High-pressure magnetic, electronic, and structural properties of $MFe_{2}O_{4}$ (M = Mg, Zn, Fe) ferric spinels

Magnetic, electronic, and structural properties of $MFe_{2}O_{4}$ (M=Mg,Zn,Fe) ferric spinels have been studied by $^{57}$Fe Mössbauer spectroscopy, electrical conductivity, and powder and single-crystal x-ray diffraction (XRD) to a pressure of 120 GPa and in the 2.4–300 K temperature range. These studies reveal for all materials, at the pressure range 25–40 GPa, an irreversible first-order structural transition to the postspinel $CaTi_{2}O_4$− type structure (Bbmm) in which the HS Fe$^{3+}$ occupies two different crystallographic sites characterized by six- and eightfold coordination polyhedra, respectively. Above 40 GPa, an onset of a sluggish second-order high-to-low spin (HS-LS) transition is observed on the octahedral Fe$^{3+}$ sites while Fe$^{3+}$ occupying bicapped trigonal prism sites remain in the HS state. Despite an appreciable resistance decrease, corroborating with the transition to the LS state, MgFe$_2$O$_4$ and ZnFeO$_4$ remain semiconductors at this pressure range. However, in the case of Fe$_3$O$_4$, the second-order HS-LS transition on the Fe$^{3+}$ octahedral sites corroborates with a clear trend to a gap closure and formation of a semimetal state above 50 GPa. Above 65 GPa, another structural phase transition is observed in Fe$_3$O$_4$ to a new Pmma structure. This transition coincides with the onset of nonmagnetic Fe$^{+2}$, signifying further propagation of the gradual collapse of magnetism corroborating with a sluggish metallization process. With this, half of Fe$^{3+}$ sites remain in the HS state. Thus, this paper demonstrates that, in a material with a complex crystal structure containing transition metal cation(s) in different environments, a HS-LS transition and delocalization/metallization of the 3d electrons does not necessarily occur simultaneously and may propagate through different crystallographic sites at different degrees of compression