Phonon and crystal field excitations in geometrically frustrated rare earth titanates

The phonon and crystal field excitations in several rare earth titanate pyrochlores are investigated. Magnetic measurements on single crystals of Gd₂Ti₂O₇ , Tb₂Ti₂O₇ , Dy₂Ti₂O₇ , and Ho₂Ti₂O₇ 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 R₂Ti₂O₇ pyrochlore structure is then used to identify crystal field excitations observed in the materials. Several crystal field excitations have been observed in Tb₂Ti₂O₇ 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 Tb³⁺ sites in the low-temperature structure. Furthermore, the crystal field level at approximately 13cm^-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 Tb₂Ti₂O₇ . The additional crystal field information on Tb₂Ti₂O₇ adds to the recent discussion on the low temperature symmetry of this system and may serve to improve its theoretical understanding.