Rietveld Analysis of High-Temperature and Controlled Po₂ Y₂Sn₂O₇ versus Y₂(Zr_0.60Ti_0.40)₂O₇ Neutron Powder Profiles. K. Eberman, B. J. Wuensch, Dept. of Materials Science and Engineering Massachusetts Institute of Technology, Cambridge MA 02139; Z. Hu, S. Short, J. D. Jorgensen, Materials Science Division, Argonne National Laboratory, Argonne, IL 60439.

A comparison is made between Y₂Sn₂O₇ and Y₂(Zr_0.60Ti_0.40)₂O₇ fast-ion conducting pyrochlores. These A³⁺₂B⁴⁺₂ O₇ pyrochlores, space group Fd3m, have fast-ion conduction properties that make them attractive candidates for applications in fuel cells. Rietveld refinement of structural models has been achieved with neutron powder diffraction data obtained at the A.N.L. spallation source. High-temperature measurements have been employed in a controlled Po₂ environment to examine the structures in the situation of their fuel cell application. A comparison of previous room temperature results from tin containing Y₂(Sn_yTi_1-y)₂O₇ (YST) and G₂(Sn_yTi_1-y)₂O₇ (GST) and zirconium containing Y₂(Zr_yTi_1-y)₂O₇ (YZT) solid solutions demonstrated that increasing the average radius of the ions that occupy the B⁴⁺ site relative to A³⁺ does not necessarily drive the system toward a disordered defect-fluorite state. Replacement of Ti⁴⁺ with larger Zr⁴⁺ in YZT does drive the pyrochlore structure rapidly toward its defect-fluorite substructure. Complete disorder was observed in the YZT:y=0.90 sample. Replacement of Ti⁴⁺ with larger Sn⁴⁺ in YST and GST, however, has only slight disordering effects. A maximum of 5% cation anti-site disorder was observed in the GST and YST y=1.00 samples. The high-temperature data show a slight increase in order in air with as temperature increases from 25 to 1400deg.C for both compositions. The fraction of Y³⁺ on the B⁴⁺ site goes as 0.08 - T[dotaccent]4[dotaccent]10^-5 and 0.04 - T[dotaccent]1.4[dotaccent]10^-5 as a function of temperature, T, in deg.C for Y₂Sn₂O₇ and Y₂(Zr_0.60Ti_0.40)₂O₇ respectively. The fractional occupation of the normally unoccupied 8b position by O^2- stays constant with temperature at zero for Y₂Sn₂O₇ but decreases with temperature as 0.725 - 2.2[dotaccent]10^-4[dotaccent]T for Y₂(Zr_0.60Ti_0.40)₂O₇. Clear linear trends with temperature are also visible among the temperature factor coefficients, u_ij, showing a slope on the order of 10^-3.

Operation of IPNS at Argonne National Laboratory is supported by the U.S. Department of Energy.