The detailed crystal structure as well as the heat capacity at low temperature and standard entropy of Ba2MoO5 are reported for the first time. High-resolution X-ray and neutron diffraction were employed to reveal the structural features of this compound. Ba2MoO5 has a six-coordinated Mo and a strongly negative excess volume with respect to the binary oxides. X-ray absorption near edge structure (XANES) spectroscopy at the Mo K-edge shows Mo to be in the oxidation state 6+. The pre-edge peak in the XANES spectrum indicates a distorted octahedral environment, in line with the results from diffraction studies and FDMNES calculations. The standard entropy and heat capacity of Ba2MoO5 at 298.15 K, determined with a thermal-relaxation technique, are calculated to be respectively 223.2 ± 7 and 184.7 ± 5 J·K–1·mol–1. The obtained thermodynamic properties are discussed in the context of the literature reports on molybdate compounds.

In this study, new insights into the solid state chemistry of the systems NaCl-RECl3 (RE = Ce, Nd) are presented, in which the intermediate compound suggested in the literature, i.e. Na3RE5Cl18, is investigated more closely. Our studies have revealed a solubility range around the intermediate composition in the form of the stoichiometry, and have allowed us to revisit the phase diagrams of the NaCl-RECl3 (RE = Ce, Nd) systems accordingly. Furthermore, we demonstrate that among the lanthanide chlorides, NdCl3 is the prime simulant candidate for the melting behaviour of PuCl3-based systems, while CeCl3 is most suited to simulate UCl3-based systems. This is corroborated in this work by comparing the melting profiles of the NaCl-MCl3, MgCl2-MCl3, and NaCl-MgCl2-MCl3 (M = Ce, Nd, U, Pu) systems. In doing so, the binary systems MgCl2-MCl3 (M = Ce, Nd) have been re-visited based on existing data in the literature and estimated mixing enthalpies. Extrapolations to the ternary systems NaCl-MgCl2-RECl3 (RE = Ce, Nd) have been made and compared to the available data in the literature, showing good agreement.

Graphical abstract: Simulant chemistry for uranium and plutonium molten fuel salts: crystallographic investigation and thermodynamic modelling assessment of the NaCl–RECl3 and NaCl-MgCl2-RECl3 (RE = Ce, Nd) systems