NEUTRON RIETVELD ANALYSIS OF FAST ION CONDUCTING Ca-DOPED (Y_0.95Ca_0.05)₂Ti₂O₇ PYROCHLORE. Kevin Eberman, Per Önnerud, Bernhardt J. Wuensch, Dept. of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge MA 02139, USA, Judith K. Stalick, Reactor Radiation Division, National Institute of Standards and Technology,

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Y₂Ti₂O₇ which crystallizes with the A₂B₂O₇ pyrochlore structure type is a good oxygen ion conductor. Ca-doping this material causes a jump in ionic conductivity of three orders of magnitude. It has been suggested that the doping creates vacancies at oxygen sites to compensate the negative relative charge of the calcium substituting for yttrium on the A site, however, Ca-doping might also promote Frenkel disorder placing oxygen on a normally unoccupied site. If doping sets the charge neutrality condition, we expect the oxygen vacancy concentration to be half the calcium concentration ( [V_o ] = [Ca_Y']/2 ). Rietveld powder-profile analysis of the data collected with 1.5405-Å thermal neutrons has been used to determine the site occupancy of oxygen. The anion array consists of three independent sites O(1), O(2), and O(3), at positions 48f, 8a, and 8b respectively. Positions 48f and 8a are completely occupied in the fully ordered pyrochlore leaving 8b completely unoccupied. The structure seems fully ordered with 0.05 Ca substituted on the A site, with no detectable occupancy of the normally unoccupied O(3) site.