Phonon anomalies and structural transition in spin-ice Dy₂Ti₂O₇: a simultaneous pressure - and temperature-dependent Raman study

Abstract

We revisit the assignment of Raman phonons of rare-earth titanates by performing Raman measurements on single crystals of O¹⁸-isotope rich spin-ice Dy₂Ti₂O₇¹⁸ and non-magnetic Lu₂Ti₂O₇¹⁸ pyrochlores and compare the results with their O¹⁶ counterparts. We show that the low wavenumber Raman modes below 250 cm^#8722;1 are not due to oxygen vibrations. A mode near 200 cm^#8722;1, commonly assigned as F2g phonon which shows highly anomalous temperature dependence, is now assigned to a disorder induced Raman active mode involving Ti⁴⁺ vibrations. Moreover, we address here the origin of the "new" Raman mode, observed earlier below Tc ~ 110 K in Dy₂Ti₂O₇, through a simultaneous pressure- and temperature-dependent Raman study. Our study confirms the "new" mode to be a phonon mode. We find that dTc/dP = + 5.9 K/GPa. Temperature dependence of other phonons has also been studied at various pressures up to ~8 GPa. We find that pressure suppresses the anomalous temperature dependence. The role of the inherent vacant sites present in the pyrochlore structure in the anomalous temperature dependence is also discussed.