W0187

Crystal Structures and Chemistry of Rare Earth Titanate Compounds. J. Matt Farmer^1,2, Lynn A. Boatner¹, Claudia J. Rawn¹, and Michael Lance¹, ¹Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA, ²Baylor Univ., Chemistry Dept., Waco, TX 76798, USA.

Rare-earth titanates, RE₂Ti₂O₇, with the pyrochlore structure are being studied for use as potential nuclear and actinide-rich waste forms. They are also a key component in the synroc-based pyrochlore-rich ceramics for the geological immobilization of surplus plutonium from dismantled nuclear weapons. Single-crystals of RE₂Ti₂O₇ (RE = Sm to Lu & Y) have been synthesized using a high-temperature flux technique, and their structures refined by single crystal X-ray diffraction. The cubic lattice parameter for RE₂Ti₂O₇ (RE = Lu to Gd) displays an approximately linear correlation with the RE-site cation ionic radius. For Sm and Eu titanate compounds, an increase of covalency between the REO₈ and TiO₆ polyhedra results in a deviation from the increasing linear lattice parameter through the series. Gd₂Ti₂O₇ exhibits the lowest oxygen x-coordinate on the 48f site, which can be attributed to the half-filled shell electronic configuration of Gd³⁺. The coefficient of thermal expansion (CTE) has been determined using high temperature X-ray diffraction (HTXRD). The CTE for the rare-earth titanate series is fairly linear vs ionic radius and has a range of 10.1 – 11.2 x 10^-6 K^-1. Raman spectroscopy indicates that the ~530 cm^-1 peak, associated with the Ti-O stretching follows a general trend of decreasing frequency shift with an increasing RE reduced mass. These crystals are a major component of an ongoing heavy-particle radiation-damage and actinide-doping project.

Research sponsored by the National Nuclear Security Administration, Office of Defense Nuclear Nonproliferation, Office of Nonproliferation Research and Engineering, and the Division of Materials Sciences, and by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of FreedomCAR and Vehicle Technologies, as part of the High Temperature Materials Laboratory User Program, Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Dept. of Energy under contract number DE-AC05-00OR22725.