JOURNAL OF SOLID STATE CHEMISTRY 83, 31-36 (1989) Structural and Dynamical Studies of S-B&O3 Oxide Ion Conductors. V. A Study of the System (Bi203)1-x(Eu20& by 151Eu Miissbauer Spectroscopy P. D. BATTLE,* T. C. GIBB, P. LIGHTFOOT, AND D. C. MUNRO School of Chemistry, The University, Leeds, LS2 9JT, Englund Received April 11, 1989; in revised form June 26, 1989 Cubic (X = 0.3) and rhombohedral (X = 0.2) phases have been prepared in the solid solution (BiZO&, (EuzO&. The cubic material transformed to the rhombohedral form when subjected to a pressure of 4 GPa at a temperature of 873 K; at 4 GPa and 1073 K the transformation was to a phase of apparently monoclinic symmetry. rS’Eu Mossbauer spectroscopy has been used to study all three forms of (Bi203)o.,(EuZOz)o~ in order to estimate the extent of anion-vacancy ordering in the solid solution. The Mossbauer linewidth from all samples is lower than that measured for C-Eu20,, suggesting that the Eu atoms in these compounds have a single, well-defined, local environment. Q 19X9 Academic Press. Inc. Introduction The high-temperature 6 phase of B&Ox (stable above 1003 K) is the best oxide ion conductor known (I) with a conductivity of -1 R-l cm-’ at 1023 K, several orders of magnitude greater than that of calcium-sta- bilized zirconia. The high conductivity is lost when the fluorite-related 6 phase trans- forms to a-B&O3 below 1003 K. However, the anion-deficient fluorite phase can be re- tained at room temperature if B&O3 is doped with the sesquioxides of the smaller rare earths or yttrium to form the solid solu- tions (Bi203)1-x(i14203)x;the exact range of dopant concentration (x) that stabilizes the cubic fluorite phase depends on the nature of M, but it is generally true that the smaller the radius of the dopant cation, the lower the concentration needed to stabilize the fluorite phase (2). The conductivity of the * To whom correspondence should be addressed. solid solutions is never as high as that of S- Bi203, but they do retain a reasonable ionic conductivity down to lower temperatures (-lo-* 0-i cm-i at 773 K). When the dop- ant concentration is lowered or when the dopant cation is relatively large, the result- ing solid solution has a rhombohedral crys- tal structure (2, 3) which also permits fast anion transport. We have previously shown (4) that the fluorite phase can be converted to the rhombohedral phase for M = Y and Er (but not Yb) by the application of high pressure. Structural studies of the fluorite-related phases have been carried out using neutron diffraction techniques (5-8). These experi- ments lead to a description of the average unit cell of the material. More recent exper- iments (9-11) have attempted to elucidate the immediate environments of the Bi3+ and M3+ cations because this local structure is likely to play a major role in determining the transport properties of the solid solu- 31 OO22-4596/8Y $3.00 Copyright 0 1989 by Academic Press, Inc. All rights of reproduction in any form reserved.