STABILITY, THERMAL-BEHAVIOR, AND CRYSTAL-STRUCTURE OF ION ORDERED BI(1-X)LN(X)O(1.5) PHASES (LN=SM-DY)

This paper deals with the bcc Bi(1-x)Ln(x)O(1.5) phases recently isola ted in the composition range 0.275 less than or equal to x less than o r equal to 0.40 depending on the lanthanide substituent (Ln = Sm, Eu, Gd, Tb, Dy). These phases are slowly obtained by long time annealing( congruent to 1500 hr) of the corresponding fcc delta-Bi3O3 type phase at about 800 degrees C, and exhibit a cell parameter almost twice the fcc parameter. X-ray diffraction in investigations of the stability an d crystal structure properties of the phases during thermal treatments have clearly shown that they are stable only in the temperature range 800-1000 degrees C, depending on the lanthanide substituent. Below 80 0 degrees C, they are metastable, but an annealing treatment or a slow dynamic thermal treatment between 600 and 800 degrees C leads to a pa rtial transformation into the true low temperature stable phases of th e systems. Above 1000 degrees C the fast transformation bce to fcc del ta-Bi2O3 type occurs systematically. The crystal structure of bcc Bi0. 65Gd0.35O1.5 has been refined with a = 11.0488(1) Angstrom, I2(1)3 spa ce group (Z= 32), using the Rietveld method. Starting from a delta-Bi2 O3 type phase, the formation of this bcc phase can be explained on the basis of a structural disorder-order transformation as a result of a long time annealing process at high temperature. (C) 1977 Academic Pre ss.