Les matériaux nanométriques ont fait l'objet d'un intérêt de recherche important car ils présentent de nouvelles propriétés physiques et chimiques par rapport aux échantillons massifs. En ce qui concerne les nanomatériaux, l'effet de taille et l'énergie de surface sont généralement invoqués, même si les concepts sous-jacents ne sont pas clairs. Dans cette thèse, la question principale à laquelle nous voulons répondre est : quels sont les principaux paramètres qui régissent la stabilité structurelle du SnO2 à l’échelle nanométrique sous haute pression comparé aux échantillons de SnO2 massifs ? La combinaison de la haute pression et de la taille des particules est particulièrement importante pour comprendre la structure de ces nanoparticules et l'effet des défauts et de l'énergie de surface sur leur stabilité de phase, car, en gardant la taille des particules constante, l'augmentation de la pression permettra l'exploration les paysages énergétiques du système. De plus, la pression et la taille sont deux paramètres qui peuvent être utilisés conjointement pour stabiliser les nouvelles phases. L'intérêt de l'étude des nanoparticules sous haute pression est au moins double : (i) acquérir une compréhension fondamentale de la thermodynamique lorsque l'énergie interfaciale devient de la même ampleur que l'énergie interne (ii) pour stabiliser de nouvelles structures potentiellement potentielles intérêt en tant que matériaux fonctionnels. Dans ce travail, nous avons utilisé la spectroscopie Raman comme principale méthode de caractérisation. Pour les échantillons de SnO2 massif, nous avons utilisé la théorie de la percolation pour expliquer la désordre « partiel » du sous-réseau oxygène qui apparaît lorsque la pression augmente, ce qu’on appelle désordre « partiel » induite par la pression. Et, en étudiant les nanoparticules de SnO2, nous avons utilisé des simulations ab initio pour expliquer l'apparition de ce type de désordre, cet à dire, le désordre du sous-réseau anionique lorsque la pression augmente. De cette façon, nous proposons d'obtenir une compréhension fondamentale du SnO2 massif et nanométrique, sous pression / Nanosized materials have been the focus of an extensive interest research as they present new physical and chemical properties in comparison to their bulk equivalent. When dealing with nanomaterials, the size effect and the surface energy are generally invoked, even though the underlying concepts are not clear. In this thesis, the main question that we want to answer is: what are the main parameters which govern the structural stability at the SnO2 nanometric under high pressure in comparison to its bulk counterpart? The combination of high pressure and particle size is particularly important in order to understand the nanoparticle structure, and the effect of the defects and of the surface energy on phase stability. By maintaining the size of the particle constant, the pressure will allow the energy landscapes of the system to be explored. In addition, pressure and size are two parameters that can be used conjointly in order to stabilize new phases. So, the interest of studying nanoparticles under the high-pressure is at least two-fold: (i) to gain a fundamental understanding of thermodynamics when the interfacial energy reaches the same magnitude as the internal energy (ii) to stabilize new structures that may have potential interest as functional materials. In this work, we used Raman spectroscopy as the main characterization method. In the study of SnO2 bulk samples, we used percolation to explain the “partial” disorder of the oxygen sublattice which appears in the powders when the pressure increases; and for studying SnO2 nanoparticles, we used ab initio simulations to explain the appearance of this kind of disorder, i.e. the anionic sublattice disorder in SnO2 nanoparticle samples. In this way, we propose to obtain a fundamental understanding of SnO2 bulk and nanoparticles under pressure

Synthesis and Characterization of Some Low and Negative Thermal Expansion Materials Tamas Varga 370 pages Directed by Dr. Angus P. Wilkinson The high-pressure behavior of several negative thermal expansion materials was studied by different methods. In-situ high-pressure x-ray and neutron diffraction studies on several compounds of the orthorhombic Sc2W3O12 structure revealed an unusual bulk modulus collapse at the orthorhombic to monoclinic phase transition. In some members of the A2M3O12 family, a second phase transition and/or pressure-induced amorphization were also seen at higher pressure. The mechanism for volume contraction on compression is different from that on heating. A combined in-situ high pressure x-ray diffraction and absorption spectroscopic study has been carried out for the first time. The pressure-induced amorphization in cubic ZrW2O8 and ZrMo2O8 was studied by following the changes in the local coordination environments of the metals. A significant change in the average tungsten coordination was found in ZrW2O8, and a less pronounced change in the molybdenum coordination in ZrMo2O8 on amorphization. A kinetically frustrated phase transition to a high-pressure crystalline phase or a kinetically hindered decomposition, are likely driving forces of the amorphization. A complementary ex-situ study confirmed the greater distortion of the framework tetrahedra in ZrW2O8, and revealed a similar distortion of the octahedra in both compounds. The possibility of stabilizing the low thermal expansion high-temperature structure in AM2O7 compounds to lower temperatures through stuffing of ZrP2O7 was explored. Although the phase transition temperature was suppressed in MIxZr1-xMIIIxP2O7 compositions, the chemical modification employed was not successful in stabilizing the high-temperature structure to around room temperature. An attempt has been made to control the thermal expansion properties in materials of the (MIII0.5MV0.5)P2O7-type through the choice of the metal cations and through manipulating the ordering of the cations by different heat treatment conditions. Although controlled heat treatment resulted in only short-range cation ordering, the choice of the MIII cation had a marked effect on the thermal expansion behavior of the materials. Different grades of fluorinert were examined as pressure-transmitting media for high-pressure diffraction studies. All of the fluorinerts studied became nonhydrostatic at relatively low pressures (~1 GPa).