Figure 1

(Color online) In-plane (electric field of light $\mathbf{E}\perp \widehat{z}$) infrared reflectivity of a $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ single crystal at 5 and $300\mathrm{K}$. The dashed lines are the data and the solid lines are fits using a dielectric function in the form of Eq. (1). The lower arrows indicate the TO frequencies used in our fits. The upper arrows are the frequencies calculated in Ref. 17.Reuse & Permissions

Figure 2

Temperature dependence of the in-plane $(\mathbf{E}\perp \widehat{z})$ transverse optical frequencies for a $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ single crystal. Due to its very small oscillator strength, the parameters for mode $E\left(3\right)$ have been kept constant above $200\mathrm{K}$. Unless indicated otherwise, the value uncertainties are within the symbol sizes.Reuse & Permissions

Figure 3

(Color online) The open triangles are the temperature dependence of the dielectric constant calculated using Eq. (2) (left axis). The solid circles depict the thermal evolution of the oscillator strength for the $E\left(1\right)$ mode (right axis). As both axis have the same range for $\epsilon$, one can see that the temperature dependence of the dielectric constant comes mostly from the softening (hence, oscillator strength increase) of the $E\left(1\right)$ phonon. The open circles and dashed lines show the temperature dependence of the $E\left(1\right)$ plasma frequency ${\Omega }_p^2\left[E\left(1\right)\right]={\left(\Delta \epsilon {\Omega }_{\mathit{TO}}^2\right)}_{E\left(1\right)}$. This value has been divided by 5500 in order to fit the data in the same scale as $\Delta \epsilon$.Reuse & Permissions

Figure 4

(Color online) The thicker line is the in-plane $(\mathbf{E}\perp \widehat{z})$ reflectivity. The thinner line is the combined response. The upward pointing arrows are the TO frequencies calculated in Ref. 17. The down pointing arrows are our proposition for the $E\parallel c$ phonons.Reuse & Permissions