Metastability of the Fluorite, Pyrochlore, and Perovskite Structures in the PbO—ZrO 2 —TiO 2 System

• Published in: Journal of the American Ceramic Society Vol. 83; no. 4; pp. 873 - 881
• Main Authors: Polli, Andrew D., Lange, Fred F., Levi, Carlos G.
• Format: Journal Article
• Language: English
• Published: 01.04.2000
• ISSN: 0002-7820; 1551-2916
• DOI: 10.1111/j.1151-2916.2000.tb01288.x

The crystallization, transformation, and partitioning of amorphous PbO—ZrO 2 —TiO 2 powders produced by pyrolytic decomposition of partially hydrolyzed, mixed alkoxide precursors were investigated. Materials have the general formulation Pb 1+χ Ti [1/(1+φ)] Zr [φ/(1+φ)] O 3+χ , where −0.2 ≤χ≤ 0.2 is the fraction of PbO nominal excess/deficiency and 0 ≤φ≤ 1 is the Zr/Ti molar ratio. Most compositions first crystallized as a metastable fluorite structure with varying degrees of pyrochlore‐like cation ordering, which transformed to a single perovskite phase upon additional heat treatment. Higher Zr/Ti ratios enhanced the retention of fluorite and reduced the incidence of cation ordering. Compositions with off‐stoichiometric amounts of PbO often yielded extended solid solutions prior to partitioning. For example, metastable perovskites with as much as 20% PbO deficiency (χ=−0.2) could be prepared for 0 ≤φ≤ 1, but only ∼10% PbO excess could be incorporated in solid solution for 0.33 ≤φ≤ 1. Increasing PbO content was found to promote crystallization, suggesting that this oxide acts as a network modifier enhancing mobility within the initial amorphous precursor powder. Higher PbO was also noted to favor cation ordering in the metastable phase and to accelerate the transformation to perovskite, as well as to promote partitioning for hyperstoichiometric compositions. The findings are discussed in light of structural relationships between the fluorite, pyrochlore, and perovskite phases, as well as current understanding of the thermodynamics of the system.