Coherence Assembly in Structures with Heavy Metal Clusters

Abstract

A crystallographic analysis of two structures, monoclinic Nа₈[{Re₄(PO)₄}(CN)₁₂]⋅18H₂O⋅CH₃OH (I) and orthorhombic [(C₆H₅)₄P]₄ [Ta₆I₁₂(CN₆)] (II), in which clusters of heavy atoms are significantly rarefied in space, so that their mutual arrangement cannot be explained only in terms of chemical interaction, has been performed. In structure I the crystallographic planes with a high atomic density (“skeletal” planes) are located in the regions with d_hkl = 10–5.5 Å and d_hkl < 3 Å. The planes in which atomic groups [Re₄(PO)₄] (playing the role of unified bulk objects) are concentrated are in fact selected in the first region. In the second region, ordering is implemented at the level of individual atoms. A crystallographic analysis showed that the structure basis is determined by the sites of heavy Re cations. A striking fact is that there are 1152 subcells and only 32 Re atoms per unit cell in this structure; i.e., only the fraction of 1/144 provides the basis of structure stability. In structure II “skeletal” planes are also absent in the range of d_hkl from ∼7 to ∼4 Å. The planes in the range of large d_hkl characterize cluster ordering, whereas the planes in the range of small d_hkl characterize ordering of separate atoms. The geometry and local symmetry of the cluster group (Та₆ octahedron) dictates the basis of translational symmetry—unified sublattice of sites, most of which are free of these atoms. The considered structures demonstrate the key role of heavy atoms in the formation of translational symmetry—the fundamental difference of the crystalline state from other condensed states. The newly formed structure retains partially local symmetry of cores (templates) of atomic groups, bound by strong chemical interactions, including the interactions between heavy and light atoms. The process of formation of a crystal structure from randomly oriented and randomly located templates—coherence assembly—is implemented according to the laws of dynamics of elastic media, where masses of atoms rather than their chemical characteristics are important.