Composition and stability constants of Np(V) fluoride complexes in chloride-fluoride melts

The absorption spectra of the NpO₂⁺ ion in the region of the ³ H ₅← ³ H ₄ magnetic dipole transition (∼1650 nm) in chloride-fluoride melts based on the eutectics NaCl-KCl-CsCl, NaCl-CsCl, and RbCl-CsCl at 650–750°C were recorded. The equilibrium concentrations of neptunium fluoride complexes in these melts were determined from the relative intensities of the NpO₂⁺ absorption bands. Existence of the complexes NpO₂Cl₃F]³⁻ and NpO₂Cl₂F₂]³⁻ was established, and the step stability constants of these complexes in melts of various cationic compositions were calculated.