Phase relations and the thermodynamic properties of phases in the Y2O3-YF3system

Variable-composition rhombic yttrium oxyfluorides were found to exist in th e Y2O3-YF3 system at high temperatures (1150-1350 K) in the concentration r ange 60-similar to 67 mol % YF3. The thermodynamic functions of formation o f the Y7O6F9 (2Y(2)O(3) . 3YF(3)) and Y4O3F6 (Y2O3 . 2YF(3)) boundary rhomb ic yttrium oxyfluorides from the constituent components (Y2O3 and YF3) were determined by the electromotive force method with a solid fluorine-ion ele ctrolyte. The thermodynamic properties of the YOF stoichiometric yttrium ox yfluoride were refined. It was shown that YOF was the thermodynamically sta blest phase in tile system under consideration. The formation of nonstoichi ometric rhombic yttrium oxyfluorides from neighbor phases in the phase diag ram was accompanied by insignificant Gibbs energy changes.