CuFe O2 at a megabar: Stabilization of a mixed-valence low-spin magnetic semiconducting ground state

The magnetic and electrical-transport properties of CuFeO2 have been studied by temperature-dependent Fe57 Mössbauer spectroscopy and resistance measurements to near megabar (∼100 GPa) pressures. Previous studies show that CuFeO2 comprises the following sublattices at P>23GPa:13[Cu(S=1/2)2+Fe(S=2)2+O2]+23[Cu(S=0)1+Fe(S=5/2)3+O2]. The magnetic ordering temperatures of the sublattices, with both Fe valences in the high-spin state, reach a maximum at ∼50GPa. At higher pressures two different magnetic components with collapsed magnetic hyperfine fields of 0 and ∼10T have been detected by the Mössbauer probe. The pressure evolution and temperature dependence of the associated hyperfine interaction parameters to ∼95 GPa identifies these as low-spin Fe(S=0)2+ and Fe(S=1/2)3+ states in the very-high-pressure phase of CuFeO2. The Fe3+ low-spin state exhibits an unusually high onset temperature for magnetic ordering, TM∼50K. Resistance-temperature dependences show CuFeO2 in the low-spin state at very high pressure to be a narrow-gap semiconductor with variable range hopping of charge carriers. The derived Mott temperature from this conductivity mechanism, T0, undergoes an appreciable decrease from ∼50GPa onwards where the spin-crossover regions nucleate and grow with increasing abundance in the lattice. An extrapolated T0 value of ∼105K at 100 GPa is interpreted as persistent charge carrier confinement in the low-spin phase. At these high densities a mixed-valence low-spin magnetic semiconducting ground state has been stabilized, in which strong electron correlations are persistent.