SYNTHESIS OF A NEW SERIES OF COMPOUNDS RE2CO2 3NB4/3O7 AND STABILITY FIELD DIAGRAM OF RE2B2/3'B-4/3''O-7 PYROCHLORE COMPOUNDS/

A new series of pyrochlore compounds RE2Co2/3Nb4/3O7 (RE = Nd, Sm and Eu) have been synthesized by using a wet chemical method and the cryst al structures of these compounds have also been determined. The crysta l structure of RE2Co2/3Nb4/3O7 belongs to a hexagonal system. The assi gnments for the absorption bands in the infrared spectra of Eu2Co2/3Nb 4/3O7 have also been given. The results of the crystal structure and t he infrared spectra can demonstrate that the compounds are pyrochlore ones. The experimentally measured density and the calculated one indic ate that they have 24 formulae in each unit cell. On the basis of the polarizability and the ionic radius of the B' ion and B'' ion, a stabi lity field diagram for the RE2B2/3'B-4/3''O-7 pyrochlore compounds wit h different symmetry is presented. The diagram shows that most pyrochl ore compounds with a formula RE2B2/3'B-4/3''O-7 will belong to a hexag onal or tetragonal system anti, especially, possess a large cell struc ture, similar to the superstructure. The electric measurements indicat e that RE2Co2/3Nb4/3O7 is an insulator and the electric resistivity of the compounds is larger than 10(5) ohm cm(-1) at room temperature. (C ) 1998 Elsevier Science S.A. All rights reserved.