Structural properties and thermal stability of Bi8Pb5O17 fast ion conducting phases

Samples with Bi8Pb5O17 stoichiometry were synthesized by ceramic techniques at temperatures ranging from 200 to 640 degrees C. Their structural properties and relative stabilities were investigated using X-ray and electron diffraction, differential scanning calorimetry, and Raman spectroscopy. The tetragonal beta(2) phase is described by an incommensurately modulated structure, whose modulation depends on the firing temperature. Heat treatments in the 200-300 degrees C range lead to the formation of a new tetragonal phase, chi, which is converted back to beta(2) above 300 degrees C, The intermediate phi phase can be obtained without the previous formation of beta(2), and its X-ray powder diffraction pattern is indexed using a triclinic cell. The present data suggest that the beta(2) phase is the low-temperature polymorph of this composition. (C) 1999 Academic Press.