Double hexagonal

The metal has a bright silvery metallic luster. Neodymium is one of the more reactive rare-earth metals and quickly tarnishes in air, forming an oxide that spalls off and exposes metal to oxidation. The metal, therefore, should be kept under light mineral oil or sealed in a plastic material. Neodymium exists in two allotropic forms, with a transformation from a double hexagonal to a body-centered cubic structure taking place at 863oC. 

Brookite is also based upon a double hexagonal close-packed arrangement of the oxygen ions. The dimensions of the unit of structure differ so much on account of distortion from those for the ideal arrangement, however, that the existence of dose-packing was recognized only after the structure had been determined. 

White lustrous metal when freshly prepared turns silvery exists in two forms as a double hexagonal closed-packed alpha form, and a closed-packed cubic structure known as beta form melts at 994°C more volatile than its neighbor elements, plutonium or curium vaporizes at 2,607°C density 13.67 g/cm soluble in dilute acids. 

Physical properties of the element are anticipated or calculated. Sdvery metal having two aUotropic forms (i) alpha form that should have a double hexagonal closed-packed structure and (ii) a face-centered cubic type beta form density 14.78 g/cm (alpha form), and 13.25 g/cm (beta form) melting point 985°C soluble in dilute mineral acids. 

The distribution of cations in a hydrated zeolite is mainly controlled by their sizes and can be described by a statistical model. In the dehydrated state, most of the cations are located on the intraframework sites their occupancies are governed by mutual repulsions and cation—framework interactions [1]. By which, the environments of the framework silicon atoms and their corresponding ssi NMR spectra are affected [2,3]. The chemical shift and lineshape of Si NMR have been found to depend on the nature and the distribution of cations in the small sodalite and double hexagonal prism (D6R) cavities of the dehydrated Y zeolites [3] The irreversible migration of La3 ions from the supercages to the small sodalite and/or D6R cavities by... 

Double hexagonal closest packing (Sec. 11-5) was first found62 for the oxygen ions in brookite, the orthorhombic form of titanium dioxide, and for the oxygen and fluoride ions in topaz 63 AUSiC F2. It has since been reported for the halogens in one modification of cadmium iodide,64 in mercuric bromide,66 and in mercuric chloride6 and for chloride and... 

The index usually indicates the sequence cubic (3), hexagonal (2), or double hexagonal (4) for the ABAC sequence. The Pearson symbols (Table 2.5) can clarify cases such as hexagonal structures with an ABC sequence and a = 3 for the index. The symbols t for tetragonal, o for orthorhombic, m for monoclinic, and h for hexagonal, rhombohedral, or trigonal indicate the type of distortion of an idealized structure. Without distortion, a = 3 is for a cubic structure and a = 2 is for a hexagonal (or rhombohedral) structure. 

TiNi3 is hexagonal with four molecules per unit cell, a = 5.101 and c = 8.3067 A. The close-packed layers are in the double hexagonal... 

Note how close together the T- and T+ sites are on each side of a P layer. However, the sites occupied are staggered. The notation is 4 2P3/4T+/,4P1/4T /4. The index 4 shows that the P layers are in the double hexagonal sequence, a more complicated sequence than for hep (2) or ccp (3), and the product gives the total of eight layers in the repeating unit. 

FIGURE 1.15. Electrohydrodynamic pattern replication, (a) double-hexagonal pattern, (b) the word nano , (c) 140 nm wide and 140 nm high lines. In (b) the line width was s300 nm. The larger columns stem from a secondary (much slower) instability of the homogeneous (not stmctured) film. Adapted from [30] and [38]. 

Flu. 6. The structure of tungsten hexafluoride showing the approximate double-hexagonal close packing of fluorine atoms. Hatched circles show atoms at x = 0, double circles those at x = j, single circles those at x - 5, and crossed circles those at x = corresponding to the sequence ABAC. Metal atoms are shown as small, dashed circles. 

Berkelium metal exhibits two stable crystallographic modifications, double hexagonal closest packed (dhcp) and face-centered cubic (fee). Thus it is isostructural with the two preceding elements, all of which exhibit the fee structure at high temperature. The room-temperature lattice constants of the dhcp form are ao = 0.3416 0.0003 nm and c0 = 1.1069 0.0007 nm, yielding a calculated density of 1.478 x 104 kg/m3 and a metallic radius (CN = 12) of 0.170 nm (119). The room-temperature fee lattice parameter is a0 = 0.4997 0.0004 nm from which the... 

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