Microscopic phase diagram of Eu(Fe1−xNix)As2 (x = 0,0.04) under pressure

To establish the microscopic P-T phase diagram of recent 112-type iron-pnictides $\mathrm{Eu}({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}){\mathrm{As}}_{2}$ (x = 0, 0.04), high-pressure synchrotron M\"ossbauer spectroscopy experiments in $^{151}\mathrm{Eu}$ and $^{57}\mathrm{Fe}$ have been performed. In ${\mathrm{EuFeAs}}_{2}$ application of pressure completely suppresses the itinerant electron magnetism from the Fe sublattice and the local-moment magnetism in Eu ions at $\ensuremath{\sim}10\phantom{\rule{0.16em}{0ex}}$ and $\ensuremath{\sim}11.6\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, respectively. High-pressure x-ray diffraction experiments in ${\mathrm{EuFeAs}}_{2}$ reveal an anomalous change in the lattice parameters and a discontinuity in volume around 10 GPa, suggesting an isostructural transition at this pressure. With Ni-doping (x = 0.04), a collapse of local magnetic order occurs at $\ensuremath{\sim}8\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, a lower critical pressure compared with the parent compound. In both systems, the suppression of local-moment magnetism is associated with a significant increase of mean valence in Eu ions.