Since the pioneering work of Zintl and Croatto, La₂Ce₂O₇ has been studied for over half a century. During this time, La₂Ce₂O₇ and more generally the La_xCe_1-xO_2-x/2 (LCO) compounds have been studied in the context of three-way automotive catalysts, as an ionic conductor in solid oxide fuel cells, and as oxygen sensor. In recent years, LCO has also become of interest as a new material for thermal barrier coatings. Although La₂Ce₂O₇ has been known for a long time, its crystal structure remains a point of discussion.

The two competing models for its crystal structure are the disordered fluorite and the pyrochlore structures. The two structures are closely related and as such difficult to distinguish experimentally. Many ternary oxides with the formula ₂B₂O₇, with +III ions A and +IV ions B, adopt a pyrochlore structure, making the latter a good candidate for the La₂Ce₂O₇ structure. Conversely, in many cases where a pyrochlore crystal structure is observed, the A and B ions are indeed lanthanides and/or transition metals. Starting from the same fluorite lattice the main difference between the two structures boils down to order. Where the pyrochlore structure is highly ordered (with monotype tetragonal cation clusters surrounding the vacancies) the disordered fluorite structure is defined as a fluorite geometry where the cation sites are randomly filled with Ce and La atoms, leading to an average filling of 0.5 Ce and 0.5 La atoms in each cation site. The oxygen vacancies are also distributed randomly leading to an average occupation of 0.875 oxygen atoms per oxygen site.

Studies of LCO show an even more interesting picture. Some authors show, using XRD experiments, that the CeO₂ fluorite structure is maintained for La concentrations up to x = 0.40 (for La₂Ce₂O₇ x = 0.50), and only at higher La concentrations the pyrochloric ordering of the cations appears. They also observe that the lattice parameter does not expand in a linear fashion as was reported earlier, and assume this reduced expansion is due to the clustering of O-vacancies. Other authors find multiple non-linear regions in the expansion of the lattice parameter and link this behavior to the presence of two phases with different La concentration.

In this thesis topic, the stability of LCO as function of the La concentration will be studied. Comparison of ordered and disordered structures will provide insight into the driving forces for stability. By also including thermal contributions to the energy, comparison with experiment becomes possible and maybe the discussion on the ground state crystal structure can finally be settled.

Interested students will be trained in the use of the required quantum mechanical codes and tools necessary for this topic.