Temperature dependent Raman and x-ray studies of spin-ice pyrochlore $Dy_2Ti_2O_7$ and non-magnetic pyrochlore $Lu_2Ti_2O_7$

TL;DR

This study investigates temperature-dependent structural and vibrational properties of Dy2Ti2O7 and Lu2Ti2O7 pyrochlores, revealing a subtle phase transition and phonon behavior changes through Raman and x-ray analyses.

Contribution

It uncovers a previously unobserved structural transformation in Dy2Ti2O7 near 110 K and confirms the phonon origin of a new Raman mode, advancing understanding of pyrochlore lattice dynamics.

Findings

Discovery of a new Raman mode below 110 K in Dy2Ti2O7

Evidence of a subtle structural transformation near 110 K

Observation of phonon red-shift due to anharmonic interactions

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 $Dy_2Ti_2O_7$ and its non-magnetic analogue $Lu_2Ti_2O_7$. Raman data show a "new" band not predicted by factor group analysis of Raman-active modes for the pyrochlore structure in $Dy_2Ti_2O_7$, appearing below a temperature of $T_c=$110 K with a concomitant contraction of the cubic unit cell volume as determined from the powder x-ray diffraction analysis. Low temperature Raman experiments on O$^{18}$-isotope substituted $Dy_2Ti_2O_7$ confirm the phonon origin of the "new" mode. These findings, absent in $Lu_2Ti_2O_7$, suggest that the room temperature cubic lattice of the pyrochlore $Dy_2Ti_2O_7$ undergoes a "subtle" structural transformation near $T_c$. We find anomalous \textit{red-shift} of some of the phonon modes in both the $Dy_2Ti_2O_7$ and the $Lu_2Ti_2O_7$ as the temperature decreases, which is attributed to strong phonon-phonon anharmonic interactions.