In situ total scattering experiments of nucleation and crystallisation of tantalum-based oxides: from highly dilute solutions via cluster formation to nanoparticles

The exact formation mechanism of tantalum oxides (and in general, metal/mixed metal oxides) from alkoxide precursors is still not fully understood, particularly when forming cluster-like or amorphous materials. The structural evolution of Ta-based oxides was studied in detail using X-ray total scattering experiments along with subsequent pair distribution function (PDF) analyses. Starting from a tantalum alkoxide precursor (Ta₂(OEt)₁₀), the formation of hydrolysed Ta_xO_yH_z clusters in highly diluted aqueous solution was analysed. From the PDF data, the connectivity and arrangement of Ta_xO_y octahedra in the cluster could be deduced as well as the approximate size of the clusters (<1 nm). Construction of cluster models allowed for identification of common structural motifs in the Ta_xO_yH_z clusters, ruling out the formation of chain- or ring-like clusters. More likely, bulky clusters with a high number of corner-sharing octahedra are formed. After separation of the amorphous solid from the liquid, temperature-induced crystallisation processes were monitored via in situ total scattering experiments. Between room temperature and 600 °C, only small rearrangements of the amorphous structure are observed. At about 610 °C, amorphous Ta_xO_yH_z transforms directly into crystalline orthorhombic L-Ta₂O₅ without formation of any crystalline intermediate structures.