Temperature-dependent Raman and X-ray studies of the spin-ice pyrochlore Dy₂Ti₂O₇ and non-magnetic pyrochlore Lu₂Ti₂O₇

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 Dy₂Ti₂O₇ and its non-magnetic analog Lu₂Ti₂O₇. Raman data show a "new" band not predicted by factor group analysis of Raman-active modes for the pyrochlore structure in Dy₂Ti₂O₇ , appearing below a temperature of T_c=110 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 O¹⁸-isotope substituted Dy₂Ti₂O₇ confirm the phonon origin of the new mode. These findings, absent in Lu₂Ti₂O₇, suggest that the room-temperature cubic lattice of the pyrochlore Dy₂Ti₂O₇ undergoes a "subtle" structural transformation near T_c. We find anomalous redshift of some of the phonon modes in both the Dy₂Ti₂O₇ and the Lu₂Ti₂O₇ as the temperature decreases, which is attributed to strong phonon-phonon anharmonic interactions.