Anomalous temperature dependence of phonons and photoluminescence bands in pyrochlore Er2Ti2O7: signatures of structural deformation at 130 K.

We report a Raman study of single crystal pyrochlore Er(2)Ti(2)O(7) as a function of temperature from 12 to 300 K. In addition to the phonons, various photoluminescence (PL) lines of Er(3+) in the visible range are also observed. Our Raman data show an anomalous red-shift of two phonons (one at ~200 cm(-1) and another at ~520 cm(-1)) upon cooling from room temperature which is attributed to phonon-phonon anharmonic interactions. However, the phonons at ~310, 330, and 690 cm(-1) initially show a blue-shift upon cooling from room temperature down to about 130 K, followed by a red-shift, indicating a structural deformation at ~130 K. The intensities of the PL bands associated with the transitions between the various levels of the ground state manifold ((4)I(15/2)) and the (2)H(11/2) as well as (4)S(3/2) excited state manifolds of Er(3+) show a change at ~130 K. Moreover, the temperature dependence of the peak position of the two PL bands shows a change in their slope (dω/dT) at ~130 K, thus further strengthening the proposal of a structural deformation. The temperature dependence of the peak positions of the PL bands has been analyzed using the theory of optical dephasing in crystals.