Modulated structure in Bi2 + xSr2 - xCu1 + yO6 + delta

The modulated structure in solid solution Bi(2+x)Sr(2-x)Cu(1+y)O(6+delta) (x greater than 0.1 and less than 0.6; y = x/4) has been investigated by means of powder X-ray diffraction, electron diffraction, and transmission electron microscopy. The 010-line component of the modulation vector decreases almost linearly with increasing x, from 5.2b (x = 0.1) to 4.2b (x = 0.5), where b is the unit length of the average structure along the 010-line direction, and is only slightly sensitive to excess oxygen content. A structural model of the modulation based on a periodic substitution of Sr for Bi and formation of Bi blocks whose size varies with x is proposed. Relations among various modulations appearing in other related phases, such as the Pb-substituted 105 K Tc phase, are discussed.