Study of electronic states of copper, bismuth, lead and oxygen atoms in some superconducting and related bismuth containing oxide phases

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PHYSICA © Physica C 282-287 (1997) 1099-1100

Study of electronic states of copper, bismuth, lead and oxygen atoms in some superconducting and related bismuth containing oxide phases V.E.Fedorov a, N.F.Zakharchuka , N.G.Naumova and U-Hyon Paek b aInstitute of Inorganic Chemistry, Russian Academy of Sciences, Siberian Branch, Novosibirsk, 630090, Russia bDepartment of Chemistry, Gyeongsang National University, Chinju 660-701, Korea

Study of electronic states of different elements in complex Bi-containing oxides by voltammetry of solids has showed that the electronic states of Bi, Cu and 0 atoms in some superconducting and related phases cannot be presented in terms of conventional oxidation states and only an adoption of unconventional oxidation states allows to describe accurately obtained experimental results.

1. INTRODUCTION

It is well known that the superconducting properties of HTSC materials are stipulated by electronic features of atoms with the variable valence (Cu, Bi, Tl, 0). Hence it is important to determine the electronic states of these atoms and to disclose the correlation between these states and superconducting properties of the phases. In this work a number of superconducting complex layered cuprates of 2201, 2212 and 2223 types and nonsuperconducting oxide phases which are formed in Bi-Pb-Ba-La-Cu-O and Bi-Pb-Sr-CaCu-O systems have been studied experimentally by the voltammetry of solids (VAS) with the aim to determine electronic states of Cu, Bi, Pb and 0 atoms.

2. EXPERIMENTAL Several complex oxide phases BhSnCaCu20s+1) , Bh-xPb xSnCaCu20s+1) (O:s;x:s; I),BhSf3-xCaxCu20s+1) (0:s;x:s;1.5) and Bh-xPbxBaLaCu06.s-xJ2 (O:s;x:s;l) solid solutions, and relevant reference samples were prepared by usual solid state reactions. All samples were carefully characterized by chemical analysis, X-ray powder diffraction and other adequate methods. Oxygen content was determined by iodometric titration. The electrochemical reactions for the some related reference compounds (CU20, CuO, PbO, 0921-4534/97/$17.00 © Elsevier Science B.Y. All rights reserved. PH S0921-4534(97)00654-0

Ca2Pb04, BaPb03, Bh03, KBi03, BaBi03) have been conducted also for the identification and assignment of various valence states of the atoms in these phases.

3. RFSUL1S AND DISCUSSION At interpretation of our experimental data the oxidation states of the alkaline earth and rare earth metals are assumed as 2+ and 3+ , respectively, i.e. as their usual oxidation states. The oxidation states of the varying valence elements have been ascribed on basis of the set of experimental VAS data for the reference substances. Experimental data for BaBi03, BhSnCaCu20s+o , Bh-xPbxBaLaCu06.s-xJ2 (09(:s;1) and BhSf3-xCaxCu20s+1) (0:s;x:s;1.5) solid solutions reveal unconventional Bi 2+ and 0- electronic states. Such results did not allow to describe these phases by simple ionic model, for example, BaBi03 as Ba2+Bi 4+(02-)3 or Ba2+(Bj3+)o.s(BP+)o.s(02-)3 and BhBaLaCu06.s as (Bi3+)2Ba2+La3+Cu 2+(02-)6.s using conventional oxidation states of ions; although the formulas satisfy charge balance they are not adequate and do not explain the experimental data. We suppose that in these phases there is the charge transfer from oxygen to bismuth providing a formation 0and Bi2+ states. The electronic states of copper differ essentially in superconducting BhSfI.sCaI.5CU20s+1) and non-

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V.E. Fedorou et al.lPhysica C 282-287 (1997) 1099-1100

superconducting Bh-xPbxBaLaCu06.5-xl2 phases. The key feature of hitter nonsuperconducting phase is the presence of copper as Cu+ in contrast to Cu 2+ detected in superconducting phase BhSr1.5Ca1.5CU208+l\. So, for example, Bi2BaLaCu06.5 and Bh-xPbxBaLaCu06.5-xl2 phases may be conceived by following formules Bi3+Bi2+Ba 2+La3+Cu+(02-)4.5(0-)2 and (Bi3+)I_xPbxBi2+Ba2+La3+Cu+(02-)4.5_0.5x(0-)2.0 because VAS experimental data showed following: I. Copper presents as Cu+, 2. There are two different states of Bi atoms: one is close to Bj3+, another can be describe as Bj2+, 3. Intensity of Bj3+ signal becomes to zero at composition BiPbBaLaCu06.0, and the oxidation state of Pb in the phase is 2+, so ratio Bi 3+/Bi 2+ is supposedly equal to I, 4. 0- states are present in the phase. It is known the presence of 0- states correlates with the superconducting properties of layered cuprates. But despite the existence of 0- states in BhBaLaCu06.5 the phase is not superconducting. Doubtless, the presence of Cu+ ions in CU02 layers changes drastically electronic properties of phase and in consequence of a superconductivity disappears. The existence in HTSC materials of unconventional 0- electronic states is one of more interesting question. In the literature there is a number of spectroscopic evidences for unusual electronic states of oxygen in the vicinity of the Fermi level which were assigned to 0(peroxytonic) states. On the other hand, it is well known that 0 2- ion is unstable by itself due to the large correlation energy in the 2p state, and is stabilized in the solid purely due to the ligand field of the surrounding cations. So, the 0- state would be more stable at apprepriate cationic surroundings. There are several elements which form peroxides very easy, among them Ba and Sf. It is very interesting that exactly all HTSC materials contain such cations which can stabilize 0- states. In structures of HTSC materials Ba and Sr cations are mainly located in the layers where the apical oxygens are lay. Therefore it seems the apex sites in the high Tc superconductors consist of oxygens with 0- electronic states. It is well known also that the superconductivity exists on subtle balance of the effects of apex oxygens. The presence of apex oxygen makes Cu02-plane easier

to dope with holes. Thus, our VAS study of electronic states of different elements with the variable valence in complex Bi-containing oxides has showed that the electronic states of Bi, Cu and 0 in some superconducting and related phases cannot be presented in terms of conventional oxidation states and only an adoption of unconventional oxidation states allows to describe accurately obtained experimental results. These results can be expressed by following schemes: Bh(BaLa) CU06.5 --- {(Bj2+)0.5 (Bj3+)0.5+(02- )1.25} 0 --- {(Bi2+)0.5 (Bj3+)0.5+(02- )1.25} 0 --- {(Ba2+)0.5(La3+)0.50-} 1.5+ --- {Cu+( 0 2-)2}3--- {(Ba2+)0.5(La3+)0.50-} 1.5+ --- {(Bj2+)0.5 (Bi 3+)0.5+(02- )1.25} 0 --- {(Bi 2+)0.5 (Bj3+)0.5+(02- )1.25} 0 (BiPb)(BaLa) CU06.0 --- {(Bi 2+)0.5 (pb 2+)0.5+(02- )I.OO} 0 2 --- {(Bi +)0.5 (pb 2+)0.5+(02- )I.oo} 0 ---{(Ba2+)0.5(La3+)0.50-} 1.5+ --- {Cu+( 0 2-)2} 3-- {(Ba2+)0.5(La3+)0.50-} 1.5+ --- {(Bj2+)0.5 (pb 2+)0.5+(02- )I.oo} 0 --- {(Bi2+)0.5 (pb 2+)0.5+(02- )I.OO} 0

---Ca2+ --- {Cu2+( 0 2-)z}2--- {Sr2+0-} + ---{Bi2+ 02-}0 ---{Bj2+ 0 2-}0 ---{Sr2+0-}+ --- {Cu2+( 0 2-)2} 2---Ca2+

ACKNOWLEDGMENTS The work was supported by grant N93203 of the Russian Government Program "High Temperature Superconductivity".