First principles study of zone centre phonons in rare-earth pyrochlore titanates, RE2Ti2O7 (RE = Gd, Dy, Ho, Er, Lu; Y)

We present first principles study of zone centre phonons in rare-earth pyrochlore titanates, RE2Ti2O7 (RE = Gd, Dy, Ho, Er, Lu; Y). The study has been made within the density functional theory and linear response formalism to calculate, understand, and assign the various Raman and infra-red mode frequencies. Our DFT calculated Raman mode frequencies are in excellent agreement with experimental results except one F2g mode frequency around 800 cm−1, which is not observed in experimental measurements, in the same manner as that of ab initio calculations of Fischer et al. [Phys. Rev. B 78 (2008) 014108] [19]. Increase in frequency of this F2g mode indicates that TiO6 octahedra becomes more tightly packed as we go from Gd → Dy → Ho → Y → Er → Lu. Experimentally observed frequency around 450 cm−1 is confirmed in our calculations, and has been assigned to F2g mode, which was assigned to TiO2 as an impurity phase in Gd2Ti2O7, Er2Ti2O7, and Y2Ti2O7 pyrochlore oxides.