Far-infrared dielectric and vibrational properties of nonstoichiometric wüstite at high pressure

The far-infrared reflectivity of ${\text{Fe}}_{0.91}\text{O}$ was investigated from atmospheric pressure up to 33 GPa at room temperature using synchrotron Fourier transform infrared reflectivity techniques in conjunction with the diamond-anvil cell from $100–700{\text{cm}}^{-1}$. The frequency of the fundamental transverse optic (TO) mode was found to be nearly independent of pressure up to 4.6 GPa, followed by an increase in the TO frequency with pressure up to the rhombohedral phase transition at ~8 GPa. In addition, a second weak mode at $583{\text{cm}}^{-1}$ at atmospheric pressure was well resolved and found to shift to higher frequency and increase in strength with pressure. This localized mode arises from the presence of vacancies in the crystal structure, and the relative strength of this mode suggests pressure-induced charge localization near the vacancy sites. The data was fit with the classical Lorentz model with the addition of a plasmon resonance. This allowed an estimation of the electrical conductivity as well as plasmon-phonon coupling energies. The pressure dependencies of the dielectric properties of wüstite have been quantified, and their pressure derivatives show a change in sign near the pressure-induced rhombohedral phase transition. Classical theories relating dielectric, vibrational, and elastic properties are evaluated, and in the case of the bulk modulus, the theory fails to reproduce accepted literature values.

Figure 1

(Color online) Reflectivity of the diamond-wüstite interface at various pressures. The dots are the raw data; the solid lines are the best fits of those data to Eq. (4).

Figure 2

(Color online) (a) Real and (b) imaginary parts of the dielectric function of ${\text{Fe}}_{0.91}\text{O}$ at various pressures.

Figure 3

(Color online) (a) Real and (b) imaginary parts of the index of refraction of ${\text{Fe}}_{0.91}\text{O}$ at various pressures.

Figure 4

(Color online) (a) Spring model for nonstoichiometric wüstite; [(b) and (c)] atomic displacements plotted in the vertical direction for the two strongest normal-mode frequencies of a 19-atom iron oxide chain with one iron vacancy $\left({\text{Fe}}_{0.9}\text{O}\right)$. Atomic displacements are exaggerated relative to atomic spacing, the displacements for the TO and localized modes are plotted on the same scale.

Figure 5

Static, ${\epsilon }_0$, and background, ${\epsilon }_{\infty }$, dielectric constants as a function of pressure. The vertical line shows the approximate structural transition pressure from rocksalt to rhombohedral under nonhydrostatic conditions. Estimated errors on ${\epsilon }_{\infty }$ are smaller than the symbols.

Figure 6

Optic and localized mode frequencies of ${\text{Fe}}_{0.91}\text{O}$ plotted against unit-cell volume. See text for details.