Temperature-dependent Raman and x-ray studies of the spin-ice pyrochloreDy2Ti2O7and nonmagnetic pyrochloreLu2Ti2O7

Abstract

We present here temperature-dependent Raman, x-ray diffraction, and specific heat studies between room temperature and 12 K on single crystals of spin-ice pyrochlore compound ${\text{Dy}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$ and its nonmagnetic analog ${\text{Lu}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$. Raman data show a ``new'' band not predicted by factor group analysis of Raman-active modes for the pyrochlore structure in ${\text{Dy}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$, appearing below a temperature of ${T}_{c}=110\text{ }\text{K}$ with a concomitant contraction of the cubic unit cell volume as determined from the powder x-ray diffraction analysis. Low-temperature Raman experiments on ${\text{O}}^{18}$-isotope substituted ${\text{Dy}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$ confirm the phonon origin of the new mode. These findings, absent in ${\text{Lu}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$, suggest that the room-temperature cubic lattice of the pyrochlore ${\text{Dy}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$ undergoes a ``subtle'' structural transformation near ${T}_{c}$. We find anomalous redshift of some of the phonon modes in both the ${\text{Dy}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$ and the ${\text{Lu}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$ as the temperature decreases, which is attributed to strong phonon-phonon anharmonic interactions.