Abstract
A series of BiRESr2O6-type compounds was synthesized and characterized through X-ray powder diffraction, sample density measurements. Bi and RE valency determination, anda.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 groupP12,/n1 and lattice parameters corresponding toa ⋟ √2ap,b ⋟ √2ap,c ⋟ 2ap, β > 90°. The La member of the series (BiLa0.833Sr2.167O6 − y y ≅ 0.276) differs from the others with an La-deficient stoichiometry. The La-deficient phase has a onoclinic unit cell in the space groupP12,/n1, but a different distortion (a = 5.985A,b = 8.496A,c = 5.994A, β = 89.67°) and the atomic positions of La and Sr mutually mixed. The deficiency is of (La1, Sr2) type (x ⋟ 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.
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