The ZrO₂-TiO₂ phase diagram

The ZrO₂-TiO₂ phase diagram was determined experimentally between 800 and 1200°C, 1 atm, extending our knowledge of this system to temperatures previously inaccessible for equilibrium experiments due to sluggish kinetics. The crystallization of the ordered (Zr,Ti) ₂O₄ phase from the oxides was facilitated by the addition of flux (CuO or Li₂MoO₄/MoO₃), and seeds. Two ordered (Zr,Ti)₂O₄ phases with different compositions were identified, and their phase relationships with TiO₂ and ZrO ₂ solid solutions investigated. Structure data, superstructure reflections and composition were used to locate the ordering phase transition of (Zr,Ti)₂O₄ in equilibrium with ZrO₂ and TiO₂. At the onset of ordering between 1130 and 1080°C, (Zr,Ti)₂O₄ is of composition X_Ti = 0.495 ± 0.02, and displays a dramatic change in b-dimension. At 1060°C and below, the composition of (Zr,Ti)₂O₄ is significantly more Ti-rich and dependent on temperature, ranging from X_Ti = 0.576 at 1060°C to 0.658 at 800°C. This variability in composition of the ordered phase contrasts with previous studies that suggested the composition to be constant at either X_Ti = 0.667 [ZrTi₂O₆] or 0.583 [Zr₅Ti₇O₂₄]. When grown at low temperatures and with lithium molybdate, the crystals of ordered (Zr,Ti) ₂O₄ are acicular to needle shape, and develop distinct square cross-sections and end facets.