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

The relative conductivity of (Formula presented)(Formula presented) 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 300 K. The pressure dependence of the Verwey transition (Formula presented) and the mechanism of conductivity below (Formula presented) 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 (Formula presented): (1) In the range 0-6 GPa the DT curves show sharp minima at (Formula presented) consistent with a first-order phase transition. In this range (Formula presented) 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≊6 GPa (Formula presented) 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; (Formula presented) becomes indiscernible. It was shown that the variation of (Formula presented) with rising P is in close analogy to (Formula presented) changes with chemical composition (δ, x, y) of (Formula presented)(Formula presented), (Formula presented)(Formula presented)(Formula presented), and (Formula presented)(Formula presented)(Formula presented) 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<(Formula presented) the conductivity σ is typical of the variable-range hopping (VRH) mechanism [σ=(Formula presented) exp((Formula presented)/T(Formula presented)] with (Formula presented) decreasing with increasing pressure. Below 30 K σ(T) deviates from the (Formula presented) law at all pressures.