Volume 222, Issue 12

High quality data are essential for crystal structure solution, particularly when Direct Methods instead of Global Optimization Techniques are used. In this work we study the performances of the variable-counting-time techniques in the two crucial steps of the phasing process: decomposition of the full diffraction pattern and direct phasing. The experimental data were collected by a conventional X-ray laboratory diffractometer: they were then submitted to the program EXPO2004 for assessing their usefulness in the phasing process. The results are compared with those obtained by using experimental diffraction data collected via the traditional fixed-counting-time technique under the same experimental conditions.

The crystal structure of erythrite from Bouzaer, south Morocco, was refined through single crystal X-ray diffraction data, within monoclinic space group C 2/ m , with the following unit cell constants: a = 10.187(1) Å, b = 13.470(1) Å, c = 4.755(1) Å, β = 104.971(2)°, V = 630.3(2) Å 3 . Transition metal and magnesium cations are randomly distributed over two special positions Me (1) and Me (2), making up single and dimerical octahedral groups, connected by (AsO 4 ) tetrahedra groups, in order to form infinite sheets down b . Such sheets are held together within the three-dimensional framework owing to the presence of an intricate hydrogen bond network. An infrared spectrum was collected in order to achieve information on the HOH bending modes of the erythrite.

The structure of bismuth triborate has been analyzed by the combination of neutron and X-ray diffraction on powder and on single-crystalline samples. Bismuth triborate exhibits a distinct anisotropic thermal expansion with the coefficient along a being negative over the wide temperature range studied, 20 to 800 K. The remarkable crystal structure of bismuth triborate with a netlike linkage of rigid borate units [BO 3 ] and [BO 4 ], is found to generate the uncommon thermal expansion behavior. Our investigations gave no evidence for a structural phase transition in bismuth triborate between 3.5 and 999 K at ambient pressure.

The crystal structure of a high pressure polymorph of CaAl 2 O 4 has been investigated using laboratory X-ray powder diffraction data. The compound was prepared from a ceramic precursor in a piston cylinder apparatus at 800 °C and 20 kbar. The quenchable so-called phase-II of monocalcium aluminate adopts the monoclinic space group P 2 1 / c ( a = 7.97187(7) Å, b = 8.62844(7) Å, c = 10.26276(10) Å, β = 94.801(4)°, V = 703.44(1) Å 3 , Z = 4, D calc = 2.99 g/cm 3 ). The main structural features of the high- P form are layers of AlO 4 -tetrahedra perpendicular to the a -axis. Stacking of the layers parallel to [100] results in a three-dimensional framework containing channels, where the calcium cations are accommodated for charge compensation. Individual sheets can be described as being built by condensation of ditrigonally-shaped six-membered rings (S6R). The sequence of up (U) and down (D) pointing apices within a single ring is UUDUDD. The calcium cations in the tunnels are coordinated by six to seven oxygen ligands. Topologically the framework belongs to the group of three-dimensional 4-connected nets. The coordination sequences (4-11-24-41-63-91-123-160-202-249) and the vertex symbols (6 1 .6 1 .6 2 .4 1 .6 1 .6 1 ) are identical for the four crystallographically independent Al-atoms. The framework density has a value of 17.06 T-atoms/1000 Å 3 .

Using synchrotron radiation, a single crystal investigation has been performed at 293 and 100 K for the structural characterization of the manganoan pectolite, Na(Ca 1.73 Mn 0.27 )[Si 3 O 8 (OH)] (from the pegmatite at Mt. Koashva, Khibiny alkaline massif, Kola peninsula, Russia). The data obtained are compared with other members of the pectolite – serandite, Na(Ca 2–x Mn x )[Si 3 O 8 (OH)], series. The strong asymmetric hydrogen bond observed in the other members of the series has been confirmed at 293 K (O3–O4 = 2.473(3) Å; H–O3 = 0.97(4) Å, H … O4 = 1.50(4) Å ; O3–H … O4 = 178(4)°). This bond exhibits strengthening (O3–O4 = 2.458(2) Å) and becomes more symmetric at 100 K (H–O3 = 1.06(4) Å, H … O4 = 1.40(4) Å, O3–H … O4 177(4)°). The data obtained for 293 K confirm the previously hinted tendency: the higher the content of Mn, the more asymmetric the H-bond. The influence of the asymmetric position of the H-bond on the environment close to the bonded O3 and O4 atoms and the H–Na interaction are discussed. The alternative topological role of the hydrogen bond is considered in comparison to a similar structure Li 2 Mg 2 [Si 4 O 11 ].

(1) C 13 H 13 N 3 O 5 , Mr = 291.26, P -1, a = 7.4629(9), b = 7.9203(9), c = 12.126(2) Å, α = 86.804(5), β = 78.471(7), γ = 69.401(8)°, V = 657.3(2) Å 3 , Z = 2, R 1 = 0.0454; (2) C 11 H 12 N 2 O 4 , Mr = 236.23, Pbca , a = 7.2713(9), b = 14.234(1), c = 20.848(3) Å, V = 2157.8(4) Å 3 , Z = 8, R 1 = 0.0504; (3) C 13 H 13 N 2 O 3 Cl, Mr = 280.70, P 2/ n , a = 17.344(2), b = 9.237(1), c = 18.398(2) Å; β = 92.61(2)°, V = 2944.4(6) Å 3 , Z = 8, R 1 = 0.0714. The conformational features of three 4-substituted-3-4-dihydropyrimidin-2(1H)-ones were investigated by computational and single crystal X-ray crystallographic studies. The geometries were optimized using semiempirical (AM1) and first principle calculations (B3LYP/6-31G**) methods, the rotational barriers for important functional groups were studied. In all structures the pyrimidinone rings are in a more or less distorted boat conformation. The phenyl and the furane rings are almost perpendicular to the best least-squares plane through the dihydropyrimidinone ring.

The crystal structure of the β polymorph of Pigment Yellow 181 was solved from laboratory X-ray powder diffraction data and Rietveld refined. In the crystal structure, the molecules are essentially planar, with extensive hydrogen bonding between the molecules.

The program BRIZ has been developed for displaying Brillouin zones and Fermi sphere configurations. The underlying physics relates to the nearly-free-electron model for simple sp-metals. A Brillouin zone boundary near the Fermi level provides formation of the energy gap and leads to the lowering of the structure energy. Consideration of the Brillouin zone with respect to a free-electron Fermi sphere and zone filling by electron states provides useful information for phase structure stability and physical properties.