The fluorite-like Ln5Mo3O16+δ family extension: LiLn4Mo3O15F (Ln = Eu, Gd, or Dy) ceramics

Abstract

Fluorine-containing rare earth compounds of the LiLn₄Mo₃O₁₅F (Ln = Eu, Gd, or Dy) composition have been obtained using solid-phase synthesis in vacuum-sealed quartz ampoules. It was found that only LiEu₄Mo₃O₁₅F samples are isostructural to cubic Ln₅Mo₃O₁₆ (Ln = La–Gd) molybdates with a fluorite-like structure. Luminescence, hygroscopic, and electrophysical characteristics of the LiEu₄Mo₃O₁₅F phase were studied for the first time. The double replacement of europium with lithium and oxygen with fluorine leads to the appearance of a reversible phase transition accompanied by a jump in conductivity by 2 orders of magnitude. The conductivity of the LiEu₄Mo₃O₁₅F compound exceeds the value of 10⁻² S cm⁻¹ at 700 °C. A relationship between the size of the Ln cation and the hygroscopic and conductive properties of LiLn₄Mo₃O₁₅F compounds has been established: as the lanthanide radius decreases, the compounds lose their ability to absorb water and transfer protons.