Dissolution of A2Ti2O7 (A= Y3+, Gd3+, or Lu3+) Pyrochlore by Experiment at pH= 2, T= 90 C: Evidence for Solubility Control Using a Linear Free Energy Model

We performed a series of dissolution experiments with well-characterized pyrochlore ceramics with the formula A2Ti2O7, where A= Y3+, Gd3+, or Lu3+ in H2O- and D2O-based solutions[pH(D)= 2] at 90 C. Normalized log10 dissolution rates (g?m-2?d-1) in H2O-based solutions increase from Lu2Ti2O7 (-3.2 to -3.3) to Gd2Ti2O7 (-2.6 to -2.9), to Y2Ti2O7 (-1.9 to -2.0). Rates in D2O-based solutions are indistinguishable from rates in H2O, indicating that release of elements is probably not diffusion controlled. A recent dissolution model, based on ligand-exchange reactions, suggests that the rate of reaction should increase in inverse order of the cation field strength: Lu < Y < Gd (where the cation denotes the appropriate pyrochlore composition), which is not observed. Evaluation of the thermodynamic stability of the three solids was performed using a linear free-energy model and reported free energies of formation. The calculations indicate that reactivity should follow in the progression Lu < Gd < Y, as observed in the dissolution experiments. Our data indicates, therefore, that dissolution models based on ligand-exchange reactions may not be strictly applicable to simple pyrochlore minerals