Raman study of the Verwey transition in Magnetite at high-pressure and low-temperature; effect of Al doping

TL;DR

This study investigates how hydrostatic pressure affects the Verwey transition in pure and Al-doped magnetite using Raman spectroscopy, revealing differences in transition behavior and entropy changes due to doping.

Contribution

It provides new insights into the pressure dependence of the Verwey transition in magnetite and the effects of Al doping on transition thermodynamics.

Findings

Al doping decreases the transition temperature linearly with pressure

Pure magnetite shows a steeper pressure-temperature slope than doped magnetite

Al doping results in smaller entropy change and larger volume expansion at transition

Abstract

We report high-pressure low-temperature Raman studies of the Verwey transition in pure and Al-doped magnetite (Fe_3O_4). The low temperature phase of magnetite displays a number of additional Raman modes that serve as transition markers. These transition markers allow one to investigate the effect of hydrostatic pressure on the Verwey transition temperature. Al-doped magnetite Fe_2.8Al_0.2O_4 (TV=116.5K) displays a nearly linear decrease of the transition temperature with an increase of pressure yielding dP/dT_V = -0.096 GPa/K. In contrast pure magnetite displays a significantly steeper slope of the PT equilibrium line with dP/dT_V = -0.18 GPa/K. The slope of the PT equilibrium lines is related to the changes of the molar entropy and molar volume at the transition. We compare our spectroscopic data with that obtained from the ambient pressure specific heat measurements and find a good agreement in the optimally doped magnetite. Our data indicates that Al doping leads to a smaller entropy change and larger volume expansion at the transition. Our data displays the trends that are consistent with the mean field model of the transition that assumes charge ordering in magnetite.