Infrared phonon dynamics of a multiferroic BiFeO3 single crystal

We discuss the infrared reflectivity measurement on a BiFeO3 single crystal between 5 K and room temperature. The nine predicted ab-plane E phonon modes are fully and unambiguously determined. The frequencies of the four A(1) c-axis phonons are found. These results settle issues between theory and data on ceramics. Our findings show that the softening of the lowest frequency E mode is responsible for the temperature dependence of the dielectric constant, indicating that the ferroelectric transition in BiFeO3 is soft-mode driven.