Phase Equilibrium of Y20 -TiO2

Abstract

The phase equilibrium relationship of Y2O2-TiO2- system in air was determined in the temperature range between 800°C and 1600°C (Table 1 and Fig.1). In this study, the coprecipitants of Y(111) and Ti(JV) hydroxides were used as the samples for temperature range below 1200°C, while the mixtures of Y208 and TiO2- powders for higher temperature range. The samples were heated for 2-30 days, quenched in liquid nitrogen and analysed by X-ray diffraction. Four compounds, ce-Y2Ti05, 13-Y2TiO5, Y2Ti207 and fluorite type compound (YsTii_xO2-x/20x/2) were identified in this system. The a-Y2TiO5 with orthorhombic lattice is stable below 1330°C. The 3-Y2TiO5 with hexagonal lattice, ao= 3.615 A, c0=11.838 A (Table 2), is stable between 1330°C and 1520°C, and at higher temperature it changes into fluorite type cubic solid solution. The solubilization of TiO2 or Y2O2- was observed neither a- nor 3-Y2TiO5. Only a small amount of TiO2 and Y20, are soluble in cubic pyrochlore type (Y2Ti2Q7) below 1400°C, while above 1450°C, Y2O2- is markedly soluble in it and the produced solid solution gradually changes its structure from pyrochlore to fluorite type. The lowest temperature at which the fluorite phase is stable is 1390°C, when the solution contains 43.7 mol % TiO2 and its region becomes wide with increase of temperature.