Predicted pyrochlore to fluorite disorder temperature for A 2 Zr 2 O 7 compositions
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C.R. Stanek Los Alamos National Laboratory, Los Alamos, New Mexico 87545
Scott Owens Nuclear Sciences and Technology Services, H260, Hinton House, Risley, Warrington WA3 6AS, United Kingdom (Received 1 March 2004; accepted 16 March 2004)
In a previous publication the order–disorder pyrochlore to fluorite transformation temperatures for a series of A2Hf2O7 pyrochlores were predicted [C.R. Stanek and R.W. Grimes: Prediction of rare-earth A2Hf2O7 pyrochlore phases. J. Am. Ceram. Soc. 2002, 85, p. 2139]. This was facilitated by establishing a relationship between these temperatures and the energy required to introduce a specific defect structure into the perfect pyrochlore lattice. Here an equivalent relationship for A2Zr2O7 pyrochlores was generated, and from this the disorder temperatures for a number of compositions including Eu2Zr2O7 were predicted.
The pyrochlore structure is closely related to that of fluorite and can be considered as a BO2 fluorite in which half of the B4+ cations have been replaced by A3+ cations. Charge compensation takes place by introducing oxygen vacancies into the lattice resulting in a change in space group from fluorite Fm3m to Fd3m pyrochlore. In a pyrochlore, the two cation species and oxygen vacancies show long-range order. At temperatures above the disorder transformation temperature, this ordering is lost; oxygen vacancies become randomly distributed on anion sites and A3+, B4+ cations distribute randomly on their sites. It has been established experimentally that some materials with the pyrochlore structure will transform at high temperature to disordered fluorite, while others remain as pyrochlore up to their melting points.1 Here we represent the pyrochlore to disordered fluorite transformation by calculating the internal energy to form a small section of the disordered lattice, namely a defect cluster consisting of both oxygen Frenkel and cation antisite defects in nearest neighbor positions. The stability of this type of cluster was previously established by Wilde and Catlow.2 We assume that the cluster formation energy is representative of the complete disorder process within a given series of pyrochlores such as A2Hf2O7 or
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0231 J. Mater. Res., Vol. 19, No. 6, Jun 2004
A2Zr2O7 (where A3+ is a rare earth in the range La to Lu). The energies required to form the defect cluster in a given perfect lattice were derived from atomistic scale computer simulations based on an ionic lattice model. The Buckingham potential was used to describe the short range interaction between pairs of ions (the potential parameters used were taken from Refs. 3 and 4). This method has met with considerable success in predicting a wide range of structures and processes for pyrochlores and ceramic materials generally.5 Here we used both the CASCADE and GULP codes.6,7 Results for the A2Zr2O7 pyrochlores are presented in Fig. 1 along with values for A2Hf2O7 published previously.8 Included in this figure are
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