Rb2Ln[Si2O6]F (Ln = Y, Eu–Lu): Flux Synthesis, Structure Determination, and DFT-Calculated Formation Enthalpies of a Series of Mixed-Anion Rare Earth Cyclosilicates

A new series of mixed-anion alkali rare earth silicate fluorides with composition Rb2Ln[Si2O6]F (Ln = Y, Eu–Lu) has been synthesized via an alkali chloride/fluoride eutectic flux synthetic route. All synthesized compositions crystallize in the tetragonal space group P42/mnm with a 3D framework consisting of LnO4F2 octahedra, tetrasilicate rings, and 1D channels containing alkali metals. A combination of powder X-ray diffraction, single-crystal X-ray diffraction, and luminescence emission spectroscopy was performed to characterize the reaction products. In addition, density functional theory (DFT) calculations were utilized to calculate the 0 K formation enthalpies of the synthesized phases and of hypothetical trivalent actinide analogues to probe the likelihood of the successful synthesis of such trivalent transuranic containing phases, specifically Am and Cm, in the future.