On the Yttrium Tantalate – Zirconia phase diagram

The phase diagram for the YTaO4-ZrO2 quasi-binary has been determined up to 1600 °C. There are three distinct compositional regimes: an extensive YTaO4 solid solution, an extensive ZrO2 solid solution and a two-phase intermediate region. The addition of ZrO2 to YTaO4 decreases the M–T transition temperature almost linearly from 1426 °C to approximately 450 °C at the solubility limit (∼28 mol% ZrO2), and then remains constant until the ZrO2 solid solution phase boundary is reached. Within the intermediate region, there exists an extensive two-phase tetragonal (T + t) phase field above the M–T transformation temperature. The transformation exhibits no hysteresis on heating and cooling but nonetheless there is a distribution with temperature in the mass fraction of the monoclinic and tetragonal phases so no unique transformation temperature can be identified. No other high temperature phases were observed but it is suggested that a higher temperature solid solution phase is likely above 1700 °C, based on the similarity in crystallographic relationship between the two tetragonal solid solution structures.