Oxycarbonates in the system
1991; Elsevier BV; Volume: 94; Issue: 2 Linguagem: Inglês
10.1016/0022-4596(91)90195-n
ISSN1095-726X
Autores Tópico(s)Advancements in Solid Oxide Fuel Cells
ResumoThe 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(OCO3)Ba(OCO3)Cu(OCO3) 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)uO11+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 pCO2 ≈ 4 Pa decreases with decreasing Ba content from 960°C for (1) to 940°C for (2) and to 830°C for (3).
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