By applying controlled-atmosphere methods to minimize the presence of carbonate and CO 2 and H 2 O contamination, subsolidus phase equilibria of the BaO-Sm 2 O 3 -CuO x system at p O 2 ¼ 100 Pa (0.1% O 2 in Ar by volume fraction) and T ¼ 810 °C were determined. Under carbonate-free conditions, the BaO-Sm 2 O 3 -CuO x phase diagram at p O 2 ¼ 100 Pa is different from that reported in the literature, as prepared in air. The BaO-Sm 2 O 3 -CuO x system consists of four ternary phases. The first phase, (Ba 2 À x Sm x )CuO 3 À z (x ¼ 0 to %0.3), occurs in the high-BaO region of the phase diagram. The second phase, Ba 4 Sm 2 Cu 2 O z , is a new insulator phase with an orthorhombic Pnn2 structure (a ¼ 11:9718ð7Þ
A, b ¼ 11:8884ð7Þ A, c ¼ 3:8465ð2Þ A, V ¼ 547:47 A 3 ) comprised of unusual CuO 5 linear chains. The structure and the X-ray reference pattern of Ba 4 Sm 2 Cu 2 O 9 À z were obtained using the Rietveld refinement technique. The third phase, the 'green phase' BaSm 2 CuO 5 , has a different structure from that of the 'brown phases' BaLa 2 CuO 5 and BaNd 2 CuO 5 , despite having the same cation stoichiometry. The fourth phase is the high T c solid solution series, Ba 2 À x Sm 1 þ x Cu 3 O z ð0:2 P x P 0Þ. The Ba 2 SmCu 3 O 6 þ z -(Ba 2 À x Sm x )CuO 3 þ z tie-line established in this study is different from the BaCuO 2 þ x -BaSm 2 CuO 5 tie-line frequently observed when samples are prepared in air. This tie-line substantially expands the field of stability of the Ba 2 À x Sm 1 þ x Cu 3 O 6 þ z superconductor phase into the relatively BaO-rich region of the phase diagram under carbonate-free conditions.