The incommensurate crystal structure of the Pd₅B_1-z phase; B ordering driven by elastic interaction between B atoms

Abstract

The atomic structure of the incommensurate Pd₅B_1-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 (_~PdB_0.18-0.19) 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 Pd₅B_1-z phase, whereas occurrence of second-nearest neighbour B-B pairs is avoided. For the ideal composition Pd₅B the crystal structure of the Pd₅B_1-z phase corresponds to that of monoclinic UCl₅ with U playing the role of B and Cl that of Pd.