LAUSR

A well-known feature of the ${\mathrm{CaFe}}_{2}{\mathrm{As}}_{2}$-based superconductors is the pressure-induced collapsed tetragonal phase that is commonly ascribed to the formation of an interlayer As--As bond. Using detailed x-ray scattering and spectroscopy, we find that Rh-doped ${\mathrm{Ca}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ does not undergo a first-order phase transition and that local Fe moments persist despite the formation of interlayer As--As bonds. Our density functional theory calculations reveal that the Fe--As bond geometry is critical for stabilizing magnetism and the pressure-induced drop in the $c$ lattice parameter observed in pure ${\mathrm{CaFe}}_{2}{\mathrm{As}}_{2}$ is mostly due to a constriction within the FeAs planes. The collapsed tetragonal phase emerges when covalent bonding of strongly hybridized Fe $3d$ and As $4p$ states completely wins out over their exchange splitting. Thus the collapsed tetragonal phase is properly understood as a strong covalent phase that is fully nonmagnetic with the As--As bond forming as a by-product.