NEW COMPLEX CUPRATES BASED ON BI

I. Afanaseva, G. Kuz'micheva

M.V.Lomonosov State Academy of Fine Chemical Technology, Moscow 117571,Russia
E-mail: kuz7micheva@glasnet.ru

Keywords: superconductivity, new bismuth cuprates, crystal structure, crystallochemical analysis

There are some series of layered perovskites in Bi-Sr-Ca(Ln)-Cu-O system: Bi₂Sr₂(Ca,Ln)_kCu_k+1O_x - 22k(k+1), BiSr₂(Ca,Ln)_kCu_k+1O_x - 12k(k+1), Bi₄(Ca_k'-1Cu_k')(Ca_k"-1Cu_k")Sr₄O_x, and similar to them as Bi₂Sr_kCu_k+1O_x phases. The well-known superconducting phases belong to 22k(k+1) series with highest critical temperature Tc=110K for Bi-2223 phase. The least studied are phases of 12k(k+1) and Bi₂Sr_kCu_k+1O_x series. The chief purpose of the investigation was to determine the crystal structure of two phases of 12k(k+1) series with k=0 - Bi(Sr,Tb)₂CuO_x; k=1 - BiSr₂(Ln,Ca)Cu₂O_x and phase Bi₂Sr_kCu_k+1O_x with k=3 and to explain the absence of superconductivity in these compounds.

The samples of BiSr₂(Ca,Ln)_kCu_k+1O_x series were synthesized from binary oxides by solid-phase sintering of the components under normal conditions t=860^oC, t =12h in air for k=0 and in two stages for k=1 (t=860^oC, t=12h - 1 stage) and under high pressure (p=5GPa, t=700^oC, t=5min - 2 stage). The 1201 and 1212 phases are tetragonal with a space group (sp.gr) of P4/mmm and lattice paramerers a=3.834(2) c=8.935(3) and a=3.79(2) c=12.46(3), respectively. The samples of Bi₂(Sr,Ca)₃Cu₄O_x and Bi₂Sr₃(Cu,M)₄O_x with (M=Sb, V, Nb) compositions have been prepared at t=860^oC =2-3h with colling in air. They crystallize in the tetragonal structure (sp. gr. I4/mmm; a=3.853.86 A, c=39.539.6 A for M=V,Nb,Cu; a=3.854(2)A, c=41,24(3)A for M=Sb) derivative from Aurivillius phases.

Refinement of the structures (DBWS 9411 program) and crystallochemical analysis of results obtained have been carried out. The non-superconducting Bi-1201 and Bi-1212 phases are almost analogous to superconducting phases in Tl-system with T_Cmax=122K for Tl-1234. The absence of superconductivity in Bi-phases is associated with non-optimum oxygen content. It may be stated that change of phase compositions will allow to obtain a superconducting Bi-12k(k+1) phases.

The principal feature of Bi₂Sr_kCu_k+1O_x phases is pair of electrons on Bi³⁺ that lead to formation of Bi-square antiprism with different Bi-O distances (4+4). The Cu polyhedra in Bi₂(Sr,Ca)₃Cu₄O_x structure are squares, on the contrary in Bi₂Sr₃(Cu,Sb)₄O_x structures the (Cu,Sb⁵⁺) polyhedra are octhaedra. We found that these phases show metallic conductivity. Although there are two-dimensional CuO2 planes of perovskite layers in Bi₂Sr_kCu_k+1O_x phases the like as in superconducting perovskite-like cuprates they differ by charge reservoir oxide layers. It is concluded that the structure of Bi-polyhadra is the main reason of superconductivity absence in phases derivative from Aurivillius phases.