Chimie douce and phase transitions

New oxides have been prepared in the WO3-MoO3 system, the structure of which can be described by corner-sharing (MO6) octahedra forming structures of hexagonal tungsten bronze-type, ReO3-type and pyrochlore-type. These oxides are metastable phases which transform irreversibly above 400°C into the stable phases. These monotropic, reconstructive and first-order transitions present special features compared with classical phase transitions. Indeed, they are exothermic with increasing temperature, the low-temperature phases having open structures with lower densities than the high-temperature phases. These results are discussed in relation to the structural and thermodynamic classifications of phase transitions and in relation to their chimie douce preparation routes.