Accurate neutron powder diffraction experiments at several temperatures allow one to monitor the reconstructive tetragonal to monoclinic phase transition as a function of the size of zirconia nanoparticles. The structure of the tetragonal phase observed in the nanocrystals is identical to that observed in micrometric zirconia above 1400 K. A uniaxial strain depending on grain size is observed. The phase transition occurs above a threshold crystal size. These results are analyzed within the Landau theory and can be understood as a mechanism of size-dependent phase transition where the primary order parameter is altered by the nanoparticle size.

Optical spectroscopy and x-ray diffraction were used to study the behavior of polycrystalline samples of pure monoclinic zirconia irradiated by low energy ions. A microscopic model based on these experiments is proposed to explain the displacive phase transition observed in this material after irradiation. Defects, produced in the oxygen sublattice, induce important strain fields on a nanometric scale. This strain field can be handled as a secondary order parameter within the Landau theory approach, leading to a decrease of the phase transition temperature and thus quenching the high temperature tetragonal phase. The model also explains the consequences of the thermal annealing for restoring the ground state monoclinic structure.

Intuitively scientists accept that order can emerge from disorder and a significant amount of effort has been devoted over many years to demonstrate this. In metallic alloys and oxides, disorder at the atomic scale is the result of occupation at equivalent atomic positions by different atoms which leads to the material exhibiting a fully random or modulated scattering pattern. This arrangement has a substantial influence on the material’s properties, for example ionic conductivity. However it is generally accepted that oxides, such as defect fluorite as used for nuclear waste immobilization matrices and fuel cells, are the result of disorder at the atomic scale. To investigate how order at the atomic scale induces disorder at a larger scale length, we have applied different techniques to study the atomic composition of a homogeneous La₂Zr₂O₇ pyrochlore, a textbook example of such a structure. Here we demonstrate that a pyrochlore, which is considered to be defect fluorite, is the result of intricate disorder due to a random distribution of fully ordered nano-domains. Our investigation provides new insight into the order disorder transformations in complex materials with regards to domain formation, resulting in a concord of chemistry with crystallography illustrating that order can induce disorder.

Based on the fractal nature of collisions, the fragmentation of displacement cascades is investigated. Introducing the caliper diameter to take into account large displacements of atoms in a collision cascade, it is possible to describe this fragmentation in terms of a threshold energy Ec. From the knowledge of Ec, the number and the volume fraction of subcascades produced is then derived. This last information allows us to classify the efficiency of impinging particles to induce structural changes of solids.