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Models for the modulation in A2B2CanCu1+nO6+2n, A, B=Bi, Sr OR Tl, Ba and n=0, 1, 2

1988; Elsevier BV; Volume: 156; Issue: 3 Linguagem: Inglês

10.1016/0921-4534(88)90756-3

1873-2143

H.W. Zandbergen, Pim Groen, F.C. Mijlhoff, Gustaaf Van Tendeloo, S. Amelinckx,

Advanced Condensed Matter Physics

A number of models for the modulation in the title compounds are postulated. For the Bi compounds periodic changes are considered due to the following structural models: (a) extra oxygen in the BiO layers, (b) partial substitution of Sr by a vacancy, (c) partial substitution of Bi by Cu, and (d) changes in the orientations of the lone pairs of Bi. Computer calculated images for these models are compared with high resolution images. Also general structural considerations and structural details from the reported subcells are taken into account. It is concluded that the best agreement with experimental data is obtained for the model with extra oxygen in the BiO layers. Several performed experiments, which support this conclusion, are reported and discussed; the most important of which are the existence of solid solutions Bi2Sr2−xBaxCuO6+δ, with x<0.5, and Bi2Sr2−xLaxCuO6+δ, with x<1.2. In the first solid solution extra oxygen can be introduced into the lattice leading to a decrease in the length of the c axis. The latter solid solution shows considerable non-linear changes in the unit cell dimensions and a change in the modulation vector from 4.7b to 3.5b for x = 0 and x = 1.2, respectively. Bi2Sr2−xLaxCaCu2O8+δ shows frequently the absence of a doubling of the subcell along a, whereas the modulation remains, suggesting both features not to be correlated. For the Tl compounds modulations are considered due to the models: (a) a periodic shift of the O ions in the TlO layers, (b) extra oxygen in the TlO layers, (c) a partial substitution of Tl3+ by Tl+ or Tl2+.