Crystallographic transitions related to high-pressure magnetic and electronic phenomena in Fe-based compounds

The main purpose of this paper is to address the issue of crystallographic transitions related to magnetic/electronic phenomena in strongly correlated transition metal (TM) compounds in a regime of very high static density. The experimental tools used were: synchrotron X-ray diffraction, Mossbauer spectroscopy and electrical resistivity. We focus on the following cases: (i) high-spin to low-spin transition which could lead to a significant reduction of the TM ionic radii and therefore even a structural transition; (ii) sluggish structural phase transitions in antiferromagnetic insulators FeI₂ and FeCl₂ attributed to the onset of a Mott transition; (iii) volume dependence of the orbital term of the moment in FeI₂ and FeCl ₂ resulting in its eventual collapse, which is accompanied by a significant lattice distortion; and (iv) pressure-induced metal-metal intervalence charge transfer in the antiferromagnetic Cu¹⁺Fe ³⁺O₂ as a result of the increase in overlap of atomic orbitals.