A study of the oxidation products of some LiRF4 phases

The effects of oxidation in both normal and moist air upon powdered single crystals of the scheelite-type compounds LiYF₄, LiErF₄, -YLF (LiY_0.434 Er_0.5 Tm_0.055 Ho_0.011 F₄) and -LuLF (LiLu_0.434 Er_0.5 Tm_0.05 Ho_0.011 F₄) have been studied by X-ray powder diffraction and differential thermal analysis. For all the materials the oxidation rate is enhanced in the presence of moisture. The oxidation process results in the formation of rare-earth oxyfluoride phases, LiYF₄, forming yttrium oxyfluoride (with a rhombohedral phase predominating) and LiErF₄ forming erbium oxyfluoride (with a tetragonal phase predominating). The LiYF₄ compound oxidizes more rapidly than LiErF₄ both in normal and moist air. The more complex -YLF compound oxidizes to form a mixture containing significant quantities of both the rhombohedral and tetragonal rare-earth oxyfluoride phases. The -LuLF compound oxidizes to form the tetragonal rare-earth oxyfluoride phase together with a very small amount of the rhombohedral variety. The melting reactions of oxidized samples are very broad thermal events which are coupled with the presence of a strong LiF/LiRF₄-type eutectic component. These observations are rationalized in terms of the appearance of excess LiF due to selective oxidation of the trifluoride component.