Melting investigation of the systemBaF2–BaO–½Y2O3–CuOx–H2O

Abstract

In order to understand low-temperature melting during the‘BaF2 process’, equilibria in the quaternary Ba, Y, Cu//O, F reciprocalsystem have been investigated using a compositional model which can berepresented as a trigonal prism. This prism is comprised of three tetrahedra:BaF2–Y F3–CuF2–CuOx, BaF2–Y F3––CuOx,and BaF2–BaO––CuOx. Systematic differential thermal analysis (DTA) studies of compositionsspaced along compositional vectors extending from the fluoride endof the prism to the oxide end gave evidence of low-melting liquids(<600 °C) near the fluorine-richregion in the BaF2–Y F3–CuF2–CuOx tetrahedron. Inthe intermediate BaF2–Y F3––CuOx tetrahedron, a low-temperature DTA peak(550–570 °C) was also identified; this has been shown to be due to a reversiblephase transformation in crystalline YOF. In the oxide-richBaF2–BaO––CuOx tetrahedron, where, in the presence of water vapour, the principal defluorinationprocess is generally thought to occur, the lowest melting temperature observed was815 °C atpO2 = 20 Paand pH2O = 2.1 kPa. However, published observations on films undergoing theBaF2 process have suggested the presence of an amorphous phase thought to be a liquid at . Based on our results, the low-melting liquids reported in theliterature do not appear to exist as a stable liquid phase in theBaF2–BaO––CuOx–H2O system under the conditions of our experiments. Rather, the formation of low-meltingliquid in Ba–Y–Cu–O–F films at may require (a) relatively fluorine-rich compositions, (b) metastable melting, (c) formationof hydroxide- or hydroxyfluoride-based liquids, or (d) some combination thereof.