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 depende nce of the Verwey transition T-V and the mechanism of conductivity bel ow T-V were the main issues addressed. Three pressure regimes were ass igned, based on the different behavior of the temperature derivative ( DT) of the conductivity curves through T-V: (1) In the range 0-6 GPa t he DT curves show sharp minima at T-V consistent with a first-order ph ase transition. In this range T-V decreases linearly with pressure fro m 122 to 107.5 K. (2) At P>6 GPa the DT minima broaden considerably, c onsistent 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 mini ma are detected; T-V becomes indiscernible. It was shown that the vari ation of T-V with rising P is in close analogy to T-V changes with che mical composition (delta, x, y) of Fe-3(1-delta)O-4, Fe3-xZnxO4, and F e3-yTiyO4 as compiled from previous studies. This information is ratio nalized in terms of phase transitions associated with ''Wigner structu res.'' Changes in the band gap with rising P are discussed. Below 16 G Pa 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 f rom the T1/4 law at all pressures.