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Aperiodicity; Electron diffraction; Neutron diffraction; Palladium-boron alloys; Powder diffraction structure analysis; Superstructures
Abstract:
The atomic structure of the incommensurate Pd5B1-z phase has been analysed in detail by selected area electron diffraction and neutron-powder diffraction. That structure is based on a close-packed cubic arrangement of Pd atoms with the B atoms occupying the octahedral sites in a long-range ordered fashion. The ordering pattern of B is an incommensurately modulated one with a modulation vector of a length which changes with the B content (similar to PdB0.18-0.19) to adapt the ordering pattern continuously to the actual composition. The structure can be conceived to be constituted from "isolated" BPd6 units (also occurring in the previously reported Pd6B phase) and edge-sharing B2Pd10 octahedra units, which occur in a ratio reflecting the actual B content. As predicted on the basis of elastic interactions between B atoms, nearest neighbour B-B pairs occur (the B2Pd10 double octahedra units) in the structure of the Pd5B1-z phase, whereas occurrence of second-nearest neighbour B-B pairs is avoided. For the ideal composition Pd5B the crystal structure of the Pd5B1-z, phase corresponds to that of monoclinic UCl5 with U playing the role of B and Cl that of Pd.
