The tetragonal-monoclinic, ferroelastic transformation in yttrium tantalate and effect of zirconia alloying

Oxide compositions of equimolar YO1.5 and TaO2.5 in the Y-Ta-Zr-O system have attractive properties for high-temperature applications, including as thermal barrier coatings. The effect of zirconia concentration, from 0 to 20 mol.% cation, on the tetragonal-to-monoclinic phase transition has been studied using high-temperature X-ray diffraction, Raman spectroscopy and electron microscopy. The transformation is reversible and the temperature variation of an order parameter based on the spontaneous strain is consistent with the transformation being ferroelastic, a critical feature for toughening at high temperatures. The presence of twin domains further supports this conclusion. Additionally, stabilization of the tetragonal phase with increasing ZrO2 is evident from the amount of partially retained tetragonal phase at room temperature.