The phase diagram of the ZrO 2-Nd 2O 3 system has been characterized showing isolated two-phase regions for a cubic fluorite-type ZrO 2 solid solution and Nd 2Zr 2O 7 with a pyrochlore-type structure. A thermodynamic analysis was carried out to elucidate the origin of this interesting phase equilibrium. A compound energy model with the formula (Zr 4+,Nd 3+) 0.5(Nd 3+,Zr 4+) 0.3(O 2-,va) 2 was applied to describe the Gibbs energy for these phases in consideration of the ordering of the cation sites in the structure. The ordering arrangement on the anion sites was not taken into account. The Gibbs energy for the liquid was described using an ionic solution model, while the binary compounds, such as tetragonal and monoclinic ZrO 2, and cubic and hexagonal Nd 2O 3, were treated as stoichiometric solid phases. The thermodynamic assessment was based on the experimental phase boundaries as well as the evaluated formation energy for the stoichiometric Nd 2Zr 2O 7 phase. The phase diagram calculations showed that the peculiar feature of this phase diagram was reproduced well in our work. The results strongly suggest that the two-phase boundaries between the cubic fluorite-type ZrO 2 solid solution and the pyrochlore-type structure occur due to the ordering of the Zr 4+ and Nd 3+ cations.
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