Defective perovskite

The compound NCu02 (N = Ca0 86Sr014) is an insulator, but its structure, which is a simple defect perovskite made of layers (CuOa) sandwiched between layers (N), can be considered as the parent structure of a large family of superconductors. The sequence. ..(Cu0 )OiC(N)e o(Cu02)OjC. .. is in fact one of the building blocks of many compounds considered in this review. The refined parameters for NCu02 are given in Table 3. 

Figure 1 Relationship between the perovskite structure ABOs (left) and the defect-perovskite superconductor YBajCugOy (right). Metal atoms are shaded. Note the missing oxygen atoms in the latter drawing that result in formation of copper-oxygen sheets (above and below the Y atoms), and copper-oxygen chains (between the Ba atoms).

Besides PTBs, A-site defective perovskite oxides are known to be formed when B = Ti. Nb.Ta and soon "13. Such compounds exhibit metallic properties and perovskite structures when the B atom occurs in a low oxidation stale. Compositions such as A0 5Nb03 (A = Ba. Pb etc.) where niobium is in the highest oxidation state adopt non-perovskite network structures. An interesting example20-21 of a A-site defective perovskite is Cu 5Ta03 which crystallizes in a pseudocubic perovskite structure. The unit cell is orthorhombic with a = 7.523, />= 7.525 and c = 7.520 A and eight formula units per cell. Tantalum atoms form... 

In spite of several investigations of defect perovskite systems in the last few years, there is much scope for study in this interesting aspect of solid state inorganic chemistry. The nature of defects as well as the ordering of defects in many systems are yet to be studied. For example, not much is known about the defect... 

Partial substitution of A and B ions is allowed, yielding a plethora of compounds while preserving the perovskite structure. This brings about deficiencies of cations at the A-or B-sites or of oxygen anions (e.g. defective perovskites). Introduction of abnormal valency causes a change in electric properties, while the presence of oxide ion vacancies increases the mobility of oxide ions and, therefore, the ionic conductivity. Thus, perovskites have found wide apphcation as electronic and catalytic materials. 

Tarascon JM, Greene LH, McKirmon WR, Htrll GW, Geballe TH (1987) Superconductivity at 40 K in the oxygen-defect perovskites La2 cSr Cu04 y. Science 235 1373-1376 Taylor M, Brown GE (1976) High-temperatrrre structrrral study of the P2 /a-A2/a phase transition in synthetic titanite, CaTiSiOs. Am Mineral 61 435-447... 

Y. Teraoka, H.-M. Zhang and N. Yamazoe, Oxygen-sorptive properties of defect perovskite-type Lai-xSrxCoi-yFeyOs-e. Chem. Lett. (1985) 1367-1370. 

S. Adler, S. Russek, J. Reimer, M. Fendorf, A. Stacy, Q. Huang, A. Santoro, J. Lynn, J. Baltisberger and U. Werner, Local structure and oxide-ion motion in defective perovskites. Solid State Ionics, 68 (1994) 193-211. 

Ullmann, H., and Trofimenko, N.J. 2001. Estimation of effective ionic radii in highly defective perovskite-type oxides from experimental data. Journal of Alloys and Compounds 316, 153-158. 

The compound I YCujOy shows an even higher superconducting transition ( 93 K) and crystallizes as a defect perovskite. The structure of I YCh Oy has been determined by neutron diffraction analysis. oThe space group is Pmmm with a = 3.8198, b = 3.8849 and c = 11.6762 A. Barium and yttrium are ordered on the A site to give a tripled cell along c and the oxygens occupy 7/9 of the anion sites. One third of the copper is in 4-fold coordination and 2/3 are five-fold coordinated (Gallagher, P.K. O Bryan, H.M. Sunshine,... 

The discovery by Wu et al. (1) of superconductivity at 9SK in a mixed-phase sample in the Y-Ba-Cu-0 system has stimulated an unprecedented amount of research effort directed at solving the structures of the phase(s) responsible for superconductivity. In this paper we will present the results of the first single-crystal structure analyses of the phases present in these samples to be carried out (2), and then review the additional structural information now available in relation to the previously determined structures of other oxygen defect perovskites. 

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