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All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Materials Data on Ho2Ti2O7 by Materials Project

Ho2Ti2O7 crystallizes in the cubic Fd-3m space group. The structure is three-dimensional. Ho3+ is bonded to eight O2- atoms to form distorted HoO8 hexagonal bipyramids that share edges with six equivalent HoO8 hexagonal bipyramids and edges with six equivalent TiO6 octahedra. There are two shorter (2.20 Å) and six longer (2.48 Å) Ho–O bond lengths. Ti4+ is bonded to six equivalent O2- atoms to form TiO6 octahedra that share corners with six equivalent TiO6 octahedra and edges with six equivalent HoO8 hexagonal bipyramids. The corner-sharing octahedral tilt angles are 49°. All Ti–O bond lengths are 1.97 Å. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded in a 4-coordinate geometry to two equivalent Ho3+ and two equivalent Ti4+ atoms. In the second O2- site, O2- is bonded to four equivalent Ho3+ atoms to form corner-sharing OHo4 tetrahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Ho2Ti2O7 by Materials Project

Ho2Ti2O7 crystallizes in the monoclinic P2_1 space group. The structure is three-dimensional. there are four inequivalent Ho3+ sites. In the first Ho3+ site, Ho3+ is bonded in a 8-coordinate geometry to eight O2- atoms. There are a spread of Ho–O bond distances ranging from 2.25–2.72 Å. In the second Ho3+ site, Ho3+ is bonded in a 7-coordinate geometry to seven O2- atoms. There are a spread of Ho–O bond distances ranging from 2.23–2.39 Å. In the third Ho3+ site, Ho3+ is bonded in a 8-coordinate geometry to eight O2- atoms. There are a spread of Ho–O bond distances ranging from 2.23–2.45 Å. In the fourth Ho3+ site, Ho3+ is bonded in a 8-coordinate geometry to eight O2- atoms. There are a spread of Ho–O bond distances ranging from 2.33–2.61 Å. There are four inequivalent Ti4+ sites. In the first Ti4+ site, Ti4+ is bonded to six O2- atoms to form corner-sharing TiO6 octahedra. The corner-sharing octahedra tilt angles range from 30–45°. There are a spread of Ti–O bond distances ranging from 1.87–2.20 Å. In the second Ti4+ site, Ti4+ is bonded to six O2- atoms to form distorted corner-sharing TiO6 octahedra. The corner-sharing octahedra tilt angles range from 26–45°. There are a spread of Ti–O bond distances ranging from 1.83–2.29 Å. In the third Ti4+ site, Ti4+ is bonded to six O2- atoms to form distorted corner-sharing TiO6 octahedra. The corner-sharing octahedra tilt angles range from 30–50°. There are a spread of Ti–O bond distances ranging from 1.78–2.28 Å. In the fourth Ti4+ site, Ti4+ is bonded to six O2- atoms to form distorted corner-sharing TiO6 octahedra. The corner-sharing octahedra tilt angles range from 26–50°. There are a spread of Ti–O bond distances ranging from 1.82–2.34 Å. There are fourteen inequivalent O2- sites. In the first O2- site, O2- is bonded to two Ho3+ and two Ti4+ atoms to form distorted OHo2Ti2 tetrahedra that share corners with four OHo3Ti tetrahedra and an edgeedge with one OHo2Ti2 tetrahedra. In the second O2- site, O2- is bonded to three Ho3+ and one Ti4+ atom to form distorted OHo3Ti tetrahedra that share corners with five OHo2Ti2 tetrahedra and an edgeedge with one OHo3Ti tetrahedra. In the third O2- site, O2- is bonded in a 3-coordinate geometry to one Ho3+ and two Ti4+ atoms. In the fourth O2- site, O2- is bonded in a 3-coordinate geometry to one Ho3+ and two Ti4+ atoms. In the fifth O2- site, O2- is bonded in a 4-coordinate geometry to three Ho3+ and one Ti4+ atom. In the sixth O2- site, O2- is bonded in a 5-coordinate geometry to three Ho3+ and two equivalent Ti4+ atoms. In the seventh O2- site, O2- is bonded in a 1-coordinate geometry to two Ho3+ and two Ti4+ atoms. In the eighth O2- site, O2- is bonded in a 3-coordinate geometry to two Ho3+ and one Ti4+ atom. In the ninth O2- site, O2- is bonded to two Ho3+ and two Ti4+ atoms to form a mixture of distorted edge and corner-sharing OHo2Ti2 tetrahedra. In the tenth O2- site, O2- is bonded in a distorted see-saw-like geometry to two Ho3+ and two Ti4+ atoms. In the eleventh O2- site, O2- is bonded to two Ho3+ and two Ti4+ atoms to form a mixture of distorted edge and corner-sharing OHo2Ti2 tetrahedra. In the twelfth O2- site, O2- is bonded to three Ho3+ and one Ti4+ atom to form a mixture of distorted edge and corner-sharing OHo3Ti tetrahedra. In the thirteenth O2- site, O2- is bonded in a 5-coordinate geometry to three Ho3+ and two equivalent Ti4+ atoms. In the fourteenth O2- site, O2- is bonded in a 1-coordinate geometry to two Ho3+ and two Ti4+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Influence of Compositional Complexity on Amorphization Resistance of Swift Heavy Ion Irradiated Titanate Pyrochlores

Compositionally complex oxides have garnered attention recently for their potential technological applications in harsh environments such as thermal barrier coatings and nuclear waste forms. Therefore, their response to extreme conditions, including high temperature and intense irradiation fields, must be thoroughly investigated. Here, the structural evolution of two pyrochlore oxides with comparable cation size ratio, r A /r B , (Yb 0.2 Er 0.2 Dy 0.2 Tb 0.2 Gd 0.2 ) 2 Ti 2 O 7 and Ho 2 Ti 2 O 7 , was evaluated after irradiation with 946 MeV Au ions up to a fluence of 8 × 10 12 ions/cm2 using synchrotron X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. The overall radiation response is comparable for both titanate oxides and is dominated by a loss of crystallinity. When compared to a series of conventional titanate pyrochlore compositions, the amorphous track diameter of (Yb 0.2 Er 0.2 Dy 0.2 Tb 0.2 Gd 0.2 ) 2 Ti 2 O 7 is slightly larger than that of Ho2Ti2O7 and more in line with the diameter of the endmember with the maximum A-site cation size (Gd 2 Ti 2 O 7 ). Density functional theory calculations suggest that this behavior may be linked to local lattice distortions and the associated energetics of cation antisite formation. TEM and Raman analyses show that a disordered, crystalline shell surrounds the amorphous ion tracks in (Yb 0.2 Er 0.2 Dy 0.2 Tb 0.2 Gd 0.2 ) 2 Ti 2 O 7 , and the corresponding short-range structure resembles a weberite-type atomic arrangement.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

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 (Ea) 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. In Memoriam: Rodney C. Ewing, Fondly Remembered.

Physics↗