The high-temperature Knudsen effusion method was used to study the vaporization processes in the DyF,-Dy,O, system at 1357i5 K. The following ions were identified in the mass spectra of the vapour over the system studied: DyF;, DyF,', DyF', Dy+, DyOF', DyO' as well as insignificant quantities of the Dy,F;, Dy,OF,', Dy,OF; and F+ ions. Analysis of the vapour mass spectra over the compositions containing 0.95; 0.85; 0.60; 0.40; 0.25; 0.10 mol fractions of DyF, was carried out. The data obtained indicate that DyOF, DyF, and Dy20F, molecules are the main vapour species over the DyF3-Dy,03 system.
Information on the vaporization processes of the rare earth oxyhalide systems is of great interest for developing a variety of high-temperature processes such as the preparation of alloys by electrolysis of oxygen-containing salts in molten fluorides," the manufacture of rare-earth alloy magnets, and for obtaining rare earth materials of the 21st century:
Data on the equilibrium between the gaseous and condensed phases of the rare earth oxyhalide systems are extremely limited and do not allow one to outline the main regularities of their vaporization.' Oxyhalides of La, Y, E u ; ~ Nd, Tm, Yb7 and Sm' are vaporized in the form of gaseous halides, while the corresponding oxides may be observed as residues in the condensed phase. More complex vaporization behaviour was observed over ThOq, UO,F,'o. '' and NPO,F,.'~ New gaseous oxyfluoride molecules UOzF, and UOF, were identified by Lau et al." On the other hand, John~on'~ mentions the existence of only the UF, molecule in the vapour over UOF,. Complex oxyfluoride gaseous molecules were also found over tungsten oxyfl~orides.'~ In general, there is not much information in the literature' concerning the existence of oxyfluoride molecules in the vapour over various oxyfluoride systems. A number of recent works have been published with identification of the following species: NbOF, molecules over NbOZF'' the NbWOF, and WzOzF, vapour species;16 the MoOF, WOF, and ReOF, gaseous molecules over corresponding com-pound~;'~ TaOF, over Ta0,F;" OsOF,, as might be expected, from the mass spectra of vapour over OSOF,;'~ the VOF?' and V z 0 , F ~' vapour species; the AlOF molecule over the CaF,-CaO-AlzO, s y ~t e m . ~~-~~ Information on the vaporization processes over the DyF,-DyzO, system was not found in the Literature. Vaporization and thermodynamic properties of DyF, have been studied recently by the high temperature mass spectrometric method and are discussed in an earlier publication.*' In this work, the determination of the vapour pressure and the enthalpy of sublimation of DyF, was carried out by measuring the total ion current of DyF; (as the sum of Dy', DyF', DyF; and DyF,' ion currents) during vaporization from effusion cells made from different materials (Ta, Mo, R) over the temperature range 1280-1440 K. This is superior to the earlier mass spectrometric studies, where only the Dy' and DyF' ion currents ' Author for correspondence.
were considered. At 1382 K, the Dy,F6 gaseous molecule was also identified. The other pure component of the DyF,-Dy,O, system, Dy203, vaporizes at significantly higher temperatures than DyF,. According to a reviewz6 summarizing the high-temperature mass spectrometric studies of Dy,O, vaporization, the DyO, Dy and 0 species were found in the vapour over Dy203 at 2500 K. The reliability of the