Structural and Electronic Studies of Pure and Lead Doped Phases in the Single Copper Oxide Layer of Bismuth Strontium Copper Oxide System

Studies of the structural and electronic properties of pure and lead doped phases in the single Cu-O layer of Bi-Sr-Cu-O system were carried out using X-ray diffractometry, magnetization, and resistivity characterization techniques. It is concluded that the Bi_2Sr _2CuO_6 compound does not exist at its nominal composition 2201, but rather requires quite a measure of Sr deficiency to form a stable structure. There is a strong structural incommensurate modulation evidenced by the presence of the satellite peaks in the X-ray diffraction patterns. This modulation is due to a combination of lattice mismatch, Bi/Sr atomic ratio and extra oxygen content. The modulation periodicity and the oxygen content vary with the Bi/Sr ratio as do the lattice parameters a and c. Substitution of lead occurs on the Bi site only. However, there is evidence that by putting Pb on the Bi site the need for excess Bi on the Sr site is reduced. This may be the key to raising the superconducting transition temperature of this compound. The substitution of lead in the series (Bi_{rm 2.12-x}Pb _{rm x})Sr _{1.88}CuO_{ rm z} has a marked effect on both the structural and the electronic properties of the material. For a moderate lead doping in the series the structural modulation is hindered and vanishes for lead content above x = 0.3. However, the pseudotetragonal structure is transformed into a distinctly orthorhombic structure. Electronically, the compound is rendered metallic and the onset of T_{ rm c} is considerably lowered to values below 4.2 K. The lead doping was found to decrease the number of excess oxygen in the polycrystalline phase, indicating that it enters in the +2 state. However the decrease in oxygen content is less than expected, accounting, perhaps for the observed increase in metallicity.