Metastable Phase Evolution in TiO 2 -YO 3/2 -ZrO 2 Tobias Schaedler 1 , Solome Girma 2,3 , Ashutosh S. Gandhi 1 , Sanjay Sampath 2 , Carlos G. Levi 1 1 Materials Department, University of California, Santa Barbara, CA, USA 2 Center for Thermal Spray Research, State University of New York, Stony Brook, NY, USA 3 RISE intern, Materials Department, University of California, Santa Barbara, CA, USA ABSTRACT The metastable-to-equilibrium phase evolution over a wide range of compositions in the TiO 2 - YO 3/2 -ZrO 2 system was investigated. The competing phases are all derivatives of the fluorite structure and compositions within the fluorite and pyrochlore fields exhibit technologically inter- esting ionic and mixed ionic-electronic conductivity. Powders of various compositions were synthesized by precursor routes, pyrolyzed and subsequently heat-treated in a stepwise manner at progressively higher temperatures to explore the sequence of phase evolution. Extended solid solutions with amorphous, fluorite and ordered pyrochlore structures were produced over signifi- cant composition ranges within the ternary. The study also sheds new light on the correct form of the phase equilibria at 1300°C. INTRODUCTION Solution synthesis routes for multicomponent ceramic oxides enable molecular-scale mixing of precursors that, upon subsequent pyrolysis, often yield amorphous phases with novel composi- tions. Upon additional heat treatment at temperatures wherein long-range diffusion is kinetically constrained the amorphous oxide may evolve through a series of metastable states before reach- ing the equilibrium configuration [1]. Systems where metastable states are kinetically stable over significant periods of time offer the opportunity to investigate the relationship between al- ternate structures of a fixed composition and its properties. The ternary TiO 2 -YO 3/2 -ZrO 2 (Fig. 1) [2] is ideally suited for fundamental studies of metastable phase selection because of the structural relationship between the competing phases. Notably, pyrochlore ( Fd 3 m ) and bixbyite ( I a 3 ) are ordered superstructures of fluorite ( Fm 3m ) [3, 4] while tetragonal (P4 2 /nmc) and monoclinic (P2 1 /c) ZrO 2 , ZrTiO 4 (Pcnb) and even rutile (P4 2 /mnm) may be envisaged as distorted forms of fluorite [5]. Moreover, the fluorite and pyro- chlore structures in this system exhibit technologically relevant ionic and mixed ionic-electronic conductivity [6]. The present paper is part of a broader effort aimed at understanding the ther- modynamics and kinetics governing phase selection in this system in the context of conventional precursor routes and precursor-based thermal spray processes. It is also intended to clarify some inconsistencies in the current description of the phase equilibria. Specifically, Fig. 1 proposes a two-phase field between fluorite and rutile whereas other works suggest equilibrium between pyrochlore and ZrTiO 4 , [7, 8], which would negate the existence of the former at the same tem- perature. The longer-term goal is to synthesize metastable structures with sufficient stability to enable the study of the effects of crystallization, ordering and phase separation on ionic/mixed conductivity. K3.11.1 Mater. Res. Soc. Symp. Proc. Vol. 835 © 2005 Materials Research Society