Study On Microstructure Of Doped Bi-based Copper Oxides

Due to its remarkable preferred orientation characteristics,Bi-based copper oxide high-temperature superconductors(HTSC)have been of great significance both in basic scientific research and in application prospects since their discovery.However,the relatively low superconducting critical transition temperature(Tc)hinders its further development and application.At present,element doping has become one of the typical methods to further improve the superconducting properties of Bi-based HTSC.The two kinds of doping systems,Ca-doped Bi2Sr2-xCaxCuOy and La-doped Bi2-xLaxSr2CaCu2Oy,are studied systematically in this paper.The effects of doping amount x on unit cell structure,incommensurate modulation structure,atomic occupancy,microstructure and defects are discussed.It aims at establishing a relationship among doping effect,crystal structure,micro structure characteristics and superconductivity.It is found that element Ca has entered the Bi-2201 orthorhombic lattice and replaced partly the positions of Sr atoms in the original unit cell in the Ca-doped Bi-2201 system.With an increase in Ca doping amount,the lattice parameters a and b of Bi-2201 unit cell have a little change,whereas,the parameter c is gradually decreasing.It is mainly due to the substitution of Ca2+ with a smaller ionic radius.As the Ca doping amount x is in the range of 0.1～0.6,the modulation wavelength qb is increasing with increasing x,and qc is decreasing;When x≥0.6,qb has no obvious change,and qc is increasing gradually,suggesting that x=0.6 should correspond to the highest Tc temperature in the system.Meanwhile,the intergrowth phase of Bi-2201 and Bi-2212 are formed commonly in the Ca-doped Bi-2201 system,resulting from the uneven distribution of doping element Ca.At the junction of Bi-2201 and Bi-2212,the different numbers of perovskite layers between the double Bi-O layers leads to the different c-length and the staggered defects of Bi-O layers in the system.In the La-doped Bi-2212 system,Rietveld refinements were carried out for the samples doped with the different La doping amounts.The results show that the increasing doping amount x gives rise to the significant change of Oxygen positions,causing structural distortion of the Cu-O layers.Both the La-O bonding and the ionic radius of La3+ can influence remarkably the lattice constants a and c of the Bi-2212 phase,which in turn changes the carrier concentration near the Cu-O layer and superconductivity.