Physica C 180 ( 1991 ) 387-393 North-Holland PHYSICA Electrochemical reduction of BizSrzCal Cu208 superconductor single crystals Joseph B. Schlenoff 1, W. Jack Rink and Lawrence Seger Department of Chemistry and Centerfor Materials Science (MARTECH), The Florida State University, Tallahassee, FL 32306, USA Received 14 May 1991 Revised manuscript received 5 July 1991 Single crystals and polycrystalline pellets of the high-temperature cuprate superconductor Bi2Sr2Ca~Cu2Oswere doped at room temperature by electrochemical reduction at > 95% Coulombic efficiency using lithium dopant ions in propylene carbonate elec- trolyte. Cyclic voltammetry and potential step measurements on single crystals suggest an unusual reduction mechanism, with a diffusion coefficient for Li + in the c-axis direction of bulk superconductor of ca. 3 × 10- ~ ~ cm2s - 1. Sintered pellets of polycrystal- line powder could be doped more rapidly, with an apparent diffusion coefficient of 7 × 10- s cmZs- l X-ray susceptibility analysis show extensive disordering occurs on heavy Li doping, with a first-order transition from a crystalline/superconducting to an amorphous/non-superconducting phase. Single, crystals of Bi2Sr2CalCu2Os exhibited a color change on reduction from metallic gray to golden bronze. The reduced material was highly air-sensitive, forming a hydroxide surface film on exposure to ambient atmosphere. I. Introduction It has been widely observed that the supercon- ducting critical temperature, T~, of the cuprate su- perconductors can be correlated to the oxidation state of the copper in the CuO2 layers [ 1,2 ]. Empirically speaking, the formal oxidation state of the copper should be more than or less than two, corresponding to p-type (hole carriers) and n-type (electron car- riers) materials, respectively. Recently, bond va- lence sum analyses on p-type superconductors have demonstrated that T~ is more closely correlated to the hole density, nil, in the CuO2 planes and that T¢ exhibits a maximum [ 3 ] at some value of nil. Regulating the oxidation state of copper is typi- cally achieved by doping with elements that remove or add electrons. Doping can be substitutional or ad- ditive. The classic example of the latter case is the oxygen doping of non-superconducting YBa2Cu306.5, where the copper has a formal oxidation state of +2.0, to superconducting YBa2fu3OT.o, increasing the copper valence to an average of +2.33 [4]. An To whom correspondence should be addressed. increase in Tc is observed with increasing oxygen content [ 5 ]. Substitutional doping is exemplified by the replacement of yttrium ( + 3) with praseodym- ium (+4) in the series Yl_xPrxBazCuaOv: super- conductivity is completely suppressed [6] when x>0.6. Certain compositions of the BiSrCaCu family of superconductors possess a surfeit of oxygen (an ex- cess of holes) in the material [ 7 ] as it is made using high-temperature firing in an oxygen atmosphere. For example, Bi2SrECalCuzOs shows increasing Tc with increasing quenching temperatures [7], and an- nealing BizSrECalCU20 s in oxygen-poor atmo- spheres optimizes Tc. Strobel et al. [8] demon- strated that the excess of hole carriers in Bi2Srl.vsCal.25CuzOs could be compensated by in- jecting electrons electrochemically (along with lith- ium counter-ions), resulting in significant increases in T~ with a maximum Tc for a Li content of y=0.1 in LiyBi2SrL75CaL25Cu208. Higher levels of lithium- doping result in a marked decrease in supercon- ducting volume fraction. A similar effect was re- cently observed on reduction of Bi2Sr2CalCU208 us- ing copper ion dopants [9]. Changes in the 0921-4534/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights res¢rved.