High-pressure magnetic, electronic, and structural properties ofMFe2O4(M=Mg,Zn,Fe) ferric spinels

Abstract

Magnetic, electronic, and structural properties of $M\mathrm{F}{\mathrm{e}}_{2}{\mathrm{O}}_{4} (M=\mathrm{Mg},\mathrm{Zn},\mathrm{Fe})$ ferric spinels have been studied by $^{57}\mathrm{Fe}$ M\"ossbauer 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 $\mathrm{CaT}{\mathrm{i}}_{2}{\mathrm{O}}_{4}\ensuremath{-}$ type structure (Bbmm) in which the HS $\mathrm{F}{\mathrm{e}}^{3+}$ 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 $\mathrm{F}{\mathrm{e}}^{3+}$ sites while $\mathrm{F}{\mathrm{e}}^{3+}$ occupying bicapped trigonal prism sites remain in the HS state. Despite an appreciable resistance decrease, corroborating with the transition to the LS state, $\mathrm{MgF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}$ and $\mathrm{ZnF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}$ remain semiconductors at this pressure range. However, in the case of $\mathrm{F}{\mathrm{e}}_{3}{\mathrm{O}}_{4}$, the second-order HS-LS transition on the $\mathrm{F}{\mathrm{e}}^{3+}$ 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 $\mathrm{F}{\mathrm{e}}_{3}{\mathrm{O}}_{4}$ to a new Pmma structure. This transition coincides with the onset of nonmagnetic $\mathrm{F}{\mathrm{e}}^{2+}$, signifying further propagation of the gradual collapse of magnetism corroborating with a sluggish metallization process. With this, half of $\mathrm{F}{\mathrm{e}}^{3+}$ 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.