Beziehungen zwischen Bindungsl�ngen und Bindungsst�rken in Oxidstrukturen

The crystal structure of tellurium tetraiodide TeL was determined from three-dimensional X-ray diffractometer data and was refined by full-matrix least-squares methods to an R index of 5.1%. The crystals are orthorhombic, space group Pnma, with a= 13.635 (5), b= 16.798 (5), c= 14.624 (5)/~, Z= 16TeI4. The structure is built of tetrameric (TeI4)4 molecules which are not isomorphous with the (TeCl4)4 molecular structure, but form a novel binary X4Y16 structure type composed of edge-sharing TeI6 octahedra. Mean Te-I bond lengths are 2.769, 3.108 and 3.232 ,~ for bonds to terminal, doubly and triply bridging iodine atoms, respectively. Intermolecular I...I distances as short as 3.870, 3"893, 3.932 ,~ indicate weak bonding interactions between the Te4116 molecules. Within the series of the TeF4, TeCL and TeL crystal structures, a strong decrease of the stereochemical activity of the non-bonding electron pair on tellurium(IV) is observed.

Section: Beschreibung Der Struktur Und Diskussionunclassified

Kristallstruktur des Tellurtetrajodids TeJ₄

Krebs¹,

Paulat²

1976

“…Equations (3) and (4) both give similar results (Donnay & Donnay, 1973;Allmann, 1975). However, due to the above-mentioned 'universality' of b, (3) has an advantage° over (4).…”

Section: Introductionmentioning

confidence: 59%

Semi-empirical groundwork of the bond-valence model

Urusov¹

1995

The bond-valence model (BVM) which relates the length R of a bond to its bond valence s is a widely used empirical approach to the interpretation and prediction of atomic arrangements in crystals. However, a theoretical foundation of the BVM appears to be fairly inadequate so far. In this paper derivations of the main expressions of R(s) Independence of the BVM from bonding character is the result of such a treatment. Interrelations between both versions of the BVM are also analyzed. Much attention is given to the question of transferability of the single bondlength parameter and to the special role of bond-specific values of the softness parameter. The latter is proved to be a simple function of the ionization potentials of bonded atoms. In turn, this explains the strong linear correlations between the single bond lengths from cations to a pair of anions and the relation between single bond lengths and atom sizes (sum of the covalent radii). It is demonstrated that a correction related to electronegativity difference is involved in this consideration. In general, one might conclude that the BVM becomes soundly based from both the empirical and semi-empirical viewpoints.

“…l(b)] des defekten Oktaeders hat dann keinen MoNachbarn mehr und l~isst sich bei Nichtbesetzung der zentralen Mo-Position statistisch durch ein isoliertes Kristallwassermolekfil [HzO(1) in Fig. l(a) (Pauling, 1947;Cotton & Wing, 1965;Donnay & Allmann, 1970;Allmann, 1971Allmann, , 1975Schr6der, 1975;B6schen, Buss & Krebs, 1974;B6schen, 1974) (Allmann, 1971(Allmann, , 1975. Da 0(2 iii) und 0(2 ,Hi) in Nachbarschaft einer unbesetzten Mo-Position das st~rkste Bindungsdefizit zur formalen Bindungssumme zwei aufweisen, ist anzunehmen, dass das Wasserstoffatom wahrscheinlich an eines dieser beiden symmetrieS.quivalenten Atome gebunden ist und der Bindungsausgleich ftir das zweite Atom fiber starke Wasserstoffbrfickenbindungen oder durch Verst~irkung der Bindungen zu den beiden verbliebenen Mo-Nachbarn erfolgt.…”

Section: Introductionunclassified

Kristallstruktur des sogenannten Kaliumdekamolybdats KMo₅O₁₅OH.2H₂O

Krebs¹,

Paulat‐Böschen²

1976

The crystal structure of the so-called potassium decamolybdate, the formula of which has to be written as KMosOlsOH. 2H20, was determined from single-crystal diffractometer data. The compound crystallizes with hexagonal symmetry in the space group P63/m, the cell parameters being a= 10.550 (8), c--3-727 (3) ,~ and Z= 1. The structure has been solved from the Patterson function and chemical considerations and refined by full-matrix least squares to a conventional R of 0.042 based on 335 independent reflexions. The structure consists of double chains made of edge-sharing, strongly distorted MoO6 octahedra. The double chains are linked by common corners of octahedra to form a threedimensional network. The double chains are not perfect due to statistical non-occupation of one of the six equivalent Mo positions in the unit cell. Along the short c axis the polymeric structure forms tunnels in which the potassium ions are located. Mo-O bond lengths in the MoO6 octahedra are: 1.698, 1.701, 1.955 (2 x ), 2.190 and 2.373/~. The chemical composition and the observed interatomic distances lead to a plausible model of the structural disorder: In the neighbourhood of a vacant Mo position the O atoms are partially replaced by coordinated H20 molecules, isolated H20 molecules (crystal water), and OH groups. The statistical distribution of the Mo vacancies can be interpreted as resulting in smaller sub-units of edge-sharing octahedra linked in the direction of the double chains only by common corners. The structure is only weakly related to the structures of the other typical polymolybdates crystallizing from aqueous solutions. Closer relations are found to the structures of ~-MoO3.H20 and of MOO3.