Chalcopyrite (CuFeS₂) is the main source of copper worldwide and is usually formed at high temperature. Its occurrence at low temperature is poorly documented, and the mechanisms controlling its formation remain uncertain. We found evidences of chalcopyrite formation in acidic pit lake sediments at ~ 12 °C. Using high-resolution electron microscopy and synchrotron-based spectroscopy, we observed aggregates of nanoscale crystals with the composition and structure of disordered chalcopyrite. Laboratory incubations at 20 °C and geochemical modelling suggest that microbial activity may contribute to chalcopyrite formation under these conditions. Particularly, crystal growth was associated with hollow structures resembling microbial cell surfaces, and on the membranes of eukaryotic-like cells, providing nucleation sites. Our findings suggest that microbial processes, including the production of hydrogen sulfide and the presence of organic surfaces, promote chalcopyrite formation at low temperature. This has implications for understanding copper and sulfur cycling and its potential biotechnological application in sulfidic environments.

To assure the safety of oxide-fuel based nuclear reactors, the knowledge of the atomic-scale properties of U_1−yM_yO_2±x materials is essential. These compounds show complex chemical properties, originating from the fact that actinides and rare earths may occur with different oxidation states. In these mostly ionic materials, aliovalent cationic configurations can induce changes in the oxygen stoichiometry, with dramatic effects on the properties of the fuel. First studies on U_1−yAm_yO_2±x indicated that these materials exhibit particularly complex electronic and local-structure configurations. Here we present an in-depth study of these compounds, over a wide compositional domain, by combining XRD, XAS and Raman spectroscopy. We provide evidences of the co-existence of four different cations (U⁴⁺, U⁵⁺, Am³⁺, Am⁴⁺) in U_1−yM_yO_2±x compounds, which nevertheless maintain the fluorite structure. Indeed, we show that the cationic sublattice is basically unaffected by the extreme multi-valence states, whereas complex defects are present in the oxygen sublattice.

The physicochemical properties of the potassium neptunate K₂NpO₄ have been investigated in this work using X-ray diffraction, X-ray absorption near edge structure (XANES) spectroscopy at the Np-L₃ edge, and low-temperature heat capacity measurements. A Rietveld refinement of the crystal structure is reported for the first time. The Np(VI) valence state has been confirmed by the XANES data, and the absorption edge threshold of the XANES spectrum has been correlated to the Mössbauer isomer shift value reported in the literature. The standard entropy and heat capacity of K₂NpO₄ have been derived at 298.15 K from the low-temperature heat capacity data. The latter suggest the existence of a magnetic ordering transition around 25.9 K, most probably of the ferromagnetic type.

Solid/liquid equilibria in the system UO₂–ZrO₂ are revisited in this work by laser heating coupled with fast optical thermometry. Phase transition points newly measured under inert gas are in fair agreement with the early measurements performed by Wisnyi et al., in 1957, the only study available in the literature on the whole pseudo-binary system. In addition, a minimum melting point is identified here for compositions near (U_0.6Z_r0.4)O_2+y, around 2800 K. The solidus line is rather flat on a broad range of compositions around the minimum. It increases for compositions closer to the pure end members, up to the melting point of pure UO₂ (3130 K) on one side and pure ZrO₂ (2970 K) on the other. Solid state phase transitions (cubic-tetragonal-monoclinic) have also been observed in the ZrO₂-rich compositions X-ray diffraction. Investigations under 0.3 MPa air (0.063 MPa O₂) revealed a significant decrease in the melting points down to 2500 K–2600 K for increasing uranium content (x(UO₂)> 0.2). This was found to be related to further oxidation of uranium dioxide, confirmed by X-ray absorption spectroscopy. For example, a typical oxidised corium composition U_0.6Zr_0.4O_2.13 was observed to solidify at a temperature as low as 2493 K. The current results are important for assessing the thermal stability of the system fuel – cladding in an oxide based nuclear reactor, and for simulating the system behaviour during a hypothetical severe accident.

The charge distributions in α-Na₂UO₄, Na₃NpO₄, α-Na₂NpO₄, Na₄NpO₅, Na₅NpO₆, Na₂PuO₃, Na₄PuO₅, and Na₅PuO₆ are investigated in this work using X-ray absorption near-edge structure (XANES) spectroscopy at the U-L₃, Np-L₃, and Pu-L₃ edges. In addition, a Rietveld refinement of monoclinic Na₂PuO₃, in space group C2/c, is reported for the first time, and the existence of the isostructural Na₂NpO₃ phase is revealed. In contrast to measurements in solution, the number of published XANES data for neptunium and plutonium solid phases with a valence state higher than IV is very limited. The present results cover a wide range of oxidation states, namely, IV to VII, and can serve as reference for future investigations. The sodium actinide series show a variety of local coordination geometries, and correlations between the shape of the XANES spectra and the local structural environments are discussed herein.