Formation enthalpies of rare earth titanate pyrochlores

High-temperature oxide melt solution calorimetry and Rietveld refinements of powder X-ray diffraction data were used to investigate the structure (Fd3m; Z=8) and energetics of a series of RE2Ti2O7 (RE-Sm-Lu) compounds with the pyrochlore structure as well as La2Ti2O7 with a layered perovskite-type structure. All of the RE-titanates were found to be stable in enthalpy with respect to their oxides. In the pyrochlore series, Lu2Ti2O7 was least stable in enthalpy (DeltaHf-ox at 298K = 56.0+/-4.0kJ/mol); the most stable materials were Gd-, Eu-, and Sm2Ti2O7 with DeltaHf-ox at 298K=-113.4+/-2.7, -106.1+/-4.2. -115.4+/-4.2kJ/mol, respectively. In general, as the radius ratio of the A- to B-site cations. R-A/R-B. decreases, the pyrochlore structure becomes less stable. The trend of ionic radius of the RE3+ cation vs. DeltaHF-ox at 298 K is non-linear and approximately parallels the increasing "resistance" to ion-beam-induced amorphization as R-A/R-B decreases. (C) 2004 Elsevier Inc. All rights reserved.