Formation enthalpies of rare earth titanate pyrochlores

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Abstract

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 (ΔHf-ox at 298 K=−56.0±4.0 kJ/mol); the most stable materials were Gd-, Eu-, and Sm2Ti2O7 with ΔHf-ox at 298 K=−113.4±2.7, −106.1±4.2, −115.4±4.2 kJ/mol, respectively. In general, as the radius ratio of the A- to B-site cations, RA/RB, decreases, the pyrochlore structure becomes less stable. The trend of ionic radius of the RE3+ cation vs. ΔHf-ox at 298 K is non-linear and approximately parallels the increasing “resistance” to ion-beam-induced amorphization as RA/RB decreases.

Introduction

Titanate pyrochlore, A2Ti2O7, materials are important because of their potential use as solid electrolytes and mixed ionic/electronic conducting electrodes [1], [2], [3], [4], [5], [6], [7], [8], [9], catalysts [10], and ferroelectric/dielectric device components [11], [12], [13], [14]. Pyrochlore is also a mineral, occurring in a wide variety of high-temperature geologic settings, including pegmatites and carbonatites, preferentially incorporating U (up to 30 wt%) and Th (up to 9 wt%) into the structure [15], [16], [17], [18]. Most naturally occurring pyrochlores are at least partially metamict, i.e., amorphous due to the cumulative effects of the radioactive decay of 238U, 235U and 232Th and their daughter products [19]. Because of the capacity of pyrochlore to incorporate actinides [20], pyrochlore and the structurally related monoclinic zirconolite, are the primary phases in the titanate-based waste form ceramics being investigated for the immobilization of plutonium from dismantled nuclear weapons [20], [21], [22], [23], [24], [25], [26], [27]. Despite the broad interest in titanate pyrochlore, only estimated thermodynamic data have been reported previously in the literature [28]. In the present work, we report the results of a detailed thermochemical investigation of RE-titanate pyrochlore, RE2Ti2O7 (where RE=Lu to Sm, or Y).

Section snippets

The pyrochlore structure

The pyrochlore, A2B2O7, structure (Fd3m, Z=8), is an anion-deficient derivative of fluorite, AX2 (Fm3m, Z=1), with two types of cations ordered on the A- and B-sites and one eighth of the anions removed. The structure can be envisioned as interpenetrating networks of BO6 octahedra and A2O chains of distorted cubes (Fig. 1) [29], [30]. The general stoichiometry is given by A2B2X6X′ where A=various large, low-valence cations such as Bi, In, Tl, Pb2+, Sc, Cd, Hg2+, Ca, Sr, Mn2+, Sn2+ or RE (the

Previous work

Calculated formation enthalpies for RE2Ti2O7 were reported based on extrapolations of measured RE-zirconate pyrochlore data [28], [36]. Formation enthalpies of the RE-hafnates and zirconates were measured by Paputskii et al. (1974) by combustion in an adiabatic calorimeter [36]. Reznitskii (1993) used their experimental values to approximate the formation enthalpies for RE-zirconate and titanate pyrochlores [28]. Reznitskii used the enthalpy values reported by Paputskii et al. (1974) to derive A

Sample synthesis and characterization

RE-titanate pyrochlore (RE=Eu to Lu) single crystals were grown by a flux technique at Oak Ridge National Laboratory (ORNL). The RE-titanate pyrochlores (RE=Sm, Eu, Gd, Y) prepared using the sol–gel method were provided by K.V.G. Kutty from the Indira Gandi Center for Atomic Research, India.

X-ray powder diffraction (XRD) data were collected using a Scintag PAD-V diffractometer with a Cu anode and an accelerating voltage of 45 kV over an angular range, 2θ=14–94° and a 0.02° step size with a dwell

Results

Drop solution experiments using a 3Na2O·4MoO3 solvent at 976 K were conducted for TiO2, rutile, RE2O3 where RE=La–Lu (excluding Ce, Pm, Pr) plus Y, and the pyrochlore samples (Table 1). The ΔHds value for TiO2 was consistent with previously reported values [51], [52], [53]. The ΔHds data for the RE-sesquioxides were previously reported and shown to be reliable through the application of multiple thermodynamic cycles and crosschecks [54], [55]. A recent thermochemical investigation of La-bearing

Comparison to previous reported data

The measured formation enthalpies, ΔHf-ox, for the RE2Ti2O7 pyrochlores are consistent with the previously reported calculated data (Fig. 4) [28]. In general, a plot of the radius ratio of the A to B-site cations against the formation enthalpies shows a decrease in stability with respect to the oxides as RA/RB decreases (Fig. 3) [57]. This is consistent with observations that pyrochlore has an increasing tendency to disorder to a defect fluorite structure as RA/RB decreases [30]. The

Conclusions

All of the RE2Ti2O7 samples were stable in enthalpy with respect to their oxides. The general trend of decreasing stability with decreasing RA/RB is consistent with an increasing propensity to disorder and a more energetically favorable fluorite structure. Deviations in linearity in the enthalpy data are consistent with the observed trends in radiation damage “resistance” as determined by shifts in the critical temperature, that is the temperature above which the material cannot be amorphized

Acknowledgements

Financial support was provided by the University of California at Davis and by the Office of Basic Energy Sciences, DOE grant DE-FG02-97ER45656 (RCE). Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the US Department of Energy under contract DE-AC05-00OR22725.

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