Mapping Cation Disorder in Irradiated Gd₂Ti₂O₇ Pyrochlore by 4D-STEM

Journal Article · Mon Aug 05 00:00:00 EDT 2019 · Microscopy and Microanalysis

DOI:https://doi.org/10.1017/s1431927619008535· OSTI ID:1602851

Janish, Matthew T. ^[1]; Schneider, Matthew M. ^[1]; Ophus, Colin ^[2]; Ciston, Jim ^[2]; Valdez, James A. ^[1]; McClellan, Kenneth J. ^[1]; Byler, Darrin D. ^[1]; Chen, Di ^[1]; Wang, Yongqiang ^[1]; Holesinger, Terry G. ^[1]; Uberuaga, Blas P. ^[1]

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1. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
2. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Molecular Foundry

Pyrochlores have the general formula A₂B₂O₇, where A is trivalent and usually a rare earth, and B is a tetravalent transition metal. The pyrochlore structure is closely related to that of fluorite; the difference stems from a systematic oxygen vacancy that compensates the lesser charge of the A cation. Arranging the charges appropriately requires a unit cell that is twice as large on a side compared to what is necessary to describe the fluorite structure, and so pyrochlores have lattice constants over 1 nm with eight formula units per cell. The structure of Gd₂Ti₂O₇ (GTO) is shown in Figure 1, with a = 1.018 nm, Gd occupying the 16c sites, Ti on 16d, and O on 8a and 48f (space group $$Fd\overline3m$$) direction is 5° off [110] to emphasize the ordering of the cation sublattice. Pyrochlores have generated interest as components of solid nuclear waste forms and as ionic conductors. Random mixing of the cation sublattice is important to both of these applications, the first because self-irradiation of a waste form will cause mixing over time, and the second because the ionic conductivity of pyrochlores is related to disorder on the cation sublattice. The more the cations mix, the less systematic the oxygen vacancy needs to be to maintain local charge neutrality, and the more mobile it is. Although GTO amorphizes readily under irradiation of any kind, it does pass through a defect-fluorite phase in the process that retains none of the pyrochlore superstructure. In this paper we describe an experiment devised to quantify and map the degree of disorder on the cation sublattice in pyrochlore materials at the nanoscale using four-dimensional scanning transmission electron microscopy (4D-STEM). A GTO single crystal was grown using the float-zone method with a (100) growth direction, oriented with a Laue diffractometer, and sectioned into samples with a {110} surface plane. The samples were polished and irradiated with 200 kV He ions to a fluence of 10¹⁷ ions/cm². TEM specimens were lifted out from the irradiated surface using a Thermo Scientific Helios 600 focused ion beam (FIB) instrument, also with a {110} foil plane. A low-magnification Z-contrast STEM image of a typical specimen is presented in Figure 2a. The 4D-STEM data was acquired on the TEAM I microscope at the National Center for Electron Microscopy. Scans were performed across the bottom interface, encompassing the pristine pyrochlore below the He ions’ range and the irradiated material above.

• Research Organization: Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States); Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22). Materials Sciences & Engineering Division
• Grant/Contract Number: AC02-05CH11231; 89233218CNA000001; LANLE4BU
• OSTI ID: 1602851
• Journal Information: Microscopy and Microanalysis, Vol. 25, Issue S2; Conference: Microscopy and Microanalysis 2019, Portland, OR (United States), 4-8 Aug 2019; ISSN 1431-9276
• Publisher: Microscopy Society of America (MSA)
• Country of Publication: United States
• Language: English

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36 MATERIALS SCIENCE