Volume 226, Issue 6 -

Even though the higher-dimensional superspace approach is an established method to describe aperiodic crystal structures, it is not yet fully considered and applied by a larger community of scientists. In this contribution the structural description of incommensurately and commensurately modulated molecular compounds is discussed and it is shown, that the higher-dimensional superspace approach is an elegant way for an exact structural description and an exact crystal-chemical analysis of such structures. While discussing several modulated molecular compounds, the idea behind the higher-dimensional superspace approach is shown. On the examples treated in the discussion it is explained, how to understand and how to interprete modulated molecular compounds. Also a short introduction is given to the higher-dimensional superspace approach itself, to the basic principles and to the applied nomenclature.

Hexagonal rare earth carbonate hydroxides, RE(CO 3 )OH, where RE = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er, were hydrothermally synthesized from formic acid and hydroxide gels of rare earth elements. The crystals exhibited bicephalous hexagonal prisms with lengths of several tens of micrometers. The crystal structures of a series of hexagonal RE(CO 3 )OH were solved using the single crystal CCD-XRD intensity data sets. The space groups of the synthetic hexagonal RE(CO 3 )OH crystals are all P -6. The present study has cast doubt upon the space group P -62 c previously reported for the natural Ce(CO 3 )OH, hydroxylbastnäsite-(Ce). The cell parameters decreased linearly with decreases in the ionic radii of the rare earth elements. La(CO 3 )OH showed the largest unit cell ( a = 12.6752(6), c = 10.0806(10) Å), while Er(CO 3 )OH showed the smallest ( a = 11.8977(4), c = 9.6978(8) Å). The rare earth atoms are in ninefold coordination with oxygen atoms to form a tricapped trigonal prism. The structure consists of layers of 2 ∞ [(OH)RE 3/3 ] 2+ ions linked by carbonate ions. Raman spectra indicate the presence of carbonate and hydroxide groups. An evolutionary shift was observed from La to Er towards higher frequency, which was associated with a decreasing RE—O bond length.

Single crystals of diammonium hexafluorogermanate(IV), (NH 4 ) 2 [GeF 6 ], grown at room temperature, have hexagonal symmetry with space group P 6 3 mc . So far, only a trigonal room temperature phase and a cubic phase grown at 50 °C were known. We refined the crystal structure, excluding the hydrogen atoms, and measured refractive indices and birefringence.

Various tin complexes containing the bidentate 2-amidoethyl ligand, H 2 NCOCH 2 CH 2 — C , O , are described. The compounds are [ 1 : (H 2 NCOCH 2 CH 2 — C , O ) 2 SnX 2 : X = Cl (two polymorphs), X = N 3 and the mixed halide, X = Cl/I] and the 1,2-dithiolates, [ 1 : X 2 = 1,2-dithiole-3-thione-4,5-dithiolate (dmt) and 1,3-dithiole-2-thione-4,5-dithiolate (dmit)]. Also discussed are the structures of co-crystalline 1 : 1 : 1 complexes, {[NBu 4 ] : [X] : ( 1) }, namely ( 2 : X = Br and I). The remaining compound considered is the hydrogen bonded 2 : 1 complex of ( 1 : X = Cl) with 18-crown-6, 3 . In all of the structures, tin is six-coordinate in an octahedral environment distorted, primarily and most markedly in ( 1 : X 2 = dmt or dmit), by the bite angle of the bidentate ligands. The creation of three-, two- and one-dimensional connectivity in the supramolecular structures is dominated by intermolecular hydrogen-bonds with the amide NH 2 as the donor species.