Incommensurate stistaite--order made to order.

The structure of stistaite-SbSn, was investigated by single crystal X-ray diffraction and EDX to yield a simple relation between the modulated structure and the composition. Stistaite is found to be incommensurately ordered over the entire stability region.

Superlattice formation in the lithiated vanadium oxide phases Li(0.67)V(6)O(13) and LiV(6)O(13).

Two new lithiated phases of V(6)O(13) were formed by carefully tuning the temperature of electrochemical lithiation in a "coffee-bag" type Li-ion battery at 2.78 V versus Li/Li(+). These were studied by single-crystal X-ray diffraction. A phase with the composition Li(2/3)V(6)O(13) was obtained at 308 K with a unit cell three times the volume of the original V(6)O(13) cell. A single crystal discharged at ambient temperature was shown to be LiV(6)O(13) and twice the unit-cell volume of the original V(6)O(13) cell. On lithiation, the structures retain their basic V(6)O(13) structure of alternating single and double layers of VO(6) octahedra. The lithium ions occupy chemically equivalent sites, where they coordinate fivefold to O atoms, and associate with the single layers of VO(6) octahedra. The insertion of lithium causes a significant elongation of one of the V-O bonds in each structure, which expands from 1.65 to 1.89 A; this is due to the charge reduction of a specific V atom.

Gallium and indium under high pressure

Ga and In crystallize in unusual open ground-state crystal structures. Recent experiments have discovered that Ga under high pressure transforms into a close-packed structure, while this has so far not been observed for In. Results from first principles calculations explain in a simple way this difference in behavior. We predict a so far undiscovered transition of In to a close-packed structure at extreme pressures and show that the structure determining mechanism originates from the degree of s-p mixing of the valence orbitals. Group-III elements are shown to strongly disobey the standard corresponding-state rule.

Elucidation of the crystal structure of oxyapatite by high-resolution electron microscopy.

High-resolution electron-microscopy (HREM) images from different hydroxyapatite (OHAp) samples showed p3 projection symmetry along [001] instead of the p6 projection symmetry compatible with the space group P6_3/m of OHAp. Image processing was used to establish without ambiguity that threefold symmetry dominates the images along [001]. OHAp crystals decompose in the transmission electron microscope and it is concluded that the threefold symmetry observed corresponds to an early step in the decomposition process, the dehydration of OHAp to oxyapatite (OAp). A structural model for OAp where every second O atom along the 6(3) axis in OHAp is removed has the maximal space-group symmetry P{\bar 6}. This is compatible with the p3 projection symmetry observed. Atomic shifts in this OAp model compared to the OHAp structure were estimated using the HREM images and geometric optimizations of the atomic structure. No refinements of the atomic coordinates against diffraction data were possible but the simulated HREM images of this crude model fit well with the experimental images.

Iodo-oxyapatite, the first example from a new class of modulated apatites.

Iodo-oxyapatite [pentadecacalcium iodide oxide nonaphosphate, Ca(15)(PO(4))(9)IO] was synthesized by a flux method and the structure was solved from single-crystal X-ray data. The crystal structure was refined in the space group P6(3)/m [a = 9.567 (1), c = 20.754 (2) Å and Z = 2] to wR on F of 0.0459. Iodo-oxyapatite has a typical hexagonal apatite structure but the unit cell is tripled along the hexad owing to ordering of the iodide and oxide ions along this direction.

Bone graft proteins influence osteoconduction. A titanium chamber study in rats.

Although it is often emphasized that the matrix of bone grafts contains several growth factors, it is not known if these factors become activated and play a role in bone grafting. We therefore compared ground defatted bone which had or had not been deproteinized by heating with water to 270 degrees C at an autogenic pressure of 55 bar. This is a careful ceramic procedure which leaves the mineral unchanged. Deproteinized and non-deproteinized bone granulae derived from cortical rat bone were placed in titanium bone conduction chambers implanted bilaterally in rat tibiae. Ingrowing bone could enter the cylindrical interior of the chamber only at one end. It then penetrated the material in the chamber, but due to the length of the cylinder, it never reached the other end. The mean distance which the ingrown bone had reached in the material was then measured on histological slides. The bone formation activity was measured by TcMDP scintimetry. With the protein-containing granulae, the mean bone ingrowth distance and the scintimetric activity were increased by 41% and 31%, respectively (p < 0.01). We conclude that there is a limited favourable effect of proteins in a graft. Our grounded material had a large fracture surface area with no osteoid lining. The leakage of growth factors from such areas may explain the extraordinarily good clinical incorporation of morselized compacted allografts.

DNA packing in chromatine, a manifestation of the Bonnet transformation.

The packing of DNA is described using the formalism of differential geometry. Winding of the DNA double helix around the histone 2-5 octamer forming a nucleosome and the condensation of the so-formed bead-on-a-string chromatine aided by histone 1 is interpreted as two consecutive isometric, i.e. Bonnet, transformations. The DNA double helix can be approximated to a helicoid which can be transformed isometrically to a catenoid, an approximation of the nucleosome. Owing to the organization of the histone octamer the extended chromatine takes a helicoidal shape allowing a second Bonnet transformation to consummate the condensation into a chromatine fibre.