Uranothorite is of geological interest because of its natural occurrence in uranium deposits 4, 5, 7, 13, 14, 15, 16, suggesting an enhanced stability field for the coffinite endmember. Uranothorite (U xTh 1− xSiO 4) is the only known natural silicate system where actinides, U and Th, form a continuous solid solution between thorite (ThSiO 4) and coffinite (USiO 4) 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12. The electronic origin accounts for the strong non-ideality in thermodynamic parameters that extends the stability field of the actinide silicates in nature and under typical nuclear waste repository conditions.
Partial covalency from the U/Th 5 f–O 2 p hybridization promotes electron transfer during mixing and leads to local polyhedral distortions. The local disorder and nanosized heterogeneous is an example of the non-ideality of mixing that has an electronic origin. Neutron total scattering implemented by pair distribution function analysis confirmed a random distribution model for U and Th in first three coordination shells however, a machine-learning algorithm suggested heterogeneous U and Th clusters at nanoscale (~2 nm).
We have investigated an actinide silicate solid solution system (USiO 4–ThSiO 4), demonstrating that thermodynamic non-ideality follows a distinctive, atomic-scale disordering process, which is usually considered as a random distribution. Non-ideal thermodynamics of solid solutions can greatly impact materials degradation behavior.