Mössbauer and resistivity studies of the magnetic and electronic properties of the high-pressure phase ofFe3O4

Abstract

The high-pressure phase of magnetite, $h\text{\ensuremath{-}}{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$, has been studied by M\"ossbauer spectroscopy, and electrical conductivity to pressure of $140\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and in the $5--573\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ temperature range. M\"ossbauer studies following annealing at $573\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and $34\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ reveal three equal-abundant iron species within two magnetic sublattices, I and II: ${\mathrm{Fe}}^{3+}(I)$ sublattice with hyperfine field typical of six-coordinated ferric ions and magnetic ordering temperature ${T}_{M}(I)>600\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ at $34\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and a combined ${\mathrm{Fe}}^{3+}(II)$ and ${\mathrm{Fe}}^{2+}$ species forming a magnetic sublattice II with ${T}_{M}(II)\ensuremath{\sim}300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ at $34\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. Starting at $50\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ the ${\mathrm{Fe}}^{3+}(I)$ moment gradually collapses becoming nonmagnetic at $P>80\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. This moment collapse is explained in terms of a charge-transfer $d\text{\ensuremath{-}}p$ gap closure mechanism. ${T}_{M}(II)$ decreases with pressure, and to $120\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, the highest pressure reached with M\"ossbauer spectroscopy, ${\mathrm{Fe}}^{3+}(II)$ remains magnetically ordered. Resistance studies with a nonannealed and highly stoichiometric sample at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ reveal a sharp increase in $R$ at the onset of the $h\text{\ensuremath{-}}{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ phase $(P>25\phantom{\rule{0.3em}{0ex}}\mathrm{GPa})$, reaching a 25-fold maximum at $\ensuremath{\sim}45\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, after which it shows a precipitous decrease in the $45--70\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ range after which it decreases gradually with pressure reaching 10-fold reduction at $140\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. The sharp increase in $R$ is attributed to a gap opening once $h\text{\ensuremath{-}}{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ is formed. Starting at $P>50\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, coinciding with partial correlation breakdown of the ${\mathrm{Fe}}^{3+}(I)$ sublattice, a sluggish onset of metallization is observed manifested by a positive $d\mathrm{R}∕d\mathrm{T}$.