Volume 209, Issue 3

X-ray diffraction (XRD), Mössbauer spectroscopy (MS), and magnetization (SQUID) data are reported for a “pyrrhotite-like” compound ( δ -FeSe), belonging to the central portion of the Fe-Se system. The δ -phase(s) extends compositionally over a range of around Fe 0.94 Se-Fe 0.86 Se at 870 K, but narrows around Fe 7 Se 8 at room temperature. Lattice parameters for the primitive hexagonal cell decrease regularly with increasing vacancy concentration x in Fe 1 − x Se; c = 5.97 – 5.87 Å, a = 3.65 – 3.62 Å. An observed regular decrease in cell axes ratio, c/a = 1.644 – 0.145 x , is described as due to increased ordering with increased vacancy concentration. The disordered high-temperature 1 c structure is maintained at room temperature for vacancy-poor compositions, while superstructures, 3 c and 4 c , form for vacancy-rich compositions. Using a bond model for the magnetic coupling, MS spectra for ordered 3 c and 4 c superstructures can be well fitted assuming three well-defined surroundings, while those for disordered 1 c show a distribution in surroundings. A spin direction change, from the c -plane towards the c -axis with decrease in temperature, is sharp at 130 K for 3 c , extended and incomplete for 4 c (∼220 – 120 K), and again rather sharp for 1 c at ∼ 220 K. It is suggested that the spin flip at 130 K for 3 c , which is also visible (but weak) in SQUID measurements of 1 c samples, is indicative of an element of short range order in the disordered structure.

Sanidine single crystals from Volkesfeld/Eifel (Germany) exhibit an unusually rapid Si, Al exchange on annealing at temperatures above 1023 K. X-ray topographic studies (Lang technique) before and after annealing show that large volumes (up to several cm 3 ) of the crystals are highly perfect. There are few defects only (growth striations, growth-sector boundaries, grown-in dislocations), and Pendellösung fringes can be observed. While the Si, Al-exchange rate is reduced by annealing, the growth striations and Pendellösung fringes vanish, and for certain reflections, e.g. (002), the diffracted intensity is increased due to reduction of the “dynamical” extinction. These changes are illustrated by selected X-ray topographs.

Dislocation configurations induced by room temperature microindentations on the (001) face of GaSb (undoped and Te-doped) have been studied using high voltage transmission electron microscopy. Perfect and partial dislocations could be found in all four arms of the dislocation rosette around the indent. Microtwins and rarely single stacking faults are associated with the partials. Contrary to other binary III–V compounds, an “inverse” glide prism along the [1[unk]0]/[[unk]10] rosette arms is created and it is bounded by {111} B planes which diverge away from the surface. Under high stress – low temperature conditions plastic deformation in undoped as well as n-doped GaSb proceeds by the movement of mobile α -dislocations, whereas β -dislocations are almost immobile. The reasons for the observed slip asymmetry of the dislocation rosette are discussed and a model for the formation of microtwins is proposed.

Large single crystals of salol were grown in various crystallographic directions by the Czochralski technique. Their growth defects were studied by X-ray topography. The typical arrangement of grown-in dislocations is described. In some cases crossing growth dislocations interact and form dislocation nodes. It is confirmed, that the Burgers vectors b of the dislocations merging into the nodes obey Frank's theorem Σ b = 0. The dislocation reactions proceed in such a way that the energy of the system is reduced.

Very slow warming and cooling sequences are made in the molecular dynamics (MD) of the true crystal to plastic crystal phase transition in SF 6 . A parallel computer with 1024 processors (the AMT DAP) is used, each processor taking charge of three molecules. The Parrinello-Rahman (PR) zero-stress algorithm is used, starting with a perfect single crystal. The MD system is warmed and cooled in sequence twice between 30 K and 190 K, indicating a transition with a hysteresis. On warming, the plastic phase appears at 129 K ± 6 K, but is better defined as 83 K ± 3 K on cooling. In both the warming and cooling sequences it can be seen that the structure change proceeds through a rhombohedral structure which is metastable in the real system under certain conditions. In the non-equilibrium MD (NEMD) the metastable phase cannot be stabilised when using a large system and the PR algorithm, but its appearance is reproducible. Although the simulations are NEMD, entropy estimates are made using the velocity scaling procedure as a heat bath, and the discrepancies introduced because of the algorithm and the quasi-static approximation are seen to be remarkably small. The latent heat at the transition is about 10kcal/mol. The extended frozen defect which appears in one cooling sequence has a very significant effect on the entropy function, showing that future studies of the entropy of defects through MD are a viable possibility.

It appears to be normal practice to exclude weak data from the least squares refinement, even when this data has already been collected on a diffractometer. What is more, there is no standard criterion for what is or is not a weak intensity. By means of a computing exercise based on two recently determined structures, it is pointed out that the effect of excluding weak data will improve the residual but this is cosmetic as it is accompanied by changes in coordinates, temperature factors, bond lengths and angles which are not only detectable but may exceed the e.s.d. in particular parameters thus leading to unquantifiable errors. In some cases changes in bond lengths and angles are apparently systematic with the degree of truncation. In addition truncating the data may lead to an increase in the computed e.s.d. in the parameters. This confirms previous work that data should not be truncated and all available data should be used for crystal structure determination.

With the use of a new computer program and following the PBC periodic bond chain concept, the morphology of triciinic potassium trihydrogen dioxalate dihydrate, KH 3 (C 2 O 4 ) 2 · 2H 2 O (also termed potassium tetroxalate, KTO) has been deduced from its crystal structure. Several values of the parameters of the potential energies for ionic and hydrogen bonds were examined. The morphology turned out to be rather independent of the choice of the potentials. The agreement with observed morphologies reported in the literature is good.

A model for a twin consisting of two domains is proposed for a new pseudo-hexagonal modification of high-density boron nitride. A structure refinement based on high resolution diffraction data taken with 0.5 Å synchrotron radiation resulted in R factors below 0.015 for each domain. The refinement against detwinned and composite data converged to R = 0.053, with domain ratio 0.462(2):0.538.

The LaF 3 -structure has been reinvestigated using diffraction data measured with twinned crystals. LaF 3 is a fluorine ion conductor with essentially 3-dimensional conductivity. The conduction perpendicular to the c -axis is shown to occur on pathways which include all three sets of fluorine positions in the structure. These pathways connect symmetrically non-equivalent nearest neighbour positions only. This conclusion is supported by the determination of anharmonic temperature factors. The temperature dependence of the harmonic Debye-Waller factors indicates that thermal vibrations are described rather than static discorder. The magnitude of the temperature parameters of fluorine is consistent with an intrinsic activation energy of 0.8 eV for fluorine conduction.

Pure phosphate bioglass ceramics of the system Na 2 O–CaO–Al 2 O 3 –P 2 O 5 –F − with the additives ZrO 2 , TiO 2 and FeO/Fe 2 O 3 were investigated by X-ray powder diffraction at temperatures up to 750°C. The glass system with ZrO 2 and TiO 2 exsolves eight crystalline phases in this temperature range. Three of them are new phosphate phases (U2, U3, U4) whereas NaZr 2 (PO 4 ) 3 and U1 (Na–Ca–(Zr, Ti)–Al-monophosphate), which is isostructural to NaTi 2 (PO 4 ) 3 with space group R [unk] c , belong to the Nasicon-structure-type. Lattice constants of these phases were determined by transmission electron microscopy. Energy dispersive X-ray analysis gave some indication of the chemical composition of the various phases.

The crystal structure of the monoclinic diammonium succinate (NH 4 ) 2 C 4 H 4 O 4 was determined at 301 K. The space group is P 2 1 / c with Z = 2 and a = 3.897(1) Å, b = 7.819(1) Å, c = 11.945(2) Å, β = 92.88(2)°, V = 363.5(1) Å 3 , calculated and experimental density D x = 1.390(1) g/cm 3 and D exp = 1.387(7) g/cm 3 . The crystals show a perfect cleavage along (10[unk]), parallel to the structural layers.

The absolute structure of nicergoline (C 24 H 26 BrN 3 O 3 ) was solved by direct methods and refined anisotropically to the R value of 0.039 for 3 145 observed Friedel pairs. The title semisynthetic ergot alkaloid crystallizes in the orthorhombic space group P 2 1 2 1 2 1 with lattice parameters a = 11.507(2) Å, b = 13.219(2) Å, c = 14.753(2) Å, Z = 4. The pyrrole moiety is nearly planar, the ergoline C ring has a half-boat conformation and the D ring is a regular chair. The absolute configuration has been found to be 5R, 8R and 10S.

(+)-Camphor reacts in basic media, with N-sulfinylaniline to give, in good yields, dicamphoquinone, which is hydrogenated to dicamphor. Crystals of dicamphor (C 20 H 30 O 2 ) belong to space group P 2 1 , with a = 6.920(1) Å, b = 11.207(1) Å, c = 11.516(1) Å, β = 99.62(1)°, z = 2, T = 293 K. The final R is 0.044 for 1113 observed reflections. The cohesion of the crystal is the result of van der Waals interactions. The two camphor moieties in this 1.4-diketone are located in chiral positions with respect to each other. The two carbonyl groups form a right handed helix. The Circular Dicroism (CD) spectrum exhibits two Cotton effects of opposite sign and equal intensities in the region of n → π * transition: positive at longer wavelength and negative at shorter wavelength. The nature of CD is discussed.

The compound Cu(p-OHC 6 H 4 COO) 2 (DENC) 2 (DENC = N,N′-diethylnicotinamide) crystallizes in the orthorhombic system, space group Pbca (No. 61), with unit cell parameters a = 44.966(1) Å, b = 16.267(1) Å, c = 14.0147(3) Å, and Z = 12. The final R value for 5528 independent reflexions is equal to 0.054. Its structure consists of centrosymmetric [Cu(p-OHC 6 H 4 COO) 2 (DENC) 2 ] units and infinite polymer chains, interconnected by hydrogen bonds. Within a monomeric unit, the Cu atom is pseudooctahedrally trans-coordinated by four carboxylic O atoms of two p-hydroxobenzoate ligands [Cu–O bond distances being 1.923(4) Å (2×) and 2.582(3) Å (2×), and by two N atoms from two DENC ligands [Cu–N bond distances are 2.050(3) Å (2×)]. Each Cu aton in the chain is hexacoordinated by three carboxylic O atoms from the monodentate and tridentate p-hydroxobenzoates [Cu–O bond distances are 1.929(4) Å, 1.962(4) Å and 2.652(3) Å], by two N atoms from two DENC ligands [Cu–N bond distances are 2.019(3) Å and 2.025(3) Å] and one hydroxyl O atom of further tridentate p-hydroxobenzoate [Cu–O bond distance is 2.378(3) Å].

The crystal structure of [Cu(Sulfathiazole) 2 (MeOH)Cl 2 ](C 19 H 22 N 6 O 5 Cl 2 CuS 4 ; triclinic; a = 7.564(6) Å, b = 11.044(6) Å, c = 16.344(7) Å, α = 108.43(1)°, β = 102.99(1)°, γ = 90.31(1)°, Z = 2, space group P [unk]) has been determined from the intensities of 3966 independent reflections (graphite monochromator Mo K α 4-circle diffractometer; r = 5.64%). The coordination polyhedron around Cu(II) ion is intermediate between square pyramidal and trigonal bipyramid.

The structures for two sugar-derived 5,6-dihydropyran-2-ones were determined: (5 R ,6 S )-3.5-bisbenzoyloxy-6-methyl-5,6-dihydro-2 H -pyran-2-one (1), C 20 H 16 O 6 , P 2 1 2 1 2 1 , a = 27.610(9), b = 10.734 (4), c = 5.972 (2) Å, Z = 4, R = 0.033, R w = 0.037, and (5 S )-3, 5-bisbenzoyloxy-5.6-dihydro-2 H -pyran-2-one (2), C 19 H 14 O 6 , P 2 1 , a = 15.93(1), b = 8.150(5), c = 6.238(5), β = 99.43(5)°, z = 2, R = 0.096, R w = 0.096. They adopt skew-boat (SB) solid-state conformations of the pyranoid ring, such that C-6, and, to a lesser degree, the ring oxygen lie above a plane made up of C-(2)–C-(3)–C-(4)–C-(5).