Equilibrium between fluorite and pyrochlore structures in the ZrO2-Nd2O3 system

• The phase diagram of the ZrO2-Nd2O3 system has been characterized showing isolated two-phase regions for a cubic fluorite-type ZrO2 solid solution and Nd2Zr2O7 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 (Zr4+,Nd3+)(0.5)(Nd3+,Zr4+)(0.5)(O2-,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 ZrO2, and cubic and hexagonal Nd2O3, 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,2Zr2O7 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 ZrO2 solid solution and the pyrochlore-type structure occur due to the ordering of the Zr4+ and Nd3+ cations.

Nyckelord

thermodynamic analysis

calculation of phase diagrams (CALPHAD) method

order-disorder transition

pyrochlore structure

zirconia ceramics

phases

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