NEUTRON-SCATTERING AND STABILITY OF CERTAIN DISORDERED MAGNELI PHASES OF YBA2CU3O6+X

We study two partially ordered phases A and B of the CuOx planes (those which contain the O vacancies) of YBa2Cu3O6+x. They consist of a random distribution of two types of parallel strips. In phase A, for x > 1 2, one strip is given by a full Cu-O chain and the other is composed of one full and one empty chain. In phase B, for x #62; 1 2 the first strip is replaced by one consisting of three chains, two of them full and the other empty. For x < 1 2, full and empty chains are interchanged. While diffraction patterns of phase B are known to consist of two peaks for x > 0.556, we obtain that in phase A only one broad peak at k = (π/a, 0, 0) exists. Comparing the stability of finite domains of both phases, we show that O-O repulsive interactions at distances of at least 2a are necessary to explain the diffraction experiments. © 1990.