Effect of local ordering around Th4+ ions in glycine-nitrate precursor gel on the powder characteristic of gel-combusted ThO2

In the present study, influence of local environment around Th4+ ions in the precursor gel on the powder characteristics of combusted product (ThO2) is investigated in detail. Partial crystallization of Th4+ complexes having different number of glycine and NO3− species in glycine-thorium nitrate aqueous gel has been confirmed by DSC and SAXS studies. Further, combining SAXS and FT-IR results with existing speciation studies, it is inferred that, varying concentrations of mono- and hexanuclear Th4+-glycine-NO3- complexes are formed in the precursor gel depending upon the fuel to oxidant ratio (F:O). Lower F:O values lead to predominance of hexanuclear Th4+-glycine-NO3- complexes with excess NO3− ions in its vicinity while higher F:O value (F:O = 1.25) results in predominant formation of monocular Th4+-glycine-NO3- complexes. Combustion of gel consisting of mononuclear complexes leads to smaller ThO2 crystallites as compared to hexanuclear complexes as the Th4+--Th4+ distances are larger in the former compared to the latter. The above mentioned complex species are responsible for the observed bimodal crystallite size distribution in nanocrystalline ThO2 powder prepared upon combustion of gel precursor. •Gels containing Th4+-glycine-NO3- undergo partial crystallization before combustion.•Fraction of mono-/hexanuclear Th4+-glycine complexes depends on fuel to oxidant ratio.•Presence of mono-/hexanuclear complexes assists bimodal size distribution of ThO2.•Nature of complexes governs the crystallite size distribution in as-combusted ThO2.•Molecular origin of crystallite size in as-combusted ThO2 has been identified.