Effect of structural collapse on electron density distribution and magnetic properties of ThCr₂Si₂-type pnictides

The structural phase transitions in the ThCr₂Si₂-type materials involve a gradual or abrupt (first-order) collapse along the tetragonal c axis. Despite many examples of such transitions in the AT₂X₂ structures, the direct experimental assessment of changes in the electron density redistribution between the A and [T₂X₂] layers upon the formation and breaking of the X–X bonding interactions is largely lacking. Earlier studies have revealed fascinating pressure-induced transitions in EuCo₂Pn₂ (Pn = P, As) which are accompanied by the change in the Eu oxidation state and the transition from the localized (4f) magnetism to itinerant (3d) magnetism. In the present contribution, we demonstrate that the changes in the electron concentration in the [Co₂Pn₂] layer defy the formal electron-counting rules that are often used for Zintl-like phases. X-ray absorption measurements offer the direct insight into the changes in the Eu oxidation state and magnetism and the associated redistribution of the electron density in the [Co₂Pn₂] layer. Calculations of the band structure and analysis of chemical bonding have provided a satisfactory interpretation for the observed changes in the X-ray absorption spectra.