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X-Ray Study of and Structural Approach to the Incommensurate Perovskite Pb2CoWO6

Pb2CoWO6 in its incommensurate phase (230 less than or equal to T less than or equal to 300 K) invariably exhibits orientational domains. Such a polydomain state has been investigated by means of X-ray diffraction at 283 K. Assuming the modulation to be one-dimensional, we have identified the orientations of four domains. In view of the pseudosymmetry of this phase, many refinements of a purely displacive modulation had to be performed in order to establish the best suited unit cell and superspace group. Two major categories of candidates have arisen from this approach. (i) In a centered monoclinic or triclinic cell, there were clear indications of disorder in the structure, but owing to numerical instabilities, it was very difficult to decide whether this phenomenon was due to an occupational modulation or to a more complex domain structure (overlapping satellites, presence of antiphase domain boundaries etc.). (ii) In a primitive monoclinic cell model, a purely displacive modulation was conclusive. In each category, the major feature is an antiparallel shift of the Pb atoms. The main component of this shift involves a rotation and/or deformation of the oxygen octahedra. The structure of Pb2CoWO6 may be described to a good approximation as displacively modulated, but occupational modulation and coherent domains will also have to be added to the model in order to reach an even more satisfactory picture of this elusive phase. [References: 19]

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