Mössbauer and resistivity studies of the magnetic and electronic properties of the high-pressure phase of Fe₃O₄

The high-pressure phase of magnetite, h-Fe₃O₄, has been studied by Mössbauer spectroscopy, and electrical conductivity to pressure of 140 GPa and in the 5-573 K temperature range. Mössbauer studies following annealing at 573 K and 34 GPa reveal three equal-abundant iron species within two magnetic sublattices, I and II: Fe³⁺(I) sublattice with hyperfine field typical of six-coordinated ferric ions and magnetic ordering temperature T_M(I)>600 K at 34 GPa and a combined Fe ³⁺(II) and Fe²⁺ species forming a magnetic sublattice II with T_M(II) ∼300 K at 34 GPa. Starting at 50 GPa the Fe ³⁺(I) moment gradually collapses becoming nonmagnetic at P > 80 GPa. This moment collapse is explained in terms of a charge-transfer d-p gap closure mechanism. T_M(II) decreases with pressure, and to 120 GPa, the highest pressure reached with Mössbauer spectroscopy, Fe ³⁺(II) remains magnetically ordered. Resistance studies with a nonannealed and highly stoichiometric sample at 300 K reveal a sharp increase in R at the onset of the h-Fe₃O₄ phase (P > 25 GPa), reaching a 25-fold maximum at ∼45 GPa, after which it shows a precipitous decrease in the 45-70 GPa range after which it decreases gradually with pressure reaching 10-fold reduction at 140 GPa. The sharp increase in R is attributed to a gap opening once h-Fe₃O₄ is formed. Starting at P > 50 GPa, coinciding with partial correlation breakdown of the Fe³⁺(I) sublattice, a sluggish onset of metallization is observed manifested by a positive dR/dt.