Structural and crystal chemical properties of rare-earth titanate pyrochlores

Rare-earth titanates, RE₂Ti₂O₇ (where RE = a rare-earth) with the pyrochlore structure continue to be investigated for use as potential stable host materials for nuclear and actinide-rich wastes. Accordingly, the present work is directed towards the elucidation of the fundamental structural, physical, and thermochemical properties of this class of compounds. Single-crystals of the rare earth pyrochlores were synthesized using a high-temperature flux technique and were subsequently characterized using single-crystal X-ray diffraction. The cubic lattice parameters display an approximately linear correlation with the RE-site cation radius. Theoretical calculations of the lattice constants and bond lengths of the subject materials were carried out using density functional theory, and the results are compared to the experimental values. The Sm and Eu titanates exhibit a covalency increase between the REO₈ and TiO₆ polyhedral resulting in a deviation from the increasing linear lattice parameter through the transition series. Gd₂Ti₂O₇ with the 4f⁷ half-filled f-orbital Gd³⁺ sub-shell exhibits the lowest 48f oxygen positional parameter. The coefficient of thermal expansion for the rare-earth titanate series is approximately linear, and it has a range of 10.1-11.2 × 10^-6 °C^-1. Raman spectroscopy indicated that the ∼530 cm^-1 peak associated with the Ti-O stretching mode follows a general trend of decreasing frequency with increasing RE reduced mass.