Substituent Influence of Yttria by Gadolinia on the Tetragonal Phase Stability for Y2O3-Ta2O5-ZrO2 Ceramics at 1300 °C

The influences of equimolar substitution of yttria by gadolinia on the phase constituent and stability for Y2O3-Ta2O5-ZrO2 ceramics have been investigated. The ceramics with the Gd content lower than 8 mol% exhibit pure tetragonal phase as well as high tetragonal phase stability. However, the high Gd containing ceramics consist of t-ZrO2, m-ZrO2 and monoclinic GdTaO4 and show complicated phase evolution behaviors. The fractions of monoclinic ZrO2 and GdTaO4 increase with increasing Gd content, indicating that the excessive substitution of Gd for Y results in a reduction in the tetragonal phase stability. In addition, the lower Gd containing ceramics show an almost invariable tetragonality, while the higher Gd containing ceramics reveal a composition-dependent tetragonality. Accordingly, an association of the tetragonal phase stability and tetragonality with the Gd concentration is established, which provides us a clue to understand the phase stability of these ceramics.