SYNTHESIS AND CHARACTERIZATION OF THE BIRESR(2)O(6)-TYPE TERNARIES (RE=LA AND LANTHANIDES)

A series of BiRESr(2)O(6)-type compounds was synthesized and character ized through X-ray powder diffraction, sample density measurements, Bi and RE valency determination, and a.c. magnetic susceptibility measur ements. It has been found that BiRESr(2)O(6)-type ternaries are formed by all the RE (including La) except Ce, Pr and Tb. A characteristic f eature of the series is the presence of pentavalent Bi and trivalent R E components. The series represents a monoclinically distorted perovsk ite-related crystal structure, with space group P12(1)/n1 and lattice parameters corresponding to a congruent to root 2a(p), b congruent to root 2a(p), c congruent to 2a(p), beta > 90 degrees. The La member of the series (BiLa0.833Sr2.167O6-y, y congruent to 0.276) differs from t he others with an La-deficient stoichiometry. The La-deficient phase h as a monoclinic unit cell in the space group P12(1)/n1, but a differen t distortion (a = 5.985 Angstrom, b = 8.496 Angstrom, c = 5.994 Angstr om, beta = 89.67 degrees) and the atomic positions of La and Sr mutual ly mixed. The deficiency is of (La1-xSr2+x) type (x congruent to 0.167 at 850 degrees C), and increases with temperature. It is shown that u nder oxidizing sintering conditions none of the tetravalent REs (Ce, P r, Tb) creates the structure type under discussion with 1:1:2 stoichio metry.