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

Abstract

We report a Raman study of single crystal pyrochlore Er2Ti2O7 as a function of temperature from 12 to 300 K. In addition to the phonons, various photoluminescence (PL) lines of Er3+ 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 (4I15/2) and the 2H11/2 as well as 4S3/2 excited state manifolds of Er3+ 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 () 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.