Phonon and crystal field excitations in geometrically frustrated rare earth titanates

Abstract

The phonon and crystal field excitations in several rare earth titanate pyrochlores are investigated. Magnetic measurements on single crystals of ${\text{Gd}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$, ${\text{Tb}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$, ${\text{Dy}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$, and ${\text{Ho}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$ are used for characterization, while Raman spectroscopy and terahertz time domain spectroscopy are employed to probe the excitations in the materials. The lattice excitations are found to be analogous across the compounds over the whole temperature range investigated (295--4 K). The resulting full phononic characterization of the ${R}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$ pyrochlore structure is then used to identify crystal field excitations observed in the materials. Several crystal field excitations have been observed in ${\text{Tb}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$ in Raman spectroscopy, among which all of the previously reported excitations. The presence of additional crystal field excitations, however, suggests the presence of two inequivalent ${\text{Tb}}^{3+}$ sites in the low-temperature structure. Furthermore, the crystal field level at approximately $13\text{ }{\text{cm}}^{\ensuremath{-}1}$ is found to be both Raman and dipole active, indicating broken inversion symmetry in the system and thus undermining its current symmetry interpretation. In addition, evidence is found for a significant crystal field--phonon coupling in ${\text{Tb}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$. The additional crystal field information on ${\text{Tb}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$ adds to the recent discussion on the low temperature symmetry of this system and may serve to improve its theoretical understanding.