Preparation and characterization of rare earth molybdatesStructure-property relation

Resumen

The work presented in this thesis focuses on the experimental study of rare earth molybdates with chemical formula RE2(MoO4)3 (RE=La-Lu, Y). These compounds crystallize in three structural types at ambient conditions: modulated scheelite-type structure (divided in two: La2(MoO4)3 and alpha-phase) and beta'- and gamma-phases. We have characterized these compounds under different conditions of temperature and pressure by using several experimental techniques, such as powder diffraction, impedance, optical and Raman spectroscopies, and thermal analysis. In addition, we complemented the high-pressure experiments using theoretical calculations. For rare earth molybdates with alpha-phase three transport mechanisms have been found: semiconductor, polaronic and ionic; in three temperature regions. These transport mechanisms were correlated with an anomalous temperature dependence of the crystal structure. Moreover, high-pressure studies revealed that the alpha-phase reaches the pressure-induced amorphization without any intermediate structural phase transition. Furthermore, the mechanisms of structural compression and amorphization were proposed. Rare earth molybdates with beta'-phase were also studied under high pressures. These materials have a phase transition to a new delta-phase and their lattice parameters were found. On the other hand, we conducted a preliminary study of the beta'-phase in several rare earth molybdates at ambient conditions, looking for correlations between the rare-earth ionic radius and their ferroelectric-ferroelastic properties. In addition, we obtained the phase diagram varying the temperature in the Ho2(MoO4)3 polymorphous with beta'- and gamma-phases in order to identify their phase transitions. Finally, we analysed the negative thermal expansion in several compounds with gamma-phase. We evaluated their crystal structures as a function of the temperature and the rare-earth ionic radius, proposing the structural mechanisms that explain the compression of the lattice parameters when the temperature increases.