Volume 217, Issue 5

A bonding model, valid for alimited group of borates, is formulated which allows the ratio of triangularly to tetrahedrally coordinated B atoms to be calculated from the chemical formula. The anion complex, which is formed by covalently bound atoms, has non-directed ionic bonds with the surrounding cations. To complete its electron octet, each O atoms of the complex forms exactly two covalent bonds either with 2 B and/or with 1 B and 1 H atom, depending on the number of OH groups. The B atoms have no preference for either triangular or tetrahedral O coordination. However, a change of a B-O base triangle to an isoelectronic base tetrahedron decreases the number of O atoms which can be used for B-O-H bonds and increases the number of shared O atoms engaged in B-O-B bonds - and vice versa. The ratio of B-O triangles to tetrahedra is adjusted such that O atoms can form the proper number of B-O-B and/or B-O-H bonds. Using the model one can also prove that a borate with OH groups and all its dehydration products (independent whether they are crystallized or a glass) should have the same ratio of three- to four-coordinated B atoms. Under the presupposition that the bonding model is applicable, one can specify a simple mixture of base triangles and/or base tetrahedra with which it should be possible to construct an anion complex. The observed complex is either built up with this simple mixture or with a mixture of base polyhedra derived from the simple one by cross-substitution which does not change the triangle to tetrahedron ratio. The realization of the bonding model is in principle possible only within a restricted composition range in which lie one quarter of the known borates. The equation to calculate the triangle to tetrahedron ratio has been tested on all borates within this range. An agreement was found for 85 percent of the 222 investigated crystal structures.

A new indium yttrium germanate presenting the thortveitite structure with symmetry described by the space group C2/ m (No. 12) has been prepared by high temperature solid state reaction as polycrystalline powder material. This crystallizes in the monoclinic system, with cell parameters a = 6.8286(1) Å, b = 8.8836(2) Å, c = 4.9045(1) Å, β = 101.8340(7)º, V = 291.195(9) Å 3 and Z = 2. The structure was characterized by X-ray powder diffraction and Rietveld refinement of the diffraction pattern. The In 3+ and Y +3 cations occupy the same octahedral site forming a hexagonal arrangement on the ab planes. In their turn, the hexagonal arrangements of InO 6 /YO 6 octahedral layers are held together by sheets of isolated diorthogroups constituted by a double tetrahedra sharing a common vertex. In this compound, the Ge 2 O 7 diorthogroup shows the C 2h symmetry implying a Ge-O-Ge angle of 180º, being an important feature of the thortveitite structure, which has been controversial in some reported papers. A remarkable photo-luminescence effect (in comparison with glass) was observed when the sample was irradiated with α -particles beam during the RBS experiments employed to analyze the chemical stoichiometry.

Correlations between crystal chemical properties of anhydrous oxoborate crystals wereanalysed using artificial neural networks. Using structuralproperties of oxoborate crystal structures described inthe literature, we developed several neural network modelsthat capture statistical relations between crystalchemical properties of the anhydrous oxoborates from the existingdata sets. This indicates the suitability of neural networks forthe prediction of structural propertiesof crystals.

Unit-cell parameters of La 3 Nb 0.5 Ga 5.5 O 14 (langanite) have been determined at a number of pressures up to 23GPa in diamond anvil high pressure cells with single crystal X-ray diffraction techniques. The anisotropic behaviour of unit cell parameters of langanite is caused by differences of bonding strength in direction parallel to the a - and c -axis. Within the investigated pressure range, the c / a -ratio increases from 0.6232 to 0.6503. The volume compressibility of trigonal langanite (space group P 321) is the 0.007 GPa -1 , with calculated bulk modulus of 145(3)GPa (B' 0 =1.4(8)). At a pressure of 12.4(3)GPa a first-order phase transition was detected, characterised by a discontinuity in the pressure dependence of lattice parameter c . The high-pressure phase (presumably monoclinic, A 2) with a volume compressibility of 0.011 GPa -1 (B 0 =93(2) GPa, B' 0 =1.9(9)) is more compressible as compared to the initial phase due to an abnormal pressure dependence of lattice parameter a .

Crystal structure of the title compound is orthorhombic, Pbca , a =7.8689(2) Å, b =9.6519(3) Å, c =31.3579(9) Å. The structure was solved by direct methods and refined by a full-matrix least squares procedure to final R (all)=0.0682 for 2853 unique diffractions merged from 24459 measured at 183(2)K. The molecule is formed by two planar moieties joined through an atom of sulphur. The short intramolecular O...O contact is 2.430(2) Å, the intramolecular hydrogen bond is slightly asymmetric. Theoretical ab initio calculations done at B3LYP/SVP and ONIOM(MP2/SVP:B3LYP/SVP) levels showed that the experimental geometry of pentanedione pseudoring is close to the geometry corresponding to a transition state on the asymmetric potential energy curve. Calculated energy difference between two minima is only –4.72 kJ/mol and the energy barrier –6.02 kJ/mol, which agrees well to ~6.6kJ/mol found in microwave spectroscopic study of malonaldehyde.