Terbium Fluoride: Oxide Converted in HF and Argon at 750 °C, then Melted in HF to Remove the Last Oxygen (Henderson, Ames 1970)

Rare-earth metals made from fluorides are only as clean as the fluoride. Henderson converted the terbium oxide in a stream of hydrogen fluoride and argon at 750 °C, then melted the product in hydrogen fluoride so the last oxide and oxyfluoride turned to fluoride as well. Result: TbF3 with under 50 ppm oxygen. Source: D. C. Henderson, The Preparation and Some High Temperature Properties of the Rare Earth Trifluorides (Iowa State University / Ames Laboratory, 1970), pp. 17-34. Anhydrous hydrogen fluoride is deadly and burns deep into skin. Picture drawn by Youblob.