STRUCTURE FAMILIES OF SUPERCONDUCTING OXIDES

J. Hauck and K. Mika

IFF, Research Center Jülich, D-52425 Jülich, Germany

The Ising type analysis of superconducting cuprates shows that the layered structures are composed from perovskite, SrO and CaCuO₂ structural units with different interactions between oxygen atoms. The 45 structures for a combination of a maximum of 8 structural units can be classified in 13 structure families which are named by the authors who synthesized the first compound or found superconducting properties like the Ruddlesden-Popper, Bednorz-Müller family with the sequence of coordination numbers (CN) of metal atoms 12 6₂ 12₂ 6₂ 9₂ 6_n 6'_n (Table 1). The formula Cu La₂ O₄ or Cu Ba₂ Tl O₅ indicate the sequence of metal atoms Cu La La, Cu or Cu Ba Tl Ba, Cu with single Cu (or Tl) atoms at mirror planes and two La or Ba atoms in between the mirror planes. Long sequences of CN values increase with the possibility of segregation e.g. of La Cu₂ La₂ O₇ (with CN values 12 6₂ 9₂) to La Cu O₃ (12 6) and Cu La₂ O₄ (6 9₂). The final members Ca Cu O₂ (8 4), Sr O(6') and La Cu O₃ or Ca Ti O₃ (12 6), which can be formed at segregation are underlined in table 1. The composition Cu (La, Sr)₂ O₄ can be obtained either with two trivalent La atoms (2 0 1) or one La and one Sr atom (1 1 1). The formal valency of divalent copper atoms (C₁) in 2 0 1 is increased at (1 1 1) composition (C₂). The underlined value corresponds to the experimental compound. Most superconducting oxides are within the first three families listed in table 1. Some compounds of other families are non-metallic as e.g. Ba (Cu, Fe)₂ YO₅. Other compounds as e.g. Pb Sr₂ Fe₂ Sr₂ O₉ have the same composition MNC₂ = 232 as Pb Fe₂ Sr₂ Bi₂ O₉. Both structures are closely related. The sequence of the CN values of the Lucas, Raveau series 6'_n 9₂ 6₂ 9₂ 6'_n can be derived from 6'_n 9₂ 6₂ (12₂ 6₂) 9₂ 6'_n of the Ruddlesden-Popper family, if the part given in brackets is omitted. The structures of the Cava, Subramanian series with CN values 4 8₂ 5₂ 9₂ 9₂ 6₂ 12 can be obtained from4 8₂ 5₂ 9₂ (6'₂) 9₂ 6₂ 12 in a similar way. Other families can be obtained, if CuSr₂O₃ (4 7₂) of the Müller-Buschbaum series 8 4₂ 7₂ 6' is substituted in 4 8₂ 5₂ 9₂ 6'₂ 9₂ 6₂ 12 for 4 8₂ 5₂ 9₂ 7₂ 4₂ 8 of the Subramanian, Cava or 8 4₂ 7₂ 9₂ 6₂ 12 of the Fu, Zandbergen series. No compounds are known for MNC₁=403 or 242 with the corresponding CN values 4 10₂ 6₂ 9₂ or 6 9₂ 6'₂ 7₂ 4. The Aurivillius phases with CN values 12 6₂ 12'₂ and the CeO₂ containing phases with 6 12₂ 5₂ 8'_n (Li, Cava), 8 4₂ 10₂ 5₂ 8' (Sawa) can be obtained, if 9₂ 6'_n of 12 6₂ 9₂ 6'_n is replaced by 6 12'₂ (WBi₂ O₆) and 8 4 (Ca Cu O₂) is replaced by 4 8'₂ (Cu(Nd, Ce)₂ O₄).

Table 1: Composition of different cuprates containing trivalent (M), divalent (N), formally divalent (C₁) or higher valent (C₂) copper atoms and coordination numbers of metal atoms