Figure 4.

a,b) Temperature evolution of the IR reflectivity spectra of EuAl₁₂O₁₉ measured in the E ^ω ∥ a configuration   and in the E ^ω ∥ c configuration, respectively. New transverse optic modes get visible upon cooling, they are pointed by arrows and labeled ${\mathrm{TO}}_a^{\mathrm{LT}}$ for E ^ω∥ a and ${\mathrm{TO}}_c^{\mathrm{LT}}$ for E ^ω∥ c . Some other modes already visible at room temperature get strongly enhanced upon cooling, they are labelled ${\mathrm{TO}}_c^{\mathrm{HT}}$. c,d) Frequency dependence of the THz real c and imaginary parts d of the complex permittivity spectra of EuAl₁₂O₁₉, measured in the E ^ω∥ c configuration. A broad relaxation R1 spreads practically across the whole THz range at T = 295 K and its absorption is so strong that there is even missing gap in the original transmittance data, filled by dashed line as guide to the eye. ${\mathrm{TO}}_c^{\mathrm{LT}}$ indicates three phonon modes appearing below T _S = 49 K.