The role and effect of rare-earth elements and Fe3+ during the crystallisation of nanostructured materials synthesised from carbonates = La influencia de los elementos de tierras raras y Fe3+ durante la cristalización de materiales nanoestructurados sintetizados a partir de carbonatos

Abstract

This PhD thesis reports the investigation of the early stages of crystallisation of carbonatebearing compounds in four different systems, using Dy3+, La3+, Nd3+ and Fe3+. The crystallisation of these compounds was initiated by the formation of poorly-ordered precursors that, depending on their chemistry and experimental conditions, followed different crystallisation pathways. All precursors were synthesised from aqueous solutions at 21 °C and characterised by solid-state, spectroscopic and microscopic techniques. The stability of rare-earth bearing precursors was studied in air (21-1000 °C) and in solution (21-220 °C). The precursors with shorter and longer lifetimes were the amorphous La3+ and Dy3+ carbonates, respectively. These different lifetimes can be understood when the different ionic potentials of the rare-earth elements (La3+ < Nd3+ < Dy3+) are taken into account. Amorphous precursors are hydrated compounds and their crystallisation involves dehydration processes. The strongest the ionic potential of the RE3+, the more energy needed to dehydrate its hydration shell, so the longer the lifetime of the precursor. Crystalline carbonates developed spherulitic morphologies in solution. These were a consequence of the fast dissolution of the amorphous precursors, promoting high supersaturation levels during the crystallisation process. The experiments carried out with Fe3+ resulted in the formation of 2-line ferrihydrite with adsorbed or co-precipitated carbonate. This carbonated 2-line ferrihydrite was dry-heated and transformed to ¿-Fe2O3 above 250 °C. The magnetic properties of ¿-Fe2O3 were studied from 350 to 1000 °C, showing a linear increase of the magnetic coercivity with temperature. Its crystallite and particle size grew linearly and logarithmically, respectively, with increasing temperature. This growth rate difference was translated into the development of a subparticle structure that explains the enhanced magnetic coercivity of this compound.