Polaron physics and crossover transition in magnetite probed by pressure-dependent infrared spectroscopy

TL;DR

This study investigates magnetite's optical properties under pressure, revealing polaronic behavior and a transition in charge distribution, using infrared spectroscopy up to 8 GPa.

Contribution

It provides new insights into pressure-induced changes in magnetite's polaronic coupling and charge distribution via infrared spectroscopy.

Findings

Polaronic coupling in magnetite is intermediate at room temperature.

Pressure causes phonon modes to harden linearly and shifts absorption bands to lower frequencies.

An abrupt change in the pressure coefficient of a phonon mode suggests charge redistribution around 6 GPa.

Abstract

The optical properties of magnetite at room temperature were studied by infrared reflectivity measurements as a function of pressure up to 8 GPa. The optical conductivity spectrum consists of a Drude term, two sharp phonon modes, a far-infrared band at around 600 cm$^{-1}$, and a pronounced mid-infrared absorption band. With increasing pressure both absorption bands shift to lower frequencies and the phonon modes harden in a linear fashion. Based on the shape of the MIR band, the temperature dependence of the dc transport data, and the occurrence of the far-infrared band in the optical conductivity spectrum the polaronic coupling strength in magnetite at room temperature should be classified as intermediate. For the lower-energy phonon mode an abrupt increase of the linear pressure coefficient occurs at around 6 GPa, which could be attributed to minor alterations of the charge distribution among the different Fe sites.