Phase relations in the BaO–Sm 2O 3–CuO x system at pO 2=100 Pa and T=810 °C

Abstract

By applying controlled-atmosphere methods to minimize the presence of carbonate and CO 2 and H 2O contamination, subsolidus phase equilibria of the BaO–Sm 2O 3–CuO x system at p O 2 =100 Pa (0.1% O 2 in Ar by volume fraction) and T=810 °C were determined. Under carbonate-free conditions, the BaO–Sm 2O 3–CuO x phase diagram at p O 2 =100 Pa is different from that reported in the literature, as prepared in air. The BaO–Sm 2O 3–CuO x system consists of four ternary phases. The first phase, (Ba 2 − x Sm x )CuO 3 − z ( x=0 to ≈0.3), occurs in the high-BaO region of the phase diagram. The second phase, Ba 4Sm 2Cu 2O z , is a new insulator phase with an orthorhombic Pnn2 structure ( a=11.9718(7) Å, b=11.8884(7) Å, c=3.8465(2) Å, V=547.47 Å 3) comprised of unusual CuO 5 linear chains. The structure and the X-ray reference pattern of Ba 4Sm 2Cu 2O 9 − z were obtained using the Rietveld refinement technique. The third phase, the `green phase' BaSm 2CuO 5, has a different structure from that of the `brown phases' BaLa 2CuO 5 and BaNd 2CuO 5, despite having the same cation stoichiometry. The fourth phase is the high T c solid solution series, Ba 2 − x Sm 1 + x Cu 3O z (0.2⩾ x⩾0). The Ba 2SmCu 3O 6 + z –(Ba 2 − x Sm x )CuO 3 + z tie-line established in this study is different from the BaCuO 2 + x –BaSm 2CuO 5 tie-line frequently observed when samples are prepared in air. This tie-line substantially expands the field of stability of the Ba 2 − x Sm 1 + x Cu 3O 6 + z superconductor phase into the relatively BaO-rich region of the phase diagram under carbonate-free conditions.