Nature of the Verwey transition in magnetite (Fe3O4) to pressures of 16 GPa

The relative conductivity of Fe3O4 single crystals very close to ideal stoichiometry has been measured as a function of pressure up to P=16 GPa and in the temperature range of 4.2 to 309 K. The pressure dependence of the Verwey transition T-V and the mechanism of conductivity below T-V were the main issues addressed. Three pressure regimes were assigned, based on the different behavior of the temperature derivative (DT) of the conductivity curves through T-V: (1) In the range 0-6 GPa the DT curves show sharp minima at T-V consistent with a first-order phase transition. In this range T-V decreases linearly with pressure from 122 to 107.5 K. (2) At P>6 GPa the DT minima broaden considerably, consistent with a second- or higher-order transition. At P approximate to 6 GPa T-V changes discontinuously from 107.5 to 100 K and between 6 and 12.5 GPa decreases linearly to 83 K. (3) At P>12.5 GPa no DT minima are detected; T-V becomes indiscernible. It was shown that the variation of T-V with rising P is in close analogy to T-V changes with chemical composition (delta, x, y) of Fe-3(1-delta)O-4, Fe3-xZnxO4, and Fe3-yTiyO4 as compiled from previous studies. This information is rationalized in terms of phase transitions associated with ''Wigner structures.'' Changes in the band gap with rising P are discussed. Below 16 GPa and at 30 K<T<T-V the conductivity sigma is typical of the variable-range hopping (VRH) mechanism [sigma=sigma(0) exp(T-0/T)(1/4)] with T-0 decreasing with increasing pressure. Below 30 K sigma(T) deviates from the T1/4 law at all pressures.