OXYCARBONATES IN THE Y(O/CO3)-BA(O/CO3)-CU(O/CO3) SYSTEM

The Ba-rich oxides in the Y(O)Ba(O)Cu(O) system have a high affinity toward CO2, which is manifested in the formation of oxycarbonates. The pseudoternary system Y( O CO3)Ba( O CO3)Cu( O CO3) is investigated at temperatures between 780 and 1000°C in atmospheres containing oxygen and ∼5, ∼40, and ∼350 ppm CO2. Three oxycarbonates are identified: (1) Tetragonal Y2Ba3(CO3)uO6-u, u ≈ 1, with a = 438.63(4) and c = 1185.9(2) pm. (2) Y1+xBa8Cu4+z(CO3)u O11+w, u ≈ 2, x ∈ (0, 0.3), z ∈ (0, 0.4), and w ∈ [0.05(2), 1.08(2)] for x = z = 0. Its structure, which accommodates vacancies on both cationic and anionic sites, is closely related to (the hypothetic perovskite) BaCuO3 and represents the actual composition for the so-called "other perovskite phase" claimed for the Y(O)Ba(O)Cu(O) system. (3) Tetragonal YBa2Cu3(CO3)uO7-u-v with a = 387.38(3) and c = 1161.2(4) pm for u ≈ 0.2, v ≈ 0.1, when carbonate-saturated at 800°C and oxygen-saturated at 320°C in an ∼40 ppm CO2 containing oxygen atmosphere. Structurally, this oxycarbonate is derived from YBa2Cu3O7 by replacement of some oxygens by carbonate groups, and this phase represents the actual composition of the so-called "high-oxygen tetragonal 123 phase," which is nonsuperconducting down to 4 K. The thermal stability of the oxycarbonates in oxygen with pCO ≈ 4 Pa decreases with decreasing Ba content from 960°C for (1) to 940°C for (2) and to 830°C for (3). © 1991.