Abstract
The relative conductivity of 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 and the mechanism of conductivity below 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 : (1) In the range 0–6 GPa the DT curves show sharp minima at consistent with a first-order phase transition. In this range 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 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; becomes indiscernible. It was shown that the variation of with rising P is in close analogy to changes with chemical composition (δ, x, y) of , , and 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< the conductivity σ is typical of the variable-range hopping (VRH) mechanism [σ= exp(/T] with decreasing with increasing pressure. Below 30 K σ(T) deviates from the law at all pressures. © 1996 The American Physical Society.
- Received 4 October 1995
DOI:https://doi.org/10.1103/PhysRevB.53.6482
©1996 American Physical Society