A temperature-dependent Raman study of ${\text{Ho}}_2{\text{Ti}}_2{\text{O}}_7$ and ${\text{Dy}}_2{\text{Ti}}_2{\text{O}}_7$ single crystals was performed in the 5–873 K temperature range. Polarized spectra allowed us to establish the symmetries of the observed bands and revise the mode assignment made in previous works. Our studies revealed also two additional bands near 287 and $300{\text{cm}}^{-1}$ for ${\text{Dy}}_2{\text{Ti}}_2{\text{O}}_7$, which can be assigned to crystal field transitions in ${\text{Dy}}^{3+}$ ions. Temperature dependent Raman studies showed large increase of linewidths. These changes have been analyzed in terms of strong third-order phonon-phonon anharmonic interactions. The Raman spectra also showed anomalous softening of the majority of phonon modes upon cooling in the whole temperature range studied. In contrast to this behavior, the ${\text{F}}_{2\text{g}}$ $\sim 310{\text{cm}}^{-1}$ and ${\text{E}}_{\text{g}}$ $\sim 330{\text{cm}}^{-1}$ phonon modes showed hardening upon cooling down to about (100–120 K) and then anomalous softening below this temperature. This anomalous behavior of phonon wave numbers has been attributed to the increase in octahedral distortion upon cooling.

• Received 29 January 2009

DOI:https://doi.org/10.1103/PhysRevB.79.214437