Pyrochlore-type lanthanide titanates and zirconates: Synthesis, structural peculiarities, and properties.

This contribution provides a thorough examination of the structural characteristics of pyrochlore-type lanthanide titanates and zirconates Ln₂Ti₂O₇ and Ln₂Zr₂O₇, across various length scales. This paper also examines their processing, interesting physical properties (electrical, magnetic, and thermal characteristics), and responses to high pressure and ion irradiation. Brief sections on the elemental oxides' crystal chemistry, pertinent phase diagrams, and energetics of defect formation are also provided. Pyrochlore-type Ln₂Ti₂O₇ and Ln₂Zr₂O₇ stand out as truly multifunctional materials. Moreover, they have emerged as fascinating materials due to magnetic geometrical frustration, arising from the ordering of magnetic Ln³⁺ and non-magnetic Ti⁴⁺ (or Zr⁴⁺) cations into separate, interpenetrating lattices of corner-sharing tetrahedra. This results in a diverse array of exotic magnetic ground states, such as spin-ice (e.g., Dy₂Ti₂O₇ or Ho₂Ti₂O₇) or quantum spin ice (e.g., Tb₂Ti₂O₇), observed at both low and room temperatures. They also exhibit varied electrical and electrochemical characteristics. Some members such as Gd₂Zr₂O₇, function as fast ion conductors with a conductivity (σ) of ≈10⁻² S·cm⁻¹ at 800 °C and activation energy (E_a) ranging from 0.85 to 1.52 eV, depending on the degree of structural disorder. Others, such as Gd₂TiMoO₇, are mixed ionic-electronic conductors with σ ≈ 25 S·cm⁻¹ at 1000 °C, making them promising candidate materials for applications in energy conversion and storage devices and oxygen separation membranes. Their exceptionally low thermal conductivity (e.g., κ ∼ 1.1–1.7 W·m⁻¹·K⁻¹ between 700 and 1200 °C for Ln₂Zr₂O₇), close to the glass-like lower limit of highly disordered solids, positions them as valuable materials for thermal barrier coatings. They can also effectively accommodate actinides (e.g., Pu, Np, Cm, Am) in solid solutions and sustain prolonged exposure to radiation due to alpha-decay events, while preserving the integrity of the periodic atomic structure. Proposed as major components in actinide-bearing ceramics, they contribute to the long-term immobilization and disposal of long-lived waste radionuclides from nuclear programs. Some of these properties are displayed simultaneously, opening avenues for new applications. Despite the wealth of data available in the literature, this review highlights the need for a better understanding of order/disorder processes in pyrochlore-type materials and the influence of the structural length scale on their physical and chemical properties. Recent experimental evidence has revealed that pyrochlore short-range structure is far more complex than originally thought. Moreover, pyrochlore local structure is now believed to include short-range, lower symmetry, ordered domains, such as the orthorhombic weberite-type of structure. Notably, short- and long-range structures appear decoupled across different length scales and temperature regimes, and these differences persist even in well-ordered samples. We believe that the pyrochlore structure offers a unique opportunity for examining the interplay between chemical composition, defect chemistry, and properties. [ABSTRACT FROM AUTHOR]