Energetics of rare-earth-doped hafnia
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The enthalpies of formation of rare-earth (RE)-doped Hf1−xRExO2−x/2 solid solutions (RE ⳱ Sm, Gd, Dy, Yb; x ⳱ 0.25 to 0.62) with respect to the oxide end members, monoclinic HfO2 and C-type REO1.5, were determined using oxide melt solution calorimetry. The enthalpies of formation fit a function quadratic in composition. The strongly negative interaction parameters in all solid solutions confirm a strong tendency for short-range order. Though strongly negative for all systems, the interaction parameters become less negative with increasing ionic potential (decreasing RE radius). Crystallization energetics were investigated for amorphous coprecipitation products with x ⳱ 0.4. The energy difference between the crystalline (fluorite and pyrochlore) and amorphous phases decreases with decreasing dopant radius. This suggests that systems doped with small RE ions have more similar local structures in the fluorite and amorphous phases. These observations may result in a smaller kinetic barrier to recrystallization and account for the greater radiation resistance of materials with smaller RE cations.
I. INTRODUCTION
HfO2 occurs in three different polymorphs at ambient pressure: monoclinic (m), tetragonal (t), and cubic fluorite (c) structure, with transition temperatures at 1700 °C (m–t) and 2600 °C (t–c).1 The high-temperature cubic fluorite phase is characterized by a high oxygen mobility leading to a high ionic conductivity.2,3 The fluorite phase can be stabilized at room temperature by substituting Hf 4+ with rare earth elements RE3+ as well as Y3+. By incorporating trivalent rare-earth cations at the Hf 4+ site, vacancies are generated at the oxygen sites to maintain the charge balance. In an ideal structure, the rare-earth elements (RE) and Hf 4+ on the cation site, as well as the oxygen ions and vacancies on the anion sites, are randomly distributed. However, extensive short-range order in RE-doped hafnia with fluorite structure is proposed by several authors.4,5 Quenchable HfO2–REO1.5 solid solutions with fluorite structure are reported for the RE sequence from Sm to Yb.6 The disordered fluorite-type structure is related to the pyrochlore structure (A2B2O7), characterized by an ordered cation sublattice and ordered oxygen vacancies. The A site is usually occupied by large, low valence cations, and the B site by any transition metal element capable of octahedral coordination. Phase stabilities of HfO2–REO1.5 systems from 1000 °C
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0133 876
J. Mater. Res., Vol. 22, No. 4, Apr 2007 http://journals.cambridge.org Downloaded: 17 Mar 2015
to their melting temperature were discussed and reviewed extensively by Glushkova et al.6 For Hf1−xRExO2−x/2 between x ⳱ 0.18 and 0.66 (10−50 mol% RE2O3, as reported in the phase diagram), fluorite and pyrochlore stability fields are mainly controlled by the radius of the RE dopant. A large RE3+ cation (La–Nd) leads to the presence of pyrochlore and pyrochlore/monoclinic phase fields, wh
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