Defect-fluorite oxides M1-yLnyO2-y/2(Ln = lanthanide; M = Hf, Zr, Ce, U, Th): Structure, property, and applications*
Akio Nakamura1, Nobuyuki Masaki2, Haruyoshi Otobe2, Yukio Hinatsu3, Junhu Wang4, and Masuo Takeda4
1Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Naka, Ibaraki 319-1195, Japan; 2Nuclear Science and Engineering Directorate, Japan Atomic Energy Agency, Tokai, Naka, Ibaraki 319-1195, Japan; 3Department of Chemistry, Hokkaido University, Sapporo 060-0810, Japan; 4Department of Chemistry, Toho University, Funabashi, Chiba 274-8510, Japan
Abstract: An up-to-date summary of our coupled macroscopic (lattice parameter, a0) and microscopic (Mössbauer) studies on the entitled systems is given, shedding new light on the basic phase and structure properties of these technologically important, highly defective oxides. Through these studies, the intermediate-pyrochlore (P)-based local structure nature of the so-called stabilized-zirconia (SZ)- and stabilized-hafnia (SH)-type defect-fluorite (DF) phases in the M4+ = Zr and Hf systems seems to be almost unambiguously established with different (from the conventional) approaches: These are (1) the presence of a broad lattice parameter (a0) hump over the extended stabilized cubic DF region (y = ~0.3-~0.7); (2) the microscopic 151Eu-Mössbauer evidence of Eu3+ isomer shifts (ISs) showing characteristic V-shaped minima around the ideal-P area (y ~ 0.50) for both the M4+ = Zr and Hf systems; and (3) rich 155Gd-Mössbauer data on the Zr1-yGdyO2-y/2 system [IS, quadrupole splitting (QS), line width, peak height, and relative absorption area (RAA)], revealing detailed features of its ordered P-disordered DF phase and structure relationships. These results are discussed with reported basic-property data of these systems.
Keywords: defect-fluorite oxides; stabilized zirconia (hafnia); lattice parameter; non-Vegardian behavior; Mössbauer spectroscopy; powder X-ray diffraction; local structure; fluorite structure; pyrochlore structure; C-type structure.
*Paper based on a presentation at the 12th International IUPAC Conference on High Temperature Materials Chemistry (HTMC-XII), 18-22 September 2006, Vienna, Austria. Other presentations are published in this issue, pp. 1635-1778.