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 $T_c=110\text{K}$ 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.

• Received 9 July 2008

DOI:https://doi.org/10.1103/PhysRevB.78.214102