Modulation structure in Bi2(La1-xCa1+x)CuO6.5+δ and (Bi2-xPbx)LaCaCuO6.5+δ systems

The microstructure of Sr-free bismuth cuprates, i.e., Bi2(La1-xCa1+x)CuO6.5+δ and (Bi2-xPbx)LaCaCuO6.5+δ, was determined by means of x-ray diffraction and electron diffraction (ED). ED analysis revealed that the two types of Sr-free compounds possess a monoclinic superstructural modulation, which is similar to Bi2Sr2CuO6+δ. In the Bi2(La1-xCa1+x)CuO6.5+δ system, the modulation wavelength along the b and c axes increased markedly with the Ca substitution for La. At the same time this substitution facilitated the commensurate-to-incommensurate transition. For the (Bi2-xPbx)LaCaCuO6.5+δ system, a similar change in the modulation wave vector was also observed. The dependence of the modulation vector on chemical composition can be best interpreted by the crystal-misfit model. In addition, Raman scattering analysis of the Bi2(La1-xCa1+x)CuO6.5+δ system showed that the vibrational properties of the oxygen atoms in the Bi-O bonding also changed with the increase in the modulation wavelength, and this behavior can be regarded as a consequence of the structural relaxation caused by the enhancement of the degree of crystal fit.