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

Abstract

The relative conductivity of ${\mathrm{Fe}}_3$ ${\mathrm{O}}_4$ 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 ${\mathit{T}}_{\mathit{V}}$ and the mechanism of conductivity below ${\mathit{T}}_{\mathit{V}}$ 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 ${\mathit{T}}_{\mathit{V}}$: (1) In the range 0–6 GPa the DT curves show sharp minima at ${\mathit{T}}_{\mathit{V}}$ consistent with a first-order phase transition. In this range ${\mathit{T}}_{\mathit{V}}$ 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≊6 GPa ${\mathit{T}}_{\mathit{V}}$ 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; ${\mathit{T}}_{\mathit{V}}$ becomes indiscernible. It was shown that the variation of ${\mathit{T}}_{\mathit{V}}$ with rising P is in close analogy to ${\mathit{T}}_{\mathit{V}}$ changes with chemical composition (δ, x, y) of ${\mathrm{Fe}}_{3\left(1\mathrm{-}\mathrm{\delta }\right)}$ ${\mathrm{O}}_4$, ${\mathrm{Fe}}_{3\mathrm{-}\mathit{x}}$ ${\mathrm{Zn}}_{\mathit{x}}$ ${\mathrm{O}}_4$, and ${\mathrm{Fe}}_{3\mathrm{-}\mathit{y}}$ ${\mathrm{Ti}}_{\mathit{y}}$ ${\mathrm{O}}_4$ 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<${\mathit{T}}_{\mathit{V}}$ the conductivity σ is typical of the variable-range hopping (VRH) mechanism [σ=${\mathrm{\sigma }}_0$ exp(${\mathit{T}}_0$/T${)}^{1/4}$] with ${\mathit{T}}_0$ decreasing with increasing pressure. Below 30 K σ(T) deviates from the ${\mathit{T}}^{1/4}$ law at all pressures. © 1996 The American Physical Society.