Stability, Thermal Behavior, and Crystal Structure of Ion Ordered Bi1-xLnxO1.5Phases (Ln=Sm-Dy)

This paper deals with the bcc Bi1-xLnxO1.5phases recently isolated in the composition range 0.275 [less-than or equal to] x [less-than or equal to] 0.40 depending on the lanthanide substituent (Ln=Sm, Eu, Gd, Tb, Dy). These phases are slowly obtained by long time annealing ( [approximately equal to] 1500 hr) of the corresponding fcc δ-Bi3O3type phase at about 800°C, and exhibit a cell parameter almost twice the fcc parameter. X-ray diffraction investigations of the stability and crystal structure properties of the phases during thermal treatments have clearly shown that they are stable only in the temperature range 800-1000°C, depending on the lanthanide substituent. Below 800°C, they are metastable, but an annealing treatment or a slow dynamic thermal treatment between 600 and 800°C leads to a partial transformation into the true low temperature stable phases of the systems. Above 1000°C the fast transformation bcc to fcc δ-Bi2O3type occurs systematically. The crystal structure of bcc Bi0.65Gd0.35O1.5has been refined witha=11.0488(1) A,I213 space group (Z=32), using the Rietveld method. Starting from a δ-Bi2O3type 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. © 1997 Academic Press.