IR-active phonons of the LiNbGeO$_5$ superionic crystal

The infrared (IR) reflectance spectra of the LiNbGeO$_5$ superionic crystal have been studied. The crystal belongs to the orthorhombic syngony, the space group $Pnma$. Measurements in polarized light made it possible to separate phonons by symmetry in accordance with the selection rules. The spectral dependences of the reflectivity $R(\nu)$ for three polarizations along the crystallographic axes were modeled using dispersion analysis. The parameters of IR-active phonons of symmetry $B_{1u}$, $B_{2u}$ and $B_{3u}$ are obtained. A group-theoretical analysis of the vibrational spectrum has been performed. The frequency dependences of the complex dielectric constant and the loss function are calculated. The frequencies of IR-active phonons are calculated from the first principles. The experimental and calculated frequencies of IR-active phonons are close to each other. Inverted phonons have been detected.