Correlation of Formation Enthalpies with Critical Amorphization Temperature for Pyrochlore and Monazite
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Correlation of Formation Enthalpies with Critical Amorphization Temperature for Pyrochlore and Monazite
K.B. Helean and A. Navrotsky, Thermochemistry Facility and NEAT-ORU, The University of California at Davis, Davis CA 95616, [email protected] J. Lian and R.C. Ewing, Department of Nuclear Engineering and Radiological Sciences, The University of Michigan, Ann Arbor MI 48109 ABSTRACT Systematic studies of a series of RE-titanate pyrochlore single crystals determined the formation enthalpies, ∆H0f-ox, and the critical amorphization temperature, Tc, the temperature above which the crystal can no longer be amorphized. A negative linear correlation was observed between ∆H0f-ox and Tc. In general as the cation radius ratio, RA/RB, decreases the formation enthalpy becomes less exothermic, i.e. the pyrochlore structure becomes less thermodynamically stable while the susceptibilty to ion beaminduced amorphization decreases. The opposite relation, i.e. a positive linear correlation between ∆H0f-ox and Tc was observed for a series of REPO4 single crystals with both the monazite and xenotime structures. Both relations can be understood in terms of pyrochlore and orthophosphate crystal chemistry, their formation of point defects, and the role of ion-irradiation induced phase transformations. INTRODUCTION Titanate pyrochlores, A2Ti2O7, that contain rare earth elements (RE = lanthanides + Y) on the A-site are of technological importance due to their potential use as solid electrolytes and mixed ionic/electronic conducting electrodes [1-5], catalysts [6], and ferroelectric/dielectric device components [7,8]. Rare earth orthophosphates, REPO4, with both the monazite and xenotime structures have potential application as scintillators for gamma-ray detection [9-11], thermophosphors [12], hosts for microlasers [13,14] and analytical standards [15]. Both pyrochlore and monazite are minerals that preferentially incorporate actinides into their structures [16-19]. 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 [20]. Naturally occurring monazite is almost always found in a crystalline state despite large (> 10 wt.% U + Th) actinide contents [21]. Because of the capacity of pyrochlore and monazite to incorporate actinides, they are candidate nuclear waste form ceramics [19,2227]. THE PYROCHLORE STRUCTURE The oxide 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 Aand 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 [28,29].
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The ionic radii of the A-and B-site cations are typically in the ranges of RA = 0.087 – 0.151 nm; RB = 0.040 – 0.078 nm [28]. Empirically, the pyrochlore structure is stable when the radius ratio RA/RB = 1.29 – 2.30. There are three anion sites,
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