\mathrm{As}}_2$, with a saturation moment $0.87(3){\mu }_B$ per Fe that is orientated along the longer $a$ axis of the $ab$ planes. The simultaneous first-order structural and magnetic transition is in contrast with the separated transitions previously reported in the 1111-type materials. The orientational relation between magnetic alignment and lattice distortion supports a multiorbital nature for the magnetic order.

The new $\alpha$-Fe(Te,Se) superconductors share the common iron building block and ferminology with the LaFeAsO and ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ families of superconductors. In contrast with the predicted commensurate spin-density-wave order at the nesting wave vector ($\pi$, 0), a completely different magnetic order with a composition tunable propagation vector ($\delta \pi$, $\delta \pi$) was determined for the parent compound ${\mathrm{Fe}}_{1+y}\mathrm{Te}$ in this powder and single-crystal neutron diffraction study. The new antiferromagnetic order survives as a short-range one even in the highest $T_C$ sample. An alternative to the prevailing nesting Fermi surface mechanism is required to understand the latest family of ferrous superconductors.

Recent investigations of the superconducting iron-arsenide families have highlighted the role of pressure, be it chemical or mechanical, in fostering superconductivity. Here we report that ${\text{CaFe}}_2{\text{As}}_2$ undergoes a pressure-induced transition to a nonmagnetic volume “collapsed” tetragonal phase, which becomes superconducting at lower temperature. Spin-polarized total-energy calculations on the collapsed structure reveal that the magnetic Fe moment itself collapses, consistent with the absence of magnetic order in neutron diffraction.