Strong hexagonal close packing

The corresponding unit cells are shown in Figure 1.1 and an examination of simple ball-and-stick models (which the reader is strongly urged to carry out) shows that the face-centred cubic (fee) and hexagonal close-packed (hep) structures correspond to the only two possible ways of close-packing spheres, in which each sphere has twelve nearest neighbours. 

SnBr2 in liquid ammonia. The anionic trigonal pyramidal [Sn(NH2)3] units are linked via interactions with the K+ cations into a strongly distorted hexagonally close-packed array (K-N = 2.907(7) and 2.949(8) A, and Sn is 2.154(7) A). 

It is thermodynamically less stable, has hexagonal close packed structure, and is characterized by a strong reflex at d = 2.78 A in the X-ray spectrum (Fig. 2 b). The a and P forms have crystallographic parameters similar to the y and a forms of TiCl3, respectively (Table 1) 29,30,31). 

Abbreviations BCC. body centered cubic DOS. density of states ESR. electron spin resonance HX.AI S, extended X-ray absorption fine structure F CC. face centered cubic (a crystal structure). FID, free induction decay FT, Fourier transform FWHM, full width at half maximum HCP, hexagonal close packed HOMO, highest occupied molecular orbital IR, Infrared or infrared spectroscopy LDOS, local density of states LUMO, lowest unoccupied molecular orbital MAS. magic angle spinning NMR. nuclear magnetic resonance PVP. poly(vinyl pyrrolidone) RF. Radiofrequency RT, room temperature SEDOR, spin echo double resonance Sf, sedor fraction SMSI, strong metal-support interaction TEM. transmission electron microscopy TOSS, total suppression of sidebands. 

The investigation of solid H2 has been of growing interest in recent years. Solid H2 exist in the two forms para-H2 and ortho-H2, corresponding to the spin states of the two nuclei. The percentage of p-H2 varies from 99.8 % at 20 K to about 25 % at room temperature. The solid is a molecular crystal with strong covalent bonds and weak intermolecular bonds. The solid phase is hexagonal close packed with an a of about 3.6 A, although a Pa3 structure is also known at very low temperatures [1]. 

As for the preparation of colloidal crystals using polymer spheres, the monodispersity of the cells strongly influences the order of the material. Hence yeast cells were carefully grown to form spherical cells of similar diameter. These cells were dip coated with a silica sol on a microscope slide [39]. A mono-layer of the cells arranged in a hexagonal close packing form on the microscope slide. The interstitial sites between the cells contained silica, an SEM image of the film is shown in Fig. 4. The cells remain alive and such films have potential applications in catalysis and as sensors. 

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