Synthesis and characterization of the BiRESr2O6-type ternaries (RE=La and lanthanides)

A series of BiRESr2O6-type compounds was synthesized and characterized through X-ray powder diffraction, sample density measurements, Bi and RE valency determination, and a.c. magnetic susceptibility measurements. It has been found that BiRESr2O6-type ternaries are formed by all the RE (including La) except Ce, Pr and Tb. A characteristic feature of the series is the presence of pentavalent Bi and trivalent RE components. The series represents a monoclinically distorted perovskite-related crystal structure, with space group P121/n1 and lattice parameters corresponding to a similar 2ap, b similar 2ap, c similar 2ap, β>90 . The La member of the series (BiLa0.833Sr2.167O6-y, y similar 0.276) differs from the others with an La-deficient stoichiometry. The La-deficient phase has a monoclinic unit cell in the space group P121/n1, but a different distortion (a=5.985 A, b=8.496 A, c=5.994 A, β=89.67 ) and the atomic positions of La and Sr mutually mixed. The deficiency is of (La1-xSr2+x) type (x similar 0.167 at 850 C), and increases with temperature. It is shown that under oxidizing sintering conditions none of the tetravalent REs (Ce, Pr, Tb) creates the structure type under discussion with 1:1:2 stoichiometry. (orig.)